Fig. 1. Omitted stimulus potentials (OSPs) from crayfish. (A) Eight successive trials (single sweeps) from the protocerebrum, a locus that gives no visual evoked potentials (VEPs) and mainly spike bursts for OSPs. The last two flashes (arrows at bottom) of a 1 s train at 15 Hz are shown. Trials 1–4, from the top, with a low flash intensity of 1 (arbitrary units proportional to lumens); trials 5–8 at a high flash intensity of 16. (B) Simultaneous recording from the optic tract at a locus that shows slow waves larger than spikes, both VEP and OSP. Averages of the optic tract recordings 1–4 and 5–8, below, reveal the consistent slow negative wave approximately 40 ms wide and peaking at approximately 75 ms after the due-time of the first missing flash (vertical dashed lines). At some electrode loci, this wave is much more conspicuous than it is here. Note that the spike burst, without a slow wave, in the protocerebral locus (A) coincides with a spike burst in the optic tract (B). Both recordings were filtered to pass 1–2000 Hz. The recordings in A and B illustrate the robustness of the OSP and the small effect of the intensity of the conditioning stimulus train. (C) Effect of frequency of conditioning flashes. This recording was taken from the protocerebrum of another preparation and shows single sweeps after trains at different flash rates. Responses are aligned by the due-time of the first missing flash. Note the near constancy of the OSP latency, if measured from the due-time. At 3 Hz, no OSP is seen, and a single spike occurs in some trials at 4 Hz. At values above 15 Hz, some preparations continue to emit a burst at this time, whereas others do not. The constant latency from a given locus, within a given preparation, shown here, is the hallmark of the OSP. Its value varies with temperature and possibly with locus. (D) Recordings from another preparation showing the lack of effect of flash intensity over a range of 8:1 (same units as in A and B). Note the VEPs after each flash. These are of the small spike burst type, with long latency. (E) Effect of stimulus frequency on the amplitude of the OSP, estimated by the root-mean-square voltage (Vrms) during the 200 ms beginning with the due-time. The arrow marks the mean Vrms under the prevailing conditions, without stimulation. The increased Vrms beginning somewhat below 10 Hz is attributable mainly to the spike burst OSPs. Data are from three preparations, each shown by an individual symbol; values are means ± standard deviations (N=6).
