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Biomedical subjects

Masud Husain

Publications and source records attributed to Masud Husain.

At least 37 records · Page 2Linked to original sources
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

Distinct cortical and collicular mechanisms of inhibition of return revealed with S cone stimuli.

Visual orienting of attention and gaze are widely considered to be mediated by shared neural pathways, with automatic phenomena such as inhibition of return (IOR)--the bias against returning to recently visited locations--being generated via the direct pathway from retina to superior colliculus (SC). Here, we show that IOR occurs without direct access to the SC, by using a technique that employs stimuli visible only to short-wave-sensitive (S) cones. We found that these stimuli, to which the SC is blind , were quite capable of eliciting IOR, measured by traditional manual responses. Critically, however, we found that S cone stimuli did not cause IOR when saccadic eye movement responses were required. This demonstrates that saccadic IOR is not the same as traditional IOR, providing support for two separate cortical and collicular mechanisms of IOR. These findings represent a clear dissociation between visual orienting of attention and gaze.

Adult↗
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↗
PubMed2004

The impact of extensive medial frontal lobe damage on 'Theory of Mind' and cognition.

The ability of humans to predict and explain other people's behaviour by attributing to them independent mental states, such as desires and beliefs, is considered to be due to our ability to construct a 'Theory of Mind'. Recently, several neuroimaging studies have implicated the medial frontal lobes as playing a critical role in a dedicated 'mentalizing' or 'Theory of Mind' network in human brains. Here, we report a patient, G.T., who suffered an exceptionally rare form of stroke-bilateral anterior cerebral artery infarction, without rupture or the complications associated with anterior communicating artery aneurysms. Detailed high-resolution neuroanatomical investigations revealed extensive damage to the medial frontal lobes bilaterally, including regions identified to be critical for 'Theory of Mind' by functional neuroimaging of healthy human subjects. For the first time in such a patient, we carried out a thorough assessment of her cognitive profile including, critically, an experimental investigation of her performance on a range of tests of 'Theory of Mind'. G.T. had a dysexecutive syndrome characterized by impairments in planning and memory, as well as a tendency to confabulate. Importantly, however, she did not have any significant impairment on tasks probing her ability to construct a 'Theory of Mind', demonstrating that the extensive medial frontal regions destroyed by her stroke are not necessary for this function. These findings have important implications for the functional anatomy of 'Theory of Mind', as well as our understanding of medial frontal function. Possible reasons for the discrepancies between our results and neuroimaging studies are discussed. We conclude that our findings urge caution against using functional imaging as the sole method of establishing cognitive neuroanatomy.

Cognition Disorders↗
PubMed2004

Auditory deficits in visuospatial neglect patients.

Since the pioneering experimental work of Bisiach et al. (1984) on deficits in sound localisation associated with unilateral brain lesions and visual neglect, a number of systematic investigations have examined auditory processing in visuospatial neglect patients. Evidence from a variety of experimental paradigms has revealed some auditory deficits in detection and identification tasks, during bilateral stimulation; plus localisation deficits for single sounds. These deficits emerge predominantly for contra-lesional sounds, although some auditory disturbances applying to both contra- and ipsilesional sounds have also been documented. Here we review evidence suggesting that some of these auditory deficits arise in relatively high-level stages of spatial processing. In addition, we present new analyses showing that auditory deficits in identification and localisation tasks often correlate with clinical measures of visual neglect, across a variety of different studies and tasks. This empirical relation suggests that a disturbance of multisensory spatial processing may often account for the joint auditory and visual spatial deficits in neglect patients, although rarer dissociations between the modalities should also be considered.

Auditory Perception↗
PubMed2004

Spatiotemporal dynamics of attention in visual neglect: a case study.

Patients with visual neglect are often unaware of contralesional visual stimuli. Recent studies have demonstrated non-spatial as well as spatial deficits in allocating attention in neglect patients. We examined the spatiotemporal dynamics of directing attention in a patient with neglect for visual objects appearing in the left side of space by using a version of the attentional blink paradigm. presenting two visual stimuli in succession. The first target (T1) was presented at fixation. The location of the second target (T2) and time between the two targets was varied. When T2 appeared to the left, the patient required more time between targets to identify both accurately, compared to when T2 appeared at fixation or to the right. Our findings demonstrate a spatial and temporal gradient of attention. The results are discussed with respect to current models of visual processing in visual neglect.

Aged↗
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↗
PubMed2003

Cerebral amyloid angiopathy and motor neurone disease presenting with a progressive supranuclear palsy-like syndrome.

We describe a 68-year-old woman who presented with falls, mild limb bradykinesia, axial rigidity, and a severe supranuclear gaze palsy, which failed to benefit from levodopa. She subsequently developed severe apraxia, progressive dysarthria, dysphagia, and a frontal cognitive impairment. Pyramidal weakness with fasciculations and widespread chronic partial denervation appeared shortly before her death from bronchopneumonia, 6 months after disease onset. A severe cerebral amyloid angiopathy diffusely involving the cerebral hemispheres and cerebellum was present at autopsy as well as a second pathological condition indicative of motor neurone disease. Cerebral amyloid angiopathy may rarely present with a progressive supranuclear palsy-like phenotype.

Aged↗
PubMed2003

Differential cortical activation during voluntary and reflexive saccades in man.

A saccade involves both a step in eye position and an obligatory shift in spatial attention. The traditional division of saccades into two types, the "reflexive" saccade made in response to an exogenous stimulus change in the visual periphery and the "voluntary" saccade based on an endogenous judgement to move gaze, is supported by lines of evidence which include the longer onset latency of the latter and the differential effects of lesions in humans and primates on each. It has been supposed that differences between the two types of saccade derive from differences in how the spatial attention shifts involved in each are processed. However, while functional imaging studies have affirmed the close link between saccades and attentional shifts by showing they activate overlapping cortical networks, attempts to contrast exogenous with endogenous ("covert") attentional shifts directly have not revealed separate patterns of cortical activation. We took the "overt" approach, contrasting whole reflexive and voluntary saccades using event-related fMRI. This demonstrated that, relative to reflexive saccades, voluntary saccades produced greater activation within the frontal eye fields and the saccade-related area of the intraparietal sulci. The reverse contrast showed reflexive saccades to be associated with relative activation of the angular gyrus of the inferior parietal lobule, strongest in the right hemisphere. The frequent involvement of the right inferior parietal lobule in lesions causing hemispatial neglect has long implicated this parietal region in an important, though as yet uncertain, role in the awareness and exploration of space. This is the first study to demonstrate preferential activation of an area in its posterior part, the right angular gyrus, during production of exogenously triggered rather than endogenously generated saccades, a finding which we propose is consistent with an important role for the angular gyrus in exogenous saccadic orienting.

Adult↗
PubMed2002

Testing memory for unseen visual stimuli in patients with extinction and spatial neglect.

Visual extinction after right parietal damage involves a loss of awareness for stimuli in the contralesional field when presented concurrently with ipsilesional stimuli, although contralesional stimuli are still perceived if presented alone. However, extinguished stimuli can still receive some residual on-line processing, without awareness. Here we examined whether such residual processing of extinguished stimuli can produce implicit and/or explicit memory traces lasting many minutes. We tested four patients with right parietal damage and left extinction on two sessions, each including distinct study and subsequent test phases. At study, pictures of objects were shown briefly in the right, left, or both fields. Patients were asked to name them without memory instructions (Session 1) or to make an indoor/outdoor categorization and memorize them (Session 2). They extinguished most left stimuli on bilateral presentation. During the test (up to 48 min later), fragmented pictures of the previously exposed objects (or novel objects) were presented alone in either field. Patients had to identify each object and then judge whether it had previously been exposed. Identification of fragmented pictures was better for previously exposed objects that had been consciously seen and critically also for objects that had been extinguished (as compared with novel objects), with no influence of the depth of processing during study. By contrast, explicit recollection occurred only for stimuli that were consciously seen at study and increased with depth of processing. These results suggest implicit but not explicit memory for extinguished visual stimuli in parietal patients.

Aged↗
PubMed2002

Control of visuotemporal attention by inferior parietal and superior temporal cortex.

The human cortical visual system is organized into major pathways: a dorsal stream projecting to the superior parietal lobe (SPL), considered to be critical for visuospatial perception or on-line control of visually guided movements, and a ventral stream leading to the inferotemporal cortex, mediating object perception. Between these structures lies a large region, consisting of the inferior parietal lobe (IPL) and superior temporal gyrus (STG), the function of which is controversial. Lesions here can lead to spatial neglect, a condition associated with abnormal visuospatial perception as well as impaired visually guided movements, suggesting that the IPL+STG may have largely a "dorsal" role. Here, we use a nonspatial task to examine the deployment of visuotemporal attention in focal lesion patients, with or without spatial neglect. We show that, regardless of the presence of neglect, damage to the IPL+STG leads to a more prolonged deployment of visuotemporal attention compared to lesions of the SPL. Our findings suggest that the human IPL+STG makes an important contribution to nonspatial perception, and this is consistent with a role that is neither strictly "dorsal" nor "ventral". We propose instead that the IPL+STG has a top-down control role, contributing to the functions of both dorsal and ventral visual systems.

Attention↗
PubMed2002

The electrophysiology of tactile extinction: ERP correlates of unconscious somatosensory processing.

We examined the electrophysiological correlates of left-sided tactile extinction in a patient with right-hemisphere damage. Computer-controlled punctate touch was presented to the left, right or both index fingers in an unpredictable sequence. The patient reported his conscious tactile percept ("left", "right" or "both"). He showed extinction on 75% of bilateral trials, reporting only right stimulation for these. Somatosensory evoked potentials for unilateral stimulation showed early components over contralateral somatosensory areas (P60 and N110) for either hand. In contrast to the results observed for age-matched controls, the patient's P60 was smaller in amplitude for left-hand touch over the right hemisphere than for right-hand touch over the intact hemisphere. Bilateral trials with extinction revealed residual P60 and N110 components over the right hemisphere in response to the extinguished left touch. These results demonstrate residual unconscious somatosensory processing of extinguished touch. They also suggest that tactile extinction can be caused by attenuation rather than elimination of somatosensory responses in the damaged hemisphere, with an underlying deficit even on unilateral trials.

Aged↗
PubMed2002

Neural correlates of conscious and unconscious vision in parietal extinction.

Brain areas activated by stimuli in the left visual field of a right parietal patient suffering from left visual extinction were identified using event-related functional magnetic resonance imaging. Left visual field stimuli that were extinguished from awareness still activated the ventral visual cortex, including areas in the damaged right hemisphere. An extinguished face stimulus on the left produced robust category-specific activation of the right fusiform face area. On trials where the left visual stimulus was consciously seen rather than extinguished, greater activity was found in the ventral visual cortex of the damaged hemisphere, and also in frontal and parietal areas of the intact hemisphere. These findings extend recent observations on visual extinction, suggesting distinct neural correlates for conscious and unconscious perception.

Aged↗
PubMed2002

Active tool use with the contralesional hand can reduce cross-modal extinction of touch on that hand.

After a unilateral brain lesion, patients may show cross-modal, visual-tactile extinction. Such patients may fail to report tactile stimuli on the contralesional hand when presented together with competing visual stimuli near the ipsilesional hand. In this work we tested the hypothesis that this cross-modal extinction may be reduced when a patient has used a tool with the contralesional hand to reach for objects in the ipsilesional visual field. Consistent with previous work, we hypothesize that active use of a tool may extend cross-modal interactions between visual stimuli at the tip of the tool and tactile stimuli on the hand wielding the tool. In the new situation of a tool connecting the contralesional hand with ipsilesional visual space, competition between stimuli on these opposite sides may be reduced, so that extinction decreases. We studied patient BV, who showed reliable cross-modal, visual-tactile extinction after right-hemisphere stroke. In two separate sessions we showed that prolonged tool use (10-20 min) with the contralesional hand in ipsilesional space reduced cross-modal extinction for up to 60-90 min post-training. We propose that an actively used tool may be effective in linking cross-modal stimuli presented along its extension. This can then overcome competition between stimuli presented on opposite sides of the body midline, thus modulating extinction.

Aged↗
PubMed2002

Cognitive neuroscience of hemispatial neglect.

INTRODUCTION: Patients with neglect often fail to be aware of stimuli located on their contralesional side--the side of space opposite their brain damage. Some patients show impairments in extrapersonal space; others within personal (body) space; and some in both. Although the neglect syndrome may be associated with a distorted perception of extrapersonal or personal space, recent investigations suggest that attentional, memory and motor deficits may be important additional components. CONCLUSION: Better understanding of these cognitive impairments holds the key to offering effective treatment for neglect, as well as providing important insights into normal brain function.

Journal Article↗