Biomedical subjects

Paresh Malhotra

Publications and source records attributed to Paresh Malhotra.

7 recordsLinked to original sources

PubMed2006

Space re-exploration in hemispatial neglect.

Exploration of the space around us is a fundamental part of human behaviour. When it breaks down there is an important opportunity to understand its underlying mechanisms. Here we show that many right-hemisphere patients with left neglect re-explore rightward locations, failing to keep track of them during search. Importantly, such re-exploration occurred despite leftward stimuli being indistinguishable in peripheral vision, so it is unlikely to result from implicit processing of neglected targets. Revisits generally occurred after visits to other targets and are therefore not immediate perseverations. Finally, manipulating the visual salience of found targets altered the degree of neglect, but not revisit rates. Space exploration appears to be modulated both by the ability to keep track of spatial locations and by stimulus salience.

Aged↗

PubMed2006

Hemispatial neglect, balance and eye-movement control.

PURPOSE OF REVIEW: Disorders of spatial awareness and balance following stroke are common but often under-diagnosed. They lead to poor outcome and frequently coexist. Here we focus on recent progress in the understanding of the mechanisms underlying these disorders and potential therapeutic advances. RECENT FINDINGS: Right-hemisphere networks are important for both spatial attention and postural awareness. Neglect patients show multiple oculomotor impairments including reduced saccade amplitude and difficulty retaining spatial locations across saccades. There has been controversy regarding the brain regions associated with neglect, although most studies show the right inferior parietal lobe to be crucial and new imaging modalities have provided insight into neglect caused by subcortical stroke. The 'pusher syndrome' is a poorly understood balance disorder where patients push towards their paretic side, resulting in falls. It may involve impairment of subjective verticality but experimental studies have reported diverse findings. Advances in treatment for neglect include the successful use of prism adaptation and pilot data suggesting noradrenergic stimulation may improve search in selected patients. SUMMARY: New experimental techniques have provided insight into the debilitating disorders of spatial and postural awareness that often follow stroke. There are currently no widely used therapies for neglect but both new behavioural techniques and pharmacological methods are promising.

Animals↗

PubMed2005

Spatial working memory capacity in unilateral neglect.

It has been proposed recently that a deficit in keeping track of spatial locations may contribute to the severity of unilateral neglect in some right hemisphere stroke patients. However, performance on traditional spatial working memory (SWM) tasks (e.g. Corsi blocks) might be confounded by failure to encode leftward locations, rather than a true deficit of maintaining locations in SWM. Here we introduced new procedures for circumventing this to measure SWM capacity in neglect. In a first experiment, 20 right hemisphere stroke patients (10 with and 10 without neglect) were tested on a computerized vertical variant of the Corsi task. Sequences of spatial locations in a vertical column were displayed and participants had to tap out the remembered sequence on a touchscreen. Patients with left neglect were impaired on this vertical SWM task compared with all control groups. However, poor performance on this task (as for Corsi blocks) might involve impaired memory for stimulus sequence, or poor visuomotor control of manual responding, rather than reduced SWM capacity per se. A second experiment therefore employed a purer measure of vertical SWM. After the displayed sequence, a single location was now probed visually, with observers judging verbally (yes/no) if it had been in the preceding sequence. Hence order no longer mattered, and no spatial motor response was required. Again, the neglect group was impaired relative to all others, now with very little overlap between the performances of individual neglect patients versus individuals in control groups. Poor performance on the second task, which provides a purer measure of SWM capacity, correlated with severity of left neglect on cancellation tasks (but not on line bisection), consistent with recent proposals that SWM deficits can exacerbate left neglect on visual search tasks when present conjointly. Lesion anatomy indicated that neglect patients with a SWM deficit were most likely to have damage to parietal white matter, plus, in the second experiment, to the insula also. These findings demonstrate that an impairment in SWM capacity can contribute to the neglect syndrome in patients with stroke involving regions within the right parietal lobe and insula.

Adult↗

PubMed2005

Priming of color and position during visual search in unilateral spatial neglect.

We examined priming of visual search by repeated target location or color in two patients with left visual neglect and extinction, following strokes centered on the right inferior parietal lobe. Both patients, like the healthy controls we tested, showed intact priming, with performance speeded when either the location or color of a singleton target was repeated over successive trials in a standard search condition (Experiment 1). This was observed both from and to targets on the contralesional (left) side. Moreover, priming of search was still observed even when a return of fixation back to display-center was required between successive trials (Experiment 2). When briefer displays were used (Experiment 3), the patients often failed to detect left targets. This situation revealed an important dissociation: Whereas location priming only arose from preceding left targets that had been consciously detected, color priming (possibly arising within the intact ventral stream) did not depend on awareness of the preceding target. There was considerable color priming from missed targets. These findings demonstrate relatively intact priming of visual search by color and location in patients with right parietal damage, and also reveal that location priming may differ from color priming in requiring awareness.

Aged↗

PubMed2004

Attention modulates the visual field in healthy observers and parietal patients.

Recent attention research suggests that factors other than low-level sensory processes modulate perception across the visual field, with right parieto-temporal cortex playing a critical role in directing visual attention to peripheral events. Here we examine how different degrees of attentional demand at fixation dynamically affect detection of abrupt visual onsets in the periphery. In young healthy subjects, peripheral detection was significantly disrupted bilaterally when there was high attention demand at fixation. Right parieto-temporal lesioned patients, tested with a simplified version of task, demonstrated bilateral shrinkage of their available visual field, worse to the contralesional side, under increased attentional demand at fixation. These findings demonstrate how the effective visual field is dynamically modulated by the deployment of attention in health and, more severely, following right parieto-temporal damage.

Adolescent↗

PubMed2003

The anatomy of visual neglect.

The brain regions that are critically associated with visual neglect have become intensely disputed. In particular, one study of middle cerebral artery (MCA) stroke patients has claimed that the key brain region associated with neglect is the mid portion of the superior temporal gyrus (STG), on the lateral surface of the right hemisphere, rather than the posterior parietal lobe. Such a result has wide-ranging implications for both our understanding of the normal function these cortical areas and the potential mechanisms underlying neglect. Here, we use novel high resolution MRI protocols to map the lesions of 35 right-hemisphere patients who had suffered either MCA or posterior cerebral artery (PCA) territory stroke. For patients with MCA territory strokes, the critical area involved in all neglect patients was the angular gyrus of the inferior parietal lobe (IPL). Although the STG was damaged in half of our MCA neglect patients, it was spared in the rest. For PCA territory strokes, all patients with neglect had lesions involving the parahippocampal region, on the medial surface of the temporal lobe. PCA patients without neglect did not have damage to this area. We conclude that damage to two posterior regions, one in the IPL and the other in the medial temporal lobe, is associated with neglect. Although some neglect patients do have damage to the STG, our findings challenge the recent influential proposal that lesions of this area are critically associated with neglect. Instead, our results implicate the angular gyrus and parahippocampal region in this role.

Adult↗

PubMedDate not supplied

Impaired spatial working memory: one component of the visual neglect syndrome?

Both impaired spatial working memory (SWM) and unilateral neglect may follow damage to the right parietal lobe. We propose that impaired SWM can exacerbate visual neglect, due to failures in remembering locations that have already been searched. When combined with an attentional bias to the ipsilesional right side, such a SWM impairment should induce recursive search of ipsilesional locations. Here we studied a left neglect patient with a right temporoparietal haemorrhage. On a nonlateralised, purely vertical SWM task, he was impaired in retaining spatial locations. In a visual search task, his eye position was monitored while his spatial memory was probed. He recursively searched through right stimuli, re-fixating previously inspected items, and critically treated them as if they were new discoveries, consistent with the SWM deficit. When his recovery was tracked over several months, his SWM deficit and left neglect showed concurrent improvements. We argue that impaired SWM may be one important component of the visual neglect syndrome.

Attention↗