Section of Neurobiology, Yale University School of Medicine, New
Haven, Connecticut 06510
Contrast adaptation is a psychophysical phenomenon, the neuronal
bases of which reside largely in the primary visual cortex. The
cellular mechanisms of contrast adaptation were investigated in the cat
primary visual cortex in vivo through intracellular recording and current injections. Visual cortex cells, and to a much
less extent, dorsal lateral geniculate nucleus (dLGN) neurons, exhibited a reduction in firing rate during prolonged presentations of
a high-contrast visual stimulus, a process we termed high-contrast adaptation. In a majority of cortical and dLGN cells, the period of
adaptation to high contrast was followed by a prolonged (5-80 sec)
period of reduced responsiveness to a low-contrast stimulus (postadaptation suppression), an effect that was associated, and positively correlated, with a hyperpolarization of the membrane potential and an increase in apparent membrane conductance. In simple
cells, the period of postadaptation suppression was not consistently
associated with a decrease in the grating modulated component of the
evoked synaptic barrages (the F1 component).
The generation of the hyperpolarization appears to be at least
partially intrinsic to the recorded cells, because the induction of
neuronal activity with the intracellular injection of current resulted
in both a hyperpolarization of the membrane potential and a decrease in
the spike response to either current injections or visual stimuli.
Conversely, high-contrast visual stimulation could suppress the
response to low-intensity sinusoidal current injection.
We conclude that control of the membrane potential by intrinsic
neuronal mechanisms contributes importantly to the adaptation of
neuronal responsiveness to varying levels of contrast. This feedback
mechanism, internal to cortical neurons, provides them with the ability
to continually adjust their responsiveness as a function of their
history of synaptic and action potential activity.
Key words: adaptation; cerebral cortex; contrast; vision; plasticity; receptive field