Art - Abbott (P)

277 Although retinal input relayed through the lateral geniculate nucleus (LGN) of the thalamus clearly drives responses in the primary visual cortex (V1), LGN afferents account for only a small fraction of the synapses onto V1 neurons1–7. The primary source of synaptic input to neurons in primary visual cortex, at least in terms of numbers, is excitatory input from other nearby cortical neurons. What role do these local recurrent connections have in shaping the responses of V1 neurons to visual stimuli? One interesting idea is that recurrent connections amplify weak feedforward input signals from the LGN8 and increase neuronal selectivity8–11. Neurons in the primary visual cortex are selective for a number of stimulus characteristics, including the orientation and direction of motion of light bars or gratings. Previous studies have suggested that cortical amplification may enhance orientation tuning9–11 or direction selectivity8. Experiments in which cortical connections are disrupted by cooling or shocking the cortex suggest that recurrent connections do indeed amplify input signals, but they do not support the idea that this cortical amplification increases orientation or direction selectivity12,13. Thus, the functional role of recurrent connections in primary visual cortex remains unknown. In previous models, cortical amplification has been used to increase neuronal selectivity8–10. Here we show that cortical amplification can also act to decrease selectivity. The effect on selectivity depends on the pattern of connectivity within the circuit. If neurons with similar selectivities excite each other, and those with different selectivities inhibit each other, selectivity is enhanced. If, on the other hand, neurons excite each other independently of their response tuning, selectivity is decreased. This is interesting with respect to the primary visual cortex where the responses of a class of neurons, the complex cells, exhibit little sensitivity to the spatial location of visual stimuli within their receptive fields. Neurons in the primary visual cortex can be divided into two classes, simple and complex, based on the spatial separation or overlap of their responses to light and dark stimuli14, and on their responses to bars14 and sinusoidal gratings15,16. Here we focus on the responses to sinusoidal gratings. Simple cells are selective not only for the orientation of a grating, but also for its spatial freComplex cells as cortically amplified simple cells