Linear mechanistic models for the dorsal lateral geniculate nucleus of cat probed using drifting-grating stimuli.

Experiments with sinusoidal visual stimuli in the early visual pathway have traditionally been interpreted in terms of descriptive filter models. We present an alternative mechanistic approach for interpretation of this type of data recorded from X cells in the dorsal lateral geniculate nucleus (dLGN) of cat. A general, linear, rate-based mathematical expression for the geniculate transfer ratio, i.e. the ratio between the first-harmonic components of the output of a geniculate relay cell and its retinal input, is derived. In linear theory this ratio is independent of the signal processing occurring at the retinal level. Further, the ratio is straightforwardly accessible in experiments due to the presence of S-potentials, representing the retinal input, in extracellular recordings from dLGN. The expression accounts for feedforward inputs from retina and intrageniculate interneurons as well as feedback inputs from cortex and the thalamic reticular nucleus and can be used to experimentally test different mechanistic models for the geniculate circuitry. Two examples of this are considered: a purely feedforward model incorporating relay cell inputs from retinal ganglion cells and interneurons, and a model including cortical feedback inhibition of relay cells via intrageniculate interneurons.