Identification of spatially quantised tachistoscopic images of faces: How many pixels does it take to carry identity?

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Abstract
Six images of human faces were quantised into isoluminant square-shaped pixels (16 grey levels) at eight different spatial levels of quantisation. The subjects had to identify the faces that were presented with different exposure durations (from 1 to 200 msec) and with one of two brightness conditions (variable brightness in Experiment 1 or isobrightness in Experiment 2). All finer quantisation levels led to better identification than the most coarse quantisation level (15 pixels per face in the horizontal dimension) at all exposure durations. The observation of an abrupt decrease in identification efficiency on moving from 18 or more pixels per face to 15 pixels per face and the approximate equality in identification efficiency within a broad range of quantisation levels above 18 pixels per face pose some problems for existing theories of face recognition. The implications of these findings for prototype-related, auto correlation and micro genetic accounts of face and pattern processing are discussed.