Sound Localization Acuity Research Articles

Can measures of sound localization acuity be related to the precision of absolute location estimates?

Studies of sound localization use relative or absolute psychoacoustic paradigms. Relative tasks assess acuity by determining the smallest angle separating two sources that subjects can discriminate, the minimum audible angle (MAA), whereas absolute tasks measure subjects’ abilities to indicate sound location. It is unclear whether or how measures from the two tasks are related, though the belief that the MAA is specifically related to the precision of absolute localization is common. The present study aimed to investigate the basis of this relationship by comparing the precision of absolute location estimates with a measure of spatial acuity computed from the same data. Three cats were trained to indicate apparent sound source locations that varied in azimuth and elevation via orienting gaze shifts (combined eye and head movements). The precision of these absolute responses, as measured by their standard deviation, was compared with acuity thresholds derived from receiver operating characteristic (ROC) analyses of the cumulative distributions. Surprisingly, the acuity measures were occasionally very poor indicators of absolute localization precision. Incongruent results were attributed to errors in mean accuracy, which are disregarded in analyses of traditional relative tasks. Discussion focuses on the potential for internal biases to affect measures of localization acuity.

Behavioral sensitivity to interaural time differences in the rabbit

An important cue for sound localization and separation of signals from noise is the interaural time difference (ITD). Humans are able to localize sounds within 1–2° and can detect very small changes in the ITD (10–20 μs). In contrast, many animals localize sounds with less precision than humans. Rabbits, for example, have sound localization thresholds of ∼22°. There is only limited information about behavioral ITD discrimination in animals with poor sound localization acuity that are typically used for the neural recordings. For this study, we measured behavioral discrimination of ITDs in the rabbit for a range of reference ITDs from 0 to ±300 μs. The behavioral task was conditioned avoidance and the stimulus was band-limited noise (500–1500 Hz). Across animals, the average discrimination threshold was 50–60 μs for reference ITDs of 0 to ±200 μs. There was no trend in the thresholds across this range of reference ITDs. For a reference ITD of ±300 μs, which is near the limit of the physiological window defined by the head width in this species, the discrimination threshold increased to ∼100 μs. The ITD discrimination in rabbits less acute than in cats, which have a similar head size. This result supports the suggestion that ITD discrimination, like sound localization [see Heffner, 1997. Acta Otolaryngol. 532 (Suppl.), 46–53] is determined by factors other than head size.

Underwater localization of pure tones by harbor seals (Phoca vitulina)

The underwater sound localization acuity of harbor seals (Phoca vitulina) was measured in the horizontal plane. Minimum audible angles (MAAs) of pure tones were determined as a function of frequency from 0.2 to 16 kHz for two seals. Testing was conducted in a 10-m-diam underwater half circle using a right/left psychophysical procedure. The results indicate that for both harbor seals, MAAs were large at high frequencies (13.5 degrees and 17.4 degrees at 16 kHz), transitional at intermediate frequencies (9.6 degrees and 10.1 degrees at 4 kHz), and particularly small at low frequencies (3.2 degrees and 3.1 degrees at 0.2 kHz). Harbor seals seem to be able to utilize both binaural cues, interaural time differences (ITDs) and interaural intensity differences (IIDs), but a significant decrease in the sound localization acuity with increasing frequency suggests that IID cues may not be as robust as ITD cues under water. These results suggest that the harbor seal can be regarded as a low-frequency specialist. Additionally, to obtain a MAA more representative of the species, the horizontal underwater MAA of six adult harbor seals was measured at 2 kHz under identical conditions. The MAAs of the six animals ranged from 8.8 degrees to 11.7 degrees , resulting in a mean MAA of 10.3 degrees .

Sound-localization acuity and its relation to vision in large and small fruit-eating bats: I. Echolocating species, Phyllostomus hastatus and Carollia perspicillata

Passive sound-localization acuity for 100-ms noise bursts was determined behaviorally for two species of bats: Phyllostomus hastatus, a large bat that eats fruit and vertebrates, and Carollia perspicillata, a small species that eats fruit and nectar. The mean minimum audible angle for two P. hastatus was 9°, and that for two C. perspicillata was 14.8°. This places their passive sound-localization acuity near the middle of the range for mammals. Sound localization varies widely among mammals and the best predictor of a species’ acuity remains the width of the field of best vision ( r = .89, p < .0001). The five echolocating bats that have been tested do not deviate from this relationship suggesting that despite their specialization for echolocation, the use of hearing to direct the eyes to the source of a sound still serves as an important selective factor for sound localization.

Evolution of mammalian hearing

The comparative study of mammals reveals systematic differences in their hearing abilities. First, the ability to hear above 10 kHz (which apparently evolved for using pinna cues to localize sound) varies inversely with the size of an animal’s head such that smaller animals usually have better high-frequency hearing than larger ones. Second, low-frequency hearing ability is bimodally distributed: Although most mammals can hear below 125 Hz, about one third do not hear much below 500 Hz and probably do not use temporal coding for pitch perception. Third, sound localization acuity varies directly with the width of the field of best vision, indicating that a major source of evolutionary pressure on hearing is to direct the eyes to the source of a sound. Finally, while some mammals do not use the binaural time-difference locus cue and a few others do not use the binaural intensity difference cue, it is likely that all mammals that possess pinnae use pinna locus cues to localize sound. Taken together, these findings suggest that some of the hearing abilities of extinct mammals may be inferred from fossil evidence bearing on their head size, eyes, and the presence of pinnae.

Underwater auditory localization by harbor seals (Phoca vitulina)

The underwater sound localization acuity of a swimming harbor seal was measured in the horizontal plane. The stimulus was either a double sound (two 6-kHz pure tones lasting 0.5 s separated by an interval of 0.2 s) or a single continuous sound of 1.2 s. Testing was conducted in a 10-m-diameter underwater half-circle with hidden loudspeakers. The animal was trained to swim along the diameter of the half-circle and to change its course towards the sound source right after the signal was given. The seal indicated the sound source by touching its assumed position at the half-circle. The deviation of the seal’s choice from the actual sound source was measured by means of video analysis. In trials with the double sound the seal localized the sound sources with a mean deviation of 2.8° and in trials with the single sound with a mean deviation of 4.5°. In a second experiment minimum audible angles of the animal were determined as a function of frequency (2, 4, 6, 8, and 16 kHz resulting in MAAs of 9.4°, 9.6°, 9.8°, 13.0°, and 13.5°, respectively). Intraspecific differences of the MAA will be presented for five additional harbor seals.

Underwater auditory localization by a swimming harbor seal (Phoca vitulina)

The underwater sound localization acuity of a swimming harbor seal (Phoca vitulina) was measured in the horizontal plane at 13 different positions. The stimulus was either a double sound (two 6-kHz pure tones lasting 0.5 s separated by an interval of 0.2 s) or a single continuous sound of 1.2 s. Testing was conducted in a 10-m-diam underwater half circle arena with hidden loudspeakers installed at the exterior perimeter. The animal was trained to swim along the diameter of the half circle and to change its course towards the sound source as soon as the signal was given. The seal indicated the sound source by touching its assumed position at the board of the half circle. The deviation of the seals choice from the actual sound source was measured by means of video analysis. In trials with the double sound the seal localized the sound sources with a mean deviation of 2.8 degrees and in trials with the single sound with a mean deviation of 4.5 degrees. In a second experiment minimum audible angles of the stationary animal were found to be 9.8 degrees in front and 9.7 degrees in the back of the seal's head.

Bilateral Cochlear Implants in Children: Localization Acuity Measured with Minimum Audible Angle

To evaluate sound localization acuity in a group of children who received bilateral (BI) cochlear implants in sequential procedures and to determine the extent to which BI auditory experience affects sound localization acuity. In addition, to investigate the extent to which a hearing aid in the nonimplanted ear can also provide benefits on this task. Two groups of children participated, 13 with BI cochlear implants (cochlear implant + cochlear implant), ranging in age from 3 to 16 yrs, and six with a hearing aid in the nonimplanted ear (cochlear implant + hearing aid), ages 4 to 14 yrs. Testing was conducted in large sound-treated booths with loudspeakers positioned on a horizontal arc with a radius of 1.5 m. Stimuli were spondaic words recorded with a male voice. Stimulus levels typically averaged 60 dB SPL and were randomly roved between 56 and 64 dB SPL (+/-4 dB rove); in a few instances, levels were held fixed (60 dB SPL). Testing was conducted by using a "listening game" platform via computerized interactive software, and the ability of each child to discriminate sounds presented to the right or left was measured for loudspeakers subtending various angular separations. Minimum audible angle thresholds were measured in the BI (cochlear implant + cochlear implant or cochlear implant + hearing aid) listening mode and under monaural conditions. Approximately 70% (9/13) of children in the cochlear implant + cochlear implant group discriminated left/right for source separations of <or=20 degrees , and, of those, 77% (7/9) performed better when listening bilaterally than with either cochlear implant alone. Several children were also able to perform the task when using a single cochlear implant, under some conditions. Minimum audible angle thresholds were better in the first cochlear implant than the second cochlear implant listening mode for nearly all (8/9) subjects. Repeated testing of a few individual subjects over a 2-yr period suggests that robust improvements in performance occurred with increased auditory experience. Children who wore hearing aids in the nonimplanted ear were at times also able to perform the task. Average group performance was worse than that of the children with BI cochlear implants when both ears were activated (cochlear implant + hearing aid versus cochlear implant + cochlear implant) but not significantly different when listening with a single cochlear implant. Children with sequential BI cochlear implants represent a unique population of individuals who have undergone variable amounts of auditory deprivation in each ear. Our findings suggest that many but not all of these children perform better on measures of localization acuity with two cochlear implants compared with one and are better at the task than children using the cochlear implant + hearing aid. These results must be interpreted with caution, because benefits on other tasks as well as the long-term benefits of BI cochlear implants are yet to be fully understood. The factors that might contribute to such benefits must be carefully evaluated in large populations of children using a variety of measures.

A comparative study of sound localization acuity of congenital blind and sighted people

A comparative study of sound localization acuity of congenital blind and sighted people

Sound localization in a new-world frugivorous bat, Artibeus jamaicensis: acuity, use of binaural cues, and relationship to vision.

Passive sound-localization acuity and its relationship to vision were determined for the echolocating Jamaican fruit bat (Artibeus jamaicensis). A conditioned avoidance procedure was used in which the animals drank fruit juice from a spout in the presence of sounds from their right, but suppressed their behavior, breaking contact with the spout, whenever a sound came from their left, thereby avoiding a mild shock. The mean minimum audible angle for three bats for a 100-ms noise burst was 10 degrees-marginally superior to the 11.6 degrees threshold for Egyptian fruit bats and the 14 degrees threshold for big brown bats. Jamaican fruit bats were also able to localize both low- and high-frequency pure tones, indicating that they can use both binaural phase- and intensity-difference cues to locus. Indeed, their ability to use the binaural phase cue extends up to 6.3 kHz, the highest frequency so far for a mammal. The width of their field of best vision, defined anatomically as the width of the retinal area containing ganglion-cell densities at least 75% of maximum, is 34 degrees. This value is consistent with the previously established relationship between vision and hearing indicating that, even in echolocating bats, the primary function of passive sound localization is to direct the eyes to sound sources.

Sound localization in an Old-World fruit bat (Rousettus aegyptiacus): acuity, use of binaural cues, and relationship to vision.

The passive sound-localization acuity of Egyptian fruit bats (Rousettus aegyptiacus) was determined using a conditioned-avoidance procedure. The mean minimum audible angle for left-right discrimination for 3 bats was 11.6 degrees--very near the mean for terrestrial mammals. The bats also were able to localize low- and high-frequency pure tones, indicating that they can use both binaural phase-difference and binaural intensity-difference cues to localize sound. Moreover, they were able to use the binaural phase-difference cue up to at least 5.6 kHz, which is higher than other mammals yet tested. The width of the Egyptian fruit bats' field of best vision was 27 degrees. This value is consistent with the hypothesis that the role of passive sound localization is to direct the eyes for visual scrutiny of sound sources. Thus, the passive localization abilities of these echolocating megachiropteran fruit bats do not deviate from the patterns established for nonecholocating mammals.

Passive sound-localization ability of the big brown bat ( Eptesicus fuscus)

The passive sound-localization ability (i.e. minimum audible angle) of the big brown bat, Eptesicus fuscus, was determined using a conditioned avoidance procedure in which the animals were trained to discriminate left sounds from right sounds. The mean threshold of three bats for a 100-ms broadband noise burst was 14°, a value that is about average for mammals. A similar threshold of 15° was obtained for one animal when it was retested with one of its own recorded echolocation calls as the stimulus. The two bats tested on pure-tone localization were able to localize high-frequency, but not low-frequency tones, even when a low-frequency tone was amplitude modulated, a result indicating that these bats are not able to use binaural time-difference cues for localization. Finally, given the width of the bat's field of best vision, as determined by a count of its ganglion-cell density, its sound-localization acuity is consistent with the hypothesis that the role of passive sound localization is to direct the eyes to the source of a sound.

Comparative Study of Sound Localization and its Anatomical Correlates in Mammals

One of the fundamental features of hearing is the ability to localize the sources of sounds, particularly brief sounds, which may warn of nearby animals. Yet not all mammals localize sound equally well with threshold acuity ranging from about 1° for elephants and humans to more than 25° for gerbils and horses and a near absence of localization in some subterranean species. During the past decade evidence has accumulated that this variation cannot be accounted for simply by the availability of the physical cues for locus. Nor does it appear to be a function of an animal's lifestyle. Rather sound-localization acuity in mammals appears to be a function of the precision required of the visual orienting response to sound. Thus the neural integration of hearing and vision in cortex, as well as in multimodal subcortical structures, is a reflection of their behavioral integration and evolutionary coupling.

Degenerate hearing and sound localization in naked mole rats (Heterocephalus glaber), with an overview of central auditory structures

Behavioral tests of absolute sensitivity and sound localization in African naked mole rats show that, despite their communal social structure and large vocal repertoire, their hearing has degenerated much like that of other subterranean species. First, their ability to detect sound is limited, with their maximum sensitivity being only 35 dB (occurring at 4 kHz). Second, their high-frequency hearing is severely limited, with their hearing range (at 60 dB sound pressure level [SPL]) extending from 65 Hz to only 12.8 kHz. Third, determination of the effect of duration on noise thresholds indicates that, compared with other animals, mole rats require a sound to be present for a much longer duration before reaching asymptotic threshold. Finally, they are unable consistently to localize sounds shorter than 400 ms and cannot accurately localize sounds of longer duration, raising the possibility that they are unable to use binaural locus cues. Thus, it seems that the essentially one-dimensional burrow system of a subterranean habitat produces severe changes in hearing comparable to the changes in vision that result from the absence of light. To explore the relation between vision and sound-localization acuity, retinal ganglion cell densities were determined. The results indicate that naked mole rats have a broad area of best (albeit poor) vision, with maximum acuity estimated at 44 cycles/degree. That mammals with wide fields of best vision have poorer sound-localization acuity than those with narrower fields is consistent with the thesis that a major function of sound localization is to direct the gaze to the source of a sound. However, the fact that subterranean mammals have little use for vision in a lightless environment suggests that they represent an extreme case in this relationship and may explain the fact that, unlike surface-dwelling mammals, they have virtually lost the ability to localize brief sounds. Finally, despite their very limited auditory abilities, the major brainstem auditory nuclei, although relatively small, appear to be present.

Hearing in large mammals: Sound-localization acuity in cattle (Bos taurus) and goats (Capra hircus).

Hearing in large mammals: Sound-localization acuity in cattle (Bos taurus) and goats (Capra hircus).

Coding of interaural time differences of transients in auditory cortex of Rattus norvegicus: Implications for the evolution of mammalian sound localization

We obtained quantitative evidence on the coding of interaural time differences (ITDs) of click stimuli by 40 single neurons in the auditory cortex of anesthetized albino rats. Most of the neurons ( 31 40 ) received an excitatory input from the contralateral ear, and an inhibitory input from the ipsilateral ear (EI cells). These neurons expressed their sensitivity to ITDs in a sigmoidal relation between spike count and ITD, with maximal responses associated with contralateral-leading ITDs. The mean ITD dynamic range was 590 μs. The dynamic ranges typically encompassed at least part of the behaviorally-relevant range (about ± 130 μs). Variations in ITD from 130 μs favoring one ear to 130 μs favoring the other ear caused spike response rate changes, on average, of 29.5%. These data are similar to those previously presented for the central auditory systems of larger mammals, whose auditory localization acuity is significantly better than that of the rat. We argue, therefore, that the sound localization mechanisms based on transient ITDs have not evolved in a fashion that covaries with interaural distance, and that there exists a mismatch between the ITDs the rat will encounter in the free field, and the ITDs which are encoded by its nervous system. This may be one reason why sound localization acuity has a roughly inverse relation to interaural distance.

"Sound localization acuity in very young infants: An observer-based testing procedure": Correction.

"Sound localization acuity in very young infants: An observer-based testing procedure": Correction.

"Sound localization acuity in very young infants: An observer-based testing procedure": Correction.

"Sound localization acuity in very young infants: An observer-based testing procedure": Correction.

Sound localization acuity in very young infants: An observer-based testing procedure.

Infants 8 to 28 weeks of age weretested to determine the smallest sound shift off midline and along the horizontal axis that could be reliably discriminated, that is, minimum audible angle. Both sound-shift and no-shift trials were presented. For each trial, an observer decided, based solely on the infant's behavior, whether the sound shifted location (i.e., an observer-based yes-no task). To isolate sensory from nonsensory factors, analyses were performed on d' scores

Sound localization acuity in very young infants: An observer-based testing procedure.

Sound localization acuity in very young infants: An observer-based testing procedure.