Abstract

Perceptual learning (PL) is defined as the improvement of perceptual judgements as a function of practice or training. Throughout PL of simple stimulus features, the discrimination of physical attributes correlates with the behavioural improvements. In more natural conditions, the complex stimuli have subtle and discrete variations in one or more physical properties. Only a few studies have addressed the PL with more complex stimuli. In a previous experiment, we showed that training with kanji or scrambled checkers stimulus pairs resulted in an initial increase of the discrimination index (d prime, d’) and the subject’ responses were equally distributed between “same” and “different” responses. On later stages of learning, d’ reached a plateau and a significant bias towards correct discrimination of “different” stimulus was observed. To test if the d’/bias progression occurs under conditions of asymmetrical, unknown same-different stimulus ratios; we trained subjects with 30 scrambled checkers stimulus during 5 sessions (480 trials per session). One group of subjects (n=8) was exposed to 80/20 ratio of “same”/”different” stimulus pairs and 8 subjects to the opposite ratio. Both groups were not aware of these ratios. We obtained d’, bias using the 1AFC same-different (differencing model; Palamedes, Prins & Kingdom 2010) and the response preference across sessions. Our results show that d’ increases progressively across sessions in both groups (p=0.45). On the contrary, the bias has an asymmetrical behaviour, the 20/80 “same”/”different” group shows a significant bias towards correct responses for different stimulus pairs (p<0.026). In addition, the response preference is significantly greater for different stimuli in the 20/80 group when compared to the 80/20 group. These results suggest that visual discrimination performed on a visual category containing numerous objects, the discrimination of differences between stimuli is facilitated when compared with the detection of similarities. The behavioural advantage, observed during learning, for the detection of differences between stimuli may reveal an evolutive mechanism for perceptual discrimination of different stimuli in natural environments. Fundación Puelma and Bicentenario CIE-05.