Leftwards error in bisecting the gap between two points: stimulus quality and hand effects

Revisiting the attentional bias in the split brain

Previous research has revealed a strong right bias in allocation of attention in split brain subjects, suggesting that a pathological attention bias occurs not only after unilateral (usually right-hemispheric) damage but also after functional disconnection of intact right-hemispheric areas involved in allocation of attention from those in the left hemisphere. Here, we investigated the laterality bias in spatial attention, as measured with the greyscales task, in two split-brain subjects (D.D.C. and D.D.V.) who had undergone complete callosotomy. The greyscales task requires participants to judge the darker (or brighter) of two left-right mirror-reversed luminance gradients under conditions of free viewing, and offers an efficient means of quantifying pathological attentional biases in patients with unilateral lesions. As predicted, the results of the two split-brain subjects revealed a pathological rightward bias in allocation of attention, suggesting strong dependence on a single hemisphere (the left) in spatial attention, which is opposite to what one expects from people with intact commissures, and is remarkable in that it occurs in free viewing. In that sense both split-brain patients are behaving as though the brain is indeed split, especially in D.D.C. who had undergone partial resection of the anterior commissure in addition to complete callosotomy, whereas the anterior commissure is still intact in D.D.V. The findings support the view that the commissural pathways play a significant role in integration of attentional processes across cerebral hemispheres.

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 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.

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.

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.

Frontal and Posterior Erps Related to Line Bisection

The aim of this study was to investigate the dynamic nature of the cortical visuospatial attention processes during the line bisection test, which is sensitive to perceptual asymmetries. EEGs of 26 normal volunteers were recorded during the administration of a computerized line bisection test, which requires participants mark the midline of lines using a mouse. Two event-related potentials subsequent and time locked to the line presentations, namely, P300 and a positive slow wave, were obtained. Findings suggested that both potentials were related to the test performance, and the right hemisphere was more active. Analysis suggested a right parietotemporal and superior parietal locus for the P300 and right prefrontal activity for the positive slow wave. A dynamic asymmetrical activity was identified, such that after primary visual perception, spatial processing is then initiated in the right parietotemporal cortex and then proceeds to the right prefrontal cortex.

Word and line bisection in typical and impaired readers and a cross-language comparison

Observers exhibit larger leftward bias when bisecting words compared with lines. According to the Attentional Scaling Hypothesis, attempting to access lexical entries involves focusing attention on the initial letters of words to establish a cohort of potential matches with entries in the mental lexicon. We test this account by examining two predictions: (1) greater leftward bias for words should be evident in English readers in which the word beginning is on the left but not in Hebrew readers. (2) Dyslexics who have lexical impairments should show greater bias. Results reveal that word length modulated bisection bias differently for Hebrew and English readers, although the bias stays always leftward. Furthermore, dyslexics exhibited an exaggerated leftward bias than controls. We propose this effect arises from an interaction between reading and spatial attention rather than from the scaling of attention relative to the beginning of the word in the service of lexical access.

The role of numerical and non-numerical cues in nonsymbolic number processing: Evidence from the line bisection task

In line bisection tasks, adults and children bisect towards the numerically larger of two nonsymbolic numerosities [de Hevia, M. D., & Spelke, E. S. (2009). Spontaneous mapping of number and space in adults and young children. Cognition, 110, 198–207. doi:10.1016/j.cognition.2008.11.003]. However, it is not clear whether this effect is driven by number itself or rather by visual cues such as subtended area [Gebuis, T., & Gevers, W. (2011). Numbers and space: Indeed a cognitive illusion! A reply to de Hevia and Spelke (2009). Cognition, 121, 248–252. doi:10.1016/j.cognition.2010.09.008]. Furthermore, this effect has only been demonstrated with flanking displays of two and nine items. Here, we report three studies that examined whether this “spatial bias” effect occurs across a range of absolute and ratio numerosity differences; in particular, we examined whether the bias would occur when both flankers were outside the subitizing range. Additionally, we manipulated the subtended area of the stimulus and the aggregate surface area to assess the influence of visual cues. We found that the spatial bias effect occurred for a range of flanking numerosities and for ratios of 3:5 and 5:6 when subtended area was not controlled (Experiment 1). However, when subtended area and aggregate surface area were held constant, the biasing effect was reversed such that participants bisected towards the flanker with fewer items (Experiment 2). Moreover, when flankers were identical, participants bisected towards the flanker with larger subtended area or larger aggregate surface area (Experiments 2 and 3). On the basis of these studies, we conclude that the spatial bias effect for nonsymbolic numerosities is primarily driven by visual cues.

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.

Spatial attention: differential shifts in pseudoneglect direction with time-on-task and initial bias support the idea of observer subtypes

Asymmetry in human spatial attention has long been documented. In the general population the majority of individuals tend to misbisect horizontal lines to the left of veridical centre. Nonetheless in virtually all previously reported studies on healthy participants, there have been subsets of people displaying rightward biases. In this study, we report differential time-on task effects depending on participants' initial pseudoneglect bias: participants with an initial left bias in a landmark task (in which they had to judge whether a transection mark appeared closer to the right or left end of a line) showed a significant rightward shift over the course of the experimental session, whereas participants with an initial right bias shifted leftwards. We argue that these differences in initial biases as well as the differential shifts with time-on task reflect genuine observer subtypes displaying diverging behavioural patterns. These observer subtypes could be driven by differences in brain organisation and/or lateralisation such as varying anatomical pathway asymmetries (Thiebaut de Schotten et al., 2011).

Visuospatial processing in schizophrenia: does it share common mechanisms with pseudoneglect?

Schizophrenia patients demonstrate behavioural and cerebral lateralised anomalies, prompting some authors to suggest they exhibit a mild form of right unilateral neglect. To better describe and understand lateralised visuospatial anomalies in schizophrenia, three experiments were run using tasks often utilised to study visuospatial processing in healthy individuals and in neglect patients: the Behavioural Inattention Test (BIT), the manual line bisection task with and without a local cueing paradigm, the landmark task (or line bisection judgement), and the number bisection task. Although the schizophrenia patients did not exhibit the full-blown neglect syndrome, they did demonstrate marked spatial biases that differentiated them from controls on all but two tasks. More specifically, schizophrenia patients showed neither a simple perceptual deficit nor an asymmetry, but demonstrated (1) lateralised anomalies on a simple manual line bisection task; (2) unilateral attentional deficits for line bisection within a local cueing paradigm; and (3) a lateralised deficit in the visuospatial representations of numbers. Altogether, these results suggest a right hemineglect-like deficit in schizophrenia in attentional, representational, and motor-intentional processes. Yet it does not appear to be as strong a phenomenon. Indeed, it could be considered as an accentuation of the normal asymmetry in visuospatial processing.

Left hand dominance affects supra-second time processing

Previous studies exploring specific brain functions of left- and right-handed subjects have shown variances in spatial and motor abilities that might be explained according to consistent structural and functional differences. Given the role of both spatial and motor information in the processing of temporal intervals, we designed a study aimed at investigating timing abilities in left-handed subjects. To this purpose both left- and right-handed subjects were asked to perform a time reproduction of sub-second vs. supra-second time intervals with their left and right hand. Our results show that during processing of the supra-second intervals left-handed participants sub-estimated the duration of the intervals, independently of the hand used to perform the task, while no differences were reported for the sub-second intervals. These results are discussed on the basis of recent findings on supra-second motor timing, as well as emerging evidence that suggests a linear representation of time with a left-to-right displacement.

Focused visual attention distorts distance perception away from the attentional locus

Several lines of evidence show that visual perception is altered at the locus of visual attention: detection is faster, performance better and spatial resolution increased. It is however not known whether attention can affect visual perception further away from its locus. In the present study, we specifically question whether and how visual attention influences spatial perception away from its locus, independently from any saccadic preparation. We use a landmark task in which subjects have to estimate the location of a bisection stimulus relative to two landmark stimuli 15° apart, while fixating one of them. This task is combined with a highly demanding discrimination task performed on one of the two landmarks. This allows us to test for the effect of spatial attention allocation on distance perception, as measured by the subject estimation of the landmarks midpoint. We show that the estimated midpoint is displaced towards the attentional locus, both when attention is instructed on the central landmark or on the peripheral landmark. These results suggest an overrepresentation of space around the attentional locus that can affect perception up to 8° away, and question the existence of an objective spatial representation. They are in line with reports of spatial distortion in hemineglect patients while they strikingly contrast with the spatial compression reported around the time of saccadic execution.

The effects of reading-writing direction on the asymmetry of space perception and directional tendencies: A comparison between French and Tunisian children

We compared the influence of reading and writing habits on the asymmetry of space perception and the directional tendencies of French and Tunisian right-handers, aged 5, 7, and 9 years. By comparing two groups of children who use the opposite direction for writing (from left to right for French, from right to left for Arabic), before and after being taught to read in school, we evaluated the impact of writing direction on these asymmetries. A bisection task, a circle-drawing task, and a dot-filling task were used to assess spatial asymmetries and directional tendencies. On the bisection task, a group difference emerged at 9 years, with the French children bisecting the line to the left of the true centre, and the Tunisian children showing no bias. On the circle-drawing task, there was a group difference from 7 years on, as the French children, but not the Tunisian children, used increasing counterclockwise movements. Finally, on the dot-filling task performed with the right hand, the French children filled in significantly more dots when going from left to right from 7 years on, whereas Tunisian children filled in more dots when going from right to left. These results show the impact of basic tendencies in younger children (ipsilateral bias in line bisection, clockwise direction in circle drawing, outward tendency for horizontal displacement in dot filling), as well as the impact of writing direction on spatial asymmetries after learning to read. The results are also discussed in reference to the differences between the two languages, the closeness of the French direction of writing to spontaneous neural-based tendencies, and the influence of learning French at age 8 for the Tunisian children.

Object-centred pseudoneglect for non-verbal visual stimuli

The rightward spatial bias shown by left neglect patients and the small leftward bias displayed by healthy subjects (pseudoneglect) have been interpreted as phenomena sharing a common attentional imbalance mechanism. Here we investigated whether pseudoneglect, similarly as neglect, can occur in an object-centred frame of reference. Thirty healthy participants repeatedly bisected the elongated caricature of a basset hound with the head on the left and the tail on the right or viceversa. In the last critical trials, the figure appeared horizontally mirrored. The bisection error reversed from the left to the right space in the critical trials. This result shows that it is possible to induce object-centred pseudoneglect on newly established knowledge about the canonical orientation of non-verbal visual stimuli.

Brain activity during landmark and line bisection tasks

Neglect patients bisect lines far rightward of center whereas normal subjects typically bisect lines with a slight leftward bias supporting a right hemisphere bias for attention allocation. We used fMRI to assess the brain regions related to this function in normals, using two complementary tasks. In the Landmark task subjects were required to judge whether or not a presented line was bisected correctly. During the line bisection task, subjects moved a cursor and indicated when it reached the center of the line. The conjunction of BOLD activity for both tasks showed right lateralized intra-parietal sulcus and lateral peristriate cortex activity. The results provide evidence that predominantly right hemisphere lateralized processes are engaged in normal subjects during tasks that are failed in patients with unilateral neglect and highlight the importance of a right fronto-parietal network in attention allocation.

Hand and hand preferences in use of a visual analogue scale

Visual analogue scales are commonly used to measure the intensity of sensations, and their validity and reliability have been reported. However, biases similar to those found in visual line bisection have not been investigated. 23 right-handed and 19 left-handed participants, with a mean age of 30.1 yr., marked three points on a visual analogue scale representing imagined pain, using both the left and right hands, corresponding to 1/4, 1/2, and 3/4 of the way across the scale. In keeping with visual line bisection literature, both right- and left-handed participants marked the scale with the left hand significantly leftward of the point marked with the right hand, thereby underreporting the intensity. Right-handed participants marked 1/4 significantly leftward and 3/4 significantly rightward of veridical points, thereby underreporting and overreporting, respectively, the intensity. However, left-handed participants did not display this bias and consistently erred leftward for 1/4, 1/2, and 3/4 positions, underreporting intensity. These findings were explained in terms of hemispheric specialisation and activation for a manual response to a visuospatial task, with the conclusion that scoring the visual analogue scale to millimetre accuracy is subject to a potential confound of these factors.

Time-on-task effect in pseudoneglect

Neurologically normal subjects systematically misbisect space during visual line-bisection or similar tasks, generally erring to the left of the veridical center when bisecting horizontal lines, a phenomenon referred to as pseudoneglect. This phenomenon is usually interpreted as enhanced attention toward the left hemispace resulting in an overestimation of the leftward extent of a line. While most studies have examined the role of attention in spatial bias using spatial cueing methods in bisection tasks, Manly et al. (Neuropsychologia 43(12):1721-1728, 2005) proposed an original paradigm in which the participants' alertness was diminished by sleep deprivation or prolonged execution of a line-bisection task. The authors reported a significant rightward shift in attention related to declining alertness, but they did not control eye movements and, consequently, modifications of scanning and fixation strategies with fatigue cannot be ruled out in their study. Here we examine whether a diminution in alertness induced by a 60-min-long Landmark task would diminish (or even reverse) this attentional bias, when eye movements are absent. Participants performed a forced-choice judgment about the location of a transaction mark in relation to the veridical center of a horizontal line. The results confirmed a significant decrease in the leftward bias over the course of the session but, in contrast to the findings of Manly et al. (2005), we did not observe a reverse bias from the left to the right hemispace. The results are discussed within the context of the hemisphere-activation model.

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.

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.

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.

An investigation of the relationship between free-viewing perceptual asymmetries for vertical and horizontal stimuli

Two experiments examine the relationship between free-viewing vertical and horizontal perceptual biases. In Experiment 1, normal participants (n=24) made forced-choice luminance judgments on two mirror-reversed luminance gradients (the 'grayscales' task). The stimuli were presented in vertical, horizontal and oblique (+/-45 degrees ) orientations. Leftward and upward biases were observed in the horizontal and vertical conditions, respectively. In the oblique conditions, leftward and upward biases combined to produce a strong shift of attention away from the lower/right space toward the upper/left. Regression analyses revealed that the oblique biases were the combined product of the vertical and horizontal biases. A lack of correlation between the vertical and horizontal biases, however, suggests they reflect the operation of independent cognitive/neural mechanisms. In Experiment 2, the same stimuli were given to right-hemisphere-lesioned patients with spatial neglect (n=4). Rightward and upward biases were observed for horizontal and vertical stimuli, respectively. The biases combined to produce a strong shift of attention away from the lower/left space toward the upper/right. While our research demonstrates that vertical and horizontal attentional biases are additive, it also appears that they reflect the operation of independent cognitive/neural mechanisms. Potential applications of these findings to the remediation of spatial neglect are discussed.

Line Bisection in the Split Brain

The authors examined line bisection in 4 patients with resection of the corpus callosum and in 22 control participants. The control participants showed a leftward bias, especially with the left hand, implying right-hemispheric dominance in spatial attention. Two patients with anterior callosotomy showed similar biases, suggesting that the anterior callosum plays only a small role. A patient with complete callosotomy showed a strong right bias, regardless of hand use. A patient with posterior callosotomy showed the opposite pattern: a strong left bias, regardless of hand use. These data suggest that the posterior corpus callosum normally plays a role in line bisection and that the resection of the posterior corpus callosum produces consistent bias. The direction of the bias depends on which hemisphere assumes control.

Can free-viewing perceptual asymmetries be explained by scanning, pre-motor or attentional biases?

Judgments of relative magnitude between the left and right sides of a stimulus are generally weighted toward the features contained on the left side. This leftward perceptual bias could be the result of, (a) left-to-right scanning biases, (b) pre-motor activation of the right hemisphere, or (c) a left hemispatial attentional bias. The relative merits of these explanations of perceptual asymmetry were investigated. In Experiment 1, English and Hebrew readers made luminance judgements for two left/right mirror-reversed luminance gradients (greyscales task). Despite different reading/scanning habits, both groups exhibited a leftward perceptual bias. English and Hebrew readers also performed a line bisection task. Scanning biases were controlled by asking participants to follow a marker as it moved left-to-right or right-to-left and then stop it as it reached the midpoint of the line. Despite controlling scanning, a leftward bias was observed in both groups. In Experiment 2, peripheral spatial cues were presented prior to the greyscales stimuli. English readers showed a reduction in the leftward bias for right-sided cues as compared to left-sided and neutral cues. Right-side cues presumably overcame a pre-existing leftward attentional bias. In both experiments, pre-motor activation was controlled using bimanual responses. Despite this control, a leftward bias was observed throughout the study. The data support the attentional bias account of leftward perceptual biases over the scanning and pre-motor activation accounts. Whether or not unilateral hemispheric activation provides an adequate account of this attentional bias is discussed.

Abnormal line bisection judgements in children with Tourette's syndrome

Tourette's syndrome (TS) has been associated with loss of normal basal ganglia asymmetry, as well as loss of normal functional asymmetry, including the leftward bias on traditional visuospatial tasks such as line bisection and turning bias tests. The aim of the present study was to examine the lateralisation of visuospatial attention in TS. We examined the effect of an irrelevant moving-dot background on line bisection judgements. Nine children with a DSM IV diagnosis of TS participated, in addition to 9 healthy controls, individually matched for age, sex and IQ. Horizontal lines of varying length were presented on a computer screen with either a blank background, or a moving, random-dot field. The dots moved either leftward or rightward across the screen at 40 or 80 mm/s, and participants were instructed to ignore these distracting stimuli when judging the lines. TS children were found to be abnormally right-biased in line bisection in a similar fashion to unmedicated ADHD children who, in a previous study, showed a similar small, yet significant, right-bias in line bisection. Matched controls showed a small, nonsignificant left bias, consistent with past research. Unlike previous findings with hemineglect patients, the irrelevant moving background had no effect on bisection performance for TS children or healthy controls. The present findings suggest a deficit in visuospatial attention consistent with the emerging picture of a lateralised dysfunction of frontostriatal circuitry in TS.

Interaction of hand use and spatial selective attention in children

It has been argued that concurrent motor action can modulate visual spatial attention. The visual spatial biases of adult patients with unilateral neglect, for example, can be ameliorated by simultaneous use of the contralesional hand. Such improvements are most dramatic when the contralesional hand is moved within contralesional space. To date, evidence of such an interaction in neurologically healthy individuals has not been presented. Line bisection is a simple task that is sensitive to attentional spatial bias. When young children are asked to bisect horizontal lines using their right hands, they show a reliable, if small, bias that is consistent with the pattern seen in adult neglect. This bias is reversed when the left hand is used. Here, we show that these effects are significantly modulated by the location of the movements relative to the body mid-line - specifically that the conjunction of hand movements within ipsilateral space is necessary for the previously reported pattern to be observed. We further demonstrate that these effects are not present in the bisections of neurologically healthy adults. In a final study, we examined whether the hand movement effects seen in children's line bisections would persist in a purely visual task (that is when the movements were made irrelevant to the response). Again, significant modulation of children's perception by concurrent hand movements - and the relative location of those movements - was observed. The theoretical and clinical implications of the results are discussed.

Performance consistency of normal observers in forced-choice tachistoscopic visual line bisection

Pseudoneglect (PN) refers to the leftward error exhibited by normal observers on line bisection tasks (Bowers and Heilman, Neuropsychologia, 18, (1980) 491-8). Although a thorough review of the literature has shown PN to be relatively robust (Jewell and McCourt, Neuropsychologia, 38, (2000) 93-110), controversy remains concerning the reliability of the phenomenon, with some studies reporting a relatively high incidence of normal subjects with rightward bisection errors. The present experiment assesses the consistency of bisection performance in normal young observers. Right-handed subjects (N=22) participated in a tachistoscopic forced-choice line bisection task. Each subject participated in 7-16 experimental sessions separated by at least 24 h (total bisection measurements=317). Individual bisection performance could thus be evaluated with respect to within-subject variability measures. An eyetracker recorded gaze position during the task in one session. A highly significant mean group bisection error of -0.26 degrees (P<0.001) was obtained (left negative), and individual subject means ranged from -0.55 degrees to +0.03 degrees. Of the 317 total bisection measurements, 9% (28) deviated rightward. Significant (P<0.05) mean leftward errors occurred in 91% (20/22) of subjects. Mean bisection error in two subjects was not significantly different from zero. No subject possessed a significant rightward error. Mean gaze deviation from screen (and line) center ranged from +/-0.9 degrees, and was positively correlated (P<0.05) with bisection error. It is concluded that forced-choice tachistoscopic line bisection measures are highly reliable; a mean correlation of +0.87 exists between mean error based on 15 trials and means estimated from a random sample of only two trials. The incidence of true rightward bisection error in the population of normal right-handed subjects is thus estimated to be less than 5%.

The influence of unimanual response on pseudoneglect magnitude

Various factors influence the degree of leftward error (pseudoneglect) (Bowers & Heilman, 1980) that typifies the performance of normal subjects in line bisection tasks (Jewell & McCourt, 2000). The results of this experiment show that unimanual responding also exerts a subtle but significant modulating influence on spatial attention, as indexed by the differential magnitude of pseudoneglect. Using a forced-choice tachistoscopic line bisection protocol, 184 subjects (92 male and 92 female) bisected horizontally oriented lines (22.3 degrees wide x 0.39 degrees height) presented to central vision in two conditions, in which bisection responses were executed via button presses using the first two fingers of either the left (LH) or right (RH) hand. Perceived line midpoint deviated significantly leftward of veridical (p <.05) in both conditions. There was no significant influence of subject sex (p >.05). A significant influence of unimanual response was revealed (p <.05) where pseudoneglect magnitude was greater in the LH than the RH condition. The results are interpreted within the framework of the activation-orientation theory of attentional asymmetry.

Contributions of object- and space-based mechanisms to line bisection errors

In two experiments, normal adults divided a horizontal line segment and an equal spatial interval that did not contain a line into eight equal-appearing segments by means of successive bisections. In the first experiment, subjects' average initial bisections erred to the left of objective center for both stimuli. Their subsequent bisections produced similar errors for the line-present stimulus, as the bisection of each progressively smaller line segment was placed to the left of true center. However, this pattern did not occur when bisecting the empty interval. The finding that the presence of a line influences bisection errors implicates an 'object-based' mechanism in the genesis of line bisection errors and suggests that this mechanism varies in its operation with visual field location. In the second experiment, subjects successively bisected longer line and interval stimuli which were presented either centered on the subjects' midlines or displaced to the right or left. Bisections tended to be placed farther to the left for the left stimuli and farther to the right for the right stimuli, with little or no bias for the centrally located stimuli. Repeated measures with the centrally located stimulus demonstrated strong individual differences in bisection biases. Errors were also found to be correlated for the line-present and line-absent stimuli in both experiments, suggesting the additional contribution of a mechanism that is not object-based.

Asymmetries of visuospatial attention are modulated by viewing distance and visual field elevation: pseudoneglect in peripersonal and extrapersonal space

Many factors influence the degree of leftward error (pseudoneglect) that typifies the line bisection performance of normal subjects. We find that viewing distance also exerts a modulating influence on spatial attention in normal subjects, as it appears to do in neglect syndrome. Using forced-choice tachistoscopic line bisection, 38 right-handed subjects (15 male, 23 female) bisected horizontal lines (13.7 degrees w x 0.24 degrees h) presented in the midsagittal plane as a function of line elevation (- 3.6 degrees, 0 degrees, and 3.6 degrees relative to horizontal midline) and viewing distance (45 and 90 cm). We find a significant main effect of viewing distance, F (1, 37) = 10.04, p = .003, where pseudoneglect is larger in peripersonal (45 cm) than in extrapersonal (90 cm) space. We replicate an effect of line elevation, F (2, 74) = 4.40, p = .016, where pseudoneglect is greatest in the superior visual field (McCourt and Jewell, 1999). The interaction was not significant, p > .05. Thus, we find evidence for independent spatiotopic (viewing distance) and retinotopic (line elevation) effects on line bisection performance in normal observers, suggesting that the allocation of visuospatial attention is modulated within multiple frameworks.

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

An exhaustive qualitative (vote-counting) review is conducted of the literature concerning visual and non-visual line bisection in neurologically normal subject populations. Although most of these studies report a leftward bisection error (i.e., pseudoneglect), considerable between-study variability and inconsistency characterize this literature. A meta-analysis of this same literature is performed in which the total quantitative data set, comprising 73 studies (or sub-studies) and 2191 subjects, is analyzed with respect to 26 performance factors. The meta-analytic results indicate a significant leftward bisection error in neurologically normal subjects, with an overall effect size of between -0.37 and -0.44 (depending on integration method), which is significantly modulated to varying degrees by a number of additional task or subject variables. For example, visual bisection tasks, midsagittal-pointing tasks and tactile bisection tasks all lead to leftward errors, while kinesthetic tasks result in rightward errors. Tachistoscopic forced-choice testing methods reveal much greater estimates of bisection error (effect size = -1.32) than do manual method-of-adjustment procedures (effect size= -0.40). Subject age significantly modulates line bisection performance such that older subjects err significantly rightward compared to younger subjects, and to veridical line midpoint. Male subjects make slightly larger leftward errors than do female subjects. Handedness has a small effect on bisection errors, with dextrals erring slightly further to the left than sinistral subjects. The hand used to perform manual bisection tasks modulated performance, where use of the left hand lead to greater leftward errors than those obtained using the right hand. One of the most significant factors modulating bisection error is the direction in which subjects initiate motor scanning (with either eye or hand), where a left-to-right scan pattern leads to large leftward errors while a right-to-left scan pattern leads to rightward errors.

Effects of stimulant medication on the lateralisation of line bisection judgements of children with attention deficit hyperactivity disorder

OBJECTIVES

Deficits in the maintenance of attention may underlie problems in attention deficit hyperactivity disorder (ADHD). Children with ADHD also show asymmetric attention deficits in traditional lateralisation and visuospatial orienting tasks, suggesting right hemispheric (and left hemispace) attentional disturbance. This study aimed to examine the lateralisation of selective attention in ADHD; specifically, the effect of a moving, random dot background, and stimulant medication in the line bisection task.

METHODS

The performance of children with ADHD, on and off methylphenidate, was examined using a computerised horizontal line bisection task with moving and blank backgrounds. Twenty children with a DSM-IV diagnosis of ADHD participated with 20 controls, individually matched for age, sex, grade at school, and IQ. Twelve of the 20 children with ADHD were on stimulant medication at the time of testing. Horizontal lines of varying length were presented in the centre of a computer screen, with either a blank background, or a moving, random dot field. The random dots moved either leftward or rightward across the screen at either 40 mm/s or 80 mm/s.

RESULTS

The children with ADHD off medication bisected lines significantly further to the right compared with controls, who showed a small leftward error. Methylphenidate normalised the performance of the children with ADHD for the task with the moving dots.

CONCLUSIONS

These results support previous evidence for a right hemispheric hypoarousal theory of attentional dysfunction, and are consistent with the emerging picture of a lateralised dysfunction of frontostriatal circuitry in ADHD.

Relationships among multiple task asymmetries. II. A large-sample factor analysis

Samples from published and unpublished factor analytic studies of multiple task asymmetry are combined to produce a large-sample factor analysis of 10 measures, prescreened for significant asymmetry and acceptable reliability, from 9 lateralized tasks. A full correlational matrix is presented, with task pairings having sample sizes ranging from 103 to 479 (mean N = 290). Both significant positive and significant negative correlations between measures are found. Extraction and oblique rotation of five factors fails to reveal any evidence of a global relationship among lateral difference measures, and principal-components analysis likewise fails to reveal a component to which all measures correlate positively. However, evidence for local structure is found. Results are interpreted in terms of dissociable hemispheric processes, and consideration is given of the possible existence of modality-specific characteristic perceptual asymmetry and of single versus multiple determination of lateral differences.

The effects of gender, menstrual phase and practice on the perceived location of the midsagittal plane

Women show menstrual phase-related cognitive changes that suggest altered hemispheric activation for a particular task, such that they demonstrate the greatest lateral performance differences on prototypical left hemisphere tasks during the luteal phase and on prototypical right hemisphere tasks during menstruation. Additionally, menstrual phase may alter total cerebral responsiveness, such that response times and performance accuracy for many tasks are best during the luteal phase and most impaired during the menstrual phase. We evaluated the effect of menstrual phase on spatial bisection (a perceptuospatial task) to help further understand hormonally-mediated changes in interhemispheric dynamics. Healthy young adult women and men blindly pointed to their midsagittal plane with either hand. Women were repeatedly tested according to menstrual phase, and men were tested at similar intervals. The mean pointing error in the luteal phase differed significantly from that of all other phases and did not differ significantly from those of men, who pointed significantly to the left across test sessions. These findings suggest that, in space bisection tasks, women are more likely to have asymmetric hemispheric activation during the luteal phase than during the menstrual phase. Thus, space bisection did not resemble other prototypical right hemisphere behaviors. The luteal phase may have nonspecifically activated both hemispheres on this task instead of suppressing right hemisphere function, and a slight functional asymmetry favoring the right hemisphere may have been promoted. In addition, intermanual pointing discrepancies in both subject groups decreased over repeated sessions. This suggests that, while practice alters an internal kinesthetic reference, it does not influence an imaginal extrapersonal spatial reference.

Cognitive and perceptual influences on visual line bisection: psychophysical and chronometric analyses of pseudoneglect

Perceptual and cognitive influences on line bisection were isolated using a tachistoscopic forced-choice paradigm. Pretransected lines were presented for 150 msec at four orientations ([symbol: see text],-,/ and [symbol: see text]). Subjects made either 'left-right' (for -,/, and [symbol: see text] lines) or 'above-below' (for [symbol: see text],/and [symbol: see text] lines) discriminations in response to each line stimulus, depending upon perceived transector location relative to veridical line midpoint. Median response time and point of subjective equality (P.S.E.) were computed for each treatment condition. P.S.E.s in 'left-right' conditions were significantly left of veridical; response time maxima were similarly displaced. Azimuthal pseudoneglect was greatest for horizontal lines. P.S.E.s in 'above-below' conditions were displaced above veridical, and response time maxima were similarly displaced. Altitudinal pseudoneglect was greatest for negative diagonal lines [symbol: see text]. Azimuthal pseudoneglect significantly exceeded altitudinal pseudoneglect. 'Left-right' responses (mean = 478.3 msec) were significantly faster than 'above-below' responses (mean = 504.6 msec). We conclude that scanning eye and/or gross limb movements do not account for pseudoneglect, and that a significant component must be purely perceptual. Chronometric and psychometric measures of pseudoneglect are in remarkable agreement. The effects of altitudinal and azimuthal pseudoneglect are neither separable nor additive, suggesting the existence of independent mechanisms governing the allocation of spatial attention to objects of differing orientation. The slopes of the psychometric functions for lines of cardinal orientation are significantly steeper than for diagonal lines, which may reflect a processing conflict between these putatively independent mechanisms at diagonal line orientations. Decision context significantly modulates the magnitude of pseudoneglect for physically identical stimuli, perhaps reflecting the selective differential engagement of the vertical or horizontal attentional mechanisms. There are significant individual differences in line bisection performance, even in a very homogeneous sample of strongly right-handed subjects.

Length representation in normal and neglect subjects with opposite reading habits studied through a line extension task

In the first part of this paper, 45 French (left-to-right readers) and 30 Israeli (right-to-left readers) normal dextrals were given half a line and requested to construct the missing other with the same length (either the left one or the right one). Using this line extension task, a significant effect of reading habits on the performance was found, with no significant bias for Israeli subjects, and a significant underconstruction when building the left half from the right one for French subjects. In the second part, two patients with opposite reading habits (one French, one Israeli) suffering from left unilateral neglect were submitted to the same protocol. Both patients were found to under-construct the right half of the line from the left given half, and to over-construct the left half from the right given one, hence reproducing the well-known line bisection bias. Results are discussed with regard to enhancement and activation hypotheses, and current theories of the neglect syndrome.

Bisection and perception of horizontal lines in normal children

Simple line bisection is a complex task in which both perceptual/attentional factors and motor factors are involved. Using a judgment task on prebisected lines it is possible to assess independently the role of perceptual/attentional factors. The judgment-bisection relationship was investigated in 37 preschool children aged 4-5 and 70 school children aged 10-12. At bisection, a clear shift of the right hand with age from right to left bisections was observed, more pronounced in right- than in left-handers, and probably related to learned patterns. At judgment, a slight but significant left side overestimation was observed. In right-handers, judgment was related to left-hand but not to right-hand bisection. It is concluded that motor (manual) factors strongly influence visuo-motor bisection in normal children, and it is suggested that the judgment-bisection relationship has to be systematically investigated when bisection is used as a task informing on space perception or on direction of attention.

Tactile rod bisection: hemispheric activation and sex differences

Blindfolded subjects estimated with either hand the center of rods positioned in either left or right hemispace. In one condition, they also performed a concurrent verbal task. Bias and variability of bisection settings were the dependent variables. Bisections performed in left hemispace were biased to the left of true center, more so when the left hand was used. In contrast, bisections performed in right hemispace were biased rightward, more so when the right hand was used. There were no significant differences in variability of bisections in any condition. Interactions of hand with hemispace in which the task was performed differed for the two sexes. Moreover, the secondary verbal task had no effect on either measure. We conclude that of several factors that may underlie bisection biases, attention was the most relevant.

Visual and motor components in simple line bisection: an investigation in normal adults

Visuomotor bisection and bisection judgement of centrally presented 10-cm horizontal lines were proposed to 43 normal adults, both on paper and on the vertical screen of a computer. Tasks on paper showed slight left-side of the line overestimation and weak link between actual bisection and judgement. Tasks on screen showed, on the contrary, right-side of the line overestimation and stronger link between actual bisection and judgement. Centrally pretransected lines were often judged wrongly. These results suggest an important role of motor factors on classical paper-and-pencil visuomotor line bisection, and the relevance of the distinction between peripersonal and extrapersonal space.

Is left always right? Directional deviations in visual line bisection as a function of hand and initial scanning direction

Directional deviations in visual line bisection were investigated using normal subjects. Significant main effects were found for hand and initial scan direction resulting from significantly greater deviations to the left by the left hand compared with the right hand and by a scan from the left compared with a scan from the right. These results suggest that the amelioration of neglect can only be inferred from the left hand deviations of neglect patients if they are significantly leftwards of the objective middle and that the degree of leftward deviation in normal subjects results from an interaction between right hemispheric activation and unilateral allocation of attention.