The receptive-field properties of neurons in the striate visual cortex of normal and reeler mutant mice were studied with single-unit recording methods in order to determine whether the connections underlying these properties are altered by the developmental abnormality in neuronal position that characterizes reeler neocortex. Neurons with a projection through the corpus callosum were selected for study because they form a physiologically identifiable class of visual cortical neurons with a characteristic distribution of receptive-field properties that can be compared for normal and reeler cortex. Transcallosal cortical neurons in area 17 near its border with area 18a were identified by antidromic stimulation delivered through bipolar electrodes in the contralateral cortex. A computer controlled the visual stimuli, data acquisition, and analysis. Transcallosal neurons were principally found in layers II-III and V in the normal cortex and in a broand band deep in the reeler cortex. These populations had similar distributions of antidromic latencies, indicating that the neurons sampled from normal and reeler cortex were taken from populations with similar axonal diameters and soma sizes. The receptive-field properties of 46 units in 22 normal mice and 28 units in 11 reeler mice were characterized. Transcallosal neurons in both normal and reeler cortex were usually binocularly responsive and dominated by input from the contralateral eye. They exhibited either nonoriented (31 and 48%, respectively) or oriented (69 and 52%) receptive fields. Tuning 10 stimulus velocity was broad, with peak velocity sensitivities ranging from 1 to 1,000 degrees/s. Directional selectivity was present in 41% of normal units ad 32% of reeler units. There was no significant difference between normal and reeler cortex in the distribution of these properties. Transcallosal neurons were also examined for the presence of an inhibitory surround by comparing their responses to moving or stationary stimuli of varying sizes. Of the tested neurons, most (11/17 in normal cortex, 6/9 in reeler) showed evidence of a decrease in response to large moving stimuli. A large proportion (16/20) of normal neurons tested with stationary flashing stimuli had some degree of surround inhibition whereas significantly fewer (5/17) neurons in reeler cortex had this property. Thus, transcallosal neurons in reeler cortex less frequently had an inhibitory surround demonstrable with stationary flashing stimuli, but this difference between normal and reeler was not apparent with a moving stimulus.(ABSTRACT TRUNCATED AT 400 WORDS)
Searching for object pointers in the visual cortex
