Figure 4. Detection thresholds increase over 100 trials of stimulation. Two 100-trial threshold measurements were made at each V1 site. A, B, Each point represents the ratio of threshold measurements between two subsequent measurements when the electrode was advanced between measurements (x-axis) compared to when both measurements were made at the same site (y-axis; A: monkey 3, 140 sessions; B: monkey 4, 69 sessions). Dashed lines indicate mean x,y values. The ratios of thresholds within site were significantly greater than those between sites (both animals: p < 0.05). C, Bar plot shows the mean difference (±1 SEM) between pairs of threshold measurements. Repeated stimulation of the same site elevated thresholds for the second measurement compared to the first (left bar). This measure corresponds the y-axis ratios in A, B. After two threshold measurements, the microelectrode was advanced by 100 μm into a new site. The difference in thresholds between the first measurement at a new site and the first measurement at the previous site was not significantly different from zero (middle bar), indicating that there was no systematic change in threshold across a session for previously unstimulated sites. Consistent with these observations, the difference between the first threshold measurement at a new site and the second threshold measurement at the previous site was negative (right bar), reflecting lower thresholds for unstimulated cortex relative to cortex that has been previously stimulated. This measure corresponds to the x-axis ratios in A, B.