Identification of band-pass filtered letters and faces by human and ideal observers

To better understand how the visual system makes use of information across spatial scales when identifying different kinds of complex patterns, we mea sured human and ideal contrast identification thresholds to estimate identi fication efficiency for 1- and 2-octave wide band-pass filtered letters and faces embedded in 2-D dynamic Gaussian noise. Varying stimulus center freq uency from 1 to 70 c/object had different effects on letter and face identi fication efficiency. In the 2-octave conditions, identification efficiencie s decreased by 0.25-0.5 log units for letters and 0.5-1.2 log units for fac es as center frequency increased from 6.2 to 49.5 c/object, but only letter s were identifiable at center frequencies below 6.2 c/object. In the 1-octa ve conditions, letter identification efficiencies increased by about 0.5 lo g units as center frequency increased from 1.1 to 2.2 c/object, and were ne arly constant from 2.2 to 35 c/object. Letters were unidentifiable by human observers at 70 c/object. Surprisingly, face identification was impossible for human observers at all center frequencies except 8.8 c/object for one observer, and 8.8 and 17.5 c/object for a second observer. Ideal observer t hresholds were obtained for both letters and faces in all conditions, so in formation was always available to perform the task. Thus, the failure to id entify faces reflects constraints on visual processing rather than a lack o f stimulus information. Selective spatial sampling may account for some of the differences between letter and face identification efficiencies. (C) 19 99 Elsevier Science Ltd. All rights reserved.