Fig. 2. Experimental paradigm. (A) The schematic diagrams illustrate the experimental procedures and the corresponding electric images when the change in contrast is maximum (r₀= and r₁=0). Note the opposite orientation of the `Mexican-hat' profile, referred in the text as `top-inward' and `top-outward', respectively. The raw record at the center of the `Mexican hat' allowed us to measure the difference (PP) between the baseline amplitude and comparison amplitude of the stimulus. The temporal course of the corresponding novelty response elicited by the change in object resistance is shown in the bottom plot. (B) Studies were performed using series of trials consisting of a baseline period followed by a comparison period. Four variables were controlled: the baseline amplitude (depending on r₀), the comparison amplitude (depending on r₁), the number of baseline images (depending on the duration of the baseline period), the number of comparison images (depending on the duration of the comparison period). (C—F) The experimental paradigms used to elucidate the following issues. The number of images different from the baseline that suffice for detection (C); the effect of the difference between baseline and comparison amplitudes (PP) on the amplitude and the probability of the novelty response (D); the effects of the baseline on amplitude and probability of the novelty response (E); and the effect of stimulus history on the amplitude and probability of the novelty response (F).