Perceptual learning in foveal and peripheral contrast discrimination: exploring the effects of stimuli on learning and the effects of learning on spatial vision

Abstract

Crowding is defined as the deleterious influence of surrounding elements on visual perception. The crowding effect is predominant in the normal periphery and in amblyopia. Perceptual learning has been shown to improve spatial vision in amblyopia but has not previously been employed to investigate crowding in amblyopia.The purpose of this work was to investigate the effect of surrounding stimuli parameters on crowding and perceptual learning (PL), and the effect of PL on discrimination thresholds and crowding in foveal and peripheral vision. Two main configurations of Gabor stimuli were employed in all experiments of the study: ISO (identically oriented target and flankers) and CROSS (orthogonally oriented target and flankers). Crowding was investigated in both orientation discrimination and contrast discrimination (CD) tasks in normally sighted and amblyopic observers, while PL was investigated only in CD in the normally sighted group. PL was conducted in five and three training groups defined on the basis of training stimuli in foveal and peripheral vision, respectively. In peripheral vision, CD thresholds were compared before and after training on untrained stimulus conditions in order to investigate any learning transfer.In general, the ISO configuration yielded crowding in most conditions, while the CROSS configuration yielded no or minimal crowding in both tasks in normally sighted observers. No crowding was found in our small group of mild amblyopes. There was an overall lack of learning in foveal vision. In peripheral vision, greater improvement was obtained on flanked training stimuli than training without flankers. Additionally, learning in the ISO configuration resulted in greater improvement than the CROSS configuration. However, there was no learning transfer and no reduction of crowding in any training group following PL in peripheral vision.The primary conclusions of the study are:• Perceptual learning in contrast discrimination is dependent on the initial level of performance of observers in foveal and peripheral vision.• Stimulus configuration has an impact on the amount of improvement in perceptual learning in peripheral contrast discrimination.• Perceptual learning in peripheral contrast discrimination neither improves thresholds on untrained stimuli nor reduces crowding, using the training paradigm of the present study