Laminar organization of receptive-field properties in lateral geniculate nucleus of bush baby (Galago crassicaudatus).

Abstract

In the galago lateral geniculate nucleus (LGN) large, medium and small cells are segregated into 3 pairs of layers. The pairs each contain 1 layer innervated by each eye. The pairs are organized such that layers 1 and 2 are magnocellular layers, 3 and 6, are parvocellular layers, and 4 and 5 contain very small cells (koniocellular layers). To determine if these cell size differences have physiological correlates, the response properties of 119 neurons localized histologically to individual LGN layers in 12 greater galagos (G. crassicaudatus) were examined. Neurons were tested for latency of responses to stimulation of the optic chiasm (OX latency), latency to antidromic stimulation of striate cortex (VC latency), concentric organization, receptive-field size, sustained or transient responses to standing contrast, responsiveness to rapidly moving targets, differences in receptive-field organization as measured with spatial-temporal (S-T) maps and linearity of spatial summation in response to counterphased bipartite and sine-wave grating stimuli. Cells in the magnocellular layers had short OX and VC latencies. They responded briskly to visual stimuli and had concentrically organized receptive fields with small to medium center diameters that increased slightly in diameter with increasing eccentricity. Most magnocellular layer neurons (87%) responded transiently to stationary stimuli and 55% responded to rapidly moving stimuli appropriate to excite the surround. All had a large to moderate phasic burst of spikes at the beginning of their response to light spots (phasic-tonic index (PTI) > 55). A few responded with excitatory doubling to either bipartite stimuli (7%) or counterphased sine-wave gratings (22%). Most either had a null region or inhibitory doubling. Neurons in the parvocellular layers had moderate OX and VC latencies. They responded to visual stimuli like magnocellular layer cells. Many parvocellular layer neurons (62%) gave sustained responses to standing contrast and very few (14%) responded to rapidly moving stimuli appropriate to stimulate the surround. Nearly all had only a small (or no) phasic burst of spikes at the beginning of their response to light spots (PTI < 55). All either had a null region or inhibitory doubling to bipartite stimuli or counterphased sine-wave gratings. Neurons in the koniocellular layers were very heterogeneous in their response properties. The OX and VC latencies were generally long. Some of the koniocellular layer neurons (48%) responded briskly to visual stimuli, but some (17%) were sluggish. Of the cells that were visually responsive, 47% had a concentric, center-surround organization, 40% had on, off, or on-off centers with suppressive surrounds, and 17% had no surround. The receptive-field center diameters were variable but on average were larger than those in the other layer pairs. Responses to standing contrast, rapidly moving targets, amount of initial phasic burst and to bipartite and grating stimuli were variable. The koniocellular layers were generally distinguished from the other layer pairs by the presence of longer latencies, larger receptive-field center diameters, and heterogeneous receptive-field properties. Cell-size segregation in LGN layers is apparently strongly correlated with differences in receptive-field properties in the galago. The magnocellular layers contain many Y-like cells, most of which do not exhibit excitatory doubling when tested with counterphased grating. The parvocellular layers contain many X-like cells. The koniocellular layers contain a physiologically mixed population, some of which are W-like cells.