Thresholds were measured along three directions in color space for detecting an equiluminant color change of a set of bars embedded in a larger field of spatio-temporal achromatic noise for observers ranging in age from 3 months to 86 years. Pre-verbal observers were assessed with a forced-choice preferential-looking technique while older observers responded orally or manually. Over the life span, thresholds could be described along each color axis tested by a curve with two trends. Thresholds decreased with each doubling of age by nearly a factor of two until adolescence. Thereafter, thresholds increased by a factor of 1.4-2 with each doubling of age. Sensitivity to chromatic differences varied similarly along all three axes tested, suggesting uniformity in the sensitivity of chromatic mechanisms across the life span.

SUMMARY1. 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.2. 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.

SUMMARY1. 1127 single units were recorded during oblique penetrations in area 17 of one normal, three monocularly deprived and four reverse sutured monkeys.2. In all animals most cells outside layer IVc were orientation-selective, and preferred orientation usually shifted from cell to cell in a regular progressive sequence.3. The presence in layer IV c of non-oriented, monocularly driven units, organized in alternating right-eye and left-eye 'stripes' (LeVay, Hubel & Wiesel, 1975) was confirmed.4. Early monocular deprivation (2-5j weeks) caused a strong shift of ocular dominance towards the non-deprived eye. However, even outside layer IVc, neural background and some isolated cells could still be driven from the deprived eye in regularly spaced, narrow columnar regions. In layer JVc the non-deprived eye's stripes were almost three times wider, on average, than the deprived.5. Later monocular deprivation (11-16 months) had no detectable influence on layer Ic but seemed to cause a small shift in ocular dominance outside IVc.Deprivation for 61 months in an adult had no such effect.6. After early reverse suturing (at 51 weeks) the originally deprived eye gained dominance over cells outside layer IVc just as complete as that originally exercised by the eye that was first non-deprived. 7. The later reverse suturing was delayed, the less effective was recapture by the originally deprived eye. Reversal at 8 weeks led to roughly equal numbers of cells being dominated by each eye; fewer cells became dominated by the newly open eye after reverse suturing at 9 weeks and most of them were non-oriented; reversal at 38j weeks had no effect.8. Binocular cells, though rare in reverse sutured animals, always had very similar preferred orientations in the two eyes. The columnar sequences of preferred orientation were not interrupted at the borders of ocular dominance columns.

SUMMARY1. We have studied the physiological properties of cells in the deprived layers of the lateral geniculate nucleus (l.g.n.) in monkeys monocularly deprived from birth for up to 27 weeks, and compared them with results from the non-deprived layers in the same animals and in a series of normal animals.2. Despite the relative shrinkage of cell bodies in the deprived layers, units were easily isolated, were visually responsive and could readily be classified as linear (X) or non-linear (Y) by means of tests of spatial summation. The laminar distribution of cell types and the proportion of Y cells did not seem to be affected by deprivation.3. The patterns and latencies of discharge produced by contrast-reversing gratings did not differ grossly between deprived and non-deprived cells. The peak firing frequencies for drifting gratings were also similar. The degree of surround antagonism (though very variable from cell to cell) seemed unaffected by deprivation.4. Most surprising of all, there was little or no deficit in the spatial resolution of the receptive fields of deprived cells. Recordings were always taken ipsilateral to the deprived eye, and neural 'acuity' tended to be slightly lower in the deprived laminae than the non-deprived. However, this nasal/temporal asymmetry in spatial resolution was not obviously more pronounced than in normal animals.5. Neural 'acuity' was not abnormally low in either contralateral or ipsilateral layers in the l.g.n. of an animal binocularly deprived from birth until a year of age.6. We have not examined chromatic properties or temporal characteristics adequately to say whether they are affected by deprivation.7. Paradoxically, although the post-natal maturation of visual acuity in normal monkeys seems to be mainly limited by peripheral factors, deprivation (which causes a profound defect of behavioural acuity) does not seem to interfere substantially with physiological development of the retina or the geniculate nucleus.