Strength and orientation tuning of the thalamic input to simple cells revealed by electrically evoked cortical suppression

Is thalamic input to the visual cortex strong and well tuned for orientation, as predicted by Hubel and Wiesel's (1962) model of orientation selectivity in simple cells? We directly measured the size of the thalamic input to single simple cells intracellularly by combining electrical stimulation of the cortex with a briefly flashed visual stimulus. In nearby cells, the electrical stimulation evoked a long-lasting inhibition that prevented them from firing in response to the visual stimulus. The visually evoked excitatory postsynaptic potentials (EPSPs) recorded during the period of cortical suppression, therefore, reflected largely the thalamic input. In 16 neurons that received monosynaptic input from the thalamus, cortical suppression left 46% of normal visual response on average (12%-86% in range). In those cells tested, this remaining visual response was as well tuned for orientation as the normal response to the visual stimulus alone. We conclude that the thalamic input to cortical simple cells with monosynaptic input from the thalamus is strong and well tuned in orientation, and that the intracortical input does not appear to sharpen orientation tuning in these cells.