Fig. 4.
Effect of number of stimulus repetitions on cVEMPs. In part A, data are from a single subject stimulated with 500 Hz, 2 ms tone bursts at 105 dB LAeq. Data from a trial with 200 stimulus repetitions are shown, separated into the first three sets of 50 repetitions (1–49, 50–99 and 100–150 stimuli). Below these are the average of all 200 repetitions (black trace) and a repeat trial with 200 stimuli (grey trace). The first two 50 repetition trials gave the impression of a present response at the appropriate time (approx. 13 and 23 ms), but averaging more stimuli showed that the response was absent and the observed peaks were part of the background EMG. In parts B and C, data are from a different single subject tested with the same stimulus. Part B shows data from a trial with 200 stimulus repetitions, separated into four consecutive sets of 50 repetitions. These short trials again gave the impression of a possible cVEMP, but are too noisy to provide convincing evidence. Part C shows the same data presented with increasing numbers of stimulus repetitions. The cVEMP becomes clearer and the peaks reliably extend beyond the baseline noise level with 150–200 stimuli. A second trace of 200 stimuli is shown in grey, showing that the subject has quite small, but reproducible, cVEMPs.