Recent research suggests that neurons in primary visual cortex (V1) are capable of integrating information from regions beyond their classical receptive field (e.g. Levitt & Lund, 1997). Here we investigate, in anaesthetized and neurosmuscularly blocked marmosets, how input from regions extending beyond the classical receptive field modulates the gain of V1 neurons.

Anaesthesia was induced with Saffan (Alphadalone/Alphaxalone acetate; 1.5 ml kg^-1 I.M.) and was maintained with a respiration mixture of 70 % N₂O and 30 % O₂ and fentanyl citrate (20 µg kg^-1 h^-1 I.V.). Skeletal muscles were neurosmuscularly blocked with vecuronium bromide (0.1 mg kg^-1 h^-1 I.V.). Appropriate levels of anaesthesia were maintained by ensuring electroencephalogram activity was synchronised and heart rate was stable. Supplementary fluothane was given if necessary (Webb et al. 2002).

We examined how drifting sinusoidal gratings that flanked or surrounded the classical receptive field modulated single neuron responses in V1 to an optimal drifting grating on the classical receptive field. The grating on the classical receptive field could have several different contrasts, in the range 0-100 %. The grating was presented alone or with a grating of the same or orthogonal orientation that either (1) surrounded the receptive field to fill a square that was 10 deg X 10 deg in size, (2) extended laterally to fill flanks that were the length of the receptive field and 10 deg wide, or (3) extended at each end of the receptive field to fill a region 10 deg in length. To characterise how gain was modulated by surround stimuli, the Michaelis-Menten equation was fitted to response versus contrast functions obtained under each stimulus condition. Surround modulation of gain was modelled best as a reduction in the maximum obtainable firing rate (response gain control). Response gain varied with the orientation of surround stimuli and was most severely reduced when the orientation of surround stimuli matched the preferred orientation of the classical receptive field. There was no laminar organization to neurons whose response gain was modulated by surround stimuli, suggesting that gain control from beyond the classical receptive field is a general property of V1.

We gratefully acknowledge the support of the Wellcome Trust.