Excitatory signal processing in early visual areas based on task-relevance and perceptual salience modulates visual perceptual learning

Abstract

Adaptive visual perceptual learning (VPL) should occur primarily for visual signals relevant to an observers task but not for task-irrelevant signals. However, it is unclear which neural mechanisms reduce VPL for task-irrelevant signals. Here, we repeatedly exposed participants to a task-irrelevant visual signal (coherent motion in one direction) that was perceptually salient (suprathreshold for coherent motion detection) or weak (near threshold for coherent motion detection). The processing of the task-irrelevant signal in early visual areas during the first and final exposure sessions was measured using functional magnetic resonance spectroscopy (fMRS). The behavioral results showed that discrimination sensitivity for the exposed coherent motion direction increased to a greater extent after near threshold than after suprathreshold exposure. The fMRS results showed that excitatory processing in early visual areas, as reflected in the concentration of glutamate, was lower during suprathreshold than near threshold exposure. The lower the level of excitatory processing in the first exposure session, the more deteriorated the discrimination sensitivity for the coherent motion direction after suprathreshold exposure. When the coherent motion direction was rendered task-relevant, levels of glutamate in early visual areas reversed such that excitatory processing was greater when the coherent motion direction was presented suprathreshold than near threshold. This suggests that the level of excitatory processing in early visual areas changes with the task relevance and perceptual salience of a visual signal, which may modulate the extent of VPL for this signal when it is repeatedly exposed. Significance StatementVisual perceptual learning (VPL) enhances the efficiency in processing visual signals and is fundamental to the brains ability to adapt to changing environmental conditions. However, little is known about neural mechanisms that reduce VPL, although such mechanisms are crucial for adaptive VPL, which occurs primarily for visual signals relevant to an observers task but not for task-irrelevant signals. Here we find that early visual areas decrease excitatory processing for a salient task-irrelevant visual signal, which is associated with reduced VPL for this signal. This suggests, first, that a decrease of excitatory activity may be a mechanism that reduces VPL for salient task-irrelevant signals and, second, that this mechanism exerts its effect already at an early stage of cortical visual processing.