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Biomedical subjects

M Sams

Publications and source records attributed to M Sams.

60 records · Page 4Linked to original sources
PubMed1978

Effect of stimulus repetition on negative sustained potentials elicited by auditory and visual stimuli in the human EEG.

Effect of repetition of auditory and visual stimuli on sensory-evoked sustained potentials was studied. The stimuli were tones and flashes of 1 sec duration presented in trains of six stimuli with an inter-stimulus-interval of 1 sec. The repetition rate of the trains was 1 train/min. The EEG was recorded from electrodes located at Cz, Pz and Oz. Both auditory and visual stimuli elicited negative sustained potentials which during the first stimuli of the trains were maximal in amplitude and shortest in latency at Cz. Repetition of the stimulus resulted in almost complete disappearance of the sustained potentials; for auditory stimulation a small negative shift was recorded only at Cz, whereas for visual stimulation a small sustained potential was seen only at Pz and Oz. The results indicate that the slow potentials evoked by short tones or lights are composed of at least two components. One of these components may reflect neural processing associated with orienting reflex. The other component seems to be related to specific neural processing of the stimulus.

Acoustic Stimulation↗
PubMedDate not supplied

Mismatch negativity (MMN) for sequences of auditory and visual stimuli: evidence for a mechanism specific to the auditory modality.

ERPs to sequences of standard and deviant sinusoidal 100 msec tone pips, high-contrast sinusoidal gratings and to their simultaneously presented combinations were recorded. Mismatch negativity (MMN), an ERP component elicited by deviant stimuli, was estimated for the different stimulus sequences in order to find out whether it reflects modality-specific processes or non-specific attentive phenomena. In addition to the auditory modality, we studied whether the mismatch response could be evoked by a deviant visual stimulus in a visual sequence or by a deviant stimulus in either modality. The results show that only auditory stimuli produced the mismatch response, suggesting that MMN is not a manifestation of a general attentional mechanism but is probably specific to the auditory modality.

Acoustic Stimulation↗
PubMedDate not supplied

Cortical sensory evoked potentials and communicative forebrain functions.

Cortical evoked potentials were measured to visual, auditory and somatosensory stimuli in 20 subjects with serious neurodevelopmental impairments due to various etiologies. The results were compared with behavioral observations to find out whether the absence/presence of the responses corresponded to the level of social functioning. No cortical evoked potentials were elicited in two subjects, responses to the stimulation of one modality were missing in three subjects (retinal b-waves and brainstem auditory and somatosensory evoked potentials were, however, preserved in them). No communicative behavior was observed in subjects with absent responses. Ten subjects had marked deviations in the evoked potentials, the behavioral observations in them, ranging from no communication to sentenced speech. Five subjects had normal response patterns and they showed a great variety of communicative skills, including speech. The results support the view that bilateral loss of cortical somatosensory, visual, and auditory evoked potentials is a sign of loss of neural substrates of communication.

Adolescent↗
PubMedDate not supplied

Deviant auditory stimuli activate human left and right auditory cortex differently.

Infrequent "deviant' auditory stimuli embedded in a homogeneous sequence of "standard' sounds evoke a neuromagnetic mismatch field (MMF), which is assumed to reflect automatic change detection in the brain. We investigated whether MMFs would reveal hemispheric differences in cortical auditory processing. Seven healthy adults were studied with a whole-scalp neuromagnetometer. The sound sequence, delivered to one ear at time, contained three infrequent deviants (differing from standards in duration, frequency, or interstimulus interval) intermixed with standard tones. MMFs peaked 9-34 msec earlier in the right than in the left hemisphere, irrespective of the stimulated ear. Whereas deviants activated only one MMF source in the left hemisphere, two temporally overlapping but spatially separate sources, one in the temporal lobe and another in the inferior parietal cortex, were necessary to explain the right-hemisphere MMFs. We suggest that the bilateral MMF components originating in the supratemporal cortex are feature specific whereas the right-hemisphere parietal component reflects more global auditory change detection. The results imply hemispheric differences in sound processing and suggest stronger involvement of the right than the left hemisphere in change detection.

Acoustic Stimulation↗