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K Sathian

Publications and source records attributed to K Sathian.

At least 19 recordsLinked to original sources
PubMed2006

Activity and effective connectivity of parietal and occipital cortical regions during haptic shape perception.

It is now widely accepted that visual cortical areas are active during normal tactile perception, but the underlying mechanisms are still not clear. The goal of the present study was to use functional magnetic resonance imaging (fMRI) to investigate the activity and effective connectivity of parietal and occipital cortical areas during haptic shape perception, with a view to potentially clarifying the role of top-down and bottom-up inputs into visual areas. Subjects underwent fMRI scanning while engaging in discrimination of haptic shape or texture, and in separate runs, visual shape or texture. Accuracy did not differ significantly between tasks. Haptic shape-selective regions, identified on a contrast between the haptic shape and texture conditions in individual subjects, were found bilaterally in the postcentral sulcus (PCS), multiple parts of the intraparietal sulcus (IPS) and the lateral occipital complex (LOC). The IPS and LOC foci tended to be shape-selective in the visual modality as well. Structural equation modelling was used to study the effective connectivity among the haptic shape-selective regions in the left hemisphere, contralateral to the stimulated hand. All possible models were tested for their fit to the correlations among the observed time-courses of activity. Two equivalent models emerged as the winners. These models, which were quite similar, were characterized by both bottom-up paths from the PCS to parts of the IPS, and top-down paths from the LOC and parts of the IPS to the PCS. We conclude that interactions between unisensory and multisensory cortical areas involve bidirectional information flow.

Adult↗
PubMed2006

Mirror-image symmetry and search asymmetry: a comparison of their effects on visual search and a possible unifying explanation.

Visual search may be affected by mirror-image symmetry between target and non-targets and also by switching the roles of target and non-target. Do different attention mechanisms underlie these two phenomena? Can a unifying explanation account for both? We conducted two experiments to decompose processing into component parts, and compared results to competing models' predictions. Mirror-image search was unimpaired after target discrimination had been balanced across search conditions-results were consistent with an unlimited-capacity, decision noise model. Search asymmetry affected higher-level processing, however, resulting in capacity limitations that necessitated serial processing. A unifying explanation can account for these two seemingly unrelated phenomena.

Adult↗
PubMed2005

Short-term visual deprivation alters neural processing of tactile form.

Blindness is known to alter the responsiveness of visual cortex. Recently, reversible visual deprivation by blindfolding has been shown to affect non-visual abilities as well as visual cortical function. Here we investigated the effect of 2 h of blindfolding on cerebral cortical activation patterns during tactile form perception, using functional magnetic resonance imaging. Two form tasks were used, one requiring discrimination of global stimulus form and the other, detection of a gap in a bar. Blindfolded subjects showed significant deactivation during these tasks in regions that are intermediate in the hierarchy of visual shape processing: probable V3A and ventral intraparietal sulcus (vIPS). These regions lacked signal changes in controls. There were also task-specific increases in activation in blindfolded relative to control subjects, favoring the form over the gap task, along the IPS and in regions of frontal and temporal cortex. We also found alterations of functional connectivity that corresponded to the activity differences, with the emergence of correlated activity between the vIPS and V3A in blindfolded subjects. We conclude that blindfolding sighted individuals for a 2-h period induces significant changes in the neural processing of tactile form, probably reflecting short-term neural plasticity.

Adult↗
PubMed2005

Visual cortical activity during tactile perception in the sighted and the visually deprived.

This article reviews studies demonstrating activity in visual cortex during tactile perception in sighted participants as well as in those who have experienced visual deprivation of varying duration. This field has been very active over the last few years, with the result that a number of exciting findings have emerged, but a unifying framework is still lacking. The first section of this article deals with investigations revealing that visual cortical activity is regularly associated with the neural processing of tactile inputs in normally sighted individuals. Next, the possible reasons underlying such visual cortical recruitment are considered. The focus then shifts to the effects of visual deprivation, examining the involvement of visual cortex in sensory and language processing in the early and late blind. The final section gives an account of studies suggesting that a remarkable degree of plasticity can be observed even after quite short-lasting visual deprivation. Overall, it appears that the nature of visual cortical activity during nonvisual tasks in the sighted can be influenced by late-onset blindness and even by brief interruptions of visual input; however, the relevant neural plasticity seems to considerably more exuberant if vision is lost very early in life or was never present, which suggests that there is a critical period for the maximal expression of such plasticity.

Animals↗
PubMed2005

Tactile discrimination of grating orientation: fMRI activation patterns.

Grating orientation discrimination is employed widely to test tactile spatial acuity. We used functional magnetic resonance imaging (fMRI) to investigate the neural circuitry underlying performance of this task. Two studies were carried out. In the first study, an extensive set of parietal and frontal cortical areas was activated during covert task performance, relative to a rest baseline. The active regions included the postcentral sulcus bilaterally and foci in the left parietal operculum, left anterior intraparietal sulcus, and bilateral premotor and prefrontal cortex. The second study examined selective recruitment of cortical areas during discrimination of grating orientation (a task with a macrospatial component) compared to discrimination of grating spacing (a purely microspatial task). The foci activated on this contrast were in the left anterior intraparietal sulcus, right postcentral sulcus and gyrus, left parieto-occipital cortex, bilateral frontal eye fields, and bilateral ventral premotor cortex. These findings not only confirm and extend previous studies of the neural processing underlying grating orientation discrimination, but also demonstrate that a distributed network of putatively multisensory areas is involved.

Adolescent↗
PubMed2005

Somatosensory processing is impaired in temporal lobe epilepsy.

PURPOSE: Growing evidence suggests that temporal lobe epilepsy (TLE) is a network disease. In this view, the seizure focus may produce measurable deficits in specific cortical functions. METHODS: A tactile grating orientation (GrOr) discrimination task associated with parietal lobe function was administered at the index fingertip to 15 subjects with medically intractable TLE and to 19 neurologically normal controls. TLE subjects were tested bilaterally at baseline while taking their usual antiepileptic drugs (AEDs), and off AEDs during inpatient video-EEG monitoring (n = 9). Three subjects also were tested after temporal lobectomy. t Tests were used to compare baseline performance between TLE subjects and controls, and between hands ipsilateral and contralateral to side of seizure onset, with Bonferroni correction for multiple comparisons. TLE subjects' baseline thresholds were compared with those obtained off AEDs by using a repeated measures analysis of variance. RESULTS: TLE subjects were severely impaired bilaterally on the GrOr task, with mean discrimination thresholds nearly twice those of controls (p </= 0.001 for each hand). No significant difference was found in baseline performance between hands (p = 0.37), or between baseline and off-AED testing (p = 0.42). The three subjects tested after temporal lobectomy demonstrated improved performance compared with baseline, but statistics were not performed because of the small subject number. CONCLUSIONS: Patients with medically intractable TLE have impaired tactile GrOr discrimination bilaterally that is not due to nonspecific effects of AEDs. This impaired perceptual ability may be reversible with surgical removal of the seizure focus.

Adult↗
PubMed2004

Task-specific recruitment of dorsal and ventral visual areas during tactile perception.

Many studies have found that visual cortical areas are active during tactile perception. Here we used positron emission tomographic (PET) scanning in normally sighted humans to show that extrastriate cortical regions are recruited in a task-specific manner during perceptual processing of tactile stimuli varying in two dimensions. Mental rotation of tactile Forms activated a focus around the anterior part of the left intraparietal sulcus. Since prior studies have reported activity nearby during mental rotation of visual stimuli, this focus appears to be associated with the dorsal visual (visuospatial) pathway. Discrimination between tactile Forms activated the right lateral occipital complex, an object-selective region in the ventral visual (visual Form) pathway. Thus, tactile tasks appear to recruit cortical regions that are active during corresponding visual tasks. Activation of these areas in both visual and tactile tasks could reflect visual imagery during tactile perception, activity in multisensory representations, or both.

Adult↗
PubMed2004

Multisensory cortical processing of object shape and its relation to mental imagery.

Here, we used functional magnetic resonance imaging to investigate the multisensory processing of object shape in the human cerebral cortex and explored the role of mental imagery in such processing. Regions active bilaterally during both visual and haptic shape perception, relative to texture perception in the respective modality, included parts of the superior parietal gyrus, the anterior intraparietal sulcus, and the lateral occipital complex. Of these bimodal regions, the lateral occipital complexes preferred visual over haptic stimuli, whereas the parietal areas preferred haptic over visual stimuli. Whereas most subjects reported little haptic imagery during visual shape perception, experiences of visual imagery during haptic shape perception were common. Across subjects, ratings of the vividness of visual imagery strongly predicted the amount of haptic shape-selective activity in the right, but not in the left, lateral occipital complex. Thus, visual imagery appears to contribute to activation of some, but not all, visual cortical areas during haptic perception.

Adult↗
PubMed2003

Neural networks active during tactile form perception: common and differential activity during macrospatial and microspatial tasks.

Prior studies have shown that tactile perception recruits activity not only in somatosensory but also in visual cortical areas. The present study used functional magnetic resonance imaging to investigate the distribution of neural activity during tactile perception of 2D form. In a macrospatial form task, raised letters (uppercase T and V) were presented upside-down. In a microspatial form task, a bar, either with or without a gap, was presented. Stimuli were applied to the immobilized right index fingerpad. Six neurologically normal volunteers were studied in a block design paradigm, with alternating blocks of rest and covert discrimination between the two alternatives for a task. Each task was studied in a separate run. Contrasting macrospatial form discrimination against rest revealed activity in an extensive, bilateral network of cortical and subcortical regions, including areas of somatosensory cortex and the intraparietal sulcus (IPS), occipito-temporal cortex, dorsal and ventral premotor cortex, medial superior frontal cortex, lateral inferior frontal cortex, thalamus and cerebellar hemispheres. Contrasting (microspatial) gap detection against rest showed activity in a similar network, with the notable exception of the occipito-temporal cortical regions. A direct contrast between the two tasks yielded greater activity for the macrospatial than microspatial task in these occipito-temporal regions bilaterally, and also in foci near the right IPS and in the right cerebellar hemisphere. The occipito-temporal cortical activations were in the lateral occipital complex, a part of the ventral visual pathway active during visual form perception. Thus, macrospatial form perception preferentially recruits this region of extrastriate visual cortex, compared to microspatial form perception.

Adult↗
PubMed2002

Feeling with the mind's eye: contribution of visual cortex to tactile perception.

Visual imagery is implicated in the normal tactile perception of certain object properties. This is an example of cross-modal interactions that characterize normal perception. Here we review recent studies from our laboratory on cross-modal interactions between vision and touch in normally sighted humans. Positron emission tomography was used to demonstrate activation of a region of extrastriate visual cortex, near the parieto-occipital fissure, during tactile discrimination of grating orientation. Transcranial magnetic stimulation (TMS) over this region interfered with performance of this tactile task. In both studies, visual cortical involvement was found for tactile discrimination of orientation but not spatial frequency. Thus, this cortical region is not only active during but also necessary for optimal tactile sensing of orientation. Recent findings implicating visual cortex in Braille-reading in the blind should be evaluated from this perspective.

Animals↗
PubMed2002

Mental rotation of tactile stimuli.

When subjects decide whether two visual stimuli presented in various orientations are identical or mirror-images, reaction time increases with the angular disparity between the stimuli. The interpretation of this well-known observation is that subjects mentally rotate images of the stimuli until they are in congruence, in order to solve the task. Here we review studies involving mental rotation of tactile stimuli. Mental rotation in tactile tasks is specifically associated with the requirement for mirror-image discrimination, as opposed to identity judgments. The key brain region mediating mental rotation of tactile stimuli seems to be the parietal cortex. Visual processing appears to facilitate task performance. We report an experiment from our laboratory addressing the nature of the reference frame for mental rotation of tactile stimuli. Our observations indicate that when the hand is directly in front of the body, with the head facing forward, the shortest reaction times for mirror-image discrimination of stimuli applied to the fingerpad are obtained when the longitudinal axis of the stimulus is in or parallel to the sagittal plane, even when this is perpendicular to the long axis of the finger. Thus, the reference frame for mental rotation of tactile stimuli is not purely hand-centered. This is consistent with other findings indicating variable assignment of reference frames for tactile perception.

Animals↗
PubMed2001

Temporal cues contribute to tactile perception of roughness.

Optimal perception of surface roughness requires lateral movement between skin and surface, suggesting the importance of temporal cues. The roughness of periodic gratings is affected by changing either inter-element spacing (groove width, G) or element width (ridge width, R). Peripheral neural responses to gratings depend quantitatively on a spatial variable, G, and a temporal variable, grating temporal frequency (F(t)), with changes in R acting indirectly through concomitant changes in F(t). We investigated, psychophysically, the contribution of temporal cues to human tactile perception of roughness, using gratings varying in either R or G. Gratings were scanned across the immobile fingerpad with controlled movement speed (S) and contact force. In one experiment, we found that roughness magnitude estimates depended on both G and F(t). In a second experiment, discrimination of the roughness of gratings varying in either R or G was affected by manipulating F(t). Overall, the effect of G on roughness judgments was much stronger than that of F(t), probably explaining why many previous studies using surfaces that varied only in inter-element spacing led to the conclusion that temporal factors play no role in roughness perception. However, the perceived roughness of R-varying gratings was determined by F(t) and not spatial variables. Roughness judgments were influenced by G and F(t) in a manner entirely consistent with predicted afferent response rates. Thus perceived roughness, like peripheral afferent responses, depends in part on temporal variables.

Acoustic Stimulation↗
PubMed2001

Feeling with the mind's eye: the role of visual imagery in tactile perception.

Cross-modal interactions are characteristic of normal perception. In this article, we discuss our work on cross-modal interactions between touch and vision in normally sighted humans. A region of extrastriate visual cortex, near the parieto-occipital fissure, is not only active during but also necessary for tactile discrimination of grating orientation (but not spatial frequency). This is consistent with a role for visual imagery in certain aspects of tactile perception. These findings have implications for the interpretation of visual cortical involvement in Braille reading by the blind.

Humans↗
PubMed2000

Intermanual referral of sensation to anesthetic hands.

This report describes a study of patients with hands rendered anesthetic by stroke or neurosurgery. Touching the normal hand of such patients triggers sensations referred contralaterally to the anesthetic hand, paralleling observations of sensory referral to phantom limbs of amputees. The referred somatic sensations are elicited by touch but not usually by other kinds of stimuli, cannot be localized precisely, and do not support spatially organized perception. These characteristics suggest that referral may depend on reorganization in parietal cortical areas other than area 3b, the primary cortical recipient of cutaneous sensory inputs.

Adult↗
PubMed2000

Visual search: bottom-up or top-down?

The aim of the experiments in this paper was to explore the relationship between top-down and bottom-up processes in visual search. Employing behavioral techniques, we first consider the possible role of the magnocellular visual pathway in visual search, and find that visual search does not necessarily depend on processing by this visual sub-system. We next use functional imaging (positron emission tomography) to explore the effect of varying top-down strategy during visual search. Our findings indicate that the neural processes underlying visual search are distributed over an extensive network of brain regions, with varying roles for different parts of the network as the dynamics of top-down vs. bottom-up influences shift. The conjunction of bottom-up processing with top-down attentional suppression of an irrelevant singleton could account for activity found in right primary visual cortex (V1). The conjunction of bottom-up processing with top-down attentional set could explain activity noted in the right superior temporal gyrus/insular cortex. The left lateral cerebellum appears to play a role in attention, either in signaling popout or in switching attention repeatedly between multiple visual attributes. Loci in left parietal cortex (parietal operculum/superior temporal gyrus, parieto-occipital fissure and precuneus) are implicated in attention-demanding search for a target shape. Returning to behavioral experiments, we find that, when multiple feature singletons compete for attention, interference between them is strongest for features closely related to the distinguishing target feature. This competition appears to be feature-level rather than object-level, and is characterized by a varying degree of specificity for different features. Task complexity modulates interference effects, even for abrupt visual onsets, which are often considered to capture attention involuntarily. Overall, our observations converge on the conclusion that visual search is extremely flexible and subject to considerable specificity of top-down control, although such specificity is clearly not absolute.

Adult↗
PubMed2000

Doing it with mirrors: a case study of a novel approach to neurorehabilitation.

Arm amputees can experience the perception of movement of a phantom limb while looking at a mirror reflection of the moving, intact arm superimposed on the perceived phantom. Such use of a mirror to provide illusory visual feedback of movement can be useful in rehabilitation of hemiparetic patients. In this case report, we describe the successful application of "mirror therapy" to the post-stroke rehabilitation of a patient with poor functional use of an upper extremity, due mainly to somatosensory deficits. Mirror therapy facilitated employment of a motor copy strategy (bimanual movements) and later progression to "forced use" of the affected arm. The end result was increased functional use of the affected upper limb.

Arm↗