Spatial frequency-specific effects on the attentional bias: Evidence for two attentional systems

Mapping the anatomy of perceptual pseudoneglect. 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.

Evidence for a common mechanism of spatial attention and visual awareness: Towards construct validity of pseudoneglect

Present knowledge of attention and awareness centres on deficits in patients with right brain damage who show severe forms of inattention to the left, called spatial neglect. Yet the functions that are lost in neglect are poorly understood. In healthy people, they might produce “pseudoneglect”—subtle biases to the left found in various tests that could complement the leftward deficits in neglect. But pseudoneglect measures are poorly correlated. Thus, it is unclear whether they reflect anything but distinct surface features of the tests. To probe for a common mechanism, here we asked whether visual noise, known to increase leftward biases in the grating-scales task, has comparable effects on other measures of pseudoneglect. We measured biases using three perceptual tasks that require judgments about size (landmark task), luminance (greyscales task) and spatial frequency (grating-scales task), as well as two visual search tasks that permitted serial and parallel search or parallel search alone. In each task, we randomly selected pixels of the stimuli and set them to random luminance values, much like a poor TV signal. We found that participants biased their perceptual judgments more to the left with increasing levels of noise, regardless of task. Also, noise amplified the difference between long and short lines in the landmark task. In contrast, biases during visual searches were not influenced by noise. Our data provide crucial evidence that different measures of perceptual pseudoneglect, but not exploratory pseudoneglect, share a common mechanism. It can be speculated that this common mechanism feeds into specific, right-dominant processes of global awareness involved in the integration of visual information across the two hemispheres.

Intra- and Inter-Task Reliability of Spatial Attention Measures in Pseudoneglect

Healthy young adults display a leftward asymmetry of spatial attention ("pseudoneglect") that has been measured with a wide range of different tasks. Yet at present there is a lack of systematic evidence that the tasks commonly used in research today are i) stable measures over time and ii) provide similar measures of spatial bias. Fifty right-handed young adults were tested on five tasks (manual line bisection, landmark, greyscales, gratingscales and lateralised visual detection) on two different days. All five tasks were found to be stable measures of bias over the two testing sessions, indicating that each is a reliable measure in itself. Surprisingly, no strongly significant inter-task correlations were found. However, principal component analysis revealed left-right asymmetries to be subdivided in 4 main components, namely asymmetries in size judgements (manual line bisection and landmark), luminance judgements (greyscales), stimulus detection (lateralised visual detection) and judgements of global/local features (manual line bisection and grating scales). The results align with recent research on hemispatial neglect which conceptualises the condition as multi-component rather than a single pathological deficit of spatial attention. We conclude that spatial biases in judgment of visual stimulus features in healthy adults (e.g., pseudoneglect) is also a multi-component phenomenon that may be captured by variations in task demands which engage task-dependent patterns of activation within the attention network.

The time course for visual extinction after a 'virtual' lesion of right posterior parietal cortex

Our understanding of the attentional networks in the human brain largely relies on neuropsychological studies in patients with lesions to the posterior parietal cortex (PPC), particularly in the right hemisphere, that may cause severe disruptions of attentional functions. However, lesion studies only capture a point in time when the dysfunctions caused by the damage have triggered a chain of adaptive responses in the brain. To disentangle deficits and ensuing cortical plasticity, here we examined the time course for one's ability to detect objects in the visual periphery after an inhibitory continuous theta-burst stimulation (cTBS) protocol to the left or right PPC. Our results showed that cTBS of right PPC caused participants to be less sensitive to objects appearing on the left side as well as to objects appearing on both sides at the same time, consistent with an overall shift of attention to the right side of space. In addition, we found that participants missed more objects during bilateral presentations similar to patients with visual extinction. Critically, extinction evolved over time; that is, visual extinction for ipsilateral objects improved after 10 min whereas contralateral extinction peaked around 15-25 min after cTBS. Our findings suggest that lesions to the PPC impair competition between the two visual hemifields, resulting in contralateral extinction as a secondary response, arguably due to ensuing disruptions in interhemispheric balance.

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.

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.

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.

A right hemisphere dominance for bimanual grasps

To find points on the surface of an object that ensure a stable grasp, it would be most effective to employ one area in one cortical hemisphere. But grasping the object with both hands requires control through both hemispheres. To better understand the control mechanisms underlying this "bimanual grasping", here we examined how the two hemispheres coordinate their control processes for bimanual grasping depending on visual field. We asked if bimanual grasping involves both visual fields equally or one more than the other. To test this, participants fixated either to the left or right of an object and then grasped or pushed it off a pedestal. We found that when participants grasped the object in the right visual field, maximum grip aperture (MGA) was larger and more variable, and participants were slower to react and to show MGA compared to when they grasped the object in the left visual field. In contrast, when participants pushed the object we observed no comparable visual field effects. These results suggest that grasping with both hands, specifically the computation of grasp points on the object, predominantly involves the right hemisphere. Our study provides new insights into the interactions of the two hemispheres for grasping.

Selective modulations of attentional asymmetries after sleep deprivation

Pseudoneglect is a slight but consistent misplacement of attention toward the left visual field, commonly observed in young healthy subjects. This leftward attentional bias is thought to result from a right hemispheric dominance in visuospatial processing. Changes in endogenous levels of alertness may modulate attentional asymmetries and pseudoneglect in particular. In line with this hypothesis, it has been shown that sleep deprived shift-workers present a reversal of their attentional bias in a landmark (LDM) task (Manly, T., Dobler, V. B., Dodds, C. M., & George, M. A. (2005). Rightward shift in spatial awareness with declining alertness. Neuropsychologia, 43(12), 1721-1728). However, circadian disturbances and fatigue effects at the end of a shift work may have contributed to this reversal effect. In a first experiment, we show that sleep deprivation (SD) under controlled conditions does not markedly change the leftward bias, observable both at 21:00 and at 07:00 after SD. In a second experiment, we tested the hypothesis that a drastic reduction or inversion in the attentional bias would be present only when both the circadian drive for sleep propensity is maximal (i.e. around 05:00) and homeostatic sleep pressure is high. Thus participants were tested at 21:00 and under SD conditions at 05:00 and 09:00. Additionally, we used the greyscales (GS) task well-known to evidence a leftward bias in luminance judgments. Although results evidenced a consistent leftward bias both in the LDM and GS, we found a suppression of the leftward bias at the circadian nadir of alertness (05:00) after SD only for the GS, but not for the LDM. Noticeably, the leftward bias in the GS vanished at 05:00 after SD but reappeared at 09:00 despite continued SD, suggesting a predominant circadian influence on attentional asymmetries in the GS. Additionally, inter-sessions correlations evidenced a reproducible, consistent bias both in the LDM and GS, with no consistent relationship between the two tasks, suggesting independence of the neural networks subtending performance in LDM and GS. Overall, our results suggest that SD per se does not impede the leftward bias both in LDM and GS, whereas circadian-related variations in vigilance may impact attentional asymmetries in luminance judgments.