Intracortical microstimulation of neurons in the visual cortex of the cat.

The response of visual cortex neurons to local intracortical microstimulation was measured in the anesthetized cat. When the recording microelectrode was very close (about 20 micrometers) to the tip of the stimulating electrode, threshold currents as low as 10 micro A were capable of firing neurons. Over a 20-fold range in distance from the site of stimulation, an 80-fold increase in threshold current was observed. The mean latency of activation for 30 neurons tested with intracortical stimulation was 2.88 +/- 0.45 msec. The majority of these cells were probably synaptically activated. The mean threshold current for these neurons was 0.55 +/- 0.12 mA (N = 30). These values were significantly smaller than the thresholds found previously when stimulating electrodes were located on the pia-arachnoid surface of the visual cortex.