SPECIFICITY OF MONOSYNAPTIC CONNECTIONS FROM THALAMUS TO VISUAL-CORTEX

In cortical area 17 of the cat, simple receptive fields are arranged i n elongated subregions that respond best to bright (on) or dark (off) oriented contours, whereas the receptive fields of their thalamic inpu ts have a concentric on and off organization(1). This dramatic transfo rmation suggests that there are specific rules governing the connectio ns made between thalamic and cortical neurons(1-3) (see ref. 4). Here we report a study of these rules in which we recorded from thalamic (l ateral geniculate nucleus; LGN) and cortical neurons simultaneously an d related their receptive fields to their connectivity, as measured by cross-correlation analysis(5,6). The probability of finding a monosyn aptic connection was high when a geniculate receptive field was superi mposed anywhere over an elongated simple-cell subregion of the same si gnature (on or off). However, 'inappropriate' connections from genicul ate cells of the opposite receptive field signature were extremely rar e. Together, these findings imply that the outline of the elongated, s imple receptive field, and thus of cortical orientation selectivity, i s laid down at the level of the first synapse from the thalamic affere nts.