Monika Harvey

Line bisection errors in visual neglect: misguided action or size distortion?

The rightward line bisection errors made by patients with visuospatial neglect can be explained as due to a spatially misdirected response, which would be predicted on either of two accounts. An alternative view, however, is that such patients actually misperceive the left half of a horizontal line as being shorter than the right half. We have tested this possibility directly in three neglect patients, by giving them prebisected lines: they were found to judge a central transection mark as lying nearer to the left end of the lines. We were also able to test one of the patients on a series of size comparisons using computer-generated patterns. She was found to judge horizontal lines as shorter in the left half of visual space than in the right. This was also true for comparisons of the areas of nonsense figures. However she did not make such constant errors when comparing the lengths of vertical lines. It is suggested that an attentional deficit in left hemispace may result in the underestimation of horizontal extent. This would act in combination with misdirected reaching to determine the magnitude of line bisection errors.

Refixation frequency and memory mechanisms in visual search

Visual search-looking for a target object in the presence of a number of distractor items-is an everyday activity for humans (for example, finding the car in a busy car park) and animals (for example, foraging for food). Our understanding of visual search has been enriched by an interdisciplinary effort using a wide range of research techniques including behavioural studies in humans [1], single-cell electrophysiology [2], transcranial magnetic stimulation [3], event-related potentials [4] and studies of patients with focal brain injury [5]. A central question is what kind of information controls the search process. Visual search is typically accompanied by a series of eye movements, and investigating the nature and location of fixations helps to identify the kind of information that might control the search process. It has already been demonstrated that objects are fixated if they are visually similar to the target [6]. Also, if an item has been fixated, it is less likely to be returned to on the subsequent saccade. This automatic process is referred to as inhibition of return (IOR [7,8]). Here, we investigated the role of memory for which items had been fixated previously. We found that, during search, subjects often refixated items that had been previously fixated. Although there were fewer return saccades than would be expected by chance, the number of refixations indicated limited functional memory, indeed the memory effects that were present may primarily be a result of IOR.

Distortion of size perception in visuospatial neglect

BACKGROUND A number of studies have shown that most patients with symptoms of unilateral (left-sided) visuospatial neglect make consistently rightward errors when attempting to bisect a horizontal line at its midpoint. One possible interpretation of this impairment is that such patients misperceive the left half of the line: that is, that they underestimate its extent relative to the right half. RESULTS We have carried out direct tests for such a perceptual distortion in three neglect patients by asking them to make matching judgements on pairs of horizontal rectangles, vertical rectangles or nonsense shapes, of varying relative size, presented on a computer screen. We report here that all of the patients tested showed a significant and substantial relative underestimation of the horizontal extent or area of stimuli presented on the left side of their egocentric space. There was no such misperception of vertical extent. CONCLUSIONS It is suggested that size perception may be partially determined by a representational system that is anatomically centred in the parieto-temporal region of the brain. The results are interpreted in terms of damage to this system in neglect patients.

Visuomotor sensitivity for shape and orientation in a patient with visual form agnosia

We have previously demonstrated that a patient with visual form agnosia (DF), who is unable to report the orientation or size of visual targets, can nevertheless use these same visual attributes to control motor acts. In the first of three new experiments, we found that DF is able to grasp everyday tools and utensils proficiently (i.e. with a well-formed hand posture) but has difficulty in visually selecting the correct part of the object to grasp (e.g. the handle) for subsequent use of that object. A second experiment revealed that DFs visuomotor system is able to adjust concurrently to variations in both the size and orientation of target objects; when these visual attributes were both varied, she adjusted both her grip aperture and the orientation of her hand well in advance of target contact. These spared visuomotor abilities do not seem to extend to shape processing per se, however. In the final experiment we found that DF was insensitive to changes in the orientation of a cross-shaped object, where no single principal axis could be extracted to control orientation of the grasp. These observations extend our knowledge of DFs residual visuomotor abilities, and suggest limitations on the visual processing capacities of the human dorsal stream.

Evidence for a systematic component within scan paths in visual search

We present evidence that scan paths in visual search can include a systematic component. The task for subjects in the experiment was to search for a target that was either present or absent. With regular grid-like displays, participants generated more horizontal saccades than vertical saccades. Disruption of the grid structure in the display modulated but did not eliminate the systematic component. This is consistent with the scan path being partly determined by a cognitive strategy. We discuss the implications of this finding for studies that use refixation to investigate memory mechanisms in visual search.

Visual size processing in spatial neglect

Abstract Evidence from the use of the landmark task and from two size-matching tasks shows that many patients with left-sided neglect systematically under-perceive visual extent in leftward parts of space. This perceptual distortion of size serves to explain the occurrence of rightward line-bisection errors in most neglect patients. One possibility is that attentional biases of a chronic nature might underlie these perceptual changes seen in neglect patients. But contrary to this idea, we have found that attentional cueing in the landmark task causes changes in neglect patients exactly opposite to those seen in healthy subjects. The distortions of size perception seen in neglect could instead be caused by a high-level alteration of visual processing rather than by an attentional bias. In order to explore which visual stream of cortical processing might be compromised in such a disorder, we have begun to examine neglect patients on visuomotor as well as perceptual tasks. We have found clear evidence in one patient for intact grip scaling for object size in the neglected half of space, despite gross perceptual underestimations of the same objects. This result suggests that neglect can occur without major disruption of the dorsal stream, and may result instead from damage to a cortical system whose predominant visual input comes from the ventral stream.

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 (100–200 ms) 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.

Perceptual biases in chimeric face processing: eye-movement patterns cannot explain it all.

Experiments using chimeric faces typically report a perceptual bias towards the viewers left. Here we show that this leftward bias can be elicited even when eye movement should be impossible. Although supporting previous studies arguing that eye movements are not necessary to generate the bias, the effect we found here was significantly reduced, compared to an earlier study which allowed eye movements. We suggest that the chimeric face bias is enhanced by eye movements.

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.

The effects of visuomotor feedback training on the recovery of hemispatial neglect symptoms: assessment of a 2-week and follow-up intervention

In patients suffering from left unilateral neglect, their right-biased attention to the phenomenal world can be ameliorated, short-term, by making motor responses to left-right extended objects (rods) that immediately reveal to them that their phenomenal world is in fact skewed. In this study the extent to which more intensive experiences of this type produced enduring and useful improvements in neglect, was assessed by first examining the effect of a 3-day experimenter-administered practice of rod lifting, then by examining the effects of a self-administered practice for a further 2-week period and a further 1 month post-training. Despite the fact that by the time the patients were able to undergo the intervention they had progressed to the chronic neglect stage, significant improvements of the intervention over the control group were found for a third of the tests given after the 3-day practice. Additionally, at the 1-month follow-up the intervention group again showed significantly better results in 46% of the direct neglect tests. As far as we are aware this is the first time that significant long-term improvements have been shown in a rehabilitation approach with neglect patients with a mean time of more than 12 months post-stroke and visuomotor feedback training can thus be seen as a most encouraging paradigm for future attempts.