WHEN SIZE DOES NOT MATTER: EFFECTS OF STIMULUS SIZE ON AFFECTIVE MODULATION

Previous studies have shown that stimulus size and distance affect autonomic responses to emotional stimuli. In particular, larger arousing pictures elicit stronger autonomic modifications as compared to small stimuli. In this study we examined the effects of varying stimulus size on affective modulation of ERPs. Stimuli belonging to four categories (animals, erotica, neutral people, and mutilated bodies) were presented in four sizes, and participants were asked to categorize them as animals or people. Pictures with 21 horizontal16 vertical degrees of visual angle (100%), 118 (50%), 54 (25%) or 32 degrees (12.5%) were presented centrally for a duration of 100 ms. Dense sensor eventrelated brain potentials were measured. Consistently with previous studies, an early difference between erotica and neutral stimuli was found in the occipitotemporal region (Early Posterior Negativity, EPN). The peak latency of this differential activity varied as a function of picture size: 195 ms (100%), 202 ms (50%), 216 ms (25%), 233 ms (12.5%). Similarly, peak amplitude was linearly related with stimulus size. Although reducing stimulus size generally decreased the late positive potential (LPP), no difference in affectivemodulation of the LPP across sizes was found. The present results suggest that affective modulation is affected by picture size at an early stage probably reflecting perceptual processing (EPN), while later the affective modulation of the LPP seems to rely on a semantic, size-invariant representation of the scene.