THE ANATOMIC ORGANIZATION OF EVOKED-POTENTIALS IN RAT PARIETAL CORTEX- ELECTRICALLY-EVOKED COMMISSURAL RESPONSES

1. Two 8 x 8 channel microelectrode arrays were positioned over 3.5 x 3.5 mm(2) areas in homologous regions of right and left parietal corte x of four rats. Potentials were evoked by delivering epicortical elect rical stimulation to each electrode on one hemisphere while mapping th e commissural response from the contralateral array. Spatial distribut ions of the electrically evoked potential (EECP) complex were compared directly with cytochrome oxidase-stained sections of the recorded reg ion. 2. Electrode sites most capable of eliciting a commissural EECP w ere arranged along a diagonal band extending medially from the rostral to caudal region of each electrode array, approximating the pattern o f dysgranular cortex separating primary auditory (Te1) from primary so matosensory (Par1) cortex. Electrode sites in the rostromedial and cau dolateral region were ineffectual in eliciting an EECP in either hemis phere. Stimulation sites within secondary visual cortex (Oc2L) also pr oduced strong responses. Only weak responses were elicited from stimul ation of Te1 and no EECP could be evoked when stimulating within Par1. 3. When an EECP in the maximally sensitive diagonal region was elicit ed, its spatial distribution was typically asymmetrical throughout the recording array; the response was largest along a diagonal region als o extending medially from the rostral to caudal area of each electrode array. Thus the pattern of EECP in each hemisphere closely matched th e pattern of electrically excitable regions in the contralateral hemis phere. 4. The EECP was usually heterogeneous. EECP distributions withi n the strongly responding diagonal area often formed two regions of ma ximum amplitude separated by a less active zone. Although responses in Te1 were significantly weaker than those in the adjacent dysgranular cortex, they also revealed a heterogeneous spatial distribution with m ultiple closely spaced maxima. Only responses in Oc2L appeared consist ently homogeneous, with a single maximum representing the EECP. 5. The se results provide functional evidence supporting a model of parietal cortex in which there are two basic types of recipient regions, densel y granular regions, which are the termination sites of specific thalam ocortical fibers, and dysgranular or agranular regions, which receive both ipsilateral and contralateral projections. The functional parceli ng of rodent parietal cortex on the basis of the spatial and temporal distribution of the epicortical evoked potential complex may be superi mposed onto the anatomic parceling into granular and dysgranular zones . Implications for stages of sensory information processing are discus sed.