Network mechanisms of the response modulation in the primary visual cortex

Hiromichi Sato

The goal of this study is to clarify the mechanisms of the integration of network activity in the primary visual cortex (V1). There is considerable evidence that the neuronal response to stimulation of the classical receptive field (CRF) is modulated by the stimulus presented outside of CRF. This modulation is generally suppressive and is dependent on the relationship among the parameters of stimuli inside and outside the CRF. This "stimulus-context-dependent response modulation" is supposed to have a functional significance in controlling the optimal cortical output, according to the distribution and intensity of input over the large-scale network in V1.

To assess the possible mechanisms underlying the modulation, we examined the effects of pharmacological blockade of intracortical inhibition as well as the effects of controlling neuronal excitation level on the response modulation by changing stimulus contrast. Microiontophoretic administration of bicuculline, a GABAa receptor antagonist, did not abolish the suppressive modulation, suggesting that the intracortical inhibition does not play a major role in it. Also, single V1 neurons exhibited similar response modulation regardless of their excitation levels, suggesting that there is a mechanism for scaling the magnitude of modulation based on the level of input to a large-scale network in V1.

Fig.1 Possible mechanisms of the stimulus-context-dependent response modulation in V1. For simplicity, only the neuronal circuitry of layers II/III and IV is illustrated. Filled cells are inhibitory neurons. The different colors denote different orientation columns (1047kb JPEG image:Click to zome up).