Effect of the richness of the environment on the cat visual cortex
Authors: Beaulieu Clermont , Colonnier Marc 
Published: January 2005

In a recent study of the cat visual cortex, it was shown that there are interindividual differences in the numerical density (Nv) of symmetrical synapses associated with flat vesicles (FS synapses) but not of asymmetrical synapses associated with round vesicles (RA synapses). Since many of the environment‐sensitive properties of visual cortex neurons are GABA‐dependent, it was suggested that the interindividual differences in FS synapses might be duo to environmental factors. To verify this possibility we estimated the Nv of both types of synapses in two groups of six cats, paired by litter and by sex, and raised either in isolation or in a colony from the time of weaning to the age of 8 months. We also measured the Nv of neurons and the thickness of the cortex and made some gross anatomical measurements.

The brains of animals raised in the enriched environment are 7% heavier, and their total body weight is 10% greater: The brain‐to‐body‐weight ratio remains unchanged. The total length of the brain is not affected, but the length and width of the cerebral hemispheres are each 5% greater in the enriched cats. As in comparable rat studies, the thickness of the cortex is 4% greater, but in the present study this difference is not significant. The numerical density of neurons is diminished by 17% in enriched animals. This is probably due to a wider separation of neuronal cell bodies in a larger cortical volume, rather than to a loss of neurons.

There are no significant changes in the numerical density of RA synapses between the two milieux, but there are nearly twice as many FS synapses per mm3 of tissue in the impoverished cortex. The coefficient of variation of FS synapses, which in the previous study was on the order of 30%, has been reduced to 10% and 7% in enriched and impoverished cats, respectively. We conclude that environmental conditions can lead to selective interindividual differences in the Nv of FS synapses, as seen in our previous study of animals whose rearing conditions were not controlled.

The average diameter of RA synaptic profiles is not affected by the environment but FS synapses are 25% wider in the enriched animals. Because of the smaller neuronal Nv in enriched animals, there are, in fact, 18% more RA synapses and 34% fewer FS synapses per neuron in the enriched condition. As a consequence the total area of RA contacts per neuron is 16% greater in enriched animals, hut changes in size of FS contacts compensate perfectly for the change in number so fiat the total area of contact per neuron is not affected.

Changes in the number of neurons and synapses are similar in all individual laminae. Modifications in the number of FS synapses occur on both dendritic spines and on dendritic trunks. It is not clear whether axosomatic synapses are also affected.

Changes in the number of neurons and synapses are similar in all individual laminae. Modifications in the number of FS synapses occur on both dendritic spines and on dendritic trunks. It is not clear whether axosomatic synapses are also affected.

We conclude that the richness of the environment modifies the excitatory‐inhibitory equilibrium on the neurons of the visual cortex through a restructuring of inhibitory circuits. We predict that this will have a profound effect on the receptive field properties of the cortical neurons. The dramatic differences in the reaction of the two types of synapses to the environment suggest that different mechanisms subserve the adaptation of excitatory and inhibitory circuits to environmental stimuli.