Auditory illusion

Abstract: Phonemic restoration is a powerful auditory illusion in which listeners "hear" parts of words that are not really there. In earlier studies of the illusion, segments of words (phonemes) were replaced by an extraneous sound; listeners were asked whether anything was missing and where the extraneous noise had occurred. Most listeners reported that the utterance was intact and mislocalized the noise, suggesting that they had restored the missing phoneme. In the present study, a second type of stimulus was also presented: items in which the extraneous sound was merely superimposed on the critical phoneme. On each trial, listeners were asked to report whether they thought a stimulus utterance was intact (noise superimposed) or not (noise replacing). Since this procedure yields both a miss rate P(intact/replaced), and a false alarm rate P(replaced/intact), signal detection parameters of discriminability and bias can be calculated. The discriminability parameter reflects how similar the two types of stimuli sound; perceptual restoration of replaced items should make them sound intact, producing low discriminability scores. The bias parameter measures the tendency of listeners to report utterances as intact; it reflects postperceptual decision processes. This improved methodology was used to test the hypothesis that restoration (and more generally, speech perception) depends upon the bottom-up confirmation of expectations generated at higher levels. Perceptual restoration varied greatly wih the phone class of the replaced segment and its acoustic similarity to the replacement sound, supporting a bottom-up component to the illusion. Increasing listeners' expectations of a phoneme increased perceptual restoration: missing segments in words were better restored than corresponding pieces in phonologically legal pseudowords; priming the words produced even more restoration. In contrast, sentential context affected the postperceptual decision stage, biasing listeners to report utterances as intact. A limited interactive model of speech perception, with both bottom-up and top-down components, is used to explain the results.

Abstract: I HERE report a novel and striking auditory illusion, which provides a paradox for theories of pitch perception and auditory localisation1,2; and which varies in correlation with the handedness of the listener. The stimulus configuration which produced the illusion consisted of a sequence of tones, alternating in pitch between 400 Hz and 800 Hz (Fig. 1a). Each tone lasted 250 ms, with no gap between tones. The sequence was presented at equal amplitude to both ears simultaneously; however, when one ear received 400 Hz the other received 800 Hz, and vice versa. Thus the same two-tone combination was presented constantly, but the ear of input for each component switched every 250 ms.

Abstract: 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 perception" (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.

Abstract: Percepts unaccompanied by a veridical stimulus, such as hallucinations, provide an opportunity for mapping the neural correlates of conscious perception. Functional magnetic resonance imaging (fMRI) can reveal localized changes in blood oxygenation in response to actual as well as imagined sensory stimulation. The safe repeatability of fMRI enabled us to study a patient with schizophrenia while he was experiencing auditory hallucinations and when hallucination-free (with supporting data from a second case). Cortical activation was measured in response to periodic exogenous auditory and visual stimulations using time series regression analysis. Functional brain images were obtained in each hallucination condition both while the patient was on and off antipsychotic drugs. The response of the temporal cortex to exogenous auditory stimulation (speech) was markedly reduced when the patient was experiencing hallucinating voices addressing him, regardless of medication. Visual cortical activation (to flashing lights) remained normal over four scans. From the results of this study and previous work on visual hallucinations we conclude that hallucinations coincide with maximal activation of the sensory and association cortex, specific to the modality of the experience.