'Where' depends on 'what': a differential functional anatomy for position discrimination in one- versus two-dimensions

The pantomime of mental rotation: Left-handers are less lateralized

INTRODUCTION

The conceptualization of skilled hand movements (praxis) may be grounded in hemispherically specialized functions. However, a left-hemispherical advantage of (tool-use) pantomime gestures and a right-hemispherical advantage of spatial gestures may be more prominent in right-handed than left-handed individuals. We therefore investigated the hypothesis that right-handed but not left-handed individuals show a superiority of the left hemisphere (/right-hand preference) for the execution of pantomime (rotation of an object) gestures as well as a right-hemispherical superiority (/left-hand preference) for gestures that depict spatial information (/positioning of an object).

METHODS

20 right- and 20 left-handed participants were asked in two experiments to demonstrate with their two hands how to move tachistoscopically (in the left (LVF) or right visual hemifields (RVF)) presented geometric objects of different rotations into an identical final position. Two independent blind raters evaluated the videotaped hand gestures employing the Neuropsychological Gesture (NEUROGES) Coding System.

RESULTS

In contrast to left-handed individuals, right-handed individuals present increased pantomime - rotation gestures with the right hand and pantomime - position gestures with the left hand during stimuli presentation in either visual field. Left-handers showed significantly increased left-hand pantomime - rotation gestures during stimulus presentation within the LVF (only).

DISCUSSION

Right-handed individuals increase their pantomime - rotation gestures with the right hand to depict motion but use their left hand for pantomime - position gestures to describe spatial relations of the objects. Left-handers do not show a clear lateralization of the right and left hand with regards to either handedness or hemispherically lateralized motor functions. The hemispherical lateralization of praxis functions is therefore more pronounced in right-handed than left-handed individuals.

Hemispheric specialization for nonverbal gestures depicting motion and space

INTRODUCTION

The right hemispheric specialisation for mental rotation suggests a left hand preference for nonverbal gestures that depict spatial information. We therefore hypothesized that nonverbal depictions of spatial information are preferentially demonstrated by the left hand, i.e., are grounded in right hemispheric functions.

METHODS

Right-handed participants were asked in two experiments to nonverbally demonstrate how to move tachistoscopically presented (in the left or right visual hemifields) geometric objects of different rotations into an identical final position. Two independent blind raters evaluated the videotaped hand gestures employing the Neuropsychological Gesture (NEUROGES) Coding System.

RESULTS

Pantomime gestures increase in order to rotate gravitationally unstable objects whereas spatial relation presentation gestures increase when to nonverbally demonstrate a gravitationally stable object. Individuals preferred the right hand for pantomime gestures but the left hand for spatial relation presentation gestures.

DISCUSSION

Individuals increase their pantomime gestures to nonverbally depict motion particularly with the right hand, i.e. the left hemisphere. In contrast, increased left hand spatial relation presentations gestures indicate that those gestures are of right hemispheric origin. Thus, the hemispherical lateralization of nonverbal gestures seems to depend on the hands' functional depiction.

Evidence for abnormal visuospatial attentional processes in the interictal migraineur

Research shows decreased brain region activity in the right temporo-parietal junction (rTPJ) in people with migraine headache relative to headache-free controls when performing an orienting visuospatial attention task. Functional inactivation of the rTPJ has been associated with rightward performance deviations on laterality-based attention Landmark (LM) and greyscale (GRE) tasks in individuals with unilateral neglect and heightened activation in the rTPJ is associated with leftward deviation, known as pseudoneglect, in controls on these tasks. Given this, we investigated whether migraineurs would lack the leftward deviation found in headache-free controls on visuospatial attention tasks. 36 migraineurs and 38 controls were presented with LM and GRE tasks. Response bias scores showed a significant difference in responses between groups (p = 0.036) on the GRE, a luminance-based task, but not on the LM, a size-based task (p = 0.826). This study is the first to show laterality-based attentional differences in migraineurs, as compared to controls. Specifically, migraineurs were found to have smaller leftward biases on luminance-based visuospatial attention tasks, as compared to controls, aligning with previous research suggesting that migraine may be having an impact on a variety of attention tasks in migraineurs in between headache attacks.

Visuospatial bias in line bisection in Williams syndrome

BACKGROUND

Recently, a study using the subjective straight-ahead task showed that individuals with Williams syndrome (WS) present a bias in the representation of body perception. The aim of the present study is to examine the horizontal midline body representation in WS participants using the bisection line task, which is an important benchmark for an egocentric frame of reference.

METHOD

Fifteen WS participants (mean age = 21.7 ± 9.5 years) were compared with two typical development control groups: one composed of 15 participants matched on chronological age and one composed of 15 children matched on mental age. The task consisted of dividing each line in a series of 18 lines into two equal halves by drawing a vertical mark with a pencil in the centre of the line.

RESULTS

Individuals with WS presented a significant leftward bias in comparison to mental age and chronological age groups.

CONCLUSIONS

The leftward deviation in WS could be linked to the body representation bias and difficulties in the development of the egocentric reference system. An early detection of such deviation should help in the development of targeted interventions for WS individuals to improve visual-spatial skills and learning.

Mapping the anatomy of perceptual pseudoneglect. A multivariate approach

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

No Interaction between tDCS Current Strength and Baseline Performance: A Conceptual Replication

Several recent studies have reported non-linear effects of transcranial direct current stimulation (tDCS), which has been attributed to an interaction between the stimulation parameters (e.g., current strength, duration) and the neural state of the cortex being stimulated (e.g., indexed by baseline performance ability, age) (see Fertonani and Miniussi, 2016). We have recently described one such non-linear interaction between current strength and baseline performance on a visuospatial attention (landmark) task (Benwell et al., 2015). In this previous study, we induced a small overall rightward shift of spatial attention across 38 participants using bi-hemispheric tDCS applied for 20 min (concurrent left posterior parietal (P5) anode and right posterior parietal (P6) cathode) relative to a sham protocol. Importantly, this shift in bias was driven by a state-dependent interaction between current intensity and the discrimination sensitivity of the participant at baseline (pre-stimulation) for the landmark task. Individuals with high discrimination sensitivity (HDS) shifted rightward in response to low- (1 mA) but not high-intensity (2 mA) tDCS, whereas individuals with low discrimination sensitivity (LDS) shifted rightward with high- but not low-intensity stimulation. However, in Benwell et al. (2015) current strength was applied as a between-groups factor, where half of the participants received 1 mA and half received 2 mA tDCS, thus we were unable to compare high and low-intensity tDCS directly within each individual. Here we aimed to replicate these findings using a within-group design. Thirty young adults received 15 min of 1 and 2 mA tDCS, and a sham protocol, each on different days, to test the concept of an interaction between baseline performance and current strength. We found no overall rightward shift of spatial attention with either current strength, and no interaction between performance and current strength. These results provide further evidence of low replicability of non-invasive brain stimulation protocols, and the need for further attempts to replicate the key experimental findings within this field.

Perceptual Biases in the Horizontal and Vertical Dimensions are Driven by Separate Cognitive Mechanisms

Perceptual attention in healthy participants is characterized by two biases, one operating in the horizontal plane, which draws attention leftward, and the other operating in the vertical plane, which draws attention upward. Given that these biases are reliably found in the same individual, and appear similar at a surface level, a number of researchers have investigated the relationship between horizontal and vertical attentional biases. To date, these investigations have failed to find an association, and this may be due to the fact that one-dimensional vertical and horizontal stimuli were presented separately rather than being measured from a single, two-dimensional stimulus. Across three experiments, two dimensional stimuli were presented, and participants marked the centre of the stimuli. In addition, the shapes of the stimuli were manipulated to determine whether this produced the same modulation of the two biases. Across 13 stimuli and three experiments there were no correlations between the vertical and horizontal biases. In addition, manipulations of stimulus shape, which affected biases in one dimension, did not affect biases in the other dimension. There were, however, consistent correlations between the degree of bias within each dimension across the different stimuli. This study has produced converging evidence that horizontal and vertical biases in spatial judgments rely on separate cognitive mechanisms. To account for these results we discuss a model whereby horizontal asymmetries rely more on space-based mechanisms whereas vertical asymmetries rely more on object-based mechanisms.

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.

A toggle switch of visual awareness?

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

A rightward shift in the visuospatial attention vector with healthy aging

The study of lateralized visuospatial attention bias in non-clinical samples has revealed a systematic group-level leftward bias (pseudoneglect), possibly as a consequence of right hemisphere (RH) dominance for visuospatial attention. Pseudoneglect appears to be modulated by age, with a reduced or even reversed bias typically present in elderly participants. It has been suggested that this shift in bias may arise due to disproportionate aging of the RH and/or an increase in complementary functional recruitment of the left hemisphere (LH) for visuospatial processing. In this study, we report rightward shifts in subjective midpoint judgment relative to healthy young participants whilst elderly participants performed a computerized version of the landmark task (in which they had to judge whether a transection mark appeared closer to the right or left end of a line) on three different line lengths. This manipulation of stimulus properties led to a similar behavioral pattern in both the young and the elderly: a rightward shift in subjective midpoint with decreasing line length, which even resulted in a systematic rightward bias in elderly participants for the shortest line length (1.98° of visual angle, VA). Overall performance precision for the task was lower in the elderly participants regardless of line length, suggesting reduced landmark task discrimination sensitivity with healthy aging. This rightward shift in the attentional vector with healthy aging is likely to result from a reduction in RH resources/dominance for attentional processing in elderly participants. The significant rightward bias in the elderly for short lines may even suggest a reversal of hemisphere dominance in favor of the LH/right visual field under specific conditions.

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

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

Representational pseudoneglect: a review

Pseudoneglect, the tendency to be biased towards the left-hand side of space, is a robust and consistent behavioural observation best demonstrated on the task of visuospatial line bisection, where participants are asked to centrally bisect visually presented horizontal lines at the perceived centre. A number of studies have revealed that a representational form of pseudoneglect exists, occurring when participants are asked to either mentally represent a stimulus or explore a stimulus using touch in the complete absence of direct visuospatial processing. Despite the growing number of studies that have demonstrated representational pseudoneglect there exists no current and comprehensive review of these findings and no discussion of a theoretical framework into which these findings may fall. An important gap in the current representational pseudoneglect literature is a discussion of the developmental trajectory of the bias. The focus of the current review is to outline studies that have observed representational pseudoneglect in healthy participants, consider a theoretical framework for these observations, and address the impact of lifespan factors such as cognitive ageing on the phenomenon.

Left-ear-driven representational pseudoneglect for mentally represented real-word scenes created from aural-verbal description

The current research explored pseudoneglect for the mental representation of real-world scenes generated from aural-verbal description in the complete absence of direct visual processing. Healthy participants listened binaurally or monaurally to aural-verbal descriptions of novel real-world scenes with familiar landmarks (e.g., 'shop', 'cafe', 'school') to be imagined on the left- or right-hand side. Participants were asked to mentally represent the street scene using a visuospatial template though it was up to participants how they mentally represented each individual landmark within the street (i.e., in terms of colour and size). There were two main tasks: a relative judgement task (which side of the street contains the most landmarks?) and a recall task (recall the landmarks on the left vs. right side of the street). When stimuli were presented monaurally to the left ear (favouring the activation of the right hemisphere) participants demonstrated representational pseudoneglect and showed a bias towards responding that there were more landmarks on the left compared to the right. However, this did not lead to enhanced recall for left side landmarks. When stimuli were presented binaurally or monaurally to the right ear, there was no evidence of representational pseudoneglect for the relative judgement or recall task. The current study discusses how the use of monaural presentation may boost right hemisphere activation in aural-verbal experimental paradigms designed to explore representational pseudoneglect.

Space representation in children with dyslexia and children without dyslexia: contribution of line bisection and circle centering tasks

Line bisection tasks (different space locations and different line lengths) and circle centering tasks (visuo-proprioceptive and proprioceptive explorations, with left or right starting positions) were used to investigate space representation in children with dyslexia and children without dyslexia. In line bisection, children with dyslexia showed a significant rightward bias for central and right-sided locations and a leftward bias for left-sided location. Furthermore, the spatial context processing was asymmetrically more efficient in the left space. In children without dyslexia, no significant bias was observed in central lines but the spatial context processing was symmetrical in both spaces. When the line length varied, no main effect was shown. These results strengthen the 'inverse pseudoneglect' hypothesis in dyslexia. In the lateral dimension of the circle centering tasks, children showed a response bias in the direction of the starting hand location for proprioceptive condition. For radial dimension, the children showed a forward bias in visuo-proprioceptive condition and more backward error in proprioceptive condition. Children with dyslexia showed a forward bias in clockwise exploration and more accurate performance in counterclockwise exploration for left starting position which may be in accordance with leftward asymmetrical spatial context processing in line bisection. These results underline the necessity to use the line bisection task with different locations as an appropriate experimental paradigm to study lateral representational bias in dyslexia. The contribution of the present results in the understanding of space representation in children with dyslexia and children without dyslexia is discussed in terms of attentional processes and neuroanatomical substrate.

Individual differences in solving arithmetic word problems

BACKGROUND

With the present functional magnetic resonance imaging (fMRI) study at 3 T, we investigated the neural correlates of visualization and verbalization during arithmetic word problem solving. In the domain of arithmetic, visualization might mean to visualize numbers and (intermediate) results while calculating, and verbalization might mean that numbers and (intermediate) results are verbally repeated during calculation. If the brain areas involved in number processing are domain-specific as assumed, that is, that the left angular gyrus (AG) shows an affinity to the verbal domain, and that the left and right intraparietal sulcus (IPS) shows an affinity to the visual domain, the activation of these areas should show a dependency on an individual's cognitive style.

METHODS

36 healthy young adults participated in the fMRI study. The participants habitual use of visualization and verbalization during solving arithmetic word problems was assessed with a short self-report assessment. During the fMRI measurement, arithmetic word problems that had to be solved by the participants were presented in an event-related design.

RESULTS

We found that visualizers showed greater brain activation in brain areas involved in visual processing, and that verbalizers showed greater brain activation within the left angular gyrus.

CONCLUSIONS

Our results indicate that cognitive styles or preferences play an important role in understanding brain activation. Our results confirm, that strong visualizers use mental imagery more strongly than weak visualizers during calculation. Moreover, our results suggest that the left AG shows a specific affinity to the verbal domain and subserves number processing in a modality-specific way.

Representational Pseudoneglect in an Auditory-Driven Spatial Working Memory Task

Two experiments explored lateralized biases in mental representations of matrix patterns formed from aural verbal descriptions. Healthy participants listened, either monaurally or binaurally, to verbal descriptions of 6 by 3 matrix patterns and were asked to form a mental representation of each pattern. In Experiment 1, participants were asked to judge which half of the matrix, left or right, contained more filled cells and to rate the certainty of their judgement. Participants tended to judge that the left side was fuller than the right and showed significantly greater certainty when judging patterns that were fuller on the left. This tendency was particularly strong for left-ear presentation. In Experiment 2, participants conducted the same task as that in Experiment 1 but were also asked to recall the pattern for the side judged as fuller. Participants were again more certain in judging patterns that were fuller on the left—particularly for left-ear presentation—but were no more accurate in remembering the details from the left. These results suggest that the left side of the mental representation was represented more saliently but it was not remembered more accurately. We refer to this lateralized bias as “representational pseudoneglect”. Results are discussed in terms of theories of visuospatial working memory.

Age-related changes in visual pseudoneglect

Pseudoneglect is a slight but consistent leftward attentional bias commonly observed in healthy young populations, purportedly explained by right hemispheric dominance. It has been suggested that normal aging might be associated with a decline of the right hemisphere. According to this hypothesis, a few studies have shown that elderly tend to exhibit a rightward attentional bias in line bisection. In the present study, we tested this hypothesis in young and older participants using a perceptual landmark task. Results yield evidence for an age-related shift, from a strong attentional leftward bias in young adults toward a suppressed or even a reversed bias in the elderly. Right hemisphere impairment coupled to a left hemispheric compensation might explain the perceptual shift observed in older adults. However, a decline in corpus callosum function cannot be excluded. Alternatively, these results may be in agreement with the hypothesis of an age-related specific inhibition of return dysfunction, an overt attentional orienting mechanism, and/or a decrease of dopamine.

Inter-hemispheric functional coupling of eyes-closed resting EEG rhythms in adolescents with Down syndrome

OBJECTIVE

We tested the hypothesis that inter-hemispheric directional functional coupling of eyes-closed resting EEG rhythms is abnormal in adolescents with Down syndrome (DS).

METHODS

Eyes-closed resting EEG data were recorded in 38 DS adolescents (18.7 years +/-0.67 SE, IQ=49+/-1.9 SE) and in 17 matched normal control subjects (NYoung=19.1 years +/-0.39 SE). The EEG data were recorded from 8 electrodes (Fp1, Fp2, C3, C4, T3, T4, O1, O2) referenced to vertex. EEG rhythms of interest were delta (2-4 Hz), theta (4-8 Hz), alpha 1 (8-10.5 Hz), alpha 2 (10.5-13 Hz), beta 1 (13-20 Hz), and beta 2 (20-30 Hz). Power of EEG rhythms was evaluated by FFT for control purposes, whereas inter-hemispheric directional EEG functional coupling was computed by directed transfer function (DTF).

RESULTS

As expected, alpha, beta, and gamma power was widely higher in NYoung than DS subjects, whereas the opposite was true for delta power. As a novelty, DTF (directionality) values globally prevailed from right to left occipital areas in NYoung subjects and in the opposite direction in DS patients. A control experiment showed that this DTF difference could not be observed in the comparison between DS adults with mild cognitive impairment and normal age-matched adults.

CONCLUSIONS

These results indicate a peculiar abnormal directional inter-hemispheric interplay in visual occipital areas of DS adolescents.

SIGNIFICANCE

Direction of inter-hemispheric EEG functional coupling unveils a new abnormal brain network feature in DS adolescents.

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.

Spatial neglect: clinical and neuroscience review: a wealth of information on the poverty of spatial attention

Hemispatial neglect (HSN) is a frequent, conspicuous neurobehavioral accompaniment of brain injury. Patients with HSN share several superficial similarities, leading earlier clinical neuroscientists to view neglect as a unitary condition associated with brain structures that mediate relatively discrete spatial cognitive mechanisms. Over the last two decades, research largely deconstructed the neglect syndrome, revealing a remarkable heterogeneity of behaviors and providing insight into multiple component processes, both spatial and nonspatial, that contribute to hemispatial neglect. This review surveys visual HSN, presenting first the means for detection and diagnosis in its manifold variations. We summarize cognitive operations relevant to spatial attention and evidence for their role in neglect behaviors and then briefly consider neural systems that may subserve the component processes. Finally, we propose several methods for rehabilitating HSN, including the challenges facing remediation of such a heterogeneous cognitive disorder.

Visuospatial interpolation in typically developing children and in people with Williams Syndrome

Visuospatial interpolation is the estimation of object position or contour shape computed from known "anchor" positions. We characterized the developmental profile of interpolation by measuring positional thresholds as a function of inter-element separation without (Experiment 1) and with (Experiment 2) the context of illusory contours in typically developing children, typical adults and individuals with Williams Syndrome (WS), a genetic disorder that causes impaired global visuospatial abilities. We found that typically developing children and WS individuals had more difficulty integrating information across distant elements than typical adults. However, illusory contours improved thresholds in all participant groups in a similar way. Our results suggest that in WS individuals, and in typically developing children, the grouping mechanisms that enable long-range spatial integration are immature. We hypothesize that WS individuals and young children can use stimulus-driven grouping cues for bottom-up integration, but have immature mechanisms for top-down integration of spatial information.

Neural basis for priming of pop-out during visual search revealed with fMRI

Maljkovic and Nakayama first showed that visual search efficiency can be influenced by priming effects. Even "pop-out" targets (defined by unique color) are judged quicker if they appear at the same location and/or in the same color as on the preceding trial, in an unpredictable sequence. Here, we studied the potential neural correlates of such priming in human visual search using functional magnetic resonance imaging (fMRI). We found that repeating either the location or the color of a singleton target led to repetition suppression of blood oxygen level-dependent (BOLD) activity in brain regions traditionally linked with attentional control, including bilateral intraparietal sulci. This indicates that the attention system of the human brain can be "primed," in apparent analogy to repetition-suppression effects on activity in other neural systems. For repetition of target color but not location, we also found repetition suppression in inferior temporal areas that may be associated with color processing, whereas repetition of target location led to greater reduction of activation in contralateral inferior parietal and frontal areas, relative to color repetition. The frontal eye fields were also implicated, notably when both target properties (color and location) were repeated together, which also led to further BOLD decreases in anterior fusiform cortex not seen when either property was repeated alone. These findings reveal the neural correlates for priming of pop-out search, including commonalities, differences, and interactions between location and color repetition. fMRI repetition-suppression effects may arise in components of the attention network because these settle into a stable "attractor state" more readily when the same target property is repeated than when a different attentional state is required.

The neural mechanisms underlying the Müller-Lyer illusion and its interaction with visuospatial judgments

Arrows terminating a line can distort the perceived line length. This so-called Müller-Lyer illusion can be used in healthy human subjects to mimic the performance of neglect patients in visuospatial judgments (e.g., in the landmark task). In this study, we investigated the neural mechanisms underlying the Müller-Lyer illusion, the landmark task, and their interaction. This was achieved by parametrically manipulating the magnitude of the Müller-Lyer illusion both in a landmark and in a luminance (control) task. As expected, the landmark task activated right posterior parietal cortex and right temporo-occipital cortex. In contrast, the neural processes associated with the strength of the Müller-Lyer illusion were located bilaterally in the lateral occipital cortex as well as the right superior parietal cortex. The data not only converge with but also extend neuropsychological data that indicate maintained line-length illusion in neglect patients. In addition, our results support the size-constancy scaling hypothesis as a putative mechanism underlying line-length illusions. Furthermore, activation that was driven by both the task and the strength of the Müller-Lyer illusion was observed in right intraparietal sulcus, thus arguing in favor of an interaction of illusory information with the top-down processes underlying visuospatial judgments in right parietal cortex.

Processing content or location: Distinct brain activation in a memory task

Objects are defined by their content ("what") and by their location ("where"). In the visual system, processing of these two types of information is segregated into distinct anatomical and functional pathways. Using H2 (15)O positron emission tomography to measure cerebral blood flow, we examined the differences in processing of "what" when compared with "where" information in human memory. We found that the detection of deviations from a previously learned image sequence activates distinct brain regions depending on whether the image's content or its location has changed. When deviations of an image's content had to be detected, the left medial temporal lobe (MTL) activation increased. In contrast, detection of deviations from the learned locations of the objects induced increased activation in the right MTL and in the right parietal cortex. These data demonstrate distinct contributions of the left and right MTL to the processing of "what" vs. "where" in memory.

Visuospatial neglect in near and far space: dissociation between line bisection and letter cancellation

The differential performance on a line bisection and a cancellation task in near and far space was studied. A group of 10 patients with severe left-sided visuospatial neglect and a group of 10 right-brain damaged patients without neglect were examined. The stimuli were presented at a distance of 60 cm (near space) and 160 cm (far space), respectively, and corrected for visual angle. In the line bisection task, patients were asked to point to the estimated line centre with a pencil (near space) or a stick (far space). In the cancellation task, patients pointed to all target stimuli they could detect using either a pencil (near space) or a stick (far space). Most patients with left hemineglect showed a more prominent neglect in far space as compared to near space for the line bisection task, whereas no difference of performance between near and far space was found in the control patients. In contrast, no group showed a distance effect in the cancellation task. The observation that only line bisection is influenced by the distance of the stimulus suggests that line bisection and cancellation are processed differentially. It is proposed that line bisection requires an allocentric reference system focusing attention on objects, whereas cancellation tasks are based on an egocentric reference system responsible for visuospatial attention. Our results indicate that distance changes perception within the allocentric but not within the egocentric system.

Reference Frames for Spatial Cognition: Different Brain Areas are Involved in Viewer-, Object-, and Landmark-Centered Judgments About Object Location

Functional magnetic resonance imaging was used to compare the neural correlates of three different types of spatial coding, which are implicated in crucial cognitive functions of our everyday life, such as visuomotor coordination and orientation in topographical space. By manipulating the requested spatial reference during a task of relative distance estimation, we directly compared viewer-centered, object-centered, and landmark-centered spatial coding of the same realistic 3-D information. Common activation was found in bilateral parietal, occipital, and right frontal premotor regions.

The retrosplenial and ventromedial occipital–temporal cortex (and parts of the parietal and occipital cortex) were significantly more activated during the landmark-centered condition. The ventrolateral occipital–temporal cortex was particularly involved in object-centered coding. Results strongly demonstrate that viewer-centered (egocentric) coding is restricted to the dorsal stream and connected frontal regions, whereas a coding centered on external references requires both dorsal and ventral regions, depending on the reference being a movable object or a landmark.

Performing allocentric visuospatial judgments with induced distortion of the egocentric reference frame: an fMRI study with clinical implications

The temporary improvement of visuospatial neglect during galvanic vestibular stimulation (Scand. J. Rehabil. Med. 31 (1999)117) may result from correction of the spatial reference frame distorted by the responsible lesion. Prior to an investigation of the neural basis of this effect in neurological patients, exploration of the neural mechanisms underlying such procedures in normals is required to provide insight into the physiological basis thereof. Despite their clinical impact, the neural mechanisms underlying the interaction of galvanic (and other) vestibular manipulations with visuospatial processing (and indeed the neural bases of how spatial reference frames are computed in man) remain to be clarified. We accordingly used fMRI in normal volunteers to investigate the effect of galvanically induced interference with the egocentric spatial reference frame on the neural processes underlying allocentric visuospatial (line bisection) judgments. A significant specific interaction of galvanic vestibular stimulation with the neural mechanisms underlying allocentric visuospatial judgments was observed in right posterior parietal and ventral premotor cortex only. Activation of these areas previously found to be damaged in visuospatial neglect suggests that these effects reflect the increased processing demands when compensating for the distorted egocentric spatial reference frame while maintaining accurate performance during the allocentric spatial task. These results thus implicate right posterior parietal and right ventral premotor cortex in the computation of spatial reference frames. Furthermore, our data imply a specific physiological basis for the temporary improvement of visuospatial neglect in patients with right hemisphere lesions during galvanic vestibular stimulation and may thus impact upon the rehabilitation of neglect: understanding the interaction of galvanic vestibular stimulation with allocentric visuospatial judgments in healthy volunteers may lead to the more effective deployment of such techniques in neurological patients.

Stimulus symmetry affects the bisection of figures but not lines: evidence from event-related fMRI

Many geometric shapes retain their symmetry when bisected, but appear asymmetrical when misbisected. We have previously shown that this correspondence can guide the accuracy and speed of perceptual bisection (Landmark) judgments. Using event-related fMRI, here we examined whether the behavioural effects of symmetry are also evident at the neural level. The data showed that the presence/absence of symmetry modulates the activity of right anterior cingulate gyrus, an area associated with a variety of higher level attentional functions. A previous visual half-field study also showed that bisected lines are apprehended more quickly and accurately than misbisected lines in right, but not left, visual field. We were able to localise this advantage to right superior temporal gyrus. Significantly, we found no evidence that symmetry played a role in apprehending the midpoint of the line stimuli traditionally used to assess visual neglect. The data clarify the effects of visual symmetry on bisection behaviour, and highlight novel dissociations within the neural systems thought to underline Landmark performance.

Right hemisphere control of visuospatial attention: line-bisection judgments evaluated with high-density electrical mapping and source analysis

The "line-bisection" task has proven an especially useful clinical tool for assessment of spatial neglect syndrome in neurological patients. Here, we investigated the neural processes involved in performing this task by recording high-density event-related potentials from 128 scalp electrodes in normal observers. We characterized a robust net negative potential from 170-400 ms poststimulus presentation that correlates with line-bisection judgments. Topographic mapping shows three distinct phases to this negativity. The first phase (approximately 170-190 ms) has a scalp distribution exclusively over the right parieto-occipital and lateral occipital scalp, consistent with generators in the region of the right temporo-parietal junction and right lateral occipital cortices. The second phase (approximately 190-240 ms) sees the emergence of a second negative focus over the right central parietal scalp, consistent with subsequent involvement of right superior parietal cortices. In the third phase (approximately 240-400 ms), the topography becomes dominated by this right central parietal negativity. Inverse source modeling confirmed that right hemisphere lateral occipital, inferior parietal, and superior parietal regions were the likeliest generators of the bulk of the activity associated with this effect. The line stimuli were also presented at three contrast levels (3, 25, and 100%) in order to manipulate both the latency of stimulus processing and the relative contributions from magnocellular and parvocellular inputs. Through this manipulation, we show that the line-bisection effect systematically tracks/follows the latency of the N1 component, which is considered a temporal marker for object processing in the ventral visual stream. This pattern of effects suggests that this task invokes an allocentric (object-based) form of visuospatial attention. Further, at 3% contrast, the line-bisection effect was equivalent to the effects seen at higher contrast levels, suggesting that parvocellular inputs are not necessary for successful performance of this task.

Spatial cognition: evidence from visual neglect

Recent work on human attention and representational systems has benefited from a growing interplay between research on normal attention and neuropsychological disorders such as visual neglect. Research over the past 30 years has convincingly shown that, far from being a unitary condition, neglect is a protean disorder whose symptoms can selectively affect different sensory modalities, cognitive processes, spatial domains and coordinate systems. These clinical findings, together with those of functional neuroimaging, have increased knowledge about the anatomical and functional architecture of normal subsystems involved in spatial cognition. We provide a selective overview of how recent investigations of visual neglect are beginning to elucidate the underlying structure of spatial processes and mental representations.

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.

Task instructions influence the cognitive strategies involved in line bisection judgements: evidence from modulated neural mechanisms revealed by fMRI

Manual line bisection and a perceptual variant thereof (the Landmark test) are widely used to assess visuospatial neglect in neurological patients, but little is known about the cognitive strategies involved. In the Landmark test, one could explicitly compare the lengths of the left and right line segments; alternatively, one could compute the centre of mass of the display. We here investigate with functional MRI if these cognitive strategies modulate the neural mechanisms underlying judgements whether pre-transected horizontal lines are correctly bisected (the Landmark test) in normal volunteers. Functional neuroimaging (fMRI) was carried out in 12 healthy volunteers who judged: (a) whether the line segments on either side of the transection mark were of equal length, and (b) whether the transection mark was in the centre of the line. Line centre judgements were made significantly faster than line length comparisons. Increased neural activity common to both strategies was observed in inferior parietal lobes bilaterally and right temporooccipital cortex. Further activations, most likely reflecting general task demands like response selection and motor control, were found in the precentral gyrus bilaterally, supplementary motor area bilaterally, right anterior cingulate, right dorsolateral prefrontal cortex, cerebellar vermis, and right thalamus and right putamen. Explicit length comparisons (relative to line centre judgements) differentially activated left superior posterior parietal cortex, with a tendency toward activation of the equivalent area on the right, while the reverse comparison revealed differential activation in the lingual gyrus bilaterally and anterior cingulate cortex. The activations observed in inferior parietal cortex during task performance using either strategy are consistent with the results of lesion studies. The differential activation of superior posterior parietal cortex following length instructions suggests that explicit comparisons of spatial extent were implicated. The differential activation of bilateral occipital cortex following centre judgements suggests that the centre of a line is extracted at an early stage of visual processing.

The neural basis of vertical and horizontal line bisection judgments: an fMRI study of normal volunteers

Bisection of horizontal lines is used as a clinical test of spatial cognition in patients with left visuospatial neglect after right hemisphere lesions. Bisection of vertical lines has also been employed, albeit less frequently. Interestingly, normal subjects often bisect horizontal lines too far left and vertical lines too high. We used fMRI to investigate whether vertical/horizontal stimulus orientation interacts with the neural mechanisms associated with line bisection judgments (the Landmark task). For control of orientation per se, subjects performed a visual detection task with the same stimuli. Statistical analysis of evoked BOLD responses employed SPM99. The Landmark task increased neural activity (P < 0.05, corrected) in the superior and inferior parietal lobes bilaterally, though predominantly on the right; early visual processing areas bilaterally; and cerebellar vermis, left cerebellar hemisphere, anterior cingulate, and prefrontal cortex bilaterally. Vertical lines (relative to horizontal lines and vice versa) increased neural activity in early visual processing areas, consistent with differential retinotopic stimulation. In addition, vertical lines activated right parietooccipital and superior posterior parietal cortex bilaterally. No significant interactions between the neural mechanisms associated with task and stimuli were observed. Increased neural activation in parietal and parietooccipital cortex associated with vertical lines may reflect increased attentional demands associated with this stimulus orientation. The right hemispheric dominance observed in posterior parietal during the Landmark task irrespective of stimulus orientation is consistent with lesion studies. Our results suggest that the behavioral patterns observed in normal subjects and neurological patients result from different stimulus effects rather than differential task demands.

Do we need the "lateral" in unilateral neglect? Spatially nonselective attention deficits in unilateral neglect and their implications for rehabilitation

This paper reviews evidence that the spatial imbalance of attention by which unilateral neglect is often defined is a necessary but not sufficient factor in the persistence of this disorder. A second, nonspatially lateralized loss of attentional capacity is required to coexist with the spatial bias for the disorder to persist in a clinically significant way, it is argued. To this end, I first review evidence from visual and auditory attention studies to show that unilateral neglect is very strongly associated with a fundamental loss of attentional capacity that is not confined to one region of space. Second, I attempt to characterize the nature of this attentional capacity in relation in particular to functional brain imaging studies of attention. Third, I demonstrate that the spatial imbalance in neglect can be reduced by manipulations of the nonlateralized attentional capacity and finish by reviewing the rehabilitation implications of these manipulations.

The effects of case mixing on word recognition: evidence from a PET study

The early stages of visual word recognition were investigated by scanning participants using PET as they took part in implicit and explicit reading tasks with visually disrupted stimuli. CaSe MiXiNg has been shown in behavioral studies to increase reaction times (RTs) in naming and other word recognition tasks. In this study, we found that during both an implicit (feature detection) task and an explicit word-naming task, mixed-case words compared to same-case words produced increased activation in an area of the right parietal cortex previously associated with visual attention. No effect of case was found in this area for pseudowords or consonant strings. Further, lowering the contrast of the stimuli slowed RTs as much as case mixing, but did not lead to the same increase in right parietal activation. No significant effect of case mixing was observed in left-hemisphere language areas. The results suggest that reading mixed-case words requires increased attentional processing. However, later word recognition processes may be relatively unaffected by the disruption in presentation.

Spatial Cognition: Where We Were and Where We Are

Some highlights of the ongoing study of visuospatial cognition from Descartes to the advent of functional neuroimaging are reviewed. We emphasize that parietal cortex contains multiple representations of space, appropriate to the demands of perception and action in near and far space. Converging evidence from the behavioral effects of relatively focal brain lesions on different aspects of spatial cognition and from the locus of maximal physiological activation when normal volunteers perform spatial tasks is described. Clinical pathologies of spatial attention, including visual extinction, simultanagnosia, and unilateral neglect, are examined for the light they cast on the basic functions of brain circuits involving the parietal lobes.

Cyto-, Myelo-, and Receptor Architectonics of the Human Parietal Cortex

Various cyto- and myeloarchitectonic maps of the human parietal cortex have been published since the beginning of the past century. However, the parietal lobe remains an uncharted region, since these anatomical findings fail to explain the much greater areal differentiation, especially in the posterior parietal cortex, which has recently been revealed by functional imaging studies. This lack of congruence does not imply a total lack of correspondence between anatomical and functional data, since several practically forgotten architectonic studies published during the first 5 decades of the past century demonstrate a much more differentiated map of the parietal cortex than the popular map of Brodmann and others. Moreover, recent receptor-architectonic studies also demonstrate a detailed architectonic pattern the functional aspects of which will be explored in the near future.

Neuroimaging of cognitive functions in human parietal cortex

Functional neuroimaging has proven highly valuable in mapping human sensory regions, particularly visual areas in occipital cortex. Recent evidence suggests that human parietal cortex may also consist of numerous specialized subregions similar to those reported in neurophysiological studies of non-human primates. However, parietal activation generalizes across a wide variety of cognitive tasks and the extension of human brain mapping into higher-order "association cortex" may prove to be a challenge.