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Abstract:
In the receptive fields of V1 simple cells, the adjacent excitatory and inhibitory subregions receive selective input from ON- and OFF-center LGN neurons. Modeling studies have shown that before eye opening this segregation of geniculate afferent is compatible with a Hebbian mechanism of synaptic plasticity that operates on spontaneous neural activity. However, it is not clear how Hebbian plasticity could account for the refinement and maintenance of simple cell receptive fields after eye opening, when neural activity is shaped by the statistical structure of the visual input. Natural scenes are characterized by broad spatial correlations that presumably coactivate large pools of geniculate cells with similar polarity, a result that appears incompatible with a Hebbian segregation of geniculate afferents.
We have recently suggested that after eye opening the modulations of neuronal activity produced by oculomotor behavior may contribute to a refinement of cortical receptive fields. In a model of the cat's LGN, when natural scenes were scanned by sequence of eye movements, the instability of visual fixation due to fixational eye movements modulated geniculate responses in a way that was compatible with a Hebbian refinement of simple cell receptive fields. To examine the possible impact of these short-living patterns of synchronous activity in the LGN on the long-term correlation between geniculate and cortical cells, in this study we simultaneously simulated the responses of populations of LGN and V1 neurons during oculomotor activity. In the model, small fixational eye movements attenuate the neuronal sensitivity to the broad correlational structure of natural visual input, decorrelate neural responses, and establish a regime of neural activity that is compatible with Hebbian segregation of geniculate afferents to the cortex. This result is highly robust and does not depend on the precise characteristics of the model.
