Gaze data of 4243 participants shows link between leftward and superior attention biases and age

Healthy individuals typically show more attention to the left than to the right (known as pseudoneglect), and to the upper than to the lower visual field (known as altitudinal pseudoneglect). These biases are thought to reflect asymmetries in neural processes. Attention biases have been used to investigate how these neural asymmetries change with age. However, inconsistent results have been reported regarding the presence and direction of age-related effects on horizontal and vertical attention biases. The observed inconsistencies may be due to insensitive measures and small sample sizes, that usually only feature extreme age groups. We investigated whether spatial attention biases, as indexed by gaze position during free viewing of a single image, are influenced by age. We analysed free-viewing data from 4,243 participants aged 5-65 years and found that attention biases shifted to the right and superior directions with increasing age. These findings are consistent with the idea of developing cerebral asymmetries with age and support the hypothesis of the origin of the leftward bias. Age modulations were found only for the first seven fixations, corresponding to the time window in which an absolute leftward bias in free viewing was previously observed. We interpret this as evidence that the horizontal and vertical attention biases are primarily present when orienting attention to a novel stimulus - and that age modulations of attention orienting are not global modulations of spatial attention. Taken together, our results suggest that attention orienting may be modulated by age and that cortical asymmetries may change with age.

Exploring bias in horizontal and vertical spatial representations using mental number lines and the greyscales task

People have a leftward bias when making visuospatial judgements about horizontally arranged stimuli ("pseudoneglect"), and a superior bias when making visuospatial judgements about vertically arranged stimuli. The leftward visuospatial bias in physical space seems to extend to the mental representation of space. However, whether any bias exists in mental representation of vertical space is unknown. We investigated whether people show a visuospatial bias in the mental representation of vertical space, and if any bias in mental representations of horizontal and vertical space related to the extent of bias in physical space. Participants (n = 171) were presented with three numbers and asked which interval was smaller/larger (counterbalanced): the interval between the first and middle, or middle and last number. Participants were instructed to either think of the numbers as houses on a street or as floors of a building, or were given no imagery instructions. Participants in the houses on a street condition showed a leftward bias, but there was no superior bias in the floors of a building condition. In contrast, we replicated previous findings of leftward and superior bias on greyscales tasks. Our findings reinforce previous evidence that numbers are represented horizontally and ascending left to right by default.

Elevated and accelerated: Locus coeruleus activity and visual search abilities in autistic children

BACKGROUND

Autistic individuals excel at visual search, however, the neural mechanism(s) underlying this advantage remain unclear. The locus coeruleus-norepinephrine (LC-NE) system, which plays a critical role in sensory perception and selective attention, has been shown to function in a persistently elevated state in individuals on the spectrum. However, the relationship between elevated tonic LC-NE activity and accelerated search in autism has not been explored.

OBJECTIVE

To examine the relationship between visual search abilities and resting pupil diameter (an indirect measure of tonic LC-NE activation) in autistic and neurotypical children.

METHODS

Participants were 24 school-aged autistic children and 24 age- and IQ-matched neurotypical children aged 8-15 years. Children completed two tasks: a resting eye-tracking task and a visual search paradigm. For the resting eye-tracking task, pupil diameter was monitored while participants fixated a central crosshair. For the visual search paradigm, participants were instructed to find the target (vertical line) embedded within an array of tilted (10°) distractor lines. The target was present on 50% of trials, and displayed within set sizes of 18, 24, and 36 items.

RESULTS

Consistent with previous studies, autistic children had significantly larger resting pupil size and searched faster and more efficiently compared to their neurotypical peers. Eye-tracking findings revealed that accelerated search was associated with fewer, not shorter, fixations in the autism group. Autistic children also showed reduced leftward search bias. Larger resting pupil size, indicative of increased tonic activation of the LC-NE system, was associated with greater search efficiency, longer fixation durations, and reduced leftward bias. Finally, within both groups reduced leftward bias was associated with increased autism symptomatology.

DISCUSSION

Together, these findings add to the existing body of research highlighting superior search in autism, suggest that elevated tonic LC-NE activity may contribute to more efficient search, and link non-social visual-spatial processing strengths to autism symptoms.

Accuracy and precision of responses to visual analog scales: Inter- and intra-individual variability

Visual analog scales (VASs) are gaining popularity for collecting responses in computer administration of psychometric tests and surveys. The VAS format consists of a line marked at its endpoints with the minimum and maximum positions that it covers for respondents to place a mark at their selected location. Creating the line with intermediate marks along its length was discouraged, but no empirical evidence has ever been produced to show that their absence does any good. We report a study that asked respondents to place marks at pre-selected locations on a 100-unit VAS line, first when it only had numerical labels (0 and 100) at its endpoints and then when intermediate locations (from 0 to 100 in steps of 20) were also labeled. The results show that settings are more accurate and more precise when the VAS line has intermediate tick marks: The average absolute error decreased from 3.02 units without intermediate marks to 0.82 units with them. Provision of intermediate tick marks also reduced substantially inter- and intra-individual variability in accuracy and precision: The standard deviation of absolute error decreased from 0.87 units without tick marks to 0.25 units with them and the standard deviation of signed distance to target decreased from 1.16 units without tick marks to 0.24 units with them. These results prompt the recommendation that the design of VASs includes intermediate tick marks along the length of the line.

Pseudoneglect during object search in naturalistic scenes

Pseudoneglect, that is the tendency to pay more attention to the left side of space, is typically assessed with paper-and-pencil tasks, particularly line bisection. In the present study, we used an everyday task with more complex stimuli. Subjects' task was to look for pre-specified objects in images of real-world scenes. In half of the scenes, the search object was located on the left side of the image (L-target); in the other half of the scenes, the target was on the right side (R-target). To control for left-right differences in the composition of the scenes, half of the scenes were mirrored horizontally. Eye-movement recordings were used to track the course of pseudoneglect on a millisecond timescale. Subjects' initial eye movements were biased to the left of the scene, but less so for R-targets than for L-targets, indicating that pseudoneglect was modulated by task demands and scene guidance. We further analyzed how horizontal gaze positions changed over time. When the data for L- and R-targets were pooled, the leftward bias lasted, on average, until the first second of the search process came to an end. Even for right-side targets, the gaze data showed an early left-bias, which was compensated by adjustments in the direction and amplitude of later saccades. Importantly, we found that pseudoneglect affected search efficiency by leading to less efficient scan paths and consequently longer search times for R-targets compared with L-targets. It may therefore be prudent to take spatial asymmetries into account when studying visual search in scenes.

A registered re-examination of the effects of leftward prism adaptation on landmark judgements in healthy people

It has long been known that active adaptation to a shift of the visual field, caused by laterally-displacing prisms, induces short-term sensorimotor aftereffects. More recent evidence suggests that prism adaptation may also stimulate higher-level changes in spatial cognition, which can modify the spatial biases of healthy people. The first reported, and most replicated, higher-level aftereffect is a rightward shift in the point of subjective equality (PSE) for a perceptual bisection task (the landmark task), following adaptation to leftward prisms. A recent meta-analysis suggests that this visuospatial aftereffect should be robustly induced by an extended period of adaptation to strong leftward prisms (15°, ∼26.8 prism dioptres). However, we have been unable to replicate this effect, suggesting that the effect size estimated from prior literature might be over-optimistic. This Registered Report compared visuospatial aftereffects on the landmark task for a 15° leftward prism adaptation group (n = 102) against a sham-adaptation control group (n = 102). The effect size for the comparison was Cohen's d = .27, 95% CI [-.01, .55], which did not pass the criterion set for significance. A Bayesian analysis indicated that the data were more than 4.1 times as likely under the null than under an informed experimental hypothesis. Exploratory analyses showed no evidence for a rightward shift of landmark judgements in the prism group considered alone, and no relationship between sensorimotor and visuospatial aftereffects. We further found no support for previous suggestions that visuospatial aftereffects are modulated by a person's baseline bias (leftward or rightward) for the landmark task. Null findings are also presented for a preliminary group of 62 participants adapted to 15° leftward prisms, and an additional group of 29 participants adapted to 10° leftward prisms. We do not rule out the possibility that leftward prisms might induce higher-level visuospatial aftereffects in healthy people, but we should be more sceptical about this claim.

Visuospatial perception is not affected by self-related information

Previous research suggests that attention is drawn by self-related information. Three online experiments were conducted to investigate whether self-related stimuli alter visuospatial perceptual judgments. In a matching task, associations were learned between labels ('Yourself'/friend/stranger's name) paired with cues. Cues were coloured outlines (Experiment 1, N = 135), geometric shapes (Experiment 2, N = 102), or coloured gradients (Experiment 3, N = 110). Visuospatial perception bias was measured with a greyscales task. Cues were presented prior to, and/or alongside greyscales. We hypothesized there would be a bias towards the self-related cue. In all experiments, we found a self-related bias in the matching task. Furthermore, there was an overall leftward visuospatial perceptual bias (pseudoneglect). However, we found anecdotal to moderate evidence for the absence of an effect of self-related cues on visuospatial perception judgments. Although self-related stimuli influence how our attention is oriented to stimuli, attention mechanisms that influence perceptual judgements are seemingly not affected by a self-bias.

Signatures of functional visuospatial asymmetries in early infancy

Adults present a large number of asymmetries in visuospatial behavior that are known to be supported by functional brain lateralization. Although there is evidence of lateralization for motor behavior and language processing in infancy, no study has explored visuospatial attention biases in the early stages of development. In this study, we tested for the presence of a leftward visuospatial bias (i.e., pseudoneglect) in 4- and 5-month-old infants using an adapted version of the line bisection task. Infants were trained to identify the center of a horizontal line (Experiment 1) while their eye gazes were monitored using a remote eye-tracking procedure to measure their potential gazing error. Infants exhibited a robust pseudoneglect, gazing leftward with respect to the veridical midpoint of the horizontal line. To investigate whether infants' pseudoneglect generalizes to any given object or is dependent on the horizontal dimension, in Experiment 2 we assessed infants' gaze deployment in vertically oriented lines. No leftward bias was found, suggesting that early visuospatial attention biases in infancy are constrained by the orientation of the visual plane in which the information is organized. The interplay between biological and cultural factors that might contribute to the early establishment of the observed leftward bias in the allocation of visuospatial attention is discussed.

Spatial asymmetries ("pseudoneglect") in free visual exploration-modulation of age and relationship to line bisection

When humans visually explore an image, they typically tend to start exploring its left side. This phenomenon, so-called pseudoneglect, is well known, but its time-course has only sparsely been studied. Furthermore, it is unclear whether age influences pseudoneglect, and the relationship between visuo-spatial attentional asymmetries in a free visual exploration task and a classical line bisection task has not been established. To address these questions, 60 healthy participants, aged between 22 and 86, were assessed by means of a free visual exploration task with a series of naturalistic, colour photographs of everyday scenes, while their gaze was recorded by means of a contact-free eye-tracking system. Furthermore, a classical line bisection task was administered, and information concerning handedness and subjective alertness during the experiment was obtained. The results revealed a time-sensitive window during visual exploration, between 260 and 960 ms, in which age was a significant predictor of the leftward bias in gaze position, i.e., of pseudoneglect. Moreover, pseudoneglect as assessed by the line bisection task correlated with the average gaze position throughout a time-window of 300–1490 ms during the visual exploration task. These results suggest that age influences visual exploration and pseudoneglect in a time-sensitive fashion, and that the degree of pseudoneglect in the line bisection task correlates with the average gaze position during visual exploration in a time-sensitive manner.

A Meta-Analysis of Line Bisection and Landmark Task Performance in Older Adults

Young adults exhibit a small asymmetry of visuospatial attention that favours the left side of space relative to the right (pseudoneglect). However, it remains unclear whether this leftward bias is maintained, eliminated or shifted rightward in older age. Here we present two meta-analyses that aimed to identify whether adults aged ≥50 years old display a group-level spatial attention bias, as indexed by the line bisection and the landmark tasks. A total of 69 datasets from 65 studies, involving 1654 participants, were analysed. In the meta-analysis of the line bisection task (n = 63), no bias was identified for studies where the mean age was ≥50, but there was a clear leftward bias in a subset where all individual participants were aged ≥50. There was no moderating effect of the participant’s age or sex, line length, line position, nor the presence of left or right cues. There was a small publication bias in favour of reporting rightward biases. Of note, biases were slightly more leftward in studies where participants had been recruited as part of a stand-alone older group, compared to studies where participants were recruited as controls for a clinical study. Similarly, no spatial bias was observed in the meta-analysis of the landmark task, although the number of studies included was small (n = 6). Overall, these results indicate that over 50s maintain a group-level leftward bias on the line bisection task, but more studies are needed to determine whether this bias can be modulated by stimulus- or state-dependent factors.

Disconnected hand avatar can be integrated into the peripersonal space

Several studies have shown that space immediately surrounding the body, or the peripersonal space is represented differently in the brain from the more distant extra-personal space. Moreover, the boundary of peripersonal space can be extended to space surrounding the tip of a tool held by the hand. However, it is not known if tools need to be connected to the body to modulate the peripersonal space. We used a line bisection task to investigate whether peripersonal space representation surrounds a virtual hand avatar that is disconnected from the body. Healthy participants conducted a line bisection task by responding with either a virtual hand avatar or a laser pointer. The to-be-bisected lines were presented either in peripersonal or extra-personal space. When the lines were placed in extra-personal space, the virtual hand avatar was presented near the line such that the hand avatar was far from participants and disconnected from their bodies. Results indicated a shift in the line bisection bias from the left to the right as the line presentation distance increased when using the laser pointer, whereas no shift in bias was observed when using the virtual hand avatar. This result indicates that objects resembling human hands presented even at a distance and disconnected from the body can be integrated into the peripersonal space, which suggests that peripersonal space representation is more flexible than previously reported.

Attentional and perceptual asymmetries in an immersive decision-making task

Pseudoneglect represents the tendency in healthy people to show a slight bias in favour of stimuli appearing in the left visual field. Some studies have shown that this leftward bias can be annulled or reserved towards a rightward bisection bias when lateral attentional biases are assessed in far space. Using an immersive simulated, ecologically valid football task, we investigated whether possible attentional and perceptual asymmetries affect sport-specific decision making. Twenty-seven sport athletes were required to judge different game situations, which involved both perceptual and attentional skills to perceive player configurations in the visual periphery. We did not find any performance differences in accuracy rate between the left and right visual field side for stimuli presented close to the screen centre in an object-detection (perception-based) and feature-recognition (attention-based) task. This result is in line with previous findings showing an absence of a left- or rightward bisection bias in far space. However, accuracy was higher for stimuli being presented at visual angles wide away from the screen centre at the left side compared to the right side of visual field. This finding cannot be explained by literature focusing on pseudoneglect in far space, but rather by previous findings on landmark judgments often showing left bias both in near and in far space. Overall, the current findings provide new perspectives on attentional and perceptual asymmetries in real-world scenarios, and different interpretations of results are discussed.

Hemispheric dominance for visuospatial attention does not predict the direction of pseudoneglect

Pseudoneglect refers to a tendency of neurologically healthy individuals to produce leftward perceptual biases during spatial tasks, which is traditionally measured using line bisection tasks. This behavioral asymmetry is often explained as a consequence of right hemispheric dominance for visuospatial attention. The present study directly tested this notion by comparing line bisection performance between left-handers with either right hemispheric dominance (RVSD, N = 40) or "atypical" left hemispheric dominance (LVSD, N = 23) for visuospatial attention as determined by fMRI. Although we expected a reversal of pseudoneglect in participants with LVSD, our results show that they equally often err to the left of the true center compared to RVSD controls (74% of LVSD participants and 80% of RVSD participants). However, the magnitude of misbisections was found to be slightly, but significantly, smaller in the LVSD subgroup.We conclude that hemispheric asymmetry for visuospatial attention is not the main determinant of pseudoneglect as is commonly thought, but rather only constitutes one of the multiple factors which (subtly) contributes to its direction and magnitude.

The reliability of pseudoneglect is task dependent

Bisection tasks that require individuals to identify the midpoint of a line are often used to assess the presence of biases to spatial attention in both healthy and patient populations. These tasks have helped to uncover a phenomenon called pseudoneglect, a bias towards the left-side of space in healthy individuals. First identified in the tactile domain, pseudoneglect has been subsequently demonstrated in other sensory modalities such as vision. Despite this, the specific reliability of pseudoneglect within individuals across tasks and time has been investigated very little. In this study, we investigated the reliability of response bias within individuals across four separate testing sessions and during three line bisection tasks: landmark, line bisection and tactile rod bisection. Strong reliability was expected within individuals across task and session. Pseudoneglect was found when response bias was averaged across all tasks, for the entire sample. However, individual data showed biases to both left and right, with some participants showing no clear bias, demonstrating individual differences in bias. Significant, cross-session within-individual reliability was found for the landmark and tactile rod bisection tasks respectively, but no significant reliability was observed for the line bisection task. These results highlight the inconsistent nature of pseudoneglect within individuals, particularly across sensory modality. They also provide strong support for the use of the landmark task as the most reliable measure of pseudoneglect.

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Reliability of pseudoneglect was assessed across 4 sessions and 3 tasks.

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The landmark task was the most reliable test for pseudoneglect across sessions.

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Responses to line bisection and tactile rod were less reliable across sessions.

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Responses to different bisection tasks in the same individuals were not reliable.

The left posterior cerebellum is involved in orienting attention along the mental number line: An online-TMS study

Although converging evidence suggests that the posterior cerebellum is involved in visuospatial functions and in the orienting of attention, a clear topography of cerebellar regions causally involved in the control of spatial attention is still missing. In this study, we aimed to shed light on this issue by using online neuronavigated transcranial magnetic stimulation (TMS) to temporarily interfere with posterior medial (Vermis lobule VII) and left lateral (Crus I/II) cerebellar activity during a task measuring visuospatial (landmark task, Experiment 1 and 2) and representational (number bisection task, Experiment 2) asymmetries in the orienting of attention. At baseline, participants showed attentional biases consistent with the literature, that is a leftward and upward bias with horizontal and vertical lines, respectively, and a leftward bias in number bisection. Critically, TMS over the left cerebellar hemisphere significantly counteracted pseudoneglect in the number bisection task, whilst not affecting attentional biases in the landmark task. In turn, TMS over the posterior vermis did not affect performance in either task. Taken together, our findings suggest that the left posterior cerebellar hemisphere (but not the posterior vermis) is a critical node of an extended brain network subtending the control of spatial attention, at least when attention needs to be allocated to an internal representational space and a certain degree of mental manipulation is required (as in the number bisection task).

The effects of hemispheric dominance, literacy acquisition, and handedness on the development of visuospatial attention: A study in preschoolers and second graders

A tendency to over-attend the left side of the space (i.e., pseudoneglect) has been repeatedly reported in Western adult populations and is supposed to reflect a right hemisphere dominance in the control of visuospatial attention. This neurobiological hypothesis has been partially challenged by growing evidence showing that pseudoneglect is profoundly triggered by cultural practices such as reading and writing habits. Accordingly, more recent theoretical accounts suggest a strict coupling between nature and nurture dimensions at the origins of such bias. To further explore this possibility, here we first administered a digitized cancellation task to right-handed Western children before and after literacy acquisition. Results showed an incremental leftward shift of attention in the cancellation of the first target and an increasing preference for a left-to-right visual search from preschoolers to second graders. Yet, despite these differences, the overall distribution of visuospatial attention was biased to the left in both groups. To explore the role of handedness in visuospatial asymmetries, we also tested a group of left-handed second graders. Results showed an impact of handedness on visuospatial performance, with an accentuated rightward-oriented visual search for left-handed children, although the overall distribution of attention was again biased to the left hemispace. Taken together, these findings do not provide support to a pure neurobiological view of visuospatial biases. Rather, our study indicates that the control of visuospatial attention is mediated by a dynamic interplay among biological (i.e., right hemisphere dominance), biomechanical (i.e., hand dominance), and cultural (i.e., reading habits) factors.

Eye dominance modulates visuospatial attention

Visuospatial attention has an inherent asymmetry: the leftward bias called pseudoneglect. In typical line bisection tasks, healthy individuals tend to judge the center of a line leftward of the true center, an effect attributed to the right hemisphere dominance in visuospatial attention. Since it has been shown that information perceived by the dominant eye strongly activates the ipsilateral visual cortex, we hypothesized that eye dominance may modulate visuospatial attention bias. Because activation of the left hemisphere induced by left eye dominance should mitigate the right hemisphere dominance in attention, we predicted that right-handed individuals with left dominant eye would show smaller amount of pseudoneglect than right-handed individuals with right dominant eye. We compared the performance at both the perceptual (Landmark) and manual line bisection task of forty right-handed healthy individuals, half of whom had a right dominant eye and the other half a left dominant eye. As predicted, the left eyed dominant group showed smaller, actually not significant pseudoneglect, which was thus greater in the right eye dominant group. The influence of eye dominance on visuospatial attention was present in the Landmark but not the manual line bisection task, in which the amount of visuospatial bias correlated with participants' degree of (right) handedness. This is the first report of the effect of eye dominance on visuospatial attention within a right-handed population. This finding, by showing the influence of eye dominance on visuospatial cognition, not only helps in better defining intact visuospatial cognition mechanism but also encourages further research to pinpoint the neural basis of such interaction.

Modulation of human visuospatial attention analysis by transcranial direct current stimulation (tDCS) in the line bisection performance

The general population shows physiologic biases in the line bisection performance for visuospatial attention, almost to the left known as pseudoneglect. Previous studies have shown that tDCS affects visuospatial attention in line bisection. This research applies tDCS over left posterior parietal cortice (P3) or right posterior parietal cortice (P4) to explore the effect on pseudoneglect. Subjects randomly were divided into five groups by stimulation distribution: (i) P3-anodal (P3A), (ii) P3-cathodal (P3C), (iii) P4-anodal (P4A), (iv) P4-cathodal (P4C), (v) sham. Participants respectively finished the post-tDCS line-bisection assignment (lines on the left/right side of the monitor (LL/LR), and lines in the center of the monitor (LC)) the same as the pre-tDCS task over the session (P3A, P3C, P4A, P4C and sham) tDCS condition. The principal findings were that P3A tDCS reduced the leftward shift in the horizontal line bisection task, as well as P4C tDCS reduced the leftward shift in LL. Sham stimulation as well as P3C and P4A stimulation didn't have systematic improvements in the line bisection tasks. Therefore, an activation-orientation model of pseudoneglect is corroborated by these findings. Activation of intact structures in the rebalance of left and right parietal cortex might impose modulating effects on tDCS.

A multivariate approach

Fundamental to the understanding of the functions of spatial cognition and attention is to clarify the underlying neural mechanisms. It is clear that relatively right-dominant activity in ventral and dorsal parieto-frontal cortex is associated with attentional reorienting, certain forms of mental imagery and spatial working memory for higher loads, while lesions mostly to right ventral areas cause spatial neglect with pathological attentional biases to the right side. In contrast, complementary leftward biases in healthy people, called pseudoneglect, have been associated with varying patterns of cortical activity. Notably, this inconsistency may be explained, at least in part, by the fact that pseudoneglect studies have often employed experimental paradigms that do not control sufficiently for cognitive processes unrelated to pseudoneglect. To address this issue, here we administered a carefully designed continuum of pseudoneglect and control tasks in healthy adults while using functional magnetic resonance imaging (fMRI). Data submitted to partial least square (PLS) imaging analysis yielded a significant latent variable that identified a right-dominant network of brain regions along the intra-occipital and -parietal sulci, frontal eye fields and right ventral cortex in association with perceptual pseudoneglect. Our results shed new light on the interplay of attentional and cognitive systems in pseudoneglect.

Improved Accuracy on Lateralized Spatial Judgments in Healthy Aging

OBJECTIVES

Healthy young adults often demonstrate a leftward spatial bias called "pseudoneglect" which often diminishes with aging. One hypothesis for this phenomenon is an age-related deterioration in right hemisphere functions (right hemi-aging). If true, then a greater rightward bias should be evident on all spatial attention tasks regardless of content. Another hypothesis is a decrease in asymmetrical hemispheric activation with age (HAROLD). If true, older participants may show reduced bias in all spatial tasks, regardless of leftward or rightward biasing of specific spatial content.

METHODS

Seventy right-handed healthy participants, 33 younger (21-40) and 37 older (60-78), were asked to bisect solid and character-letter lines as well as to perform left and right trisections of solid lines.

RESULTS

Both groups deviated toward the left on solid line bisections and left trisections. Both groups deviated toward the right on right trisections and character line bisections. In all tasks, the older participants were more accurate than the younger participants.

CONCLUSIONS

The finding that older participants were more accurate than younger participants across all bisection and trisection conditions suggests a decrease in the asymmetrical hemispheric activation of these specialized networks important in the allocation of contralateral spatial attention or spatial action intention.

Lateral biases in aesthetic and spatial location judgments: differences between tasks and native reading directions

People exhibit consistent leftward spatial biases across a variety of tasks. However, individuals with a native reading direction other than left-to-right (LTR) show an attenuation of the leftward bias. The current study used procedurally similar tasks to examine spatial ability and aesthetic preferences in LTR and right-to-left (RTL) groups. In the spatial task participants viewed a centred rectangle partially occluded by an overlapping circle and estimated the centre of the circle with a single mouse click. In the aesthetic task participants used the mouse to control a "virtual flashlight" to light images of abstract paintings in the most aesthetically pleasing way. Contrary to predictions, smaller errors were made for circles on the right and estimations were progressively less accurate as circle size increased in the spatial task. On the aesthetic task, light placements of LTR participants were biased to the left and significantly different from the slightly rightward placements of RTL participants. As predicted, when completing the aesthetics task amounts of time scanning left or right visual space were different between groups. Findings support the theory that directional scanning biases attenuate leftward lateral biases and further, the nature of the visuospatial task may vary the strength of lateral bias.

Endogenous orientation of visual attention in auditory space

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Facilitation was observed for right-sided auditory stimuli in a new visuo-audio task.

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Auditory space has dynamic nature, which adapts to changes in visual space.

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Sound localization was enhanced by visual cues.

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Crossmodal links in spatial attention were found between audition and vision.

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These findings have theoretical and translational implications for future studies.

Visuospatial attention is asymmetrically distributed with a leftward bias (i.e. pseudoneglect), while evidence for asymmetries in auditory spatial attention is still controversial. In the present study, we investigated putative asymmetries in the distribution of auditory spatial attention and the influence that visual information might have on its deployment. A modified version of the Posner task (i.e. the visuo-audio spatial task [VAST]) was used to investigate spatial processing of auditory targets when endogenous orientation of spatial attention was mediated by visual cues in healthy adults. A line bisection task (LBT) was also administered to assess the presence of a leftward bias in deployment of visuospatial attention. Overall, participants showed rightward and leftward biases in the VAST and the LBT, respectively. In the VAST, sound localization was enhanced by visual cues. Altogether, these findings support the existence of a facilitation effect for auditory targets originating from the right side of space and provide new evidence for crossmodal links in endogenous spatial attention between vision and audition.

The Influence of Focused and Sustained Spatial Attention on the Allocation of Spatial Attention

OBJECTIVE

The objective of this study was to evaluate the impact of directed and sustained attention on the allocation of visuospatial attention. Healthy people often have left lateral and upward vertical spatial attentional biases. However, it is not known whether there will be an increase in bias toward the attended portion of the stimulus when volitional spatial attention is allocated to a portion of a stimulus, whether there are asymmetrical spatial alterations of these biases, and how sustained attention influences these biases.

METHODS

We assessed spatial bias in 36 healthy, right-handed participants using a variant of horizontal and vertical line bisections. Participants were asked to focus on one or the other end of vertical or horizontal lines or entire vertical or horizontal lines, and then to bisect the line either immediately or after a 20 second delay.

RESULTS

We found a significant main effect of attentional focus and an interaction between attentional focus and prolonged viewing with delayed bisection. Focusing on a certain portion of the line resulting in a significant deviation toward the attended portion and prolonged viewing of the line prior to bisection significantly enhanced the degree of deviation toward the attended portion.

CONCLUSIONS

The enhanced bias with directed and sustained attention may be useful modifications of the line bisection test, particularly in clinical populations. Thus, future studies should determine whether prolonged viewing with delayed bisection and spatially focused attention reveals attentional biases in patients with hemispheric lesions who perform normally on the traditional line bisection test. (JINS, 2019, 25, 65-71).

Left-shifting prism adaptation boosts reward-based learning

Visuospatial cognition has an inherent lateralized bias. Individual differences in the direction and magnitude of this bias are associated with asymmetrical D2/3 dopamine binding and dopamine system genotypes. Dopamine level affects feedback-based learning and dopamine signaling asymmetry is related to differential learning from reward and punishment. High D2 binding in the left hemisphere is associated with preference for reward. Prism adaptation (PA) is a simple sensorimotor technique, which modulates visuospatial bias according to the direction of the deviation. Left-deviating prism adaptation (LPA) induces rightward bias in healthy subjects. It is therefore possible that the right side of space increases in saliency along with left hemisphere dopaminergic activity. Right-deviating prism adaptation (RPA) has been used mainly as a control condition because it does not modulate behavior in healthy individuals. Since LPA induces a rightward visuospatial bias as a result of left hemisphere modulation, and higher dopaminergic activity in the left hemisphere is associated with preference for rewarding events we hypothesized that LPA would increase the preference for learning with reward. Healthy volunteers performed a computer-based probabilistic classification task before and after LPA or RPA. Consistent with our predictions, PA altered the preference for rewarded versus punished learning, with the LPA group exhibiting increased learning from reward. These results suggest that PA modulates dopaminergic activity in a lateralized fashion.

The attentional repulsion effect and relative size judgments

Rapid shifts of involuntary attention have been shown to induce mislocalizations of nearby objects. One pattern of mislocalization, termed the Attentional Repulsion Effect (ARE), occurs when the onset of peripheral pre-cues lead to perceived shifts of subsequently presented stimuli away from the cued location. While the standard ARE configuration utilizes vernier lines, to date, all previous ARE studies have only assessed distortions along one direction and tested one spatial dimension (i.e., position or shape). The present study assessed the magnitude of the ARE using a novel stimulus configuration. Across three experiments participants judged which of two rectangles on the left or right side of the display appeared wider or taller. Pre-cues were used in Experiments 1 and 2. Results show equivalent perceived expansions in the width and height of the pre-cued rectangle in addition to baseline asymmetries in left/right relative size under no-cue conditions. Altering cue locations led to shifts in the perceived location of the same rectangles, demonstrating distortions in perceived shape and location using the same stimuli and cues. Experiment 3 demonstrates that rectangles are perceived as larger in the periphery compared to fixation, suggesting that eye movements cannot account for results from Experiments 1 and 2. The results support the hypothesis that the ARE reflects a localized, symmetrical warping of visual space that impacts multiple aspects of spatial and object perception.

Further to the right: Viewing distance modulates attentional asymmetries ('pseudoneglect') during visual exploration

Previous studies showed that the small leftward bias found in healthy humans' spatial judgments of lines ("pseudoneglect") shifts to the right with increasing distance between stimuli and observer. In this study, we investigated whether such a modulation of attentional asymmetry can also be observed in free visual exploration. Participants freely explored photographs of naturalistic scenes for 7 s in near (60 cm) and far (140 cm) space. After an initial leftward bias, followed by a compensatory rightward bias, gaze positions were significantly more leftward in near compared to far space (around 4 s from scene onset). Our results show that the modulation of attentional asymmetries by viewing distance previously reported for spatial judgments generalizes to free visual exploration, and we revealed the temporal dynamics of these asymmetries by fine-grained eye movement analysis. In contrast, an effect of viewing distance was reduced or absent when eye movements are under strong top-down control, as in systematic serial visual search (Sensitive Negelct Test). Finally, there was no effect of viewing distance in the landmark task (as also reported in a minority of other studies), suggesting that this effect may depend on specific, yet unidentified task characteristics.

The Trajectory of Pseudoneglect in Adults: A Systematic Review

Neurologically healthy adults tend to display a reliable leftward perceptual bias during visuospatial tasks, a phenomenon known as pseudoneglect. However, the phenomenon in older adults is not well understood, and a synthesis of research that examines pseudoneglect using the line bisection task, as well as other tasks, in the context of aging is lacking. The aim of the current systematic review is to integrate the available research on pseudoneglect in late adulthood, and to discuss the association between age and a bias to the left hemispace. The systematic search revealed that five different tasks have been used to examine pseudoneglect in younger and older adults, and that participants over 60 years of age have demonstrated inconsistent perceptual biases (e.g., enhanced leftward bias, suppressed leftward bias, and rightward bias). Based on current evidence, although some age-related trends in the perceptual bias can be identified within each task, no firm conclusions about the effects of age on pseudoneglect can be drawn. A number of recommendations for future research are outlined throughout the review, including use of smaller age ranges within age groups, differentiating between neurologically healthy participants and those with clinical diagnoses, continued examination of gender, and consistent use of stimuli and methods of analyses within each task to improve internal comparability.

A leftward perceptual asymmetry when judging the attractiveness of visual patterns

Perceptual judgements concerning the magnitude of a stimulus feature are typically influenced more by the left side of the stimulus than by the right side. This research examined whether the leftward bias also applies to judgements of the attractiveness of abstract visual patterns. Across four experiments participants chose between two versions of a stimulus which either had an attractive left side or an attractive right side. Experiments 1 and 2 presented artworks and experiments 3 and 4 presented wallpaper designs. In each experiment participants showed a significant bias to choose the stimulus with an attractive left side more than the stimulus with an attractive right side. The leftward bias emerged at age 10/11, was not caused by a systematic asymmetry in the perception of colourfulness or complexity, and was stronger when the difference in attractiveness between the left and right sides was larger. The results are relevant to the aesthetics of product and packaging design and show that leftward biases extend to the perceptual judgement of everyday items. Possible causes of the leftward bias for attractiveness judgements are discussed and it is suggested that the size of the bias may not be a measure of the degree of hemispheric specialization.

Pseudoneglect is maintained in aging but not in mild Alzheimer's disease: new insights from an enumeration task

Neurologically healthy young adults display a behavioral bias, called pseudoneglect, which favors the processing of stimuli appearing in the left visual field. Pseudoneglect arises from the right hemisphere dominance for visuospatial attention. Previous studies investigating the effects of normal aging on pseudoneglect in line bisection and greyscale tasks have produced divergent results. In addition, scarce systematic investigations of visual biases in dementia have been reported. The aim of the present study was to evaluate whether the leftward bias appearing during an enumeration task in young adults would be preserved in normal aging and at different stages of severity of Alzheimer's disease. In Experiment 1, young and older healthy adults showed a comparable pseudoneglect, performing better when targets appeared in the left visual field. In Experiment 2, the leftward bias was maintained in amnesic mild cognitive impairment patients (aMCI), but it vanished in mild Alzheimer's disease patients (AD). The maintenance of pseudoneglect in normal aging and in aMCI patients is consistent with compensatory phenomena involving the right fronto-parietal network, which allow maintaining the right hemisphere dominance. Conversely, the lack of pseudoneglect in the sample of AD patients likely results from a loss of the right hemisphere dominance, caused by the selective degeneration of the right fronto-parietal network. These results highlight the need of further systematic investigations of visuospatial biases along the continuum of normal and pathological aging, both for a better understanding of the changes characterizing cognitive aging and for improvements in the evaluation of neglect in Alzheimer's disease.

Biased Visuospatial Attention in Cervical Dystonia

OBJECTIVES

There is increasing evidence of non-motor, sensory symptoms, mainly involving the spatial domain, in cervical dystonia (CD). These manifestations are likely driven by dysfunctional overactivity of the parietal cortex during the execution of a sensory task. Few studies also suggest the possibility that visuospatial attention might be specifically affected in patients with CD. Therefore, we asked whether non-motor manifestations in CD might also comprise impairment of higher level visuospatial processing.

METHODS

To this end, we investigated visuospatial attention in 23 CD patients and 12 matched healthy controls (for age, gender, education, and ocular dominance). The patients were identified according to the dystonia pattern type (laterocollis vs. torticollis). Overall, participants were right-handers, and the majority of them was right-eye dominant. Visuospatial attention was assessed using a line bisection task. Participants were asked to bisect horizontal lines, using their right or left hand.

RESULTS

Participants bisected more to the left of true center when using their left hand to perform the task than when using their right hand. However, overall, torticollis patients produced a significantly greater leftward deviation than controls.

CONCLUSIONS

These data are consistent with preliminary findings suggesting the presence of biased spatial attention in patients with idiopathic cervical dystonia. The presence of an attentional bias in patients with torticollis seem to indicate that alterations of attentional circuits might be implicated in the pathophysiology of this type of CD. (JINS, 2018, 24, 23-32).

The asymmetrical effect of leftward and rightward prisms on intact visuospatial cognition

Rightward prismatic adaptation (RPA) can reduce neglect symptoms in patients whereas adaptation to leftward deviating prisms (LPA) can induce neglect-like behavior in healthy subjects. One influential anatomo-functional model of prismatic adaptation (PA) postulates that it inhibits activity of the posterior parietal cortex (PPC) contralateral to the prismatic deviation. By hypo-activating the PPC and thus eventually acting on interhemispheric balance, both LPA and RPA could possibly affect visuospatial perception in healthy subjects, however, such behavioral modulation has seldom been reported after RPA. In the light of recent evidence showing that LPA-induced visuospatial shift need time to develop we hypothesized that RPA might induce significant changes in visuospatial cognition on a longer time scale. We thus assessed the Landmark task, as well as sensorimotor aftereffects, several times over 8 h after a single session of either LPA or RPA. In agreement with previous reports, sensorimotor effects were symmetrical and long-lasting, with both LPA and RPA inducing shifts of comparable amplitudes in the direction opposite to the deviation that lasted up to 8 h. Visuospatial cognition assessed by Landmark performance, was also significantly modulated for up to 8 h, but only after LPA. Interestingly, the timing and direction of this modulation differed according to participants' baseline bias. An initial leftward bias led to a rapid, but short-lasting rightward shift, whereas an initial rightward bias led to a slower-developing and longer-lasting leftward shift. These findings shed new light on a so-far relatively overlooked feature of spatial cognition that may interact with the effect of PA: the state of the visuospatial system prior to PA should be taken into account when attempting to understand and modulate visuospatial cognition in healthy and brain-damaged populations. This highlights the need for refining current models of PA's mechanisms of action.

Reduced Pseudoneglect for Physical Space, but not Mental Representations of Space, for Adults with Autistic Traits

Neurotypical individuals display a leftward attentional bias, called pseudoneglect, for physical space (e.g. landmark task) and mental representations of space (e.g. mental number line bisection). However, leftward bias is reduced in autistic individuals viewing faces, and neurotypical individuals with autistic traits viewing 'greyscale' stimuli, suggestive of atypical lateralization of attention in autism. We investigated whether representational pseudoneglect for individuals with autistic traits is similarly atypically lateralized by comparing biases on a greyscales, landmark, and mental number line task. We found that pseudoneglect was intact only on the representational measure, the mental number line task, suggesting that mechanisms for atypical lateralization of attention in individuals with autistic traits are specific artefacts of processing physically visual stimuli.

Anticlockwise or clockwise? A dynamic Perception-Action-Laterality model for directionality bias in visuospatial functioning

Orientation bias and directionality bias are two fundamental functional characteristics of the visual system. Reviewing the relevant literature in visual psychophysics and visual neuroscience we propose here a three-stage model of directionality bias in visuospatial functioning. We call this model the 'Perception-Action-Laterality' (PAL) hypothesis. We analyzed the research findings for a wide range of visuospatial tasks, showing that there are two major directionality trends in perceptual preference: clockwise versus anticlockwise. It appears these preferences are combinatorial, such that a majority of people fall in the first category demonstrating a preference for stimuli/objects arranged from left-to-right rather than from right-to-left, while people in the second category show an opposite trend. These perceptual biases can guide sensorimotor integration and action, creating two corresponding turner groups in the population. In support of PAL, we propose another model explaining the origins of the biases - how the neurogenetic factors and the cultural factors interact in a biased competition framework to determine the direction and extent of biases. This dynamic model can explain not only the two major categories of biases in terms of direction and strength, but also the unbiased, unreliably biased or mildly biased cases in visuosptial functioning.

Social modulation of spatial judgment: The case of line bisection task

Our actions are influenced by the social context in which they are performed, specifically it has been shown that observing others' actions influences the execution of the same action. In the present study, we examined whether and to what extent observers are influenced by the presence and performance of another person in a visual spatial task, using a line bisection paradigm in which two participants performed the task in turns while sitting in front of each other. Thirty pairs of participants took part in the experiment, which was divided into a non-social and a social session. In the latter, each participant was alternately an agent (performing the task) and an observer (evaluating covertly the other's performance). Results show that the leftward bias (pseudoneglect) in the line bisection task was significantly reduced when the task was performed in the social session, although the bias (both in the non-social and in the social session) was observed only when the left hand was used. Moreover, a dissociation between performance and perception was observed: the judgment given to the other's performance (which visually deviated in the direction opposite to one's own bias due to the spatial arrangement of participants and their facing vantage points) was significantly in disagreement with one's own performance. Overall, our results demonstrate that the other's presence influences our own action during a line bisection task and that spatial judgments on other's performance can modulate our own performance, even when coordination between participants is not required. Results are discussed in relation to social influence and perspective taking in the general framework of interpersonal resonance.

Dissociation between line bisection and mental-number-line bisection in healthy adults

Healthy adults bisect visual horizontal lines slightly to the left of their true center. This bias has been termed "pseudoneglect" and is considered to reflect right hemisphere dominance in the orienting of spatial attention. A previous investigation reported a positive correlation between pseudoneglect and a corresponding negative bias towards numbers lower than the true midpoint, i.e. supposedly to the left of the midpoint, during the mental bisection of number intervals that were defined by two visual arabic digits presented one to the left and one to the right of a horizontal line (Longo and Lourenco, 2007, Neuropsychologia, 45, 7, 1400-1407). Here, studying a sample of 60 healthy participants we verified whether this correlation still holds when the endpoints of number intervals are defined verbally, i.e. with no visual-spatial cues suggesting their left-to-right arrangement. Participants bisected horizontal lines (2 cm, 10 cm and 20 cm), short number intervals (3-, 5-, 7- and 9-unit) and large number intervals (16-, 24-, 32-, 40-, 48-, 56-, and 64-unit). Pseudoneglect was observed both in line and number interval bisection, confirming the results of Longo and Lourenco (2007). Nonetheless, the study of correlations between bisection biases averaged across different line and number intervals lengths and between all possible pairings of line and number interval lengths revealed no significant or systematic pattern. During line bisection pseudoneglect increased as a function of line length while with short number intervals pseudoneglect decreased and turned into an opposite positive bias as a function of interval length. With large number intervals no linear relationship was present between bisection bias and interval length and, as in Longo and Lourenco (2007), the higher was the starting point of the number interval the larger was pseudoneglect. These results show that verbally defined number intervals are not mentally inspected with the same mechanisms that are engaged by the bisection of horizontal visual lines. This suggests that number intervals are not inherently arranged along the mental equivalent of a left-to-right oriented horizontal line. This spatial representation seems rather adopted when, as in the case of the SNARC task, "left" vs. "right" codes must be used for the selection of responses associated with numbers or when, as in the case of Longo and Lourenco (2007), the numerical material to be processed is arranged in left-to-right order.

The effect of hand movements on numerical bisection judgments in early blind and sighted individuals

Recent evidence suggests that in representing numbers blind individuals might be affected differently by proprioceptive cues (e.g., hand positions, head turns) than are sighted individuals. In this study, we asked a group of early blind and sighted individuals to perform a numerical bisection task while executing hand movements in left or right peripersonal space and with either hand. We found that in bisecting ascending numerical intervals, the hemi-space in which the hand was moved (but not the moved hand itself) influenced the bisection bias similarly in both early blind and sighted participants. However, when numerical intervals were presented in descending order, the moved hand (and not the hemi-space in which it was moved) affected the bisection bias in all participants. Overall, our data show that the operation to be performed on the mental number line affects the activated spatial reference frame, regardless of participants' previous visual experience. In particular, both sighted and early blind individuals' representation of numerical magnitude is mainly rooted in world-centered coordinates when numerical information is given in canonical orientation (i.e., from small to large), whereas hand-centered coordinates become more relevant when the scanning of the mental number line proceeds in non-canonical direction.

Non-linear effects of transcranial direct current stimulation as a function of individual baseline performance: Evidence from biparietal tDCS influence on lateralized attention bias

Transcranial direct current stimulation (tDCS) is a well-established technique for non-invasive brain stimulation (NIBS). However, the technique suffers from a high variability in outcome, some of which is likely explained by the state of the brain at tDCS-delivery but for which explanatory, mechanistic models are lacking. Here, we tested the effects of bi-parietal tDCS on perceptual line bisection as a function of tDCS current strength (1 mA vs 2 mA) and individual baseline discrimination sensitivity (a measure associated with intrinsic uncertainty/signal-to-noise balance). Our main findings were threefold. We replicated a previous finding (Giglia et al., 2011) of a rightward shift in subjective midpoint after Left anode/Right cathode tDCS over parietal cortex (sham-controlled). We found this effect to be weak over our entire sample (n = 38), but to be substantial in a subset of participants when they were split according to tDCS-intensity and baseline performance. This was due to a complex, nonlinear interaction between these two factors. Our data lend further support to the notion of state-dependency in NIBS which suggests outcome to depend on the endogenous balance between task-informative 'signal' and task-uninformative 'noise' at baseline. The results highlight the strong influence of individual differences and variations in experimental parameters on tDCS outcome, and the importance of fostering knowledge on the factors influencing tDCS outcome across cognitive domains.

Testing the inter-hemispheric competition account of visual extinction with combined TMS/fMRI

Theoretical models of visual neglect and extinction entail claims about the normal functioning of attention and parietal cortex in the healthy brain: (1) 'pseudoneglect', a commonly observed attentional bias towards left space, reflects the greater dominance of parietal cortex activity of the right versus left hemisphere; (2) the capacity to distribute attention bilaterally depends causally on the relative balance of parietal activity between the hemispheres; (3) disruption of the dominant right parietal cortex shifts this inter-hemispheric balance leftward, causing a rightward shift in attentional bias. We tested these claims using low-frequency offline transcranial magnetic stimulation (TMS) to transiently inhibit activity in the right angular gyrus/intra-parietal sulcus, followed by a visual detection task to assess changes in attentional bias, and functional magnetic resonance imaging (fMRI) to test for the predicted leftward shift in brain activity. The task required participants to covertly monitor both hemifields to detect and report the location of upcoming transient visual targets that appeared on the left, right or bilaterally. In the behavioural experiment, participants exhibited a leftward attentional bias ('pseudoneglect') at baseline, which was abolished by TMS. In the fMRI experiment, participants activated an expected network of visual, parietal and frontal cortex bilaterally during the period of covert bilateral attention. TMS shifted the relative hemispheric balance of parietal activity from right to left. The consistent direction of TMS-induced behavioural and functional change indicates a causal role for parietal inter-hemispheric balance in distributing visual attention across space.

Spatial distortions in localization and midline estimation in hemianopia and normal vision

Studies have shown that individuals with hemianopia tend to bisect a line toward their blind, contralesional visual field, termed the hemianopic line bisection error (HLBE). One theory proposes that the HLBE is a perceptual distortion resulting from expansion of the central region of visual space. If true, perceptual expansions of the central regions in the intact hemifield should also be present and observable across different tasks. We tested this hypothesis using a peripheral localization task to assess localization and midpoint estimation along the horizontal axis of the visual field. In this task, participants judged the location of a target dot presented inside a Goldmann perimeter relative to their perceived visual field boundary. In Experiment 1, we tested neurologically healthy participants on the peripheral localization task as well as a novel midpoint assessment task in which participants reported their perceived midpoint along the horizontal axis of their left and right visual fields. The results revealed consistency in individual biases across the two tasks. We then used the peripheral localization task to test whether two patients with hemianopia showed a selective expansion of central visual space. For these patients, three axes were tested: the spared temporal horizontal axis and the upper and lower vertical axes. The results support the notion that the HLBE is due to expansion of perceived space along the spared temporal axis. Together, the results of both experiments validate the use of these novel paradigms for exploring perceptual asymmetries in both healthy individuals and patients with visual field loss.

The implications of state-dependent tDCS effects in aging: Behavioural response is determined by baseline performance

Young adults typically display a processing advantage towards the left side of space ("pseudoneglect"), possibly as a result of right parietal dominance for spatial attention. This bias is ameliorated with age, with older adults displaying either no strongly lateralised bias, or a slight bias towards the right. This may represent an age-related reduction of right hemispheric dominance and/or increased left hemispheric involvement. Here, we applied anodal transcranial direct current stimulation (atDCS) to the right posterior parietal cortex (PPC; R-atDCS), the left PPC (L-atDCS) and a Sham protocol in young and older adults during a titrated lateralised visual detection task. We aimed to facilitate visual detection sensitivity in the contralateral visual field with both R-atDCS and L-atDCS relative to Sham. We found no differences in the effects of stimulation between young and older adults. Instead the effects of atDCS were state-dependent (i.e. related to task performance at baseline). Relative to Sham, poor task performers were impaired in both visual fields by anodal stimulation of the left posterior parietal cortex (PPC). Conversely, good performers maintained sensitivity in both visual fields in response to R-atDCS, although this effect was small. We highlight the importance of considering baseline task ability when designing tDCS experiments, particularly in older adults.

Native reading direction influences lateral biases in the perception of shape from shading

Although neurologically normal individuals often exhibit leftward biases of perception and attention, known as pseudoneglect, factors such as lighting, spatial location and native reading direction have been found to modulate these biases. To investigate lighting and spatial biases in left-to-right and right-to-left readers search times were measured in a target finding task where lighting and target locations were manipulated. Target search times under upper-left lighting were significantly shorter than lower-left, upper-right and lower-right lighting among left-to-right readers. Right-to-left readers did not display the same leftward bias, even displaying significantly shorter search times under upper-right lighting than those of left-to-right readers. Significantly shorter search times for targets located in the upper-left quadrant (compared to other quadrants) were observed for left-to-right readers, while search times for upper-right located targets were significantly shorter for right-to-left readers compared to those of left-to-right readers. Participant scan times of stimuli divided into equal quadrants were monitored by an eye-tracking camera. Both groups displayed greater scan times in upper quadrants. These findings suggest that native reading direction modulates spatial and light perception biases resulting in weaker leftward, or a lack of lateral biases among right-to-left readers.

A toggle switch of visual awareness?

Major clues to the human brain mechanisms of spatial attention and visual awareness have come from the syndrome of neglect, where patients ignore one half of space. A longstanding puzzle, though, is that neglect almost always comes from right-hemisphere damage, which suggests that the two sides of the brain play distinct roles. But tests of attention in healthy people have revealed only slight differences between the hemispheres. Here we show that major differences emerge if we look at the timing of brain activity in a task optimized to identify attentional functions. Using EEG to map cortical activity on a millisecond timescale, we found transient (20-30 ms) periods of interhemispheric competition, followed by short phases of marked right-sided activity in the ventral attentional network. Our data are the first to show interhemispheric interactions that, much like a toggle switch, quickly allocate neural resources to one or the other hemisphere.

The influence of emotional faces on the spatial allocation of attention

OBJECTIVES

Studies suggest that the right hemisphere is dominant for emotional facial recognition. In addition, whereas some studies suggest the right hemisphere mediates the processing of all emotions (dominance hypothesis), other studies suggest that the left hemisphere mediates positive emotions the right mediates negative emotions (valence hypothesis). Since each hemisphere primarily attends to contralateral space, the goals of this study was to learn if emotional faces would induce a leftward deviation of attention and if the valence of facial emotional stimuli can influence the normal viewer's spatial direction of attention.

METHODS

Seventeen normal right handed participants were asked to bisect horizontal lines that had all combinations of sad, happy or neutral faces at ends of these lines. During this task the subjects were never requested to look at these faces and there were no task demands that depended on viewing these faces.

RESULTS

Presentation of emotional faces induced a greater leftward deviation compared to neutral faces, independent of where (spatial position) these faces were presented. However, faces portraying negative emotions tended to induce a greater leftward bias than positive emotions.

CONCLUSIONS

Independent of location, the presence of emotional faces influenced the spatial allocation of attention, such that normal subjects shift the direction of their attention toward left hemispace and this attentional shift appears to be greater for negative (sad) than positive faces (happy).

Behavioural asymmetries on the greyscales task: The influence of native reading direction

Reliable leftward attentional and perceptual biases demonstrated in a variety of visuospatial tasks have been found to deviate from the left in research examining the influence of scanning habits. The aim of the current research was to examine the influence of native script direction on pseudoneglect during the greyscales task in a representative sample of native right-to-left readers. Fifty-four native left-to-right readers and 43 right-to-left readers completed the greyscales task, which required judging the darker of two left-right mirrored brightness gradients. Native left-to-right readers demonstrated a left response bias on the greyscales task, whereas right-to-left readers failed to demonstrate a bias, however, both groups responded more quickly when making leftward choices. The research suggests that the strength of attentional biases are influenced by behavioural biases, such as scanning habits, and neural and anatomical asymmetries in the right parietal and frontal cortices. Thus, to improve the clinical utility of the greyscales task for diagnosing neglect, right-to-left readers should be examined to fully understand the normal range of biases displayed by neurologically healthy individuals.

Behavioral and electrophysiological evidence of opposing lateral visuospatial asymmetries in the upper and lower visual fields

Neurologically healthy individuals typically exhibit a subtle bias towards the left visual field during spatial judgments, known as "pseudoneglect". However, it has yet to be reliably established if the direction and magnitude of this lateral bias varies along the vertical plane. Here, participants were required to distribute their attention equally across a checkerboard array spanning the entire visual field in order to detect transient targets that appeared at unpredictable locations. Reaction times (RTs) were faster to left hemifield targets in the lower visual field but the opposite trend was observed for targets in the upper field. Electroencephalogram (EEG) analyses focused on the interval prior to target onset in order to identify endogenous neural correlates of these behavioral asymmetries. The relative hemispheric distribution of pre-target oscillatory alpha power was predictive of RT bias to targets in the lower visual field but not the upper field, indicating separate attentional mechanisms for the upper and lower visual fields. Analysis of multifocal visual-evoked potentials (MVEP) in the pre-target interval also indicated that the opposing upper and lower field asymmetries may impact on the magnitude of primary visual cortical responses. These results provide new evidence of a functional segregation of upper and lower field visuospatial processing.

An evidence for lack of pseudoneglect in patients with schizophrenia: an ERP study

Studies have reported an altered expression of pseudoneglect in patients with schizophrenia, but no study has examined pseudoneglect in schizophrenia at the neural level. We investigated pseudoneglect using the visual P3 event-related potential and the mental number bisection (MNB) task in 21 patients and 25 controls. Using an oddball task, participants were asked to discriminate an infrequent ('one' or 'nine') from a frequent written number ('five'). The P3 ERP components were delayed to the targets on the right of the MNL ('nine') compared to the targets on the left ('one') in controls. The effect of number magnitude on the P3 latency was not observed in the patients. In MNB task, the patients did not show the normal leftward bias observed in healthy individuals. Our findings indicate a lack of pseudoneglect and the presence of an anomalous brain asymmetry in schizophrenia.

On the neural origin of pseudoneglect: EEG-correlates of shifts in line bisection performance with manipulation of line length

Healthy participants tend to show systematic biases in spatial attention, usually to the left. However, these biases can shift rightward as a result of a number of experimental manipulations. Using electroencephalography (EEG) and a computerized line bisection task, here we investigated for the first time the neural correlates of changes in spatial attention bias induced by line-length (the so-called line-length effect). In accordance with previous studies, an overall systematic left bias (pseudoneglect) was present during long line but not during short line bisection performance. This effect of line-length on behavioral bias was associated with stronger right parieto-occipital responses to long as compared to short lines in an early time window (100-200ms) post-stimulus onset. This early differential activation to long as compared to short lines was task-independent (present even in a non-spatial control task not requiring line bisection), suggesting that it reflects a reflexive attentional response to long lines. This was corroborated by further analyses source-localizing the line-length effect to the right temporo-parietal junction (TPJ) and revealing a positive correlation between the strength of this effect and the magnitude by which long lines (relative to short lines) drive a behavioral left bias across individuals. Therefore, stimulus-driven left bisection bias was associated with increased right hemispheric engagement of areas of the ventral attention network. This further substantiates that this network plays a key role in the genesis of spatial bias, and suggests that post-stimulus TPJ-activity at early information processing stages (around the latency of the N1 component) contributes to the left bias.

Prism adaptation in the healthy brain: the shift in line bisection judgments is long lasting and fluctuates

Rightward prism adaptation has been shown to ameliorate visuospatial biases in right brain-damaged patients with neglect, and a single session of prism adaptation can lead to improvements that last up to several hours. Leftward prism adaptation in neurologically healthy individuals induces neglect-like biases in visuospatial tasks. The duration of these effects in healthy individuals, typically assumed to be ephemeral, has never been investigated. Here we assessed the time-course of the adaptation-induced modifications in a classical perceptual line bisection task that was repeatedly administered for approximately 40min after a single session of adaptation to either a leftward or rightward prismatic deviation. Consistent with previous reports, only adaptation to leftward-deviating prisms induced a visuospatial shift on perceptual line bisection judgments. The typical pattern of pseudoneglect was counteracted by a rightward shift in midline judgments, which became significant between 5 and 10 min after adaptation, fluctuated between being significant or not several times in the 40 min following adaptation, and was present as late as 35 min. In contrast, the sensorimotor aftereffect was present immediately after adaptation to both rightward and leftward deviating prisms, decayed initially then remained stable until 40 min. These results demonstrate that both the sensorimotor and visuospatial effects last for at least 35 min, but that the visuospatial shift needs time to fully develop and fluctuates. By showing that the effects of prism adaptation in the undamaged brain are not ephemeral, these findings reveal the presence of another, so-far neglected dimension in the domain of the cognitive effects induced by prism adaptation, namely time. The prolonged duration of the induced visuospatial shift, previously considered to be a feature of prism adaptation unique to brain-damaged subjects, also applies to the normal brain.

Vertical line quadrisection: "what" it represents and who gets the upper hand

BACKGROUND

Normal adults demonstrate a slight upward bias (vertical pseudoneglect) when attempting vertical line bisection. The mechanism for this bias is unknown. Activation of the allocentric (object-centered) ventral visual system during attempted bisection may induce this bias. This object-centered ventral stream may also mediate focal attention. As compared to bisection, when normal participants perform a vertical line quadrisection task that requires more focal attention, they may have a greater upward bias.

METHODS

Sixteen participants bisected and quadrisected vertical lines.

RESULTS

The mean upward bias (deviation error) for the quadrisection tasks was significantly higher than the mean error for the line bisections.

CONCLUSIONS

These results are consistent with the hypothesis that activation of the ventral stream by a task that requires focal allocentric attention can induce an upward vertical bias that is greater than the upward bias observed with allocentric line bisection, a task that requires more global attention.