Different mechanisms underlie three inhibitory phenomena in cat area 17

Area of Science:

• Neuroscience
• Visual Neuroscience
• Computational Neuroscience

Background:

• A unified mechanism for visual suppression in the primary visual cortex (V1) has been proposed, suggesting inhibition strength depends on stimulus contrast, not cell firing rate.
• This hypothesis posits that inhibitory neuron pools with diverse stimulus specificities mediate all suppressive phenomena.

Purpose of the Study:

• To test the hypothesis of a single inhibitory mechanism in V1 by examining different types of visual suppression.
• To differentiate the effects of cross-orientation, surround, and interocular inhibition on neuronal contrast-response functions (CRFs).

Main Methods:

• Neurons in cat V1 were measured for their contrast-response functions (CRFs).
• Inhibition was induced by superimposed orthogonal gratings (cross-orientation), surrounding iso-oriented gratings (surround), and orthogonal gratings in the other eye (interocular).
• Hyperbolic ratio functions were fitted to quantify the effects on CRFs.

Main Results:

• Cross-orientation inhibition primarily caused 'contrast-gain control' (rightward CRF shift).
• Surround and interocular suppression mainly induced 'response-gain control' (downward CRF shift), with a component of contrast-gain control also observed.
• Layer 4 predominantly showed response-gain control, while layers 2/3, 5, and 6 mainly exhibited contrast-gain control.
• Surround gratings could enhance responses to low-contrast stimuli, effectively lowering detection thresholds.

Conclusions:

• Visual suppression in V1 is not mediated by a single mechanism; distinct gain control modes exist.
• The type of gain control (contrast vs. response) and its laminar distribution differ between suppression types.
• Surround inhibition exhibits complex effects, potentially improving sensitivity to low-contrast stimuli.