Ken Miller
Our work has focused mainly on cat primary visual cortex (V1), the single piece of cortex about which the most is known. The most salient features of V1 are orientation selectivity and ocular dominance of the receptive fields of individual cells and their organization into periodic maps across the cortical surface. We have developed models demonstrating how these features can arise from simple synaptic learning rules that are likely to be operating in early development. These models predict the patterns of activity in thalamus that might exist when these features arise, some of which are only now being tested. We have also developed models of the circuitry underlying the mature visual response properties of cells in layer 4 of V1, the input-recipient layer. These models emphasize the need for an untuned component of feedforward inhibition (meaning inhibition that is driven by the inputs from thalamus, although arising from cortical inhibitory cells) in order to explain both orientation and temporal frequency tuning. Experiments from other labs now support this prediction. We argue that the combination of tuned feedforward excitatory input and strong but untuned feedforward inhibitory input is likely to be a general feature of the circuitry of cortical layer 4. This combination can endow layer 4 cells with the ability to recognize a particular input activity pattern in a manner that is invariant to the overall magnitude of input activity. We also study how cortical neurons integrate their input. Our experiments show that the noise (trial-to-trial variability) in voltage responses gives cortical neurons an effective power-law input/output function. This in turn provides a novel explanation of the gain changes seen in tuning curves in many cortical systems under modulation by other influences.
In addition to ongoing work on cortical development, circuitry, and integration, current work includes the development and application of information-theoretic methods to characterize the responses of cortical and thalamic cells.
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