Visual perceptual learning of a task-irrelevant feature of the stimulus

Awareness, focused attention, and task-relevance were thought to be necessary for perceptual learning (PL): a Feature of the Stimulus (FoS) on which participants perform a task is learned, while a task-irrelevant FoS is not learned. This view has been challenged by the discovery of task- irrelevant PL, occurring for subthreshold task-irrelevant stimuli presented at an unattended, peripheral location. Here, we proof further evidence for task-irrelevant PL by showing that it can occur for subthreshold task-irrelevant FoS presented in the fovea (hence spatially attended). Our experiment was divided into 3 stages: pre-test, training, and post-test. During pre- and post- tests, participants performed a 3-dot Vernier task and a 3-dot bisection task. During training, participants performed an unrelated task (luminance discrimination) on the same stimulus. The task-irrelevant FoS, manipulated during training, was the position of the middle dot: either a subthreshold left/right offset (Experimental Group) or in perfect alignment with the outer dots (Control Group). The Experimental Group showed performance improvements in the Vernier task but not in the bisection task; while the Control Group showed no effect on performance in either task. We suggest that PL can occur as an effect of mere exposure to a subthreshold task- irrelevant FoS, which is spatially attended.