How the brain learns to see objects and faces in an impoverished context

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Abstract
A degraded image of an object or face, which appearsmeaningless when seen for the first time, is easily recognizableafter viewing an undegraded version of the same image1. The neural mechanisms by whichthis form of rapid perceptual learning facilitates perception are notwell understood. Psychological theory suggests the involvementof systems for processing stimulus attributes, spatial attentionand feature binding2,as well as those involved in visual imagery3. Here we investigate where andhow this rapid perceptual learning is expressed in the human brain byusing functional neuroimaging to measure brain activity duringexposure to degraded images before and after exposure to thecorresponding undegraded versions (Fig. 1). Perceptuallearning of faces or objects enhanced the activity of inferiortemporal regions known to be involved in face and object recognitionrespectively46. In addition, both faceand object learning led to increased activity in medial and lateralparietal regions that have been implicated in attention7 and visual imagery8. We observed a strong couplingbetween the temporal face area and the medial parietal cortexwhen, and only when, faces were perceived. Thissuggests that perceptual learning involves direct interactions betweenareas involved in face recognition and those involved in spatialattention, feature binding and memoryrecall. Binarized images of an object (top row) and face (bottom row) and their associated full grey-scale versions. The process of binarizing images involved transforming grey-scale levels into either black or white (two-tone) with values of either 0 or 1. Exposure to the associated grey-scale version took place 5 min before a second exposure to the two-tone version in study 1. In the pre- and post-learning scans, the subjects were told that they might see faces or objects, respectively, in the stimuli. A significant behavioural learning effect, operationally defined as the facilitation of performance by prior exposure to the grey-scale version, was evident for the object and face conditions. Behavioural data were collected for all conditions at the end of each individual scan. The mean number of resolved percepts pre- and post-exposure to the grey-scale images were 13 and 87% in the object, and 55 and 93% in face conditions, respectively. In study 2, the same sequence and stimuli were used but with pre- and post-exposure being interposed with non-related grey-scale images of objects and faces. Here the mean number of resolved percepts pre- and post-exposure to non-associated grey-scale images were 8 and 10% in the object, and 19 and 25% in the face conditions, respectively.