Top-down and bottom-up effects on the visual N1 category differences

Visual ERPs are modulated by both top-down factors (e.g. attention) and low-level differences (e.g. amplitude spectrum). However, it is not clear how these factors modulate the animal/non-animal category differences appearing in the visual N1 component. The aim of our study was to examine the role of amplitude spectrum and the effect of categorization task on the N1 component. Stimuli were images of animals and vehicles containing either an “X” or an “O” in the background. The stimulus set was presented in two versions: they were equalised either only for luminance or both for luminance and amplitude spectrum. Thirty-five participants had to categorize whether images depicted an animal or a vehicle or whether they contained an “X” or an “O”. The N1 amplitude was larger for vehicles compared to animals but only for the stimuli that were equalised for both luminance and amplitude spectrum. This indicates that amplitude spectrum differences between categories did not affect the N1 category difference. Furthermore, the categorization task modulated the N1 amplitudes for animals and vehicles. Therefore, the results indicate that both top-down and bottom-up processes modulate the N1 category difference. However, the amplitude spectrum does not play an important role in the animal-vehicle categorization.