Measurement of head movement during auditory localization

Head movement can have a significant effect on the ability to locate the direction of a sound source. A system has been designed to track the head movement in response to sound originating at different azimuth locations with respect to the head. A videotape record is made of a light approximating a point source carried on a lightweight “beanie” mounted on the listener’s head. Movement of the light is monitored by the video camera and recorded on tape, along with the sound stimulus and information concerning loudspeaker location and time. The horizontal and vertical coordinates of the light-spot image are determined in relation to the video synch pulses defining the field borders. Synch signals are available from a video monitor either in real-time or from tape replay to define each TV frame and horizontal scan line. The circuitry interfaces to a computer programmed to take the information, apply a calibration, and process the data into records of time-varying head position and velocity. Examples of both digital and graphic printouts of head movement are given. The system is capable of expansion to three-axis operation.     

Infants' localization of sounds in the median vertical plane: estimates of minimum audible angle

Infants 6, 9, 12, 15, and 18 months of age were seated in a dark room directly facing an array of nine loudspeakers positioned along the median vertical plane. One loudspeaker was positioned at ear level, 0 degree, and four others each were positioned above and below 0 degree. To examine infants' resolution of auditory space in the median vertical plane we sought to determine the smallest angular shift in the vertical location of a sound that infants could reliably detect (i.e., minimum audible angle). A two-alternative forced-choice procedure was used in which a sequence of white noise bursts was presented initially at 0 degree, and then shifted vertically (i.e., above or below 0 degree) and continued to be presented until the infant made a directional response; correct responses were visually reinforced. The smallest angular shift in vertical location that was reliably detected systematically decreased with increasing age between 6 months (15 degrees) and 18 months (4 degrees), suggesting a finer partitioning of auditory space along the vertical axis over this age range. By 18 months infants' performance matched that of a group of adults tested under the same circumstances.     

Tactile localization of the direction and distance of sounds

In a previous paper, experiments were reported which demonstrated that human subjects can judge accurately the azimuthal direction of sounds using a tactile localization device. It was also demonstrated that a tactile analogue of selective auditory attention was possible with this system. Three additional experiments, reported here, indicate that subjects are also able to judge the distance of the sound source and can concurrently judge both the azimuthal direction and distance of a source. Comparisons were made between conditions where head movements were permitted lactive~ and conditions where the head was held still Ipassive, and between normal auditory judgments and tactile judgments. Active tactile performance was essentially similar to auditory performance. Active performance was superior to passive in both directional and distance judgment, but different components of the motor-sensory complex were found to contribute to active superiority in the two tasks. The implication of these experiments for the design of auditory prosthetic devices is discussed.     

Effect of distractor sounds on the auditory attentional blink

Four experiments were conducted to determine whether or not the presence and placement of distractors in a rapid serial auditory stream has any influence on the emergence of the auditory attentional blink (AB). Experiment 1 revealed that the presence of distractors is necessary to produce the auditory AB. In Experiments 2 and 3, the auditory AB was reduced when the distractor immediately following the probe was replaced by silence but not when the distractor following the target was replaced by silence. Finally, in Experiment 4, only a very small auditory AB was found to remain when all distractors following the probe were replaced by silence. These results suggest that the auditory AB is affected both by the overwriting of the probe by the distractors following it and by a reduction in discriminability generated by all of the distractors presented in the sequence.     

The influence of the acoustic context on vertical sound localization in the median plane

The influence of background sounds (frames) on vertical localization of single sound sources (targets) was examined in four experiments. Loudspeakers (five targets and four frames) were positioned in the median plane, ranging from +30 degrees to -30 degrees above and below the subject's ear level. The subjects determined the vertical position of the targets by either verbal judgments or manual pointing. Frame and target sounds were presented concurrently or successively with a 1-sec interval; both consisted of (1) 300-Hz square waves, (2) noise, or (3) targets of noise and frames of 300-Hz square waves. Particularly in the second condition, the subjects consistently shifted the apparent target positions away from the frame locations. This contrast effect persisted even 1 sec after the offset of the frames. No effect was found with different waveforms for the frame and the target. Results are related to recent findings indicating a similar effect in the azimuthal dimension. Possibly the effect is based on a mechanism in which the auditory system adapts to recently heard sound source positions.     

Infants' perception of illusions in sound localization: reaching to sounds in the dark.

Sixteen infants each at 4, 6, and 8 months of age were tested for reaching to sounding toys in the dark under two auditory illusion conditions: the Haas-effect, which creates the illusion of a single lateralized sound based on an interaural intensity difference (the toy was visible and invisible under some test conditions); and the midline illusion, which creates the illusion of a single sound at midline due to an absence of any interaural time or intensity differences (invisible toy condition only). No-sound control trials indicated the level of spontaneous reaching in the dark. Results indicate that by 4 months infants perceive both the Haas-effect and midline illusions. The ability to reach both for invisible and visible sounding objects in the dark was well developed by 4 months of age, although developmental changes in aspects of reaching behavior were observed and, at all ages, object contact was most frequent when visual localization cues accompanied sound localization cues. The incidence of spontaneous reaching in the dark was low and did not vary with age. Theoretical and methodological implications of this research are discussed.     

Effect of subvocalization on memory for speech sounds.

Subjects in two groups were presented with sequences of 6 consonant-vowel syllables for ordered recall. Subjects in both groups were trained to suppress subvocalization. Only subjects in the feedback group were asked to suppress subvocalization during the experiment, while subjects in the no-feedback group were allowed to subvocalize during the memory task. Analysis of the electromyographic records showed a large decrease in subvocalization in the feedback condition, and results of the memory task revealed an increase in errors for this group. However, a comparison of errors within each group revealed an identical pattern of overall errors and intrusion errors on subvocalization and nonsubvocalization trials, and these results were identical in both groups. The results suggest that requiring subjects to simultaneously suppress subvocalization and remember syllables depresses performance slightly, but encoding of speech sounds in short-term memory occurs independently of subvocal activity during the memory task.     

Additive hypotheses of the effect of head and eye movement on dark convergence

Dark vergence depends on the vertical direction of gaze; it decreases with raised gaze and increases with lowered gaze. The vertical direction of gaze can be varied by means or raising or lowering the eyes or by way of tilting the head forward or backward. The effects of both manipulations on dark vergence are different. According to Heuer (1988) the effects of head tilt and eye inclination on dark vergence are almost, but not exactly, additive. In Exp. 1 the hypothesis of additive effects of gaze direction and eye inclination was tested and could not be rejected. The two additive hypotheses (head tilt and eye inclination vs. gaze inclination and eye inclination) result in different predictions for dark vergence with "compensatory" head and eye inclinations, which leave the direction of gaze in space invariant. In Exp. 2 it was shown that predictions from both hypotheses deviated from the observed values of dark vergence. Thus none of the two additive hypotheses provides exact predictions of dark vergence for all possible combinations of head tilt and eye inclination. For practical purposes the approximation might be sufficient. In particular, although mean dark vergence cannot be predicted exactly, individual differences can be predicted quite accurately.     

Spectral cues provided by the pinna for monaural localization in the horizontal plane

Six subjects located, monaurally, 1.0-kHz-wide noise bursts whose source originated on the side of the functioning ear and whose center frequency ranged from 4.0 through 9.0 kHz (Part 1). Irrespective of their actual locations, the stimuli appeared to migrate from the frontal sector of the arc toward the side as the center frequency was increased above 4.0 kHz. For some subjects, the sounds appeared again in front at the higher center frequencies. Comparable data were obtained with noise bursts 2.0 kHz in width. We referred to these constellations of location judgments, influenced by the frequency composition of the stimuli, as spatial referent maps. In Part 2, we measured, by means of a miniature microphone placed at the entrance of the external ear canal, the pinna amplification function for these same stimuli emanating from the same locations. The results showed a positive relation between the apparent location of noise bursts centered at 6.0 kHz and above and the relative amplification provided by the pinna. Localization performances by two subjects, chosen on the basis of their noncorresponding spatial referent maps, were examined for stimuli of wider bandwidths IPart 3). Their proficiency differed markedly from one another, which we accounted for in terms of different spatial referent maps that were associated with differences in the pinna amplification function.     

Speech-related body movement in aphasia: period analysis of upper arms and head movement.

The effects of aphasia on coverbal body movement have important implications for the understanding of both normal and pathological speech processes. The related findings were often inconsistent, partly due to inherent methodological difficulties which could be reduced by the use of advanced techniques of movement monitoring (Hadar, 1991). The present study employed a new computerized system, CODA-3, which locates small prismatic markers and computes by triangulation their three-dimensional position at 100 Hz. Movement of the head and the upper arms was monitored in 15 aphasic and normal subjects engaged in speech during a naturalistic interview. Movement analysis was based on automatized identification of successive movement extrema ("period analysis") and the computation of amplitude, duration, and velocity of each period. The results showed higher incidence and amplitude of all body movement in the aphasic population. Fluent aphasics showed this particularly with "symbolic," content-bearing movements, while nonfluent aphasics were higher than controls in both symbolic and "motor" (simple and small) movements. No deficit in the internal organization of movement was seen in the aphasic population. These results indicate that aphasics increase their coverbal movement in compensation for their speech impairment: fluent aphasics compensate primarily for a symbolic impairment, while nonfluent aphasics compensate more for a motor impairment.     

Effect of movement on the cardiac conditional reflex

The cardiac component of the motor conditional reflex to a faradic stimulus to the hind leg in three dogs was measured. The leg was paralyzed by crushing 4 to 5 anterior roots of the lumbar nerves (second to sixth). The cardiac experiments were repeated beginning four days after operation and measurements were taken of the cardiac conditional reflex. A comparison of the increase in cardiac rate to the conditional stimulus before and after operating showed no significant difference in spite of the fact that the dog was unable to lift his leg to the stimulus. From this we conclude that the execution of the movement is not necessary for the increase of heart rate in the cardiac conditional reflex.     

Performance and reaction times in monaural localization of first names in the horizontal plane

Many studies have been conducted to measure monaural azimuthal sound localization performance with different sounds varying in frequency and complexity, but few have used linguistic stimuli. The present experimental design used subjects' first names in a monaural azimuthal localization task. Analysis of response accuracy showed that subjects are not more accurate in localizing their own first name than in localizing other first names and that there was no significant advantage of one ear over another. Reaction times were shorter when the subjects localized their own first name than when they localized any other first names and there was no significant ear advantage, but localizing other first names took more time with the right than with the left ear. All stimuli were better and more quickly localized on the side of the open ear, and there was no difference in acuity or velocity of localization with the two different speaker voices used. These results suggest that first names are processed through the controlateral auditory pathway and can be analyzed in the right hemisphere.     

Voluntary head movement and allocentric perception of space

Although visual input is egocentric, at least some visual perceptions and representations are allocentric, that is, independent of the observer's vantage point or motion. Three experiments investigated the visual perception of three-dimensional object motion during voluntary and involuntary motion in human subjects. The results show that the motor command contributes to the objective perception of space: Observers are more likely to apply, consciously and unconsciously, spatial criteria relative to an allocentric frame of reference when they are executing voluntary head movements than while they are undergoing similar involuntary displacements (which lead to a more egocentric bias). Furthermore, details of the motor command are crucial to spatial vision, as allocentric bias decreases or disappears when self-motion and motor command do not match.     

Mechanisms controlling head stabilization in the elderly during random rotations in the vertical plane

Frequency-related response characteristics of the mechanisms controlling stabilization of the head in 10 elderly subjects were compared with response characteristics in 8 young adults. Angular velocity of the head with respect to the trunk and EMG responses of 2 neck muscles were recorded in 10 seated subjects during pseudorandom rotations of the trunk in the sagittal plane at frequencies of 0.35 to 3.05 Hz. Subjects were required to actively stabilize their heads with (VS) and without (NY) visual feedback so that voluntary mechanisms and the influence of vision could be tested. Reflex mechanisms were examined when subjects were distracted by a mental calculation task during rotations in the dark (MA). Age emerged as an influential factor in the performance of head stabilization mechanisms, and decrements in performance were even more pronounced in the older as compared with younger elderly subjects. Age effects could be seen in the (a) diminished ability to voluntarily stabilize the head, particularly with the absence of vision, (b) impaired ability to stabilize the head when cognitively distracted, and (c) appearance of a resonant response of the head. Control of head stabilization shifted from reflex mechanisms to system mechanics, probably as a result of age-related changes in the integrity of the sensory systems. The elderly's system mechanics could not effectively compensate for the disturbances, however, and instability was the result.     

The role of overt head movement and attention in persuasion

This study examined the effect of overt head movement on attitudes toward a product. In a headphones test, participants were required to listen to music and to either nod or shake their heads. Some participants listened to a CD of music; other participants listened to a CD of the same music and a persuasive message about the headphones. Overt head movement affected participants' product choice and price perception when they were presented with the music and a persuasive message. The findings are interpreted to suggest that head movement can be instrumental in participants' product evaluation if the head movement is directed or focused on the attitude object.