Charles Shagass

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.

Quantitative EEG in elderly depressives.

SummaryThirty-one elderly depressive patients were evaluated with topographic, quantitative EEG using relative measures, absolute measures, and computations of interhemispheric asymmetry and interhemispheric coherence in four frequency bands: delta, theta, alpha and beta. Patients were found to have lower than normal delta, higher than normal theta, higher than normal alpha and lower than normal beta values. EEG values were greater over the left than the right hemisphere in theta, alpha and beta bands. Lower than normal anterior interhemispheric coherence was found in all four frequency bands.

Problems associated with application of the contingent negative variation to psychiatric research.

As a first step in applying Walters contingent negative variation (CNV) in comparative studies of psychiatric patients, data were obtained concerning the issues of contamination by activity arising in the orbit and of individual variability between different test paradigms. Subjects were 20 college students and 10 psychiatric inpatients. Looking only at the conventional CNV recording leads (vertex-mastoid), a significant CNV was obtained in two experimental paradigms. However, analysis of the spatial distribution of the potential strongly suggested that much of the recorded vertex-mastoid negative shift can be accounted for by activity associated with eye movement. The distribution of CNV closely paralleled the distribution of vertical eye movement potentials. Results in patients and nonpatients were similar. The vertex-mastoid lead placement appears to be a poor one for recording relatively uncontaminated CNV; vertex-temporal derivation appears promising, although a smaller negative shift was recorded there. CNV measurements of the same subjects obtained under two experimental conditions were poorly correlated with each other, suggesting that the CNV responsiveness under any one condition does not characterize the individual. It was concluded that application of CNV recording to psychiatric research presents serious methodological and interpretive difficulties.

Combinations of evoked potential amplitude measurements in relation to psychiatric diagnosis

The purposes of this investigation were (1) to determine the extent to which single nonredundant measures, derived from optimal combinations of evoked potential (EP) amplitude measurements, could differentiate between various groups of psychiatric patients and between patients and nonpatients, and (2) to assess the replicability of such discriminations. Somatosensory, visual, and auditory EPs were recorded from 15 locations in 253 unmedicated patients and 99 nonpatients. Multivariate statistical methods were used to reduce the amplitude measurements to sets of factor scores that met specified criteria for entry into discriminant analyses between pairs of the following groups: nonpatients, neuroses, personality disorders, schizophrenias, schizotypal/borderlines, major depressives, and manics. Sensitivity, specificity, and predictive value were assessed for discriminant scores. The discriminant scores yielded many differences between groups, most of which were replicable in split-half analyses. Among these were differences between all six patient groups and nonpatients, and between schizophrenics and nonpsychotics. Split-half discriminant analyses were also performed with 12 measures that were obtained by taking the means of factor scores grouped by sensory modality, time, and spatial location; these variables, used without preselection, provided several replicable diagnostic discriminations.

Attention-Related Effects on Somatosensory Evoked Potentials in College Students at High Risk for Psychopathology

Attention directed to a stimulus elicits a late positive wave in the evoked potential, which has consistently been shown to be of lower than normal amplitude in schizophrenic patients. This study aimed to determine whether a similar deviation from normal occurs in nonpatient college students with extremely high scores on Chapmans Physical Anhedonia (AN) and Perceptual Aberration (PA) Scales. Individuals with such high scores have been shown to deviate from control subjects in ways analogous to schizophrenic patients and are considered to be at high risk for this disorder. Somatosensory evoked potentials (SEPs) were recorded from five scalp locations while subjects counted stimuli to one of four randomly stimulated fingers; SEPs to counted stimuli contained the attention-related late positive wave (P400). The AN (n = 10) and PA (n = 10) groups were compared with normal control subjects (n = 10); in addition, the AN group was compared with schizophrenic subjects (n = 7). P400 amplitudes of ANs were lower than those of normal control subjects matched for age and sex, whereas P400 amplitudes of PAs and their matched control subjects did not differ. P400 amplitudes of ANs did not differ significantly from those of the schizophrenic group, although AN means were somewhat higher. Measurements of SEP peaks preceding P400 yielded few consistent group differences with respect to attention-related changes. The results suggest that ANs, but not PAs, display a pattern of P400 deviation similar to that found in schizophrenic patients.

The attention-related somatosensory evoked potential late positive wave in psychiatric patients

Somatosensory evoked potential (SEP) late waves (P400) associated with selective attention were elicited by having subjects count electrical stimuli to one of four randomly stimulated fingers. P400 amplitude was lower in a diagnostically heterogeneous group of 20 psychiatric patients than in 16 nonpatients. Among patients, a general psychopathology factor derived from the Millon Clinical Multiaxial Inventory was significantly correlated with a factor accounting for most of the P400 amplitude variance. In psychiatric patients, attention-related late activity amplitude is apparently reduced from normal in evoked potentials of all modalities; degree of reduction appears quantitatively related to level of psychopathology.

Early cognitive components of somatosensory event-related potentials

Somatosensory event-related potentials (ERPs) were recorded during a selective attention task involving electrical stimuli delivered to index fingers or the left and right median nerves at the wrist. In 11 healthy, young subjects, ERPs were recorded from 6 scalp locations while they mentally counted the electrical stimuli designated as target. Sequential ERP events measured included N20 (negativity at 20 ms), P30, P45, N60, P100, N140, P180, and P400. Analysis of amplitude data indicated modifications of both early and late ERP events with selective attention. While electrical stimulation at the wrist yielded early ERP amplitudes that were larger overall and latencies that were generally shorter, the selection attention effects did not differ on the basis of site of stimulation. The early ERP selective attention effects had differing scalp topography, with the P30/P45 effect of maximal over postcentral gyrus and N60 effect maximal over prerolandic gyrus. The data further elucidate the temporal features and spatial distribution of somatosensory ERP processes involved in attentional activity.

Relationship between Pretreatment Electroencephalographic Coherence Measures and Subsequent Response to Electroconvulsive Therapy: A Preliminary Study

Quantitative analyses of EEG recordings taken prior to a course of ECT in elderly depressed patients showed that normal anterior interhemispheric coherence in the delta frequency band was associated with more favorable clinical response to ECT, whereas lower coherence values were associated with incomplete therapeutic response. Clinical implications of these findings are discussed.

Some Quantitative EEG Findings in Unmedicated and Medicated Major Depressives

To confirm previous quantitative EEG findings in major depressive disorder and to assess effects of medication, EEGs were recorded under eyes closed and open conditions in 68 patients (34 unmedicated and 34 medicated) and nonpatients matched for age and sex. Time series analyses of amplitude, frequency and wave symmetry were performed; differences between eyes open and closed were adjusted for eyes closed values to obtain measures of reactivity. These reactivity measures yielded the main differences between unmedicated patients and nonpatients; depressives were more reactive. Reactivity differences were eliminated or reversed in medicated patients. The EEGs of unmedicated depressives appear to be overreactive, while medications decrease EEG reactivity.

Neurophysiological evidence for different types of depression

A brief, selective review of a broad range of evidence bearing upon biological distinctions between neurotic and psychotic forms of depression is presented. It includes a somewhat detailed presentation of clinical neurophysiological findings obtained in the authors laboratory over three decades of research. The available evidence is taken to support the discontinuity position of the continuity-discontinuity controversy about neurotic vs psychotic depression. It is concluded that more than one process underlies clinical depression, and that there may be several kinds of depressive disorders.