Richard M. Warren

Perceptual Restoration of Missing Speech Sounds

When an extraneous sound (such as a cough or tone) completely replaces a speech sound in a recorded sentence, listeners believe they hear the missing sound. The extraneous sound seems to occur during another portion of the sentence without interfering with the intelligibility of any phoneme. If silence replaces a speech sound, the gap is correctly localized and the absence of the speech sound detected.

Measurement of sensory intensity

The measurement of sensory intensity has had a long history, attracting the attention of investigators from many disciplines including physiology, psychology, physics, mathematics, philosophy, and even chemistry. While there has been a continuing doubt by some that sensation has the properties necessary for measurement, experiments designed to obtain estimates of sensory intensity have found that a general rule applies: Equal stimulus ratios produce equal sensory ratios. Theories concerning the basis for this simple psychophysical rule are discussed, with emphasis given to the physical correlate theory, which considers judgments of subjective magnitudes to be based upon estimates of physical dimensions that vary regularly with changes in degree of stimulation. For the most thoroughly investigated sensory scales, brightness and loudness, the physical correlate is considered to be distance. Our “tacit knowledge” of the sensory effects of changing distance plays an essential role in matching motor activities to environmental conditions and in ensuring accurate perceptual evaluations through brightness and loudness constancies. In psychophysical experiments, subjects apparently use this same tacit knowledge when required to estimate relative subjective magnitudes. The evidence related to the physical correlate theory is summarized, and it is concluded that, while under appropriate conditions we demonstrate considerable skill in evaluating environmental relationships, we are quite unable to estimate the neurophysiological nature or quantity of sensory response. A psychophysics devoted to studying conditions required for accuracy and conditions producing errors in the perception of environmental relationships would seem to be more valuable than one devoted to subjective magnitudes.

Auditory Sequence: Confusion of Patterns Other Than Speech or Music

Accurate perception of temporal order is essential for many auditory tasks. Yet the temporal pattern of four successive sounds (for example, hisses, buzzes, and tones) could not be recognized even when the duration of each sound was considerably longer than either the average phoneme in normal discourse or the notes of melodies. Although each of the stimuli was perceived, their order remained frustratingly elusive.

Spectral redundancy: Intelligibility of sentences heard through narrow spectral slits

The intelligibility of word lists subjected to various types of spectral filtering has been studied extensively. Although words used for communication are usually present in sentences rather than lists, there has been no systematic report of the intelligibility of lexical components of narrowband sentences. In the present study, we found that surprisingly little spectral information is required to identify component words when sentences are heard through narrow spectral slits. Four hundred twenty listeners (21 groups of 20 subjects) were each presented with 100 bandpass filtered CID ( “everyday speech ”) sentences; separate groups received center frequencies of 370, 530, 750, 1100, 1500, 2100, 3000, 4200, and 6000 Hz at 70 dBA SPL. In Experiment 1, intelligibility of single 1/3-octave bands with steep filter slopes (96 dB/octave) averaged more than 95% for sentences centered at 1100, 1500, and 2100 Hz. In Experiment 2, we used the same center frequencies with extremely narrow bands (slopes of 115 dB/octave intersecting at the center frequency, resulting in a nominal bandwidth of l/20 octave). Despite the severe spectral tilt for all frequencies of this impoverished spectrum, intelligibility remained relatively high for most bands, with the greatest intelligibility (77%) at 1500 Hz. In Experiments 1 and 2, the bands centered at 370 and 6000 Hz provided little useful information when presented individually, but in each experiment they interacted synergistically when combined. The present findings demonstrate the adaptive flexibility of mechanisms used for speech perception and are discussed in the context of the LAME model of opportunistic multilevel processing.

Basis for Judgments of Relative Brightness

The hypothesis is offered that relative brightness judgments are based upon experience with the way the amount of light reflected by objects changes with their distance from the light source.The experimental results support this hypothesis. One group of 40 subjects judged half brightness, and another 40 estimated the change in luminance corresponding to moving a hidden point light source to twice the distance from an illuminated standard field. Judgments of the two groups were equivalent.Under stimulus conditions designed to represent the common visual situation (stimuli subtending wide visual angles, adaptation approximating stimulus levels), one quarter the standard luminance was correctly chosen for the effect of doubling distance from the source, and the same fraction was chosen for half brightness for all standard intensities (0.00086 to 87 millilamberts).Under less familiar conditions similar to those employed for the bril scale (small stimuli with black backgrounds, indeterminate levels of adaptation) half brightness judgments were again equivalent to estimates of the effect of doubling distance from object to light source. These estimates were less than one-quarter standard luminance.The hypothesis is discussed in terms of sensory scaling in general, and the neutral value and bril scales in particular.

Speech perception and phonemic restorations

When a speech sound in a sentence is replaced completely by an extraneous sound (such as a cough or tone), the listene restores the missing sound on the bases of both prior and subsequent context. This illusory effect, called phonemic restoration (PhR), causes the physically absent phoneme to seem as real as the speech sounds which are present. The extraneous sound seems to occur along with other phonemes without interfering with their clarity. But if a silent gap (rather than an extraneous sound) replaces the same phoneme, the interruption in the sentence is more readily localized in its true position and PhRs occours less frequently. Quantitative measures were taken both of the incidence of PhRs and of the direction and extent of temporal mislocalizations of interruptions for several related stimuli under a variety of experimental conditions. The results were connected with other auditory illusions and temporal confusions reported in the literature, and suggestions were made concerning mechanisms employed normally for verbal organization.

Auditory temporal discrimination by trained listeners

Abstract Previous reports from this laboratory have shown that untrained listeners cannot name the order of sounds in extended sequences of unrelated items such as hisses, tones, and buzzes at item durations of 200 msec, even though identification of order for the sounds of speech and music is accomplished readily at much shorter item durations. The present study indicates that under appropriate conditions subjects can be trained to distinguish between and to identify permuted orders within sequences of nonrelated sounds each lasting only 10 msec or less. Evidence is presented suggesting that much of what passes for direct identification of order is actually based on prior identification of a larger pattern. Two principal mechanisms seem to mediate discrimination of auditory temporal order: (1) fine resolution (requiring prior training) permitting identification of over-all patterns and capable of operating with item durations of only a few msec; (2) coarse resolution (special prior training not required) allowing naming of order of items having minimal durations usually above 200 msec for unfamiliar sequences.

Phonemic restorations based on subsequent context

Earlier experiments have shown that when one or more speech sounds in a sentence are replaced by a noise meeting certain criteria, the listener mislocalizes the extraneous sound and believes he hears the missing phoneme(s) clearly. The present study confirms and extends these earlier reports of phonemic restorations under a variety of novel conditions. All stimuli had some of the context necessary for the appropriate phonemic restoration following the missing sound, and all sentences had the missing phoneme deliberately mispronounced before electronic deletion (so that the neighboring phonemes could not provide acoustic cues to aid phonemic restorations). The results are interpreted in terms of mechanisms normally aiding veridical perception of speech and nonspeech sounds.

HELMHOLTZ ON PERCEPTION: ITS PHYSIOLOGY AND DEVELOPMENT.

Marco Mumenthaler, Director der Neurologischen Universitatsklinik Inselspital, Bern. Price,

Identification of temporal order within auditory sequences

2.45 (DM 9,80). Pp 383. Georg Thieme Verlag, Stuttgart, Germany. A veritable flood of neurology texts continues to appear and one has difficulty in distinguishing one from another. This soft covered book on neurology by Dr. Mumenthaler is, however, a welcome exception to the usual dreary lot. It is easily carried in a pocket and so can be consulted when needed. The first part has a most excellent review of clinical methods and of the localizing value of abnormal neurological signs. Superb diagrams, tables, and line drawings of common neurological abnormalities encountered at the bedside make this section unique among pocket-sized neurology texts. The diagrams of normal and pathological roentgenographic and electromyographic findings, and of other special tests, are far superior to the often encountered unintelligible photographs. In the second part, neurologic diseases are adequately described, and judiciously selected references are given for those who might want further information, but this should not be necessary for undergraduates. The understanding of neurological disease is again facilitated by drawings and tables, some of which should be of use