ELECTRICALLY-EVOKED SACCADES FROM THE DORSOMEDIAL FRONTAL-CORTEX AND FRONTAL EYE FIELDS - A PARAMETRIC EVALUATION REVEALS DIFFERENCES BETWEEN AREAS

Using electrical stimulation to evoke saccades from the dorsomedial fr ontal cortex (DMFC) and frontal eye fields (FEF) of rhesus monkeys, pa rametric tests were conducted to compare the excitability properties o f these regions. Pulse frequency and pulse current, pulse frequency an d train duration, and pulse current and pulse duration were varied to determine threshold functions for a 50% probability of evoking a sacca de. Also a wide range of frequencies were tested to evoke saccades, wh ile holding all other parameters constant. For frequencies beyond 150 Hz, the probability of evoking saccades decreased for the DMFC, wherea s for the FEF this probability remained at 100%. To evoke saccades rea dily from the DMFC, train durations of greater than 200 ms were needed ; for the FEF, durations of less than 100 ms were sufficient. Even tho ugh the chronaxies of neurons residing in the DMFC and FEF were simila r (ranging from 0.1 to 0.24 ms) significantly higher currents were req uired to evoke saccades from the DMFC than FEF. Thus the stimulation p arameters that are optimal for evoking saccades from the DMFC differ f rom those that are optimal for evoking saccades from the FEF. Although the excitability of neurons in the DMFC and FEF are similar (due to s imilar chronaxies), we suggest that the density of saccade-relevant ne urons is higher in the FEF than in the DMFC.