Bruce H. Deatherage

Auditory Localization of Clicks

Previous investigations on the auditory localization of sounds have established that interaural differences in time and in intensity are both important. There have also been attempts to set temporal and intensive differences against each other, so that a lead in time at one ear accompanied by an increase in intensity at the other ear would cause a sound to be localized in the midline. On the basis of hypotheses suggested by physiological observations, two experiments were carried out. The first showed that the intensive difference required to offset a difference in time at the two ears depends upon the level of the sounds. Clicks at low level require smaller differences in intensity to offset a given temporal difference than do clicks at a high level. The relation between intensive differences (decibels) and temporal differences (microseconds) is not linear at any of the levels studied. In the second experiment, high‐frequency masking noise interfered with reception in the basal turn of the cochlea. Although this noise did not mask the clicks, it produced a temporal lag that required compensation by a time delay at the opposite ear. The relations between these psychophysical results and physiological findings are discussed.

Effect of Interaural Correlation on the Precision of Centering a Noise

Subjects were asked to center a noise presented via earphones, by adjusting a delay line controlling interaural time differences. Various noise correlations were employed ranging from 1.00 to 0.10. Even in the latter case, which represents a signal to noise difference of −9.5 dB the subjects were able to perform well above a chance level.

Binaural Masking: Backward, Forward, and Simultaneous Effects

This study sought to examine the temporal relations of binaural masking as compared with monaural masking for conditions of forward, simultaneous, and backward masking. Five conditions of different signal‐masker configurations and a number of temporal relations were examined to determine the masking effects on a tone by a tonal masker. The results indicate that forward, simultaneous, and backward masking are all the result of time‐dependent properties of the neural mechanism, but the dependency for all three types of masking is wholly or in part mediated by the central nervous system. The concept of peripheral adaptation as an explanation for forward masking is not supported. A discussion of the various contralateral masking conditions is presented.

Examination of Binaural Interaction

The study of binarual interaction may be conveniently divided into three areas: anatomy, physiology, and psychology. These areas, while furnishing large amounts of data, have not yet provided enough information to make possible a complete theory of binarual interaction. Several partial theories are discussed, two experiments are reported, the results of the experiments set in perspective, and the problems of the construction of a full theory of binarual interaction are presented. A Bibliography is appended.

Masking and Interaural Phase. II. 167 Cycles

Masking level differences obtained for 167 cps are presented. Some involved shifting the phase of the signal through various angles, and others, shifting the noise in interaural time. Where the experimental conditions are comparable to Hirshs at 200 cps, the data agree very closely with his. The data obtained with various interaural time differences in the noise are shown to differ sharply from comparable data taken at 500 cps.

Effects of Intensity on Critical Bands for Tonal Stimuli as Determined by Band Limiting

Four subjects were run in a 2‐AFC psychophysical study in auditory masking. The signal consisted of a 100‐msec tone burst delivered inphase to the two ears. Signal frequencies between 250–4000 Hz were used. Masking noise had spectrum levels of 45, 25, and 15 dB SPL. For each signal frequency, the signal level was found which produced P(c) of .54 in wideband noise at each spectrum level. The high‐ and low‐frequency cutoffs of the noise were then varied independently (band‐limited), and changes in detectability were observed. Detectability remained constant until the cutoff of the noise was raised or lowered to a critical value; from that point on, detectability improved. The function traced by improvement in detectability represents the “critical band” for the stimulus conditions used. The width of the critical band was directly related to stimulus intensity for all signal frequencies. [Work supported through a contract from the U. S. Navy Office of Naval Research, Physiological Psychology Branch.]

Remote Masking in Selected Frequency Regions

Masked audiograms for six different masking stimuli were obtained from five observers. The audiograms show that remote masking can be controlled with respect to frequency region by controlling the envelope of the masking sound. Regular variations in the envelope at, say, 500 cps produce remote masking in the 500‐cps region; while irregular, “random,” variations produce equal amounts of masking everywhere outside the frequency region corresponding to the masking band or tone. It is argued that the constant masking seen in remote masking is related to the fact that signal frequencies are differentially attenuated in their transmission from the tympanum to the cochlear partition, while the masking frequencies are not since they are actually generated at the cochlear partition itself.

Binaural Interaction of Clicks of Different Frequency Content

Results from previous experiments have suggested the notion that the neural information for the lateralization of brief sounds comes largely from the basal turn of the cochlea. An examination of that notion which uses stimuli at the two ears of different frequency content shows that the relation is not so simple. When stimulus clicks to the two ears are identical, then approximate simultaneity places a unitary click‐image in the center of the head; and when the stimulus click to one ear differs only moderately in frequency content from the click to the other, then a single click‐image is still heard but the stimulus click of high‐frequency content must be delivered later than the low‐frequency click in order to place the image in the center of the head. If the frequency difference is great, however, a unitary click‐image is no longer heard. Instead, the sound breaks up into two images, one of high and one of low pitch, which may be independently brought to the median plane of the head by appropriate adjus...

The effect of frequency on the auditory evoked response

This study investigated the effects of frequency on the amplitude of the cortical evoked response as recorded from the cranial vertex. The stimuli were monaurally presented tones, 200 msec in duration, with frequencies of 500, 1000, 2000, or 4000 Hz. Stimulus intensities of 90-dB hearing level (HL) and 45-dB hearing level (HL) were examined. A negative relationship was found between the amplitude of the evoked response and the frequency of the stimulus. At both intensity levels, the amplitude of the response decreased progressively as the frequency increased.

Psychophysical Investigation of Auditory Sensitization

The fact of “sensitization” is well known. In the auditory system, it is demonstrated by a larger amplitude for N1 in response to a click having a condensation followed by rarefaction as opposed to the reverse (Peake and Kiang). The Bekesy audiogram also shows fluctuations in threshold as a gated tonal stimulus is slowly moved in phase along a very low‐frequency masking tone (Shickman). We have attempted to demonstrate sensitization in a controlled psychophysical setting in which a very low frequency, usually 50 Hz, was adjusted in level to be 5–10 dB below classical absolute threshold. This served as the background; it cannot strictly be called a masker. A brief high‐frequency tone was gated coherently with the low frequency and placed in various phase relations to the low‐frequency tone. Percent‐correct judgments in two‐interval forced‐choice experiments was the dependent variable. Sensitization was demonstrated. [Work supported in part by the U. S. Office of Naval Research.]