Organization and post-natal development of the monkey's lateral geniculate nucleus.

We have studied the properties of neurones in the lateral geniculate nucleus (l.g.n.) of Old World monkeys, both in mature animals and throughout post-natal development. Cells were classified as X (linear) or Y (non-linear) on the basis of their responses to contrast-reversing achromatic gratings ('null position test'). In older animals virtually all parvocellular neurones and the majority of magnocellular units were X cells; only about 15% of magnocellular neurones displayed highly non-linear spatial summation, with no 'null position', typical of Y cells. X cells could not reliably be distinguished from Y cells, nor magnocellular from parvocellular, on the basis of their temporal patterns of discharge. Some Y cells responded transiently to contrast reversal of a grating far from the receptive field but X cells showed little or no such 'shift effect'. The spatial resolution of mature l.g.n. cells varied with the eccentricity of their receptive fields such that the best of them, at each point in the visual field, resolved drifting achromatic gratings about as well as a human observer. X cells in parvocellular and magnocellular layers had similar 'acuities', even in the central foveal representation, but Y cells generally had poorer resolution. Receptive fields in the temporal retina tended to have lower resolution than those at comparable eccentricities in the nasal retina. Even on the day of birth all cells we studied responded to visual stimulation and virtually all could be classified as X or Y. The laminar distribution of cell types and the general morphological appearance of the nucleus seemed very similar to those in the adult, but neurones in very young animals had low spontaneous activity, sluggish responses, and latencies to visual stimulation longer than any we saw in the adult. Until 3 weeks of age or so, many neurones suffered cumulative 'fatigue' when visually stimulated over several minutes. Visual latency was essentially mature by about 10 weeks. In the l.g.n. of the neonatal monkey there was little variation in neuronal 'acuity' with eccentricity: even in the foveal area the best cells could resolve only about 5 cycles/deg. Over the first year or more of life there is a gradual increase in responsiveness and about a 7-fold improvement in spatial resolution for foveal l.g.n. cells, correlating roughly with the behavioural maturation of visual acuity.