STRENGTH AND ORIENTATION TUNING OF THE THALAMIC INPUT TO SIMPLE CELLSREVEALED BY ELECTRICALLY-EVOKED CORTICAL SUPPRESSION

Is thalamic input to the visual cortex strong and well tuned for orien tation, as predicted by Hubel and Wiesel's (1962) model of orientation selectivity in simple cells? We directly measured the size of the tha lamic input to single simple cells intracellularly by combining electr ical stimulation of the cortex with a briefly flashed visual stimulus. In nearby cells, the electrical stimulation evoked a long-lasting inh ibition that prevented them from firing in response to the visual stim ulus. The visually evoked excitatory postsynaptic potentials (EPSPs) r ecorded during the period of cortical suppression, therefore, reflecte d largely the thalamic input. In 16 neurons that received monosynaptic input from the thalamus, cortical suppression left 46% of normal visu al response on average (12%-86% in range). In those cells tested, this remaining visual response was as well tuned for orientation as the no rmal response to the visual stimulus alone. We conclude that the thala mic input to cortical simple cells with monosynaptic input from the th alamus is strong and well tuned in orientation, and that the intracort ical input does not appear to sharpen orientation tuning in these cell s.