One eye in each of five adult Wistar hooded rats was covered with opaque contact occluders for three or nine months. The rat retinae were studied with light and electron microscopy. No changes in retinal thickness or in gross structure were seen. However, quantitative differences in synaptic organisation were found.

(1) First comparison. A montage of a normal retina was compared with a similar montage of a three month lighted-deprived retina. Taking equal areas of inner plexiform layer, the number of amacrine (conventional) synapses in the light-deprived montage was larger by a factor of about 2.4 relative to the normal one. No significant difference in the number of bipolar (ribbon) synapses was found.

(2) Second comparison. Samples were compared from five normal and five light-deprived retinae. The light-deprived retinae showed a significantly larger number of amacrine synapses compared with the normal mean (p<0.02).

It was not possible to determine if the changes in synaptic number are related to changes in vesicle dimensions reported herein.

The increased number of conventional synapses seems evidence that central nervous tissue is capable of forming new synapses without changing the organisation of neural processes.