DIRECTIONAL ASYMMETRIES IN THE LENGTH-RESPONSE PROFILES OF CELLS IN THE FELINE DORSAL LATERAL GENICULATE-NUCLEUS

1. The visual cortex provides a major synaptic input to the dorsal lat eral geniculate nucleus (dLGN). Cortical layer VI cells giving rise to this projection are strongly influenced by stimulus orientation, leng th and direction of motion. In the dLGN, a significant component of th e strong length tuning exhibited by most cells follows from the cortic ofugal influence. We have now checked whether there are directional bi ases in geniculate cell responses, and whether such biases are influen ced by stimulus length. 2. The responses of A-laminae dLGN cells were assessed by single-unit extracellular recording. Length preference was examined by plotting multihistogram length-tuning curves to moving ba rs of light of various length. 3. Over half of the cells tested (100/1 83) exhibited directional bias and in many cases, this bias was highly dependent on bar length, resulting in radically different length resp onse profiles for the two directions of motion. These asymmetries are similar to those documented for cortical hypercomplex cells, but do no t equate to any known facet of the centre-surround organization of dLG N cell receptive fields. 4. We suspected the directional biases follow ed from the influence of the corticofugal projection. To test this, we recorded from preparations where areas 17 and 18 of the visual cortex had been removed. Surprisingly, a similar proportion of cells exhibit ed directional biases after removal of the corticofugal input, suggest ing that the biases are generated subcortically. 5. The widespread pre sence of systematic biases in the response profiles of dLGN cells furt her underlines the possibility that geniculate mechanisms may make a f ar greater contribution to visual processing than hitherto suspected.