Task-irrelevant learning occurs only when the irrelevant feature is weak

Abstract

The role of attention in perceptual learning has been controversial. Numerous studies have reported that learning does not occur on stimulus features that are irrelevant to a subject's task [1,2] and have concluded that focused attention on a feature is necessary for a feature to be learned. In contrast, another line of studies has shown that perceptual learning occurs even on task-irrelevant features that are subthreshold, and concluded that attention on a feature is not required to learn that feature [3-5]. Here we attempt to reconcile these divergent findings by systematically exploring the relation between signal strength of the motion stimuli used during training and the resultant magnitude of perceptual learning. Our results show that performance improvements only occurred for the motion-stimuli trained at low, parathreshold, coherence levels. The results are in accord with the hypothesis that weak task-irrelevant signals fail to be 'noticed', and consequently to be suppressed, by the attention system and thus are learned, while stronger stimulus signals are detected, and suppressed [6], and are not learned. These results provide a parsimonious explanation of why task-irrelevant learning is found in some studies but not others, and could give an important clue to resolving a long-standing controversy.

Figure 1. Exposure stage

A display consisted of a sequence of eight items — two digits as targets and six letters as distractors — in the center and dots moving coherently or in random directions in the periphery (white arrows). Red arrows represent coherent motion directions paired with task targets. Black arrows indicate other coherent motion directions paired with task distractors.

Figure 2. Correct improvement as a function of the exposed coherent motion ratio

Correct improvement (%) is defined as the subtraction of the summed performance (% correct) across coherence levels in the pre-test from that in the post-test [14] (see Supplementary Experimental Procedures in the Supplemental data for details). The horizontal blue arrow represents the mean motion threshold changed from before exposure (from 15.3 ± 0.8%) to after exposure (13.0 ± 0.8%). Error bars show standard error.