Extended previous research on perceptual grouping in infancy (Thorpe, Trehub, Morrongiello, & Bull, 1988). Infants' detection of temporal increments to silent intervals between groups of tones was compared with their detection of comparable silent increments within groups of tones. Ss were presented with standard 6-tone sequences, with the fust 3 tones differing from the last 3 in harmonic structure, frequency, or intensity, and contrasting sequences with increments of 80 or 100 ms to the silent interval between the 3rd and 4th tone (XXX OOO, between groups) or between the 4th and 5th tone (XXXO OO, within group). Infants discriminated temporally altered sequences from the standard only when the increments occurred within a group. The authors conclude that perceptual grouping processes are operative in infancy and that infants perceive the silent increments between groups of sounds as longer than identical intervals within a group, as do adults. In the visual domain, illusions have traditionally been regarded as mistakes made by the perceptual system. This is reflected in the terminology applied to such perception, such as deficient (Gibson, 1966, chap. 14) and misperception (Hochberg, 1981, p. 257), and in the observation that perception can go wrong** (Gregory, 1972, p. 131). In the recent literature, however, there are suggestions that illusions may result from the application of normal operating principles in circumstances where ambiguous or impoverished cues prevail (e.g., Bartley, 1980; Rock, 1986). In contrast to visual illusion, the study of auditory illusion is a relatively uncommon and comparatively recent endeavor. Some contemporary investigators of auditory-perceptual organization regard the discrepancy between proximal stimulation and perceptual experience as an accomplishment of the nervous system (Bregman, 1978, p. 391) rather than as a breakdown of perceptual mechanisms. Indeed, perceptual processes may distort sensory information to emphasize the structure of the stimulus, and such altered (i.e., illusory) information may be present in most, if not all, percepts. Perceivers are typically unaware of the lack of veridicality because of the absence of objective measurements for comparison with perceptual information. This conceptual framework depicts illusions as manifestations of perceptual organization, because the resulting percepts, although nonveridical, promote useful or meaningful interpretations of stimulus input. In this light, the demonstration of illusions in infancy may provide important insights into the development of pattern perception. Thus, it is of interest to specify which illusions are experienced, when (i.e., at what age) they appear, and what mechanisms are responsible for their appearance.