A comparison of evoked potentials and high frequency (Gamma-band) oscillating potentials in rat auditory cortex. Franowicz, Matthew N., and Daniel S. Barth. Department of Psychology, University of Colorado, Campus Box 345, Boulder, CO 80309-0345 (U.S.A.).
APStracts 2:0056N, 1995.
SUMMARY AND CONCLUSIONS
1. Transient and steady-state (40-Hz) evoked potentials, as well as spontaneous and click-evoked gamma-band oscillations, were recorded from 15 lightly anesthetized rats using an 8 x 8 electrode epipial array covering auditory cortex and adjacent areas to determine and compare the spatiotemporal distributions of these four phenomena. 2. The transient evoked response replicated earlier findings in our laboratory, consisting of an initial biphasic sharp wave in area 41 (P1a/N1a), a similar but delayed biphasic sharp wave in area 36 (P1b/N1b), and more widely distributed slow wave components (P2, N2, P3). Spatiotemporal analysis supported a model of parallel and asynchronous activation of distinct groups of thalamocortical projections underlying the neurogenesis of these temporal components of the middle latency auditory evoked potential (MAEP) complex. 3. The 40-Hz response to click trains was superimposed on a steady potential shift (SP), both of which were localized within primary auditory cortex, in contrast to a subcortical neurogenesis originally proposed by Galambos (1982). Epipial distributions of the SP were similar to those of the N1a recorded in the same animals, suggesting similar neural generators. The 40-Hz response was more focal and dissimilar from the SP and any other temporal components of the MAEP complex, suggesting a unique subpopulation of cells underlies its neurogenesis. 4. Spontaneous gamma-band activity, as assessed by power spectrum analysis, was localized to primary and secondary auditory cortex but had a variable spatial distribution between rats that did not conform to the cytoarchitectonic boundaries within subdivisions of this region. Digital movies computed for individual bursts of gamma indicated a high degree of spatiotemporal variability within and between bursts. 5. Single trial spectral analysis of click responses indicated an inhibition of gamma-band oscillations during most of the MAEP complex, with subsequent enhanced gamma during the P3 component and outlasting the MAEP by approximately 500 msec. The epipial distribution of pre-stimulus and enhanced post-stimulus gamma oscillations were the same. In contrast to the 40-Hz response to click trains, phase-locking of gamma oscillations by the single click stimulus was not observed. 6. These results suggest that both the MAEP complex, and the steady-state 40-Hz response with its associated SP, are highly stereotyped in lightly anesthetized rodent cortex. Their spatiotemporal distributions are probably determined in large part by asynchronous activation of parallel thalamocortical projection systems. Our data suggest no direct link between either the MAEP or the steady-state 40-Hz response to spontaneous or evoked gamma-band oscillations in auditory cortex. The notable spatiotemporal variability of gamma oscillations, with rapid spread within areas 41, 36 and 20, suggest a neural generator within the cortex independent of direct thalamocortical pacemaker influence. This hypothesis is further supported by the observation that neither the spatial distribution nor the phase of ongoing gamma oscillations is influenced by the MAEP. However, post-inhibitory enhancement of gamma oscillations following auditory stimulation may indicate a role in auditory information processing that is not reflected in either a stimulus specific spatial distribution or phase relationship. 

Received 18 November 1994; accepted in final form 26 February 1995.
APS Manuscript Number J732-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on  3 April 1995.