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The perceptual skills of the radiological expert are acquired through cumulative exposure to large numbers of radiographs. This extensive practice probably engenders two kinds of perceptual learning: 'low- level' feature learning, and 'search-learning'. We report here two experiments aimed at establishing the relative importance of these two mechanisms. The first experiment investigated detection of high spatial frequency - low contrast 'targets' in radiographs, and sought to locate the level at which learning occurred by using transfer of training across eyes and across stimuli as indicators of the locus of learning. Detection times improved after extensive practice (200 trials a day for 8 days) with no loss in accuracy. This improvement transferred across eyes, but transferred just partially to targets at new levels of contrast. This pattern of results suggests that both low-level feature learning and some more general, higher level learning, had occurred. Experiment two investigated this possibility in more detail using a visual search paradigm. The possibility that perceptual learning included a change from serial to parallel processing was explored, and measures of transfer of learning to new distractor sets and to novel stimulus dimensions were used to explore the likely locus of learning. Reaction times to the presence of absence of targets among distractors on a random noise background improved as the result of practice (1440 training trials). This learning transferred across eyes, to a new stimulus dimensions set and to different distractor sets. Further, true positive detection rates showed an increase for stimuli with a homogenous distractor set whilst there was no change in false positive rates. The implications of the results for training programs are discussed.
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