Different rules of spatial summation from beyond the receptive field for spike rates and oscillation amplitudes in cat visual cortex.

We measured spike rates in parallel with visually induced oscillations of multi-unit activity (MUA) and local field potentials (LFP) from cortical areas 17 and 18 of anesthetized cats. Variations in the three response types were systematically correlated with stimulus size and placement. Oscillation amplitudes of both MUA and LFP were on average low with stimuli covering just the receptive field and they increased progressively with larger stimuli, whereas average spike rates rather decreased monotonically with stimulus sizes beyond the receptive field (area 18) or reached a plateau with stimuli in the far surround (area 17). Thus, spike rates and oscillation amplitudes follow different rules of spatial summation. Since the spatial spread of the synchronized components of oscillations roughly matches the horizontal divergence zone of the pyramidal cells' axonal collaterals in area 17 and 18, the interconnected system of neighbouring columns seems to constitute a functional unit, within which the oscillations could exert their functional role.