Characterizing exploration behavior in spatial neglect: omissions and repetitive search

Spatial working memory predicts re-cancellation behaviour in neglect

The lateralised bias of spatial neglect can be modulated by concurrent non-lateralised impairments. For instance, people with left neglect may have spatial working memory deficits that prevent them from keeping track of locations visited in visual search tasks such as target cancellation. Not only do they omit targets in some parts of the array but they may revisit and re-cancel targets in other parts, and this re-cancellation behaviour increases dramatically in 'invisible' conditions, in which touching a target leaves no visible trace. It has been proposed that spatial memory deficits are the main reason for the rise of re-cancellation errors in invisible cancellation conditions. This idea predicts that spatial memory abilities should correlate with re-cancellation behaviour; but this expected relationship has never been demonstrated. The present study takes an exploratory approach to describing the behaviour of 18 people with left visual neglect, following right hemisphere stroke, on touchscreen tests of spatial working memory and target cancellation. We show that people with neglect who are less able to remember locations in a spatial memory task tend to make more re-cancellation errors in invisible cancellation conditions. We also describe an apparent trade-off, in which some people with neglect make many more re-cancellation errors, whilst others make many more target omissions. We suggest that the influence of spatial memory deficits on invisible cancellation tasks can be more fully captured by considering both types of errors, rather than re-cancellations only.

Visual neglect: is there a relationship between impaired spatial working memory and re-cancellation?

In visual search tasks, neglect patients tend to explore and repeatedly re-cancel stimuli on the ipsilesional side, as if they did not realize that they had previously examined the rightward locations favoured by their lateral bias. The aim of this study was to explore the hypothesis that a spatial working memory deficit explains these ipsilesional re-cancellation errors in neglect patients. For the first time, we evaluated spatial working memory and re-cancellation through separate and independent tasks in a group of patients with right hemisphere damage and a diagnosis of left neglect. Results showed impaired spatial working memory in neglect patients. Compared to the control group, neglect patients cancelled fewer targets and made more re-cancellations both on the left side and on the right side. The spatial working memory deficit appears to be related to re-cancellations, but only for some neglect patients. Alternative interpretations of re-exploration of space are discussed.

Disentangling the neuroanatomical correlates of perseveration from unilateral spatial neglect

Perseverative behavior, manifest as re-cancelling or re-visiting targets, is distinct from spatial neglect. Perseveration is thought to reflect frontal or parietal lobe dysfunction, but the neuroanatomical correlates remain poorly defined and the interplay between neglect and perseveration is incompletely understood. We enrolled 87 consecutive patients with diffusion-weighted, perfusion-weighted imaging, and spatial neglect testing within 24 hours of right hemisphere ischemic stroke. The degrees of spatial neglect and perseveration were analyzed. Perseveration was apparent in 46% (40/87) of the patients; 28% (24/87) showed perseveration only; 18% (16/87) showed both perseveration and neglect; and 3% (3/87) showed neglect only. Perseverative behaviors occur in an inverted "U" shape: little neglect was associated with few perseverations; moderate neglect with high perseverations; and in severe neglect targets may not enter consciousness and perseverative responses decrease. Brodmann areas of dysfunction, and the caudate and putament, were assessed and volumetrically measured. In this study, the caudate and putamen were not associated with perseveration. After controlling for neglect, and volume of dysfunctional tissue, only Brodmann area 46 was associated with perseveration. Our results further support the notion that perseveration and neglect are distinct entities; while they often co-occur, acute dorsolateral prefrontal cortex ischemia is associated with perseveration specifically.

Repetitive exploration towards locations that no longer carry a target in patients with neglect

About 50% of neglect patients show ipsilesional target re-exploration on neglect tasks and in daily life. The present study examines whether omissions and revisitings are due to disengagement failure from visible stimuli on the ipsilesional side. Thirteen patients with neglect and nine healthy controls were tested with three versions of the Bells test on touch screen, i.e. a standard cancellation in which targets have to be marked, an erase cancellation in which targets have to be erased, and a condition in which all items (including distracters) have to be erased. Whereas omissions decreased in the full-erase condition, revisitings were the most prominent in this condition. Our study shows that neglect patients also return to previously visited locations which no longer carry a target.