| Abstract: | Cortical evoked potentials were recorded from the vertex of subjects performing a similarity rating task. Proximity data were analysed according to INDSCAL (Carroll and Chang, 1970). Brain activity was averaged separately for different stimulus attributes and different stages of the judgement process. The amplitudes of the two positive deflections of the averaged evoked potential, peaking around 160 and 330 msec after stimulus onset respectively, were systematically influenced by two factors: The weight, which the evoking attribute received in the overall dissimilarity judgement, and the position of the evoking attribute in the stimulus train. The results support the hypothesis that differences in the perception of similarity are due to differences in attentional set. Reference is made to the ‘additive-difference model’ of scaling behaviour (Tversky and Krantz, 1970) and Broadbent's (1970) distinction between stimulus set and response set attention. |
