Stimulus roving and flankers affect perceptual learning of contrast discrimination in Macaca mulatta.

Xing Chen,Mehdi Sanayei,Alexander Thiele,Leonardo Chelazzi

doi:10.1371/journal.pone.0109604

Abstract

‘Stimulus roving’ refers to a paradigm in which the properties of the stimuli to be discriminated vary from trial to trial, rather than being kept constant throughout a block of trials. Rhesus monkeys have previously been shown to improve their contrast discrimination performance on a non-roving task, in which they had to report the contrast of a test stimulus relative to that of a fixed-contrast sample stimulus. Human psychophysics studies indicate that roving stimuli yield little or no perceptual learning. Here, we investigate how stimulus roving influences perceptual learning in macaque monkeys and how the addition of flankers alters performance under roving conditions. Animals were initially trained on a contrast discrimination task under non-roving conditions until their performance levels stabilized. The introduction of roving contrast conditions resulted in a pronounced drop in performance, which suggested that subjects initially failed to heed the sample contrast and performed the task using an internal memory reference. With training, significant improvements occurred, demonstrating that learning is possible under roving conditions. To investigate the notion of flanker-induced perceptual learning, flanker stimuli (30% fixed-contrast iso-oriented collinear gratings) were presented jointly with central (roving) stimuli. Presentation of flanker stimuli yielded substantial performance improvements in one subject, but deteriorations in the other. Finally, after the removal of flankers, performance levels returned to their pre-flanker state in both subjects, indicating that the flanker-induced changes were contingent upon the continued presentation of flankers.

Highlights

Perceptual learning (PL) refers to a long-lasting improvement in one’s perceptual abilities, which occurs with repeated exposure to the relevant stimuli during a period of training on a perceptual task [1,2,3,4,5,6]
We observed improvements in contrast discrimination performance over the course of training, indicating that it was possible- even in adult primates with well-developed visual perception- to hone their ability to make fine contrast discriminations. This corroborates reports from the human psychophysics literature, which show that healthy adult humans show similar improvements due to training in a CD task [2,3,7,8,9,10]
Preceding studies from the human psychophysics literature have examined the effects of various task manipulations on the ability of adult subjects to fine-tune their contrast discrimination faculties

Summary

Introduction

Perceptual learning (PL) refers to a long-lasting improvement in one’s perceptual abilities, which occurs with repeated exposure to the relevant stimuli during a period of training on a perceptual task [1,2,3,4,5,6]. We demonstrated that PL takes place in a contrast discrimination (CD) task in adult macaques [1], using a non-roving paradigm. This involved a comparison of contrast levels between two consecutively presented stimuli per trial, where the contrast of the stimulus presented in the first interval (the ‘sample’) was fixed at 30% across trials, whereas the contrast of the stimulus presented in the second interval (the ‘test’) varied from trial to trial. We observed improvements in contrast discrimination performance over the course of training, indicating that it was possible- even in adult primates with well-developed visual perception- to hone their ability to make fine contrast discriminations. It has been hypothesized that a roving paradigm impairs task performance through the continual disruption of memory traces [2,9], thereby preventing observers from constructing and maintaining internal reference templates of stimulus contrasts

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