Biomedical subjects

Suresh D Muthukumaraswamy

Publications and source records attributed to Suresh D Muthukumaraswamy.

6 recordsLinked to original sources

PubMed2006

Long-term enhanced desynchronization of the alpha rhythm following tetanic stimulation of human visual cortex.

It had been shown previously that a photic tetanus induces LTP-like changes in the visual cortex, as indexed by an enhancement of the N1b component of the visual evoked potential, recorded non-invasively by electroencephalography. This potentiation was shown to last over 1 h. In the present study, the effect of a photic tetanus on oscillatory activity is investigated. EEGs were collected from eight healthy subjects in three conditions while visual checkerboards were displayed. Following baseline presentations in two conditions a lateralized visual tetanus was given, either to the left or right visual field, and in a third condition no tetanus was given. This was followed by a return to baseline presentations, both immediately after the tetanus/control block, and 1 h later. Enhanced event-related desynchronization (ERD) of the alpha rhythm lasting 1 h was seen following the photic tetanus over occipital electrodes. Because ERD of the alpha rhythm is thought to represent active cortex, these results suggest that the visual tetanus induces long-lasting cortical changes, with stronger neuronal assemblies and increased neuronal output.

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PubMed2006

Neural processing of observed oro-facial movements reflects multiple action encoding strategies in the human brain.

In this experiment, the oscillatory responses of the MEG were characterized during the observation of four viewing conditions: (a) observation of mouth movements, (b) observation of a non-biological motion stimulus (a mechanical aperture opening and shutting), (c) observation of object-directed mouth movements and (d) observation of speech-like mouth movements. Data were analyzed using synthetic aperture magnetometry (SAM) in three frequency bands, beta (15-35 Hz), gamma (35-70 Hz) and alpha/mu (8-15 Hz). Results showed that observations of biological motion resulted in beta desynchronization over lateral sensorimotor areas, while observations of non-biological motion resulted in a more medial desynchronization, an effect that may be related to the processing of a structured event sequence. Observation of linguistic movements resulted in less alpha/beta desynchronization in posterior brain regions in comparison to biological motion stimuli, suggesting that linguistically-relevant stimuli are processed with different neuronal systems than those recruited by normal action observation. We suggest that non-linguistic actions recruit dorsal systems while linguistic actions engage ventral processing systems. Object-directed movements showed the largest sensorimotor activations, suggesting that, as is the case for observations of hand movements, motoric processing is particularly sensitive to the viewing of goal-directed actions. Taken together, the results indicate that the brain utilizes multiple action encoding strategies, tailored to the function of the observed movement.

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PubMed2006

Stimulus equivalence: testing Sidman's (2000) theory.

Sidman's (2000) theory regarding the origin of equivalence relations predicts that a reinforcing stimulus common to distinct equivalence classes must drop out of the equivalence relations. This prediction was tested in the present study by arranging class-specific reinforcers, R1 and R2, following correct responding on the prerequisite conditional discriminations (Ax-Bx, Cx-Bx) for two stimulus classes, A1B1C1 and A2B2C2. A class-common reinforcer, R3, was presented following correct responding on the prerequisite conditional discriminations for a further two stimulus classes, A3B3C3 and A4B4C4. Sidman's theory predicts reinforcer inclusion within Classes 1 and 2 only, given this training arrangement. Experiment 1 tested for the emergence of four equivalence classes and of stimulus-reinforcer and reinforcer-stimulus relations in each class. Four of the 6 subjects demonstrated the reinforcer-based relations in all four equivalence classes, rather than in only those classes with a class-specific reinforcer, as Sidman's theory predicts. One of the remaining 2 subjects showed the reinforcer-based relations in three of the four classes. Experiment 2 extended these findings to document the emergence of interclass matching relations based on the common reinforcer R3, in 5 of 6 subjects, such that a Class 3 sample occasioned the selection of a Class 4 sample when the Class 3 comparison was absent, and similarly, a Class 4 sample occasioned the selection of a Class 3 comparison when the Class 4 comparison was absent. These interclass relations emerged despite the simultaneous maintenance of Class 3 and 4 baseline conditional discriminations, so that the Class 3 and 4 stimuli and reinforcer simultaneously were, and were not, part of a single larger equivalence class. These data are irreconcilable with Sidman's theory, and question the utility of the application of the equivalence relation in describing derived stimulus relations.

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PubMed2004

Primary motor cortex activation during action observation revealed by wavelet analysis of the EEG.

OBJECTIVE: We characterised the spectral response of the EEG to median nerve stimulation using wavelet analysis, and compared the relative magnitudes of effect of several different action-observation conditions on the beta and mu 'rebound' rhythms. METHODS: EEG responses to median nerve stimulation were recorded from 8 normal adult subjects during baseline or action-observation conditions. Analysis was performed by convolution of the EEG with a family of wavelets. RESULTS: Decreased power in the mu and beta bands characterized the EEG following median nerve stimulation until 500 ms post-stimulus, followed by increased amplitudes ('rebound') of both rhythms. Execution of movement, observation of object-directed movement and observation of somatosensory stimulation all caused a decreased rebound of the beta rhythm whereas observation of aimless thumb movement did not. CONCLUSIONS: Wavelet analysis of the EEG extracted similar features reported in previous studies using bandpass filtering with respect to the activation state of the motor cortex during action observation. Further, our results show that observation of somatosensory stimulation alone is sufficient to cause significant modulation of motor cortex activity. SIGNIFICANCE: These results add further details as to what stimuli can activate the human mirror neuron system and the analytical techniques used may be useful for future studies of clinical populations such as autistic patients.

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PubMed2004

Mu rhythm modulation during observation of an object-directed grasp.

Recent electrophysiological studies have shown that the human electroencephalographic mu rhythm is suppressed during the observation of actions performed by other persons, an effect that may be functionally related to the behaviour of so-called "mirror neurons" observed in area F5 of nonhuman primates. Because mirror neuron activity has been reported to be functionally specific to object-oriented actions, the present study was designed to determine if the human mu rhythm also exhibits this property. EEG measurements were obtained from 12 normal subjects while they observed either a precision grip of a manipulandum or an empty grip using the same hand position. Our results showed that the magnitude of the mu rhythm was significantly lower for the object grip condition than for the empty grip condition. These data support the notion that the human mu rhythm indexes a brain system that is functionally comparable to the monkey mirror neuron system. We propose that nonobject-directed actions may result in representational schemas that are either different or less salient than motorically equivalent actions that are directed toward objects.

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PubMed2003

A high density ERP comparison of mental rotation and mental size transformation.

To compare mental rotation and mental size transformation, 128-channel EEG was recorded while subjects performed both tasks using random two-dimensional shapes as stimuli. Behavioural results showed significant linear effects of both size transformation and mental rotation on reaction times. Rotation ERPs showed experimental effects at two latencies: a bilateral component distributed over posterior parietal electrodes at a latency of approximately 232-300ms and a second component at approximately 424-492ms distributed over right anterior parietal electrodes. The latency and spatial distribution of this second effect is consistent with previous research indicating a functional connection between this component and mental rotation. ERPs for the size-transformation task showed an effect at 180-228ms distributed bilaterally over occipital-temporal electrodes. These results are consistent with previous hemodynamic imaging studies that show involvement of parietal cortex in mental rotation and also the involvement of BA 19 in size-transformation tasks. However, the superior temporal resolution of the present data indicates that BA 19 activation may occur at a latency that is more likely related to apparent motion than to the size-transformation operation per se.

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