ANATOMIC ORGANIZATION OF EVOKED-POTENTIALS IN RAT PARIETOTEMPORAL CORTEX - SOMATOSENSORY AND AUDITORY RESPONSES

1. Two 8 x 8-channel microelectrode arrays were used to map epicortica l field potentials from a 3.5 x 3.5-mm2 area in homologous regions of right and left parietotemporal cortex of four rats. Potentials were ev oked with bilaterally presented click stimuli and with bilateral tacti le stimulation of the 25 major vibrissae. The spatial distribution of temporal components of the somatosensory evoked potential (SEP) and au ditory evoked potential (AEP) complex were compared directly with cyto chrome oxidase-stained sections of the recorded region. 2. Epicortical responses in both hemispheres to bilateral vibrissal stimuli consiste d of a biphasic sharp wave (P1a-N1) constrained to the vibrissa/barrel granular region of primary somatosensory cortex (SmI). A slightly lat er sharp positive wave (P1b) was localized to secondary somatosensory cortex (SmII) and to perigranular cortex medial to the vibrissa/barrel field. The SEP complex ended with a biphasic slow wave (P2-N2). The P 2 was centered on SmI and spread to dysgranular lateral cortex, caudal to but excluding SmII. The N2 was centered on SmII and spread to dysg ranular cortex caudal to but excluding SmI. 3. The anatomic organizati on of the AEP in many ways approximated that of the SEP in the same an imals. The timing and morphology of the AEP were nearly identical to t he SEP. The AEP consisted of a P1a-N1 sharp wave constrained to the es timated region of primary auditory cortex (AI) in the lateral parietot emporal region, a later P1b localized to secondary auditory cortex (AI I), and subsequent slow waves (P2 and N2) that were centered on AI and AII, respectively, and spread to dysgranular regions overlapping the distributions of the P2 and N2 of the SEP complex. 4. These data sugge st that the basic neural generators for the SEP and AEP in parietotemp oral cortex are quite similar, and provide evidence for the functional anatomy of each temporal component of the sensory evoked potential co mplex. It is concluded that the early fast waves of the SEP and AEP ar e modality specific and may represent the parallel activation of prima ry and secondary sensory cortex through established parallel afferent projections from lateral and medial thalamic nuclei. The later slow wa ves of the SEP and AEP appear to selectively involve primary and secon dary sensory cortex but are more widely distributed, possibly reflecti ng a less modality-specific level of information processing in dysgran ular cortex.