Marco Amadeo

Spatial distribution of potentials evoked by half-field pattern-reversal and pattern-onset stimuli

Spatial distributions of visual potentials (VEPs) evoked by half-field checkerboard pattern-reversal and pattern-onset stimuli were studied in 13 subjects, using an 11 lead unipolar array. The main aim was to confirm findings, obtained by previous workers with bipolar recordings, that half-field pattern-reversal VEPs are confined to the contralateral hemisphere and that half-field pattern-onset VEPs are asymmetrical, with greater right hemisphere involvement. Pattern-reversal VEPs contained four consistent peaks, designated here by polarity and peak latency as: (a) P95, positivity contra-lateral and negativity ipsilateral to the field stimulated; (b) P125, predominantly ipsilateral positivity; (c) N165, predominantly ipsilateral negativity; (d) P225; predominantly midline positivity. Pattern-onset VEPs contained three consistent peaks: (a) P125, mainly contraleteral positivity; (B) N175, mainly contralateral negativity; (c) P225, midline positivity. Distributions of pattern-reversal and pattern-onset peaks resembled one another only for P225, suggesting different cortical representation for the other events of the two kinds of VEP. Bipolar pattern-reversal VEPs were largely contralateral, but unipolar recordings showed that this was due to steeper contralateral potential gradients, as ipislateral activity was widespread. Pattern-onset peaks did not differ in amplitude with respect to the half-field stimulated. Previously reported asymmetries were not confirmed. The P125 and N175 pattern-onset peaks were almost entirely restricted to the contralateral hemisphere, but the distributions by half-field were mirror-images of one another. Half-field pattern-onset stimuli could be used to investigate the responsiveness of each hemisphere, although differential hemispheric involvement was not shown. Several differences in amplitude and distribution resulted from varying the width of the vertical central dark strip.

Spatial Distribution of Sensory Evoked Potentials in Psychiatric Disorders

The topographic dimension has received little attention in evoked potential (EP) investigations of psychiatric patients. Although EPS have been recorded most often from a single lead derivation, some studies involving recordings from more than one site have yielded findings which suggest that the spatial distribution of EPs may be of psychiatric interest. For example, Rodin et al. (1968) in a study of visual evoked potentials (VEPs) observed that assessments of psychopathology in schizophrenics were more often correlated with right than with left hemisphere VEP characteristics. Perris (1974) found that amplitudes of VEPs from the left occiput were lower than those from the right in psychotic depressives while they were ill. Buchsbaum et al. (1977) reported that, in a rapidly cycling manic-depressive patient, a VEP wave was decreased in amplitude at the vertex and increased at the occiput with mania and conversely with depression. Such observations encourage further exploration of EP topography with respect to possible psychiatric correlates.

Evoked response latency recovery cycles: Changes during sleep in man☆

Abstract Somatosensory evoked response recovery cycles were measured during sleep. Latency recovery of the initial negative response peak during sleep followed a biphasic cycle, different from that of waking. In the first phase, the second response was relatively accelerated; in the second phase it was slowed. The cycle was less pronounced in stage I REM than in other sleep stages.