Group study of an "undercover" test for visuospatial neglect: invisible cancellation can reveal more neglect than standard cancellation

Visual neglect is a relatively common deficit after brain damage, particularly strokes. Cancellation tests provide standard clinical measures of neglect severity and deficits in daily life. A recent single-case study introduced a new variation on standard cancellation. Instead of making a visible mark on each target found, the patient made invisible marks (recorded with carbon paper underneath, for later scoring). Such invisible cancellation was found to reveal more neglect than cancellation with visible marks. Here we test the generality of this. Twenty three successive cases with suspected neglect each performed cancellation with visible or invisible marks. Neglect of contralesional targets was more pronounced with invisible marks. Indeed, about half of the patients only showed neglect in this version. For cases showing more neglect with invisible marks, stronger neglect of contralesional targets correlated with more revisits to ipsilesional targets for making additional invisible marks upon them. These results indicate that cancellation with invisible marks can reveal more neglect than standard cancellation with visible marks, while still providing a practical bedside test. Our observations may be consistent with recent proposals that demands on spatial working memory (required to keep track of previously found items only when marked invisibly) can exacerbate spatial neglect.

Impaired spatial working memory across saccades contributes to abnormal search in parietal neglect

Visual neglect of left space following right parietal damage in humans involves a lateral bias in attention, apparent in many search tasks. We hypothesized that parietal neglect may also involve a failure to remember which locations have already been examined during visual search: an impairment in retaining searched locations across saccades. Using a new paradigm, we monitored gaze during search, while simultaneously probing whether observers judged they had found a new target, or judged instead that they were re-fixating a previously examined target. A patient with left neglect following focal right parietal infarction repeatedly re-fixated right locations. Critically, he often failed to remember that these locations had already been searched, treating old targets as new discoveries at an abnormal rate. In comparison, healthy age-matched control subjects rarely re-fixated targets, and mistook old targets as new targets even more rarely. The frequency of such mistakes in the parietal patient, for different conditions, correlated with the severity of his neglect. Control experiments indicated no perceptual localization deficit in non-search tasks. These results suggest a deficit in retaining searched locations across saccades in parietal neglect, in addition to the lateral spatial bias. Moreover, the former deficit exacerbates the latter, such that patients do not realize that the rightward locations favoured by their bias have already been examined during previous fixations and, for this reason, they saccade back to them repeatedly. The combination of the two deficits (a lateral bias plus a deficit in retaining locations already searched) may thus explain the pathological pattern of search that characterizes parietal neglect: why stimuli on the right are re-examined recursively, as if being searched for the first time, and hence why stimuli on the left continue to be ignored even with unlimited viewing time. These proposals accord with recent electrophysiological and functional imaging data, demonstrating posterior parietal involvement in the retention of target locations across saccades.

Spatial working memory deficit in unilateral neglect

Based on the similarity of brain areas lesioned in neglect and those activated by spatial working memory (WM) tasks in normals, we hypothesized that neglect may involve spatial WM impairments. A left neglect patient with right inferior frontal and basal ganglia damage performed cancellation tasks, making either highly visible marks (to provide a reminder of visited items), or invisible marks (so only spatial WM could represent cancelled items). Invisible marks led to repeated cancellations for targets that differed only in location, but not for targets with memorable unique identities, suggesting a deficit of spatial WM, with non-spatial WM spared. Neglect was greater for cancellation with invisible marks, consistent with a role for deficient spatial WM in cancellation deficits, but contrary to account solely in terms of attention capture by salient visible marks made in ipsilesional space.

Visual attention: insights from brain imaging

We are not passive recipients of the information that impinges on our retinae, but active participants in our own perceptual processes. Visual experience depends critically on attention. We select particular aspects of a visual scene for detailed analysis and control of subsequent behaviour, but ignore other aspects so completely that moments after they disappear from view we cannot report anything about them. Here we show that functional neuroimaging is revealing much more than where attention happens in the brain; it is beginning to answer some of the oldest and deepest questions about what visual attention is and how it works.

Unconscious activation of visual cortex in the damaged right hemisphere of a parietal patient with extinction

Visual extinction is a sign classically associated with right parietal damage. The patient can see a single stimulus presented in the ipsilesional or contralesional visual field, but is characteristically unaware of the same contralesional stimulus during simultaneous stimulation of both fields. The ipsilesional stimulus is said to 'extinguish' the contralesional stimulus from awareness during bilateral stimulation, perhaps due to a pathological bias in attention towards the ipsilesional side. Recent psychophysical evidence suggests that, although extinguished stimuli are not consciously seen, they may undergo residual processing and exert implicit effects on performance. However, the neural structures mediating such residual processing for extinguished stimuli remain unknown. Here we studied the neural activity evoked by an extinguished visual stimulus, using event-related functional MRI (fMRI), in a patient with circumscribed right inferior parietal damage and profound left-sided extinction. Monochrome objects (faces or houses) were presented in the left or right field, either unilaterally or bilaterally on each trial, with the patient indicating by button press whether he saw an object on the left, the right or on both sides. He usually saw only the right object on bilateral trials, yet the fMRI data showed activation of visual cortex contralateral to the extinguished left stimulus on these trials (compared with right-only stimulation), in both striate and early extrastriate areas of the right hemisphere. This activity had a similar location and time-course to that resulting from a single stimulus in the left versus right visual field. Cortical pathways involved in the normal processing of a single seen stimulus can thus still be activated by an unseen, extinguished stimulus after right parietal damage. Comparison of fMRI responses for faces versus houses revealed some category-specific activation for extinguished stimuli in right fusiform regions, but only at low statistical threshold. These results are discussed in terms of theoretical accounts for parietal extinction and, more generally, for the neural substrates of visual awareness.

The generality of parietal involvement in visual attention

Functional magnetic resonance imaging (fMRI) was used to determine whether different kinds of visual attention rely on a common neural substrate. Within one session, subjects performed three different attention experiments (each comparing an attentionally demanding task with an easier task using identical stimuli): (1) peripheral shifting, (2) object matching, and (3) a nonspatial conjunction task. Two areas were activated in all three experiments: one at the junction of intraparietal and transverse occipital sulci (IPTO), and another in the anterior intraparietal sulcus (AIPS). These regions are not simply involved in any effortful task, because they were not activated in a fourth experiment comparing a difficult language task with an easier control task. Thus, activity in IPTO and AIPS generalizes across a wide variety of attention-requiring tasks, supporting the existence of a common neural substrate underlying multiple modes of visual selection.

Covert visual attention modulates face-specific activity in the human fusiform gyrus: fMRI study

Several lines of evidence demonstrate that faces undergo specialized processing within the primate visual system. It has been claimed that dedicated modules for such biologically significant stimuli operate in a mandatory fashion whenever their triggering input is presented. However, the possible role of covert attention to the activating stimulus has never been examined for such cases. We used functional magnetic resonance imaging to test whether face-specific activity in the human fusiform face area (FFA) is modulated by covert attention. The FFA was first identified individually in each subject as the ventral occipitotemporal region that responded more strongly to visually presented faces than to other visual objects under passive central viewing. This then served as the region of interest within which attentional modulation was tested independently, using active tasks and a very different stimulus set. Subjects viewed brief displays each comprising two peripheral faces and two peripheral houses (all presented simultaneously). They performed a matching task on either the two faces or the two houses, while maintaining central fixation to equate retinal stimulation across tasks. Signal intensity was reliably stronger during face-matching than house matching in both right- and left-hemisphere predefined FFAs. These results show that face-specific fusiform activity is reduced when stimuli appear outside (vs. inside) the focus of attention. Despite the modular nature of the FFA (i.e., its functional specificity and anatomic localization), face processing in this region nonetheless depends on voluntary attention.