Altered attentional modulation in visual cortex during perceptual learning

Abstract

Practising a visual task commonly results in improved performance. Often the improvement does not transfer to a new retinal location, suggesting that learning is mediated by changes occurring in early visual cortex, while neuroimaging and neurophysiological studies demonstrate that perceptual learning is associated with altered activity in visual cortex. Theoretical treatments tend to invoke neuroplasticity that refines early sensory processing. An alternative possibility is that learning involves an altered attentional state or strategy, and that the changes in early visual areas reflect locally altered attentional modulation. To test this idea, we used functional MRI to measure attention-related signals in visual cortex while participants practised an orientation-discrimination task. The two grating patches to be discriminated were presented simultaneously, one on each side of fixation. Four levels of task difficulty (fixed orientation differences) were intermixed. By recording activity in visual cortex during a preparatory period, we isolated attention-related activity in eight participants. On each trial, a cue indicating an upcoming stimulus pair and conveying the task difficulty was presented and the participant then attended the expected stimulus locations. After a variable interval, the two targets appeared for 1 s and the participant responded via button presses. The experiment continued for about 40 mins with short breaks. The behavioural data (percent correct trials for a given difficulty) showed a gradual improvement in performance over the session. Preparatory BOLD activity declined as learning progressed. This change was accompanied by a reduction of stimulus-evoked activity, as previously demonstrated during learning of similar tasks. Both effects were seen only at the locations of the stimuli. The change in preparatory (attention-related) activity mirrored learning, occurring most quickly for the easiest stimuli. The results suggest that spatial attention alters in some way during perceptual learning and that this may account for some of the concomitant changes seen in visual cortex.