Receptive field properties and latencies of cells in the lateral geniculate nucleus of the North American opossum (Didelphis virginiana)

Relay cells in the lateral geniculate nucleus (LGN) of the North American opossum were classified as types 1, 2, and 3 on the basis of their receptive field properties and afferent latencies to optic nerve (ON) and optic chiasm (OX) stimulation and antidromic latencies to stimulation of cortex. Type 1 and type 3 cells gave transient responses and type 2 cells gave sustained responses to appropriate standing contrast in the receptive field center. The differences in response pattern were quantified with a phasic-tonic index (PTI); the PTI values for type 2 cells (PTI less than 63) did not overlap those for type 1 cells (PTI greater than 68) or type 3 cells (PTI greater than 80). With a homogeneous field (1.3 cd/m2), the maintained discharge rates (spikes/s) of type 3 cells (less than 1-11) were significantly lower than those of type 1 cells (3-23) and of type 2 cells (1-22). For all type 2 cells tested with a counterphased sine-wave grating, a null position of the grating was found and the cells were classified as linear. The type 1 and type 3 cells tested were nonlinear (i.e., exhibited excitatory doubling and did not have a null grating position). The maximum velocity of movement that reliably elicited responses (cut-off velocity) was low for type 3 cells (mean = 29.1 degrees/s) and relatively high for type 1 cells (mean = 70.3 degrees/s). Cut-off velocities for type 2 cells (mean = 61.2 degrees/s) were slightly lower than for type 1 cells. Type 1 cells had relatively short afferent (ON and OX) latencies, fast afferent conduction velocities, and short antidromic (cortex) latencies; type 2 cells had intermediate afferent and antidromic latencies and intermediate afferent conduction velocities; and type 3 cells had relatively long afferent and antidromic latencies and slow afferent conduction velocities. The receptive field center diameters were similar for type 1 (4.2-25.5 degrees; mean = 11.4 degrees) and type 3 cells (2.8-23.7 degrees; mean = 11.0 degrees), whereas the receptive field centers for type 2 cells (2.9-15.1 degrees; mean = 6.9 degrees) were significantly smaller. The majority of type 1 (66.7%) and type 3 cells (63.3%) had on-center receptive fields, whereas the proportion of on-center fields was even greater for type 2 cells (83.0%). Only a few of the cells encountered in the opossum LGN (6%) had on-off receptive fields, and a portion of these could not be shown to be relay cells.(ABSTRACT TRUNCATED AT 400 WORDS)