Rewards evoke learning of unconsciously processed visual stimuli in adult humans

Abstract

The study of human learning is complicated by the myriad of processing elements involved in conducting any behavioral task. In the case of visual perceptual learning, there has been significant controversy regarding the task processes that guide the formation of this learning. However, there is a developing consensus that top-down, task-related factors are required for such learning to take place. Here we challenge this idea by use of a novel procedure in which human participants, who were deprived of food and water, passively viewed visual stimuli while receiving occasional drops of water as rewards. Visual orientation stimuli, which were temporally paired with the liquid rewards, were viewed monocularly and rendered imperceptible by continuously flashing contour-rich patterns to the other eye. Results show that visual learning can be formed in human adults through stimulus-reward pairing in the absence of a task and without awareness of the stimulus presentation or reward contingencies.

Figure 1

General procedure and results of Experiment 1. a, Participants received occasional drops of water as rewards while passively viewing visual orientation stimuli (20% signal; 2 c/deg; 4 deg diameter) without any task (see Supplementary Figure 1 for task schematics). b, Results for trained-orientation, blue curve indicates psychometric function from the first sensitivity test and the red curve that of the second test; shaded regions indicate standard error. A clear learning effect is evident by the separation of the curves. c, Results for untrained orientation indicate no reliable change in performance.

Figure 2

Training procedure for Experiment 2. Alternating blocks of 15s duration CFS stimuli to were presented to each eye. The CFS stimuli consisted of a sequence of full-screen textured pattern images that were presented at a rate of 10 Hz. For the trained-eye a sequence of 2Hz noise and orientation patterns were presented while CFS was shown to the other eye. For the untrained-eye, a grey screen was presented while CFS was presented to the trained-eye. After every 5 minutes, participants took a 3 minutes break. These sequences repeated 8 times in each of the 20 training sessions.

Figure 3

Results of Experiment 2. a, Results for trained-orientation, blue curve indicates psychometric function from the first sensitivity test and the red curve that of the final test; shaded regions indicate standard error. A clear learning effect is evident by the separation of the curves. b, Results for untrained orientation indicate no reliable change in performance. c, d, Results for untrained eye showed no reliable change in performance for either the trained (c) or control (d) orientations.