performed the first visible true response in cat a is a positive peak at 38 ms a second positive peak follows this wave at 54 ms the average latency of the negative and positive peaks for cat b and the two positive peaks for cat a are shown in table 1 by determining the threshold current to elicit a response at the visual cortex a strength duration curve was obtained for biphasic monopolar extraocular electrical stimulation of the retina fig1 for phase durations greater than 200 u03bcs threshold currents for eliciting a visual cortex response were below 1 5 ma we also determined the peak amplitude of the positive component of the evoked response in cat a for a range of stimulus current levels at a pulse width of 250 u03bcs fig1 higher response amplitudes were obtained with increasing current up to 5 ma after which there was a response plateau the effect of pulse trains on cortical response amplitude was investigated the amplitude of the positive wave of the electrical evoked response was recorded for 2 ma 250 u03bcs cathodal first biphasic stimuli to single pulses and pulse trains of 2 3 or 4 pulses presented at 200 hz these responses were also compared to the amplitude of the cortical potential evoked by a double pulse at 100 hz fig5 the highest cortical responses.