Behavioural and electrophysiological correlates of lightness contrast and assimilation

Lightness contrast and lightness assimilation are examples of the perception of a surface being influenced by surrounding areas. In contrast, a grey target is perceived lighter when neighbouring a dark surface, and darker when neighbouring a light surface. The reverse is true for assimilation. The general aims of this thesis were to investigate contrast and assimilation in parallel, using behavioural and electrophysiological methods to examine the responses. The first part of the project used a matching-chart method to assess the effect of depth separation on the perception of stimuli shown to elicit either contrast or assimilation, making a direct comparison between the effect on contrast and on assimilation. The second part of the project developed a forced-choice (lighter/darker) task to investigate the electrophysiological (ERP) responses associated with contrast and assimilation, thus investigating the time course of the associated neural processing, and whether contrast and assimilation result from different underlying processing. Throughout the studies, contrast effects were stronger with white inducers, whereas assimilation effects were stronger with black inducers. Both contrast and assimilation effects were disrupted when the target and inducer were separated by depth. When comparing contrast and assimilation responses, there was a difference in N1 amplitude in the left occipital area, but this was only apparent in one condition. Within other conditions, the P1 amplitude decreased as the strength of contrast effects increased and increased as the strength of assimilation increased. When comparing between conditions, a stronger contrast effect (white inducers) resulted in smaller amplitude than a weaker contrast effect (black inducers). The observation that both effects change after depth separation despite the 2D retinal image remaining equivalent, and the ERP activity throughout occipital and parietal areas suggest that contrast and assimilation require processing at the cortical level, rather than retinal processing alone.