Pseudoneglect: a review and meta-analysis of performance factors in line bisection tasks

Space perception and body morphology: extent of near space scales with arm length

Numerous studies have found that the near space immediately surrounding the body is represented differently than more distant space. In a previous study, we found a gradual shift in attentional bias (on a line bisection task) between near and far space (Longo and Lourenco in Neuropsychologia 44:977-981, 2006). The present study concerns the possibility that arm length relates systematically to the rate at which this gradual shift between near and far space occurs. Participants bisected lines using a laser pointer at eight distances (within and beyond arm's reach), and the rate of shift was estimated by the slope of the least-squares regression line. A negative correlation was found between the slopes and arm length; participants with longer arms showed a more gradual shift in bias with increasing distance than those with shorter arms. These results suggest that, while near space cannot be considered categorically as that within arm's reach, there is a systematic relation between the extent ("size") of near space and arm length. Arm length may constitute an intrinsic metric for the representation of near space.

Rightward shift of the auditory subjective straight ahead in right- and left-handed subjects

While spatial asymmetries have been extensively investigated in the visual modality and to a lesser degree in the tactile modality, there are no reports on auditory spatial asymmetries in neurologically normal subjects. The subjects in this study performed an auditory midline task in which they had to adjust the level of two sounds that were presented simultaneously in different hemispaces so as to make the perceived fused sound appear to emanate from the midsagittal plane. The results showed that subjects reported a sound to be central when it was lateralized to the left (i.e., more intense at the left ear), as if there were a rightward shift in the perceived location. This rightward shift was more pronounced in left-handed than in right-handed subjects, even though all the subjects had to use both hands to adjust the sound level. Gaze direction significantly modulated the shift of the perceived auditory midline: the rightward shift was maximal when the subjects' gaze direction was central and 30 degrees to the right, while it disappeared when the gaze was directed 30 degrees to the left side. These results are compared with data obtained from neglect patients in previous studies and are discussed in terms of the interaction between asymmetrical hemispheric activations in auditory spatial tasks and an asymmetrical allocation of attention between the left and right hemispaces.

Influences of time and spatial frequency on the perceptual bias: Evidence for competition between hemispheres

Perceptual biases for various visual features, such as size, luminance and numerosity, have been implicated with a right-hemisphere dominance in spatial and attentional functions and/or an asymmetrical competition between the two hemispheres. However, the mechanisms underlying these biases are poorly understood. For example, it has been largely ignored that processing of those features is closely interconnected with spatial frequency filters. To probe the influence of spatial frequencies on perceptual biases, here we used a new gratingscales task in neurologically healthy participants. We found that perceptual bias was strongly influenced within a bandwidth of spatial frequencies and that this bias correlated with a bias for luminance depending on presentation time. Furthermore, our participants, divided into two subgroups of perceptually "sensitive" and "insensitive" performers, showed considerably different, presentation time-dependent patterns of perceptual bias. While both groups were biased to the left, insensitive performers more than sensitive performers, these biases varied in a mirror-symmetric manner such that one group showed peaks of bias at times when the other group showed minima and vice versa. Our data suggest that perceptual bias results from an interhemispheric competition within a right-dominant system responsive to spatial frequencies, luminance and perhaps other magnitudes including abstract ones.

Mental representation of space: Insights from an oblique distribution of hallucinations

Three-dimensional spatial distributions of hypnagogic and hypnopompic hallucinations associated with sleep paralysis were used to investigate the internal representation of space. Left-right asymmetries in human preferences and abilities are well established. Parallel effects are also observed as lower-upper asymmetries. These parallels could reflect common underlying mechanisms or additive effects of independently evolved horizontal and vertical asymmetries. This study adds to the growing literature on multidimensional spatial biases in a context free from the influence of task-related factors. We present evidence of an oblique bias in the projection of both sensory and motor hallucinations toward lower-left and especially upper-right external space exceeding that accounted for by an additive model of separate horizontal and vertical biases. These observations are consistent with theories regarding a systematic functional relation of hemispheric with ventral and dorsal cerebral organization.

Rightward bisection errors for letter lines: The role of semantic information

When bisecting words in their middle, people reveal leftward bisection errors. This tendency might emerge from an attentional bias towards the beginning of the word. However, when longer meaningless letter strings are presented, people reveal a rightward bisection bias. To test the role of semantic information on leftward or rightward bisection biases, we tested letter line bisection performance in healthy right-handed students in four independent experiments. A third of the letter lines contained an embedded four-letter word to the left of true centre, another third contained an embedded four-letter word to the right of true centre, while the remaining lines contained no words. Half of these words were emotional words, the other half were neutral words. Results across experiments revealed a stronger rightward bisection bias: (i) for letter lines containing emotional as compared to neutral words, (ii) for letter lines containing words in the left as compared to right half of the lines, and (iii) for those experiments in which the spatial position of letter lines remained within a narrow body-centred space. Findings from this study suggest that letter line bisection performance might be only minimally determined by visuo-spatial attention. Rather, letter line perception might activate the left hemisphere more than the right hemisphere, shifting the subjective midpoint to the right of true centre. Leftward bisection biases for words only, as had been described in the literature, may thus have resulted from automated reading strategies rather than from attentional biases towards the left hemispace.

Spatial attention and the mental number line: evidence for characteristic biases and compression

Numbers are often proposed to be represented spatially as lying along a mental number line. The present study examined whether the direction of spatial attention operates similarly in physical and numerical space. Participants bisected physical lines by indicating the perceived center and "bisected" the mental number line by estimating (without calculating) the number midway between two others. Healthy participants generally show a slight leftward bias (pseudoneglect) when bisecting physical lines. In the present study, pseudoneglect was also observed on mental number line bisection and, importantly, was greater for participants who showed stronger pseudoneglect on physical line bisection. This finding suggests that hemispheric asymmetries in spatial attention operate similarly in physical and numerical space. Furthermore, this bias increased with the average of the numbers, consistent with the proposal that the spatial representation of the mental number line is nonlinearly compressive, with pairs of numbers lying closer together as their magnitude increases.

Developmental changes in visual line bisection in women throughout adulthood

It has been suggested that some hemispheric asymmetries change in a systematic way from young adulthood to older age. However, little is known whether these changes are due to differential aging of a single hemisphere or based on age-related alterations of interhemispheric interactions. A sample of 281 right-handed neurologically healthy participants (151 women), ranging from age 20 to 79, was investigated with a line-bisection task. Previous studies indicate the midpoint estimation shows a consistent leftward bias from the veridical center, which is accentuated when the left hand is used to bisect lines. These findings support the view of a right hemispheric superiority in spatial attention. This study revealed this pattern to be stable in men throughout adulthood. However, women from 50 to 69 years of age showed a reduced leftward bias and a reduced hand effect compared to men and younger women. The results suggest that developmental changes in hemispheric asymmetry of spatial attention are more pronounced in women and support the view that neuromorphological changes during adulthood differ between sexes.

Callosal neglect in hydrocephalus

A functional disconnection of the corpus callosum (CC) can induce a form of spatial neglect where each hand (e.g., left) when attempting to bisect lines in the opposite (e.g., right) hemispace deviates toward its own (e.g., left) hemispace. Patients with hydrocephalus often show thinning of the CC but callosal neglect has not been reported in this condition. Two right-handed patients with hydrocephalus and thinning of the CC, as well as six matched controls, were assessed for neglect by performing the line bisection task in left, right and center space with their right and left hands. When compared to controls neither patient, using either their right or left hands, demonstrated a bias in the center or left space conditions, but with lines in right space both subjects' left hand deviated significantly to the left. Thus, patients with hydrocephalic interhemispheric functional disconnection might show a form of callosal neglect. This hemispatial-hand asymmetry of deviation, however, also might be related to the disinhibition of the attentionally dominant right hemisphere.

Postictal But Not Interictal Hemispatial Neglect in Patients with Seizures of Lateralized Onset

PURPOSE

Unilateral spatial neglect, defined as a failure to report, respond, or orient to stimuli that are presented contralaterally, has been widely documented after brain damage to right, and to a lesser degree, left frontotemporoparietal networks. Group studies involving patients with seizures with a lateralized focus have demonstrated transient dysfunctions in memory and language; however, so far, only two case reports have described transient neglect after an epileptic seizure.

METHODS

To assess the existence and consistency of this phenomenon, we evaluated 33 epilepsy patients on a line-bisection task in interictal and postictal states as compared with an age- and sex-matched control group.

RESULTS

Spatial neglect, as determined by this test, was found in the postictal but not interictal examination in patients with right parietal epileptic foci and was maximal for the left-positioned lines, whereas no neglect was found in other groups.

CONCLUSIONS

Our findings indicate that patients with right parietal foci can present a transient neglect phenomenon on the line-bisection task in the postictal period, even in the absence of overt clinical neglect signs. These findings might be useful in establishing the laterality and even localization of epileptic foci based on the postictal neuropsychological evaluation.

Distance perception in a natural outdoor setting: is there a developmental trend to overconstancy?

The main purpose of the present study was to investigate whether in natural environment, using very large physical distances, there is a trend to overconstancy for distance estimates during development. One hundred and twenty-nine children aged 5 to 13 years old and twenty-one adults (in a control group), participated as observers. The observer's task was to bisect egocentric distances, ranging from 1.0 to 296.0 m, presented in a large open field. The analyses focused on two parameters, constant errors and variable errors, such as measuring accuracy and precision, respectively. A third analysis focused on the developmental pattern of shifts in constancy as a function of age and range of distances. Constant error analysis showed that there are two relevant parameters for accuracy, age, and range of distances. For short distances, there are three developmental stages: 5-7 years, when children have unstable responses, 7-11, underconstancy, and 13 to adulthood, when accuracy is reached. For large distances, there is a two-stage development: 5-11 years, with severe underconstancy, and beyond this age, with mild underconstancy. Variable errors analyses indicate that precision is noted for 7 year-old children, independently of the range of distances. The constancy analyses indicated that there is a shift from constancy (or slightly overconstancy) to underconstancy as a function of physical distance for all age groups. The age difference is noted in the magnitude of underconstancy that occurs in larger distances, where adults presented lower levels of underconstancy than children. The present data were interpreted as due to a developmental change in cognitive processing rather than to changes in visual space perception.

Fragments of the Roelofs effect: A bottom-up effect equal to the sum of its parts

The Roelofs effect is a distortion of perceived space that occurs when a large frame whose center is offset left or right from the objective midline is presented visually to an observer and causes a bias in the observer's subjective judgment of midline. Experiments were designed to test whether an isolated fragment (left or right end) of a Roelofs-inducing frame was capable of generating the Roelofs effect and to determine whether prior experience with intact frames would provide a top-down influence that would bias the Roelofs effect resulting from fragment presentation. Although the fragments did induce an effect, top-down information did not play a significant role even after a 5-day training paradigm. Instead, we found that the effect generated by an intact frame was equal in magnitude to the sum of the effects generated by the individual fragments. In addition, perception was found to be differentially affected by the two ends of the frame, with fragments falling in the right visual field causing a larger effect than those falling in the left.

Alpha-band electroencephalographic activity over occipital cortex indexes visuospatial attention bias and predicts visual target detection

Covertly directing visual attention toward a spatial location in the absence of visual stimulation enhances future visual processing at the attended position. The neuronal correlates of these attention shifts involve modulation of neuronal "baseline" activity in early visual areas, presumably through top-down control from higher-order attentional systems. We used electroencephalography to study the largely unknown relationship between these neuronal modulations and behavioral outcome in an attention orienting paradigm. Covert visuospatial attention shifts to either a left or right peripheral position in the absence of visual stimulation resulted in differential modulations of oscillatory alpha-band (8-14 Hz) activity over left versus right posterior sites. These changes were driven by varying degrees of alpha-decreases being maximal contralateral to the attended position. When expressed as a lateralization index, these alpha-changes differed significantly between attention conditions, with negative values (alpha_right < alpha_left) indexing leftward and more positive values (alpha_left < or = alpha_right) indexing rightward attention. Moreover, this index appeared deterministic for processing of forthcoming visual targets. Collapsed over trials, there was an advantage for left target processing in accordance with an overall negative bias in alpha-index values. Across trials, left targets were detected most rapidly when preceded by negative index values. Detection of right targets was fastest in trials with most positive values. Our data indicate that collateral modulations of posterior alpha-activity, the momentary bias of visuospatial attention, and imminent visual processing are linked. They suggest that the momentary direction of attention, predicting spatial biases in imminent visual processing, can be estimated from a lateralization index of posterior alpha-activity.

Exaggerated leftward bias in the mental number line of patients with schizophrenia

Several visuo-motor tasks can be used to demonstrate biases towards left hemispace in schizophrenic patients, suggesting a minor right hemineglect. Recent studies in neglect patients used a new number bisection task to highlight a lateralized defect in their visuo-spatial representation of numbers. To test a possible lateralized representational deficit in schizophrenia, we used the number bisection task in 11 schizophrenic patients compared to 11 healthy controls. Participants were required to orally indicate the central number of an interval orally presented. Whereas healthy subjects showed no significant bias, schizophrenic patients presented a significant leftward bias. Therefore, these results suggest an impairment in higher order representations of the number space in patients with schizophrenia, an impairment that is qualitatively similar to the deficit described in neglect patients.

Paired pulse TMS over the right posterior parietal cortex modulates visuospatial perception

OBJECTIVE

We previously observed a relative contralateral neglect by right parietal single-pulse TMS given 150 ms after visual stimulus presentation. Here we investigated the effects of parietal paired TMS in normal subjects performing a visuospatial task.

METHODS

Thirteen right-handed healthy subjects underwent a line-length judgement task during single-pulse and paired (1, 3, 5, 10 ms ISIs) TMS, delivered on the right parietal cortex 150 ms after visual stimulus.

RESULTS

Single pulse TMS over the right parietal cortex induced a significant rightward bias compared to the baseline condition. At 1 and 3 ms ISIs, paired-pulse TMS did not show any effect in comparison with single pulse TMS. More importantly, 5 ms ISI was able to restore baseline levels, thus inducing a significant improvement of the performance compared to single-pulse TMS and 1-3 ms ISIs.

CONCLUSIONS

Paired TMS seems able to modulate activity of the right posterior parietal cortex in healthy subjects performing a cognitive visuospatial task.

Patients with horizontal gaze palsy and progressive scoliosis due to ROBO3 E319K mutation have both uncrossed and crossed central nervous system pathways and perform normally on neuropsychological testing

BACKGROUND

Horizontal gaze palsy and progressive scoliosis (HGPPS) is caused by mutations of the ROBO3 gene, which encodes a receptor associated with axonal guidance during development. Although there is evidence for uncrossed cuneatal and corticospinal tracts in HGPPS, it is unclear whether other central nervous system pathways are involved.

OBJECTIVE

To study two patients with HGPPS homozygotic for the ROBO3 E319K mutation using a variety of neurophysiological and neuropsychological tests.

METHODS

A battery of neuropsychological tests was applied to assess various cognitive and perceptual functions. The corticospinal, somatosensory and auditory pathways were evaluated using appropriate neurophysiological tests. To access motor pathways to the neck muscles, electromyographic recordings were obtained from the sternocleidomastoideus and splenius capitis muscle during active head rotation.

RESULTS

Both patients performed normally on manual dexterity, complex sensory and visuospatial functions, reading and general intelligence tests. Motor evoked potentials in both patients showed uncrossed corticospinal tracts for the extremities, although in one patient, electromyography indicated pyramidal tract crossing for the neck muscles. Although somatosensory evoked potentials showed uncrossed somatosensory fibres subserving proprioception and light touch, right median nerve somatosensory evoked potential in one patient indicated a partial lemniscal crossing. Sympathetic skin response and blink reflex showed a midline crossing of the spinothalamic and quintothalamic tracts. Brain stem auditory evoked potentials indicated a lack of crossing in the level of the trapezoid body.

CONCLUSIONS

Our patients with the ROBO3 E319Kappa mutation show normal perceptual and cognitive functions and have both crossed and uncrossed motor, sensory and auditory pathways.

Pseudoneglect: Evidence for both perceptual and attentional factors

When neurologically normal individuals bisect a horizontal line as accurately as possible, they reliably show a slight leftward error. This leftward inaccuracy is called pseudoneglect because errors made by neurologically normal individuals are directionally opposite to those made by persons with visuospatial neglect (Jewell & McCourt, 2000). In the current study, normal right-handed observers bisected horizontal lines that were altered to bias line length judgments either toward the right or the left side of the line. Non-target dots were placed on or near the line stimuli using principles derived from a theory of visual illusions of length called centroid extraction (Morgan, Hole, & Glennerster, 1990). This theory argues that the position of a visual target is calculated as the mean position of all stimuli in close proximity to the target stimulus. We predicted that perceptual alterations that shifted the direction of centroid extraction would also shift the direction of line bisection errors. Our findings confirmed this prediction and support the idea that both perceptual and attentional factors contribute to the pseudoneglect effect.

Schizophrenia and rightward bias in line bisection

This study investigated line bisection in 10 males with schizophrenia and 15 controls. There was an overall leftward bias, consistent with slight right "pseudoneglect", but the schizophrenia group showed more variation with experimental conditions, suggesting impaired interhemispheric transfer. Specifically, the rightward bias was especially marked when the lines were positioned on the right side of the page, when the right hand was used, and when a right to left scan was adopted. The rightward bias was associated with the predominance of negative symptoms. Findings are discussed with reference to the role of the corpus callosum in the transfer of spatial attention.

Radial Character-line Bisection

BACKGROUND AND OBJECTIVES

With conventional solid radially oriented-line bisections (SLB), normal subjects tend to bisect distal to the true midpoint (distal bias). This bias is thought to reflect a distal attentional bias in the radial plane. Whereas the right hemisphere, which mediates distributed-global attention, has a distal bias, the left hemisphere, which mediates focal attention, has a proximal bias. When performing conventional radial-line bisections, participants might primarily use right hemisphere-mediated global-distributed attention and thus develop a distal bias. The character-line bisection (CLB) task consists of strings of target and nontarget characters (letters or symbols) that are linearly aligned, and subjects are required to bisect a character that is closest to the midline. Thus, when performing the CLB subjects must use focal attention and detect symbols and both these activities are mediated by their left hemisphere. This left hemisphere activation might either reduce or reverse the distal bias observed on the SLB. The purpose of this study is to test this hypothesis.

METHODS

Fifteen normal subjects performed CLB (letter-line and symbol-line) and SLB tasks.

RESULTS

Overall, the subjects revealed a distal bias in both tasks, but the attempted bisections in the CLB tasks were more proximal than those in the SLB tasks.

CONCLUSIONS

These result are consistent with the hypothesis that the CLB task, which requires both focal attention and the detection of symbols, activates the left hemisphere that has a greater proximal bias than the right hemisphere, thus reducing the distal bias observed with the SLB task.

Putting attention on the line: investigating the activation-orientation hypothesis of pseudoneglect

Neurologically healthy participants systematically misbisect horizontal lines to the left of centre, a phenomenon termed 'pseudoneglect'. According to the activation-orientation hypothesis, the distribution of attention is biased in the direction opposite to the more activated hemisphere. Since visuospatial tasks involve activation of the right hemisphere, the hypothesis suggests that a leftward line-bisection bias might be explained by the uneven distribution of attention to the left and right line segments. A crucial assumption of this explanation is that the more attended half of the line will be perceived as longer than the less attended half. This study uses a tachistoscopic Landmark test and an attention cueing paradigm to explore this assumption. Three conditions were met to demonstrate the relative elongation of the more attended half of the line: (1) attention was biased to the cued end of the line, (2) subjective line midpoint was shifted towards the cued end, and (3) alternative biasing factors were ruled out. The results also demonstrate that increased hemispheric activation, resulting from presentation of stimuli in one or the other visual field, leads to subjective midpoints that are biased away from the more activated hemisphere.

Disturbed line bisection is associated with posterior brain lesions

Neglect patients classically fail to orient and respond to stimuli appearing on their contralesional side. Traditionally, the neglect syndrome has been associated with damage to the right inferior parietal lobule (IPL) and the right temporo-parietal junction (TPJ). Neglect is popularly assessed by two different tasks: line bisection and cancellation. In a previous study (S. Ferber, H.-O. Karnath, How to assess spatial neglect-line bisection or cancellation tasks. J. Clin. Exp. Neuropsychol. 23 (2001) 599-607), we observed that performance on the cancellation task correlates well with the characteristic behavioral disorders used to clinically diagnose spatial neglect, while line bisection was a poor predictor. This might indicate that the disability to correctly bisect lines is a distinct disorder separable from spatial neglect. Here, we assess the anatomy of the patients investigated in that study, and reveal that damage to the temporo-occipital junction correlates with poor performance in the line bisection task. This work extends previous work by Binder et al. (J. Binder, R. Marshall, R. Lazar, J. Benjamin, J.P. Mohr, Distinct syndromes of hemineglect. Arch. Neurol. 49 (1992) 1187-1194) suggesting that line bisection and cancellation identify distinct syndromes. The data suggest that these two tasks dissociate both in terms of behavior and anatomy. This anatomical distinction may help reconcile our recent finding that spatial neglect is associated with damage to the superior temporal cortex and insula, while others have identified the IPL and TPJ. Specifically, we note that our previous anatomical studies did not use the line bisection task to select neglect patients, while many others used this task. We suggest that anatomical studies that combine patients from both of these two distinct groups may result in misleading findings.

How to choose a seat in theatres: Always sit on the right side?

Right seat-side preference in a cinema has been explained by suggesting that it allows for emotional information processing by the right hemisphere (Karev, 2000). In order to determine seat-side preferences, participants in our experiments had to choose seats on cinema, theatre, and restaurant maps. The maps varied with respect to cinema screen, theatre stage, and restaurant entrance positions as viewed by the participants: on top, to the right, to the left, and at the bottom. Seat-side preference was found to be modified by the map's arrangement: With screen, stage, or entrance on top, participants preferred seats to the right of the screen, stage, or entrance, while for the other positions the results were not conclusive. Based on these results it cannot be concluded that the preference for a cinema seat is solely mediated by the preparation for an upcoming event to optimise emotional processing by the right hemisphere. Rather, seat choice seems to be determined by basic behavioural tendencies, such as which side to turn to when entering a room, or by visual attentional orientation.

Parameter-based assessment of spatial and non-spatial attentional deficits in Huntington's disease

A major challenge for neuropsychological research on Huntington's disease is the identification of biomarkers for the disease at the level of cognitive functions. Given that cortical-striatal-thalamic circuits are particularly vulnerable, possible markers loading functionally on these brain regions should be particularly significant. We investigated whether parametric values derived from a 'theory of visual attention' (TVA) can serve that purpose. They are derived as mathematically independent, quantitative measures of attentional components, and the tasks require only non-speeded vocal responses. As such, the methodology seems well suited for testing patients with motor problems and general cognitive decline. Accumulating neuroanatomical evidence suggests that striatal atrophy in Huntington's disease is asymmetrical with a more pronounced left-sided degeneration. We applied a partial-report paradigm to analyse whether this results in a pathological (leftward) bias of the spatial distribution of attention. In partial report, red target letters are presented either alone or accompanied by either a second target or a green distractor letter presented in the same or in the opposite hemi-field. Since basal ganglia lesions have also been shown to cause spatially non-lateralized impairments, that is, reduced perceptual processing speed and visual working memory (WM) storage capacity within both hemi-fields, we tested possible reductions in these parameters with a whole-report paradigm. Here, columns of five red or green letters are briefly presented and the subject has to report as many as possible. Eighteen patients and 18 matched control subjects performed a partial- and a whole-report task with briefly presented letter displays. In partial report, Huntington's disease patients demonstrated a pathological bias, indicating increased attentional weighting to the left hemi-field. The extent of lateralization was strongly related to age at onset and to the number of cytosine-adenine-guanine (CAG) triplet repeats on gene IT15. In contrast, the extent of lateralization was not related to disease progression as reflected by the duration of the disease since onset of the first symptoms. In whole report, the non-lateralized attentional parameters processing speed and visual WM storage capacity were reduced bilaterally in both hemi-fields. The extent of the reduction was related to the disease duration since onset, whereas no significant correlation with CAG repeats or age at onset was found. Laterality of attentional weighting may, therefore, represent a possible trait marker reflecting the intensity of the pathogenic mechanisms, while the reduction of visual processing speed and storage capacity may be state markers for the stage of disease progression.

Peripersonal spatial attention in children with spina bifida: associations between horizontal and vertical line bisection and congenital malformations of the corpus callosum, midbrain, and posterior cortex

Horizontal and vertical line bisection was studied in 129 children and adolescents between 8 and 19 years of age, one group (n=32) of typically developing controls and one group (n=97) with spina bifida (SBM), a neurodevelopmental disorder associated with dysmorphology of the corpus callosum, posterior cortex, and midbrain. For each participant, structural brain MRIs were analyzed qualitatively to identify beaking of the midbrain tectum and corpus callosum agenesis and hypoplasia and quantitatively by segmentation and volumetric analyses of regional cortical white and gray matter. Each group showed the line length effect, whereby greater estimation errors are made with longer lines. The group with SBM differed from controls in terms of both accuracy and variability of line bisection. Children with SBM showed pseudoneglect, attending more than controls to left hemispace. The extent of rightward bisection bias was unrelated to right posterior brain volumes, although an intact corpus callosum during development moderated and normalized the exaggerated leftward line bisection bias. More children with SBM than controls attended to inferior hemispace. A normal midbrain and greater posterior cortex volume during development moderated and normalized the downward bias. Children with SBM showed more intra-subject variability than controls. Line bisection in children with SBM reflects three deficits: an exaggerated attentional bias to left hemispace, an abnormal attentional bias to inferior hemispace; and a larger zone of subjective uncertainty in bisection judgments.

A stimulus-dependent dissociation between the cerebral hemispheres under free-viewing conditions

Under free-viewing conditions, the leftward stimulus features are frequently overestimated (pseudoneglect). This asymmetry could reflect the operation of: (a) spatial attention mechanisms in the right hemisphere (RH) or, (b) stimulus-specific activation. To test these propositions, we attempted to induce a stimulus-specific dissociation between the hemispheres under free-viewing conditions. In two experiments (n=23, n=22) dextrals carried out two tasks. The 'grayscales' task required luminance judgments between two mirror-reversed luminance gradients. The flicker task required temporal frequency judgments between two mirror-reversed temporal gradients. The grayscales and flicker tasks suited the processing styles of the right and left hemispheres, respectively. For both experiments, a strong leftward bias was observed for the grayscales task, which could be the result of both of the mechanisms outlined above. In Experiment 1, there was a rightward bias for the flicker task-but only for participants with longer reaction times. In Experiment 2, where all responses were delayed, a rightward bias was found for the flicker task for shorter stimuli. The data demonstrate that stimulus-specific dissociations can be induced under free-viewing conditions. However, the fact that the rightward bias was: (a) weaker than the leftward bias and, (b) reduced by increases in length, demonstrates that RH spatial attention mechanisms are also important.

The Right Arm Likes to be Close

A prior study revealed that the left (vs. right) hemisphere has a visual attentional bias toward the body, a finding that is consistent with the observation that most of the visual cognitive activities performed by the left hemisphere (e.g., reading) take place in proximal (peripersonal) space. In addition, when right-handed people perform bimanual motor tasks their right hand is often held more proximal to the body than their left hand. In this study, we wanted to test the hypothesis that the right hand is also more biased toward the body than is the left hand. Thus, we had blindfolded right-handed subjects attempt to draw straight horizontal lines (at the junction of the transverse and coronal planes), with their left and right hands, on sheets of paper placed in front of them and then measured their deviation from the coronal plane. We found that these subjects' right hands deviated closer to their body than did their left hands. These results support the postulate that in addition to an attentional bias, the left hemisphere-right hand has a proximal motor bias. This bias might account for means by which right-handed people position their hands when using both hands on tools and implements.

The Effect of Body and Environment-Centred Coordinates on Free-Viewing Perceptual Asymmetries for Vertical and Horizontal Stimuli

Leftward and upward perceptual biases are commonly reported for horizontal and vertical stimuli, respectively. It is unclear, however, whether these biases are based upon body or environment-centred coordinates. Two experiments examined the contribution of these coordinate systems to free-viewing vertical and horizontal perceptual biases. In Experiment 1, normal participants (n = 35) made forced-choice luminance judgments on two mirror-reversed luminance gradients (the 'greyscales' task) presented in vertical and horizontal orientations. Body and environment-based coordinates were dissociated by tilting participants' heads to the left or right. A leftward and upward bias, which was observed in the horizontal and vertical conditions (respectively) when the head was upright, was extinguished when the head was tilted. Results indicated a dual reliance on body and environmental coordinates with some suggestion that the upward bias was more dependent on environmental coordinates. In Experiment 2 the same stimuli were presented as participants (n = 24) adopted an upright or supine pose. Once again, leftward and upward biases were observed in the upright condition. The leftward bias persisted in the supine condition whereas the upward bias was eliminated. Results demonstrate that the leftward bias is based predominantly on body coordinates whereas the upward bias is reliant on environmental/gravitational coordinates. The possibility that the neural basis for the biases lies in the inter-modal centres of the intraparietal region of the right hemisphere is discussed.

Unilateral Neglect is not Unilateral: Evidence for Additional Neglect of Extreme Right Space

Six patients with visuospatial neglect following right hemisphere lesions were given three tasks that assessed performance in areas of space ranging from extreme left to extreme right. A line bisection task required the patients to detect and bisect lines of four different lengths at seven left-right spatial locations, a number report task required the patients to name 11 two-digit numbers in a left-right array, and a tiling task required patients to place small black tiles over the black squares of a grid that stretched from 65 degrees left to 65 degrees right. Performance was compared with that of 20 age-matched controls. The patients showed the characteristic signs of left-side neglect in left space, extending to the central midline. Performance was relatively normal in centre-right space but all 6 patients showed signs of neglect of extreme right space (60 degrees to the right of the midline and beyond). We propose that neglect is best characterised as a bilateral, asymmetrical compression of experienced space in which the constriction extends further from the left than from the right but nevertheless affects both sides of space.

Parietal rTMS distorts the mental number line: Simulating ‘spatial’ neglect in healthy subjects

Patients with left-sided visuospatial neglect, typically after damage to the right parietal lobe, show a systematic bias towards larger numbers when asked to bisect a numerical interval. This has been taken as further evidence for a spatial representation of numbers, perhaps akin to a mental number line with smaller numbers represented to the left and larger numbers to the right. Previously, contralateral neglect-like symptoms in physical line bisection have been induced in healthy subjects with repetitive transcranial magnetic stimulation (rTMS) over right posterior parietal lobe. Here we used rTMS over parietal and occipital sites in healthy subjects to investigate spatial representations in a number bisection task. Subjects were asked to name the midpoint of numerical intervals without calculating. On control trials subjects' behaviour was similar to performance reported in physical line bisection experiments. Subjects underestimated the midpoint of the numerical interval. Repetitive transcranial magnetic stimulation produced representational neglect-like symptoms in number bisection when applied over right posterior parietal cortex (right PPC). Repetitive TMS over right PPC shifted the perceived midpoint of the numerical interval significantly to the right while occipital TMS had no effect on bisection performance. Our study therefore provides further evidence that subjects use spatial representations, perhaps akin to a mental number line, in basic numerical processing tasks. Furthermore, we showed that the right posterior parietal cortex is crucially involved in spatial representation of numbers.

The evolutionary psychology of left and right: Costs and benefits of lateralization

Why do the left and right sides of the vertebrate brain play different functions? Having a lateralized brain, in which each hemisphere carries out different functions, is ubiquitous among vertebrates. The different specialization of the left and right side of the brain may increase brain efficiency--and some evidence for that is reported here. However, lateral biases due to brain lateralization (such as preferences in the use of a limb or, in animals with laterally placed eyes, of a visual hemifield) usually occur at the population level, with most individuals showing similar direction of bias. Individual brain efficiency does not require the alignment of lateralization in the population. Why then are not left--and right-type individuals equally common? Not only humans, but most vertebrates show a similar pattern. For instance, in the paper I report evidence that most toads, chickens, and fish react faster when a predator approaches from the left. I argue that invoking individual brain efficiency (lateralization may increase fitness), evolutionary chance or direct genetic mechanisms cannot explain this widespread pattern. Instead, using concepts from mathematical theory of games, I show that alignment of lateralization at the population level may arise as an "evolutionarily stable strategy" when individually asymmetrical organisms must coordinate their behavior with that of other asymmetrical organisms. Thus, the population structure of lateralization may result from genes specifying the direction of asymmetries which have been selected under "social" pressures.

Examining the relationship between rightward visuo-spatial bias and poor attention within the normal child population using a brief screening task

BACKGROUND

Some previous studies have linked Attention Deficit Hyperactivity Disorder (ADHD) with a bias in spatial awareness away from the left. As genetic research suggests that ADHD may be better viewed as an extreme on a continuum rather than a distinct entity, here we examined this issue in boys from the normal population.

METHOD

From an initial sample of 1811, two groups of boys characterised by very high (n = 58) or very low (n = 68) levels of ADHD-type behaviours were formed. The groups completed the spatially sensitive Line Bisection test and more general measures of (non-spatial) attention and intellectual function.

RESULTS

Boys whose bisections were consistent with relative inattention to the left indeed had higher ratings of ADHD-type behaviours and performed significantly more poorly on tests of sustained attention and executive function than boys whose bisections were in the normal range. In contrast, boys who showed extreme bisections in the opposite direction were not unusual either in ratings or test performance.

CONCLUSIONS

The results support an association between poor attention and a relative rightward bias in visual awareness that may stem from right hemisphere inefficiency.

Weights and measures: A new look at bisection behaviour in neglect

Horizontal line bisection is a ubiquitous task in the investigation of visual neglect. Patients with left neglect typically make rightward errors that increase with line length and for lines at more leftward positions. For short lines, or for lines presented in right space, these errors may 'cross over' to become leftward. We have taken a new approach to these phenomena by employing a different set of dependent and independent variables for their description. Rather than recording bisection error, we record the lateral position of the response within the workspace. We have studied how this varies when the locations of the left and right endpoints are manipulated independently. Across 30 patients with left neglect, we have observed a characteristic asymmetry between the 'weightings' accorded to the two endpoints, such that responses are less affected by changes in the location of the left endpoint than by changes in the location of the right. We show that a simple endpoint weightings analysis accounts readily for the effects of line length and spatial position, including cross-over effects, and leads to an index of neglect that is more sensitive than the standard measure. We argue that this novel approach is more parsimonious than the standard model and yields fresh insights into the nature of neglect impairment.

Rightward biases in free-viewing visual bisection tasks: implications for leftward responses biases on similar tasks

Neurologically normal individuals exhibit strong leftward response biases during free-viewing perceptual judgments of brightness, quantity, and size. When participants view two mirror-reversed objects and they are forced to choose which object appears darker, more numerous, or larger, the stimulus with the relevant feature on the left side is chosen 60-75% of the time. This effect could be influenced by inaccurate judgments of the true centre-point of the objects being compared. In order to test this possibility, 10 participants completed three visual bisection tasks on stimuli known to elicit strong leftward response biases. Participants were monitored using a remote eye-tracking device and instructed to stare at the subjective midpoint of objects presented on a computer screen. Although it was predicted that bisection errors would deviate to the left of centre (as is the case in the line bisection literature), the opposite effect was found. Significant rightward bisection errors were evident on two of the three tasks, and the leftward biases seen during forced-choice tasks could be the result of misjudgments to the right of centre on these same tasks.

Interactions between lateralized choices of hand and target

Few studies have examined interactions between hand preference and target laterality preference when subjects are permitted unrestricted choices of both hand and target at the same time. We therefore trained three monkeys to perform a task in which different types of choices were presented in different trials. In some trials the monkey chose between targets presented simultaneously to the right and left, while being required to use a specific hand. In these trials, the monkeys preferentially chose the target ipsilateral to the required hand. In other trials, the monkey chose between its right or left hand to retrieve a single target presented either on the right or on the left. In these trials, each monkey showed a consistent pattern of hand preference whether the target was presented on the right or left. In still other trials, the monkey chose both which hand to use and which target to retrieve. In these trials, the choices made by each monkey reflected its individual pattern of hand preference, plus a tendency to choose the target ipsilateral to the preferred hand, with little additional evidence of preference for target laterality. These patterns of laterality preference were similar regardless of whether a cue instructing which hand(s) the monkey could use preceded or followed a cue indicating on which side(s) the target(s) would appear. If the monkeys were permitted to use a strategy in which retrieving the target on a given side could increase the amount of food retrieved per trial, however, the choices made were biased strongly by that strategy. We conclude that when lateralized targets are equivalent, lateralized choices are biased primarily by hand preference and secondarily by a preference to retrieve a target ipsilateral to the preferred hand.

Usability of a theory of visual attention (TVA) for parameter-based measurement of attention I: evidence from normal subjects

The present study investigated the usability of whole and partial report of briefly displayed letter arrays as a diagnostic tool for the assessment of attentional functions. The tool is based on Bundesen's (1990, 1998, 2002; Bundesen et al., 2005) theory of visual attention (TVA), which assumes four separable attentional components: processing speed, working memory storage capacity, spatial distribution of attention, and top-down control. A number of studies (Duncan et al., 1999; Habekost & Bundesen, 2003; Peers et al., 2005) have already demonstrated the clinical relevance of these parameters. The present study was designed to examine whether (a) a shortened procedure bears sufficient accuracy and reliability, (b) whether the procedures reveal attentional constructs with clinical relevance, and (c) whether the mathematically independent parameters are also empirically independent. In a sample of 35 young healthy subjects, we found high intraparameter correlations between full- and short-length tests and sufficient internal consistencies as measured via a bootstrapping method. The clinical relevance of the TVA parameters was demonstrated by significant correlations with established clinical tests measuring similar constructs. The empirical independence of the four TVA parameters is suggested by nonsignificant or, in the case of processing speed and working memory storage capacity, only modest correlations between the parameter values.

The influence of illusory motion on line bisection performance in normal subjects

The present study examines whether illusory movement (IM) of a horizontal line, induced by a moving background (MB), influences line-bisection performance in normal subjects. The first experiment attempted to identify the speeds of MB that induce IM. We found that when speed is increased from 1.53 degrees to 13.3 degrees/sec, IM increases, but that with further speed increases, IM decreases. Leftward MB induces rightward IM, and vice versa. In the second experiment, we had subjects bisect lines at MB speeds that had been shown to induce IM in the first experiment. We found that leftward MB induced a rightward bias, and vice versa. We also found that there was a relationship between the magnitude of IM and the degree of bias. In the third experiment, by making the target line larger than the MB, we made the conditions where IM was presumably absent. Unlike the results of bisection performed with IM, subjects showed a bias in the direction of the MB. Overall, these experiments demonstrated that the perception of motion induces subjects to attend in the direction of movement.

Biases in attentional orientation and magnitude estimation explain crossover: neglect is a disorder of both

Crossover refers to a pattern of performance on the line bisection test in which short lines are bisected on the side opposite the true center of long lines. Although most patients with spatial neglect demonstrate crossover, contemporary theories of neglect cannot explain it. In contrast, we show that blending the psychophysical construct of magnitude estimation with neglect theory not only explains crossover, but also addresses a quantitative feature of neglect that is independent of spatial deficits. We report a prospective validation study of the orientation/estimation hypothesis of crossover. Forty subjects (17 patients with and without neglect following unilateral brain injury and 23 normal controls) completed four experiments that examined crossover using line bisection, line bisection with cueing, and reproducing line lengths from both memory and a standard. Replicating earlier findings, all except one subject group exhibited crossover on the standard line bisection test, all groups showed a spontaneous preference to orient attention to one end of the lines, and all groups overestimated the length of short lines and underestimated long lines. Biases in attentional orientation and magnitude estimation are exaggerated in patients with neglect. The truly novel finding of this study occurred when, after removing the line from the bisection task, the direction of crossover was completely reversed in all subject groups depending on where attention was oriented. These findings are consistent with our hypothesis of crossover: (1) crossover is a normal component of performance on line bisection; (2) crossover results from the interplay of biases in attentional orientation and magnitude estimation; and (3) attentional orientation predicts the direction of crossover, whereas a disorder of magnitude estimation, not previously emphasized in neglect, accounts for the quantitative changes in length estimation that make crossover more obvious in neglect subjects. Paradoxically, we observed that the traditional line bisection test is suboptimal for exploring crossover because lines elicit spontaneous orientation responses from subjects that confound experimental manipulations of attention. We conclude that attentional orientation and magnitude estimation are necessary and sufficient to explain crossover and that bias in magnitude estimation is a core component of neglect.

On the nature of near space: effects of tool use and the transition to far space

Many researchers have proposed that the near space immediately surrounding the body is represented differently than more distant space. Indeed, it has often been suggested that near space encompasses that within arm's reach. The present study used a line bisection task in healthy adults to investigate the effects of tool use on space perception, and the nature of the transition between near and far space. Subjects bisected lines at four distances controlled for both veridical and angular size using a laser pointer and a set of sticks. When the laser pointer was used, a left to right shift in bias was observed as stimuli were moved from near to far space. When a tool was used, however, a leftward bias was observed at all distances, similar to that observed with the laser pointer in near space. These results suggest that the tool expanded the range of near space. Additionally, the transition from near to far space was gradual, with no abrupt shift at arm's length (or at any other distance). In contrast to theories describing near space as that within arm's reach, these findings suggest that the representation of near space is less rigid, extending with tool use and gradually transitioning into far space.

The effects of local and global processing demands on perception and action

The line-bisection task, adapted to utilise a wooden rod as the bisection stimulus, has revealed that patients with visuo-spatial neglect may be more accurate at bisection when asked to pick up the rod, compared to pointing to its centre. We recently reported that neurologically intact participants show a similar dissociation on this task-demonstrating a rightward bias when pointing to the centre, which was not present when grasping the rod by the centre. The current paper examined how pointing and grasping responses were affected by adapted rod-bisection tasks that emphasised local or global processing. In Experiment 1, 26 participants completed four rod-bisection tasks. The rods were compound stimuli and the participants directed to focus on either the local or global level. The results demonstrated that when participants focused on the global level, the previous dissociations found for pointing and grasping conditions were evident. However, the perception of centre did change when participants focused on the local level: both the pointing and grasping responses were rightward biased. In Experiment 2, 42 participants completed three bisection tasks which again emphasised either the local or global level, but in different sets of stimuli. The results of this task further support the findings in Experiment 1: the rightward bias in the local-bisection task was again evident and in addition, the global-bisection task resulted in no bias and no difference between the pointing and grasping bisections. These results demonstrate how task demands can similarly affect the pointing and grasping responses, and indicate that local and global processing may be involved in perception/action dissociations on rod bisection.

The measurement of postoperative pain: A comparison of intensity scales in younger and older surgical patients

The psychometric properties of pain intensity scales for the assessment of postoperative pain across the adult lifespan have not been reported. The objective of this study was to compare the feasibility and validity of the Numeric Rating Scale (NRS), Verbal Descriptor Scale (VDS), and Visual Analog Scale (horizontal (VAS-H) and vertical (VAS-V) line orientation) for the assessment of pain intensity in younger and older surgical patients. At 24h following surgery, 504 patients, who were receiving i.v. morphine via patient-controlled analgesia, completed the pain intensity measures and the McGill Pain Questionnaire (MPQ) in a randomized order. They were asked which scale was easiest to complete, the most accurate measure, and which they would most prefer to complete in the future, as an index of face validity. The amount of opioid self-administered was recorded. Age differences in postoperative pain intensity were not found. However, elderly patients obtained lower MPQ scores and self-administered less morphine than younger people. Psychometric analyses suggested that the NRS was the preferred pain intensity scale. It had low error rates, and higher face, convergent, divergent and criterion validity than the other scales. Most importantly, its properties were not age-related. The VDS also had a favourable profile with low error rates and good face, convergent and criterion validity. Finally, difficulties with VAS use among the elderly were identified, including high rates of unscorable data and low face validity. Its use with elderly postoperative patients should be discouraged.

Stereotyping, self-affirmation, and the cerebral hemispheres

The authors used the processing characteristics of the left and right cerebral hemispheres to gain some insight into the relation between self-affirmation and stereotyping. In Study 1, self-affirmation led to greater stereotyping (of a librarian) and to greater left hemisphere activation, which in turn mediated the self-affirmation/stereotyping relationship. Study 2 replicated these results but also found that self-affirmation decreased stereotyping for a stigmatized target. However, relative hemisphere activation did not mediate this self-affirmation/stereotyping relationship. These studies showed that self-affirmation can either increase or decrease stereotyping depending on the status of the target and that relative hemisphere activation may provide clues as to underlying processes of stereotyping. In both studies, relative hemisphere activation was assessed using a line bisection task. Discussion focuses on possible mechanisms of different kinds of stereotyping and on the ways in which a consideration of relative hemisphere activation could help researchers gain insights into those mechanisms.

Hemispheric biases and the control of visuospatial attention: an ERP study

BACKGROUND

We examined whether individual differences in hemispheric utilization can interact with the intrinsic attentional biases of the cerebral hemispheres. Evidence suggests that the hemispheres have competing biases to direct attention contralaterally, with the left hemisphere (LH) having a stronger bias than the right hemisphere. There is also evidence that individuals have characteristic biases to utilize one hemisphere more than the other for processing information, which can induce a bias to direct attention to contralateral space. We predicted that LH-biased individuals would display a strong rightward attentional bias, which would create difficulty in selectively attending to target stimuli in the left visual field (LVF) as compared to right in the performance of a bilateral flanker task.

RESULTS

Consistent with our hypothesis, flanker interference effects were found on the N2c event-related brain potential and error rate for LH-biased individuals in the Attend-LVF condition. The error rate effect was correlated with the degree of hemispheric utilization bias for the LH-Bias group.

CONCLUSION

We conclude that hemispheric utilization bias can enhance a hemisphere's contralateral attentional bias, at least for individuals with a LH utilization bias. Hemispheric utilization bias may play an important and largely unrecognized role in visuospatial attention.

The wording of conclusions in relational reasoning

Literature on relational reasoning mainly focuses on the performance question. It is typically argued that problem difficulty relies on the number of "mental models" compatible with the problem. However, no study has ever investigated the wording of conclusions that participants formulate. In the present work, we analyze the relational terms that people use in drawing conclusions from spatial relation problems (A is to the left of B, B is to the left C, D is in front of A, E is in front C, What is the relation between D and E?). We observed a general preference for expressing conclusions with 'left' rather than conclusions with 'right'. We also found that three factors had an influence on the wording of the conclusions: the linguistic form of premises, the question type and the presentation format. On the other hand, the number of models and premise order did not affect the wording of conclusions. Our study shows that the type of conclusion produced provides a new key to identifying the mental processes involved in spatial reasoning. Implications for the two main approaches to reasoning processes (i.e. the analogical and the propositional approaches) are discussed.

The effects of focal and global attentional systems on spatial biases

The background page on which a stimulus is presented can influence the allocation of attention to that stimulus. The purpose of this study was to learn if there are hemispheric asymmetries in how background distraction affects attentional processing. Asymmetries were investigated by having right eye dominant subjects perform line bisections and manipulating the side of background distraction (right versus left), the eye of regard (right versus left), and the type of attention allocated (focal versus global). Overall subjects bisected lines to the left of center (pseudoneglect) and when viewing with the right eye (versus left) deviated more to the left. Subjects had more background distraction when viewing symbol than solid lines. Although overall, bias did not differ with the side of background distraction or the line being on one side or the other, when subjects viewed symbol, but not solid lines, this leftward bias was increased when the line was displaced to the right, thereby increasing the size of the left sided background. These findings suggest that when engaging the left hemisphere by using focused attention and placing the line on the right side, there is more distraction than when the right hemisphere is engaged.

Numbers and space: a cognitive illusion?

This study investigated the relationships between numerical and spatial representations by means of a bisection task, exploring the view that the core representation of number meaning is spatially organized as a mental number line. In Experiment nos. 1 (bisection of digit strings) and 2 (bisection of flanked lines) spatial biases towards the larger digit were found to be related only to processing of relative magnitude. Experiment nos. 3 (bisection of an unfilled space) and 4 (bisection of flanked lines/unfilled spaces) aimed at disclosing perceptual, attentional, and numerical constraints on the bias induced by the position of the larger digit. This effect is interpreted in terms of a cognitive illusion of length, whereby a spatial bias compensates for the numerical disparity. This seems to operate in a categorical fashion ("small/large"), and to be congruent with the assumption that relatively large numbers are associated with the right side of a mental representational space.

A deficit in discriminating gaze direction in a case with right superior temporal gyrus lesion

The superior temporal sulcus (STS) region is well recognized as being heavily involved in detecting and discriminating gaze. Lesions confined to this area are quite rare in humans, and so the research has mainly depended on animal studies and functional neuroimaging in normal human subjects. We report one such rare case, a 54-year-old Japanese female with a possible congenital s anomaly who, after a cerebral hemorrhage, demonstrated a lesion almost completely confined to the entire right superior temporal gyrus (STG). In the subacute phase, the patient showed evidence of left hemispatial neglect, from which she gradually recovered. In the chronic phase, she showed a puzzling difficulty in obtaining eye-contact. We have conducted, in conjunction with conventional neuropsychological evaluations, experimental assessment of her ability in gaze cognition. Her performance on neuropsychological testing demonstrated no compromise in intellect, memory, or language skills, and a close-to-full recovery from neglect. On gaze cognition experiments, she was repeatedly shown to perceive left gaze as straight, and to a lesser degree, straight gaze as right. We suggest that the function of the STG in detecting gaze, together with the directional information it receives from earlier visual areas, may be associated, when damaged, with this deficit in detecting contra-directional gaze. We have demonstrated for the first time that a single circumscribed lesion to the STG results in both gaze processing deficit and concurrent aberrant gaze behavior of the victim herself, implicating a mechanism within the STG as an interface between gaze of others and gaze of self.

Visual line bisection in sinistrals and dextrals as a function of hemispace, hand, and scan direction

Visual line bisection was investigated in 26 sinistral and 24 dextral subjects as a function of hemispace, hand and scan direction. An ANOVA revealed significant main effects for hand preference, due to the mean bisection errors of dextral subjects being significantly leftward of those of sinistral subjects; for hand, due to the bisection errors of the left hand being significantly to the left of the right hand; and for scan, due to the bisection errors following a left scan being significantly to the left of a right scan. One significant interaction was found, that between hand and direction of scan, due to a significant difference between left and right hands following a scan from the left but not following a scan from the right. For dextral subjects the leftward bisection errors of the left and right hands following a scan from the left, but not for a scan from the right, differed significantly from the midpoint. For sinistral subjects the leftward bisection errors following a scan from the left and rightward bisection errors following a scan from the right differed significantly from the midpoint for the left hand but not for the right hand. No significant main effect or interactions for hemispace were found. This confirms that both sinistral and dextral subjects display pseudoneglect when using their preferred hand and scanning from the left. However, sinistrals, but not dextrals, will display reversed pseudoneglect when using their preferred hand and adopting a scan direction from the right. These results are discussed in terms of the interaction between three factors, whose influence can jointly and severally produce misbisections, hemispheric specialisation for visuospatial function, hemispheric activation for a manual response, and the allocation of visual attention.