Paul B. Johnson

Abnormalities of short-latency somatosensory evoked potentials in Parkinsonian patients

Twenty-two patients (16 affected by parkinsonian syndromes, 6 by other neurological diseases) and 12 age-matched controls were examined. Short-latency somatosensory evoked potentials were recorded from 30 scalp electrodes in the 45-52 msec following separate left and right median nerve stimulation at the wrist. Bit-colour maps were generated on a 4096 pixel matrix via quadratic interpolation. Peak latencies and amplitudes of the parietal, central and frontal components were evaluated. Moreover, the amplitude ratios between parietal and frontal components on the same hemiscalp and between peaks on homologous right and left scalp districts were taken into account. The unique significant difference between parkinsonians and controls was represented by a depressed frontal N30 wave. This peak was absent in 3 and reduced in 7 out of 16 parkinsonians, with an overall abnormality rate of 47% of the examined arms. Average maps pooling data of parkinsonians and controls confirmed the presence of reduced evoked activity for the whole duration of wave N30 on those mid- and parasagittal frontal districts where this peak is maximally represented in normals. A similar abnormality was found in 1 of the 6 non-parkinsonian neurological patients suffering from a meningioma of the falx compressing the left supplementary motor area. Possible pathophysiology of such wave N30 abnormalities in parkinsonians is discussed.

REACHING TO VISUAL TARGETS: COORDINATE SYSTEMS REPRESENTATION IN PREMOTOR AND MOTOR CORTICES

Individual arm-related neurons in both motor (area 4) and premotor (area 6) cortices of the monkey are directionally tuned. We studied these directional neurons while monkeys made arm movements of similar directions within different parts of 3-D space. The behavioral task was aimed at dissociating the direction of movement, which remained similar across the work space, from the pattern of muscular activity and joint rotations underlying these movements. Within a given part of space, motor and premotor cortical cells fired most for a given preferred direction and less for other directions of movement. These preferred directions covered the directional continuum in a uniform fashion across the work space. As movements of similar directions were made within different parts of the work space, the cells’ preferred directions in both motor and premotor cortices changed their orientation. Although these changes had different magnitudes for different cells, at the population level, they followed closely the changes in orientation of the arm necessary to move the hand from one part of the work space to another.

Computerized mapping system of cerebral evoked potentials

The introduction of computerized analysis systems in the study of bioelectrical signals is enhancing the understanding of the physiological mechanisms which underlie cerebral evoked potentials (EPs) in response to externally applied stimuli. In the present study, short latency (0-50-ms) and long latency (0-500-ms) somatosensory evoked potentials (SEPs) were recorded by 32 scalp electrodes from normal and pathological subjects during median nerve stimulation. An interpolation procedure for estimating data values between the neighboring electrodes allowed the mapping of cortical activity across the scalp. Time signals were also transformed by an FFT algorithm and frequency maps obtained following the same interpolation procedure. Temporal and frequency maps were graphically displayed using color and three-dimensional plots. The usefulness of computerized topographical analysis is discussed; the time and frequency computer maps obtained from the same subjects are compared and their relative advantages are evaluated.

Callosal and association neurons in the cortical space: A spectral analysis approach

The tangential distributions of callosal neurons of area 5 projecting homotopically to the contralateral hemisphere and of association neurons of areas 4 and 6 projecting to ipsilateral area 5 were determined in the macaque monkey by using neuroanatomical methods based on the retrograde transport of horseradish peroxidase. Both distributions were studied qualitatively through 2-dimensional reconstructions of the cortical areas of origin and quantitatively through a spectral analysis. This approach facilitated the characterization of the spatial periodicities contained in these distributions revealing that, in area 5, callosal neurons were organized in bands of various shapes and width; these bands were composed of more discrete clusters of cells. In the frontal lobe, association neurons projecting to ipsilateral area 5 were arranged similarly. This study suggests that a common principle underlies the tangential organization of both callosal and association projecting cells in different cortical areas and emphasizes a basic similarity of interhemispheric and intrahemispheric connections.