Monocular deprivation normally alters ocular dominance in the visual cortex only during a postnatal critical period (20 to 32 days postnatal in mice). We find that mutations in the Nogo-66 receptor (NgR) affect cessation of ocular dominance plasticity.

2. Eleven colour coded units were investigated. They all gave on responses to blue light in the centre of their receptive field and off responses to green light in the periphery of their receptive field. The blue pigment had a spectral sensitivity peaking at about 465 nm. The other pigment peaked near 500 nm, like the rods, but gave a response at high mesopic and probably photopic levels. In some cases there was evidence for an excitatory input from the green receptors to the centre ofthe receptive field. All the colour coded cells had rapidly conducting axons and were on centre X cells by all criteria.3. Eighty-five cells of various types other than colour coded were tested for their thresholds at 420 nm and 590 nm. In all cases the results were explained by a pigment peaking close to 500 nm, even at high mesopic and low photopic levels, which suggests the existence of cones with a cyan pigment in them.

1. Cats were monocularly deprived for 3 mo starting at 8-9 mo, 12 mo, 15 mo, and several years of age. Single cells were recorded in both visual cortexes of each cat, and the ocular dominance and layer determined for each cell. Ocular dominance histograms were then constructed for layers II/III, IV, and V/VI for each group of animals. 2. There was a statistically significant shift in the ocular dominance for cells in layers II/III and V/VI for the animals deprived between 8-9 and 11-12 mo of age. There was a small but not statistically significant shift for cells in layer IV from the animals deprived between 8-9 and 11-12 mo of age, and for cells in layers V/VI from the animals deprived between 15 and 18 mo of age. There was no noticeable shift in ocular dominance for any other layers in any other group of animals. 3. We conclude that the critical period for monocular deprivation is finally over at approximately 1 yr of age for extragranular layers (layers II, III, V, and VI) in visual cortex of the cat.

SUMMARY1. The effects of picrotoxin and strychnine were tested on the receptive fields of direction sensitive cells, orientation sensitive cells, local edge detectors, uniformity detectors and large field units in the rabbit retina.2. Picrotoxin eliminated the direction specificity and size specificity of 'on-off' and 'on' directionally sensitive cells for both black and white objects. Picrotoxin also made 'on' directionally sensitive cells responsive to faster velocities.3. Picrotoxin eliminated the orientation specificity of orientation sensitive cells, and changed the bar-flank arrangement of the receptive field into a centre surround arrangement. Thus, the orientation specificity is due to inhibitory rather than excitatory mechanisms.4. Picrotoxin altered the speed sensitivity of large field units so that they responded to slow speeds as well as fast ones, like centre surround Y cells.5. Strychnine abolished the size specificity of local edge detectors and changed their speed specificity so that they responded to faster speeds.6. Picrotoxin changed a uniformity detector into a sustained on centre cell. 7. Strychnine did not affect the direction specificity of directionally sensitive cells, the orientation specificity of orientation sensitive cells, or the speed specificity of large field units. Picrotoxin did not affect the size specificity of local edge detectors.8. Picrotoxin and strychnine usually had opposing effects on the transient responses of these units to spots and annuli. In general picrotoxin prolonged and enhanced these responses at both on and off, and strychnine shortened them.9. The effect of these drugs for every type of ganglion cell with complex receptive field properties was to make the receptive field more simple. The orientation selective cells, large field cells, 'on' direction selective cells and uniformity detectors seem to be centre surround cells with special properties that are abolished by these drugs. The 'on-off' direction selective cells and local edge detectors still have on-off receptive fields, but in each case one of the drugs abolished the feature that was the basis for the cell's name.

uring the past 20 years, basic science has shown that there are different critical periods for different visual functions during the development of the visual system. Visual functions processed at higher anatomical levels within the system have a later critical period than functions processed at lower levels. This general principle suggests that treatments for amblyopia should be followed in a logical sequence, with treatment for each visual function to be started before its critical period is over. However, critical periods for some visual functions, such as stereopsis, are not yet fully determined, and the optimal treatment is, therefore, unknown. This article summarizes the current extent of our knowledge and points to the gaps that need to be filled.