Decreased contrast sensitivity of visual cortical cells to visual stimuli accompanies a reduction of intracortical inhibition in old cats

Psychophysical experiments on human and animal subjects have proven that aged individuals show significantly reduced visual contrast sensitivity compared with young adults. To uncover the possible neural mechanisms, we used extracellular single-unit recording techniques to examine the response of V₁ (primary visual cortex) neurons as a function of visual stimulus contrast in both old and young adult cats (Felis catus). The mean contrast sensitivity of V₁ neurons to visual stimuli in old cats decreased significantly relative to young adult cats, consistent with findings reported in old primates. These results indicate that aging can affect contrast sensitivity of visual cortical cells in both primate and non-primate mammalian animals, and might contribute to the reduction of perceptual visual contrast sensitivity in aged individuals. Further, V₁ cells of old cats exhibited increased responsiveness, decreased signal-to-noise ratio, and enlarged receptive field (RF) size compared with that of young adult cats, which indicated that decreased contrast sensitivity of V₁ neurons accompanied a reduction of intracortical inhibition during senescence.