Effect of the richness of the environment on neurons in cat visual cortex. II. Spatial and temporal frequency characteristics

Abstract

The quantitative distribution of synaptic contacts in the cat cerebral cortex is affected by the richness of the environment: the number of round-asymmetrical (RA) synapses per neuron is higher while the number of flat-symmetrical (FS) contacts is lower in the visual cortex of cats raised in an enriched environmental condition (EC) compared to those reared in an impoverished condition (IC). The size of FS synaptic contacts is also affected by the complexity of the animal's environment. It has been suggested that these morphological synaptic changes induced by the richness of the environment correlate with differences in physiological properties of the visual cortex. This question has been explored by assessing the cortical unit contrast sensitivity as a function of spatial and temporal frequency of area 17 cells in cats raised either in isolation (IC) or in a colony with ample environmental stimulation (EC). Contrast sensitivity is affected by the richness of the environment: at the preferred spatial frequency, an average enriched unit can detect lower contrasts (mean = 0.6%) than an average impoverished cell (0.9%; P < 0.002). In addition, the mean highest spatial frequency that can be resolved (acuity) is greater in EC than in IC cells (1.7 and 1.2 cycles per degree, respectively, P < 0.0001), whereas the spatial frequency at which units respond best is not statistically affected by the environment. The preferred temporal frequency also show a significant difference in EC (1.0–6.5 Hz) and IC units (0.9–4.0 Hz; P < 0.01). These differences in the contrast sensitivity function of units in the visual cortex of enriched and impoverished cats demonstrate that the richness of the environment, e.g. the complexity of sensory stimuli, has an effect on the development of spatiotemporal response properties of neurons in the visual cortex.