Figure 6.

Stimuli presented to a single electrode (primary stimulation electrode; ●) yielded much-reduced responses in the first 20 msec after stimulus when the stimulation rate was increased. (We focused on short-latency responses, because the majority of response spikes occurred at short latencies and because responses cannot be defined unambiguously beyond one interpulse interval, i.e., 25 msec for the highest stimulation frequency.) In fact, at a stimulation frequency of 40 stim/sec, the response was not much higher than the spontaneous firing rate (arrow at left). Each stimulation series lasted 5 min, and we discarded the responses recorded during the first 30 sec to measure the sustained response rate. Results are mean ± SEM from 53 electrode pairs in four cultures. During these experiments, we presented stimulus pulses to a second electrode every 5 sec. The responses in the first 20 msec after these latter stimuli (◯) were not affected by the rate at which the first electrode was stimulated. Response strength in all cases was normalized to the results obtained from single-electrode stimulation at 0.2 stim/sec. The response strengths are plotted as a function of the frequency at which the primary electrode was stimulated. Inset and associated arrows indicate the stimulation protocol. Regardless of the frequency of the primary stimulation electrode, the secondary electrode was stimulated once every 5 sec.