Development of auditory localization in dogs: single source and precedence effect sounds

Development of Sound Localization in Infants and Young Children with Cochlear Implants

Cochlear implantation as a treatment for severe-to-profound hearing loss allows children to develop hearing, speech, and language in many cases. However, cochlear implants are generally provided beyond the infant period and outcomes are assessed after years of implant use, making comparison with normal development difficult. The aim was to study whether the rate of improvement of horizontal localization accuracy in children with bilateral implants is similar to children with normal hearing. A convenience sample of 20 children with a median age at simultaneous bilateral implantation = 0.58 years (0.42−2.3 years) participated in this cohort study. Longitudinal follow-up of sound localization accuracy for an average of ≈1 year generated 42 observations at a mean age = 1.5 years (0.58−3.6 years). The rate of development was compared to historical control groups including children with normal hearing and with relatively late bilateral implantation (≈4 years of age). There was a significant main effect of time with bilateral implants on localization accuracy (slope = 0.21/year, R2 = 0.25, F = 13.6, p < 0.001, n = 42). No differences between slopes (F = 0.30, p = 0.58) or correlation coefficients (Cohen’s q = 0.28, p = 0.45) existed when comparing children with implants and normal hearing (slope = 0.16/year since birth, p = 0.015, n = 12). The rate of development was identical to children implanted late. Results suggest that early bilateral implantation in children with severe-to-profound hearing loss allows development of sound localization at a similar age to children with normal hearing. Similar rates in children with early and late implantation and normal hearing suggest an intrinsic mechanism for the development of horizontal sound localization abilities.

The Precedence Effect in Three Species of Birds (Melopsittacus undulatus, Serinus canaria, and Taeniopygia guttata)

The perceived locations of paired auditory images, simulating direct sounds and their echoes, have been recently studied in budgerigars (Melopsittacus undulatus; M. L. Dent & R. J. Dooling, 2003a, 2003b). In this article, the authors extend those experiments to include measurements of the precedence effect using a discrimination paradigm in two additional bird species: canaries (Serinus canaria) and zebra finches (Taeniopygia guttata). Although time courses of summing localization, localization dominance, and echo thresholds were similar across all species, budgerigars had slightly higher overall levels of discrimination. The results from these experiments add further support that the precedence effect in birds is similar to that found in other animals and that the ability to suppress echoes that might degrade localization and auditory object perception may be a general property of the vertebrate auditory system.

Instructed learning in the auditory localization pathway of the barn owl

A bird sings and you turn to look at it a process so automatic it seems simple. But is it? Our ability to localize the source of a sound relies on complex neural computations that translate auditory localization cues into representations of space. In barn owls, the visual system is important in teaching the auditory system how to translate cues. This example of instructed plasticity is highly quantifiable and demonstrates mechanisms and principles of learning that may be used widely throughout the central nervous system.

Echo suppression in the human cortex is affected by the spatial and temporal proximity of the primary sound and echo

Echo suppression in the human auditory cortex was studied with auditory middle latency evoked potentials (AMEP) using virtual reality acoustic stimuli, including distance and elevation cues, presented by earphones. The purpose of the study was to assess the effect of proximity of the source sound and echo on the degree of echo suppression. Sixteen subjects were presented with source-echo pairs in which the preceding source sound was always at the vertex, and the echo varied among ten positions on the coronal plane. Positions varied in elevation, distance and time lag between source and echo. The psychoacoustic location judgment of the fused source-echo pair was closer to the source sound (more echo suppression) the nearer the echo drew to the source in its elevation and time. The equivalent dipole magnitudes of the cortical components of AMEP were significantly reduced (more suppression) with shorter echo lags and when echo elevation was similar to that of the source sound. The distances used in this study did not significantly affect echo suppression. These results indicate that echo suppression in the auditory cortex is more pronounced the closer are the primary sound and echo in locational attributes and timing. As source sound and echo draw apart, echo suppression in the cortex decreases and the perceived localization of the fused source-echo is more biased toward the echo.

The precedence effect

In a reverberant environment, sounds reach the ears through several paths. Although the direct sound is followed by multiple reflections, which would be audible in isolation, the first-arriving wavefront dominates many aspects of perception. The "precedence effect" refers to a group of phenomena that are thought to be involved in resolving competition for perception and localization between a direct sound and a reflection. This article is divided into five major sections. First, it begins with a review of recent work on psychoacoustics, which divides the phenomena into measurements of fusion, localization dominance, and discrimination suppression. Second, buildup of precedence and breakdown of precedence are discussed. Third measurements in several animal species, developmental changes in humans, and animal studies are described. Fourth, recent physiological measurements that might be helpful in providing a fuller understanding of precedence effects are reviewed. Fifth, a number of psychophysical models are described which illustrate fundamentally different approaches and have distinct advantages and disadvantages. The purpose of this review is to provide a framework within which to describe the effects of precedence and to help in the integration of data from both psychophysical and physiological experiments. It is probably only through the combined efforts of these fields that a full theory of precedence will evolve and useful models will be developed.

Stimulus duration and repetition rate influence newborns' head orientation toward sound

Three experiments evaluated the effects of stimulus duration and repetition rate on newborns' head orientation responses. In Experiment 1, 28 infants turned toward a 20-sec continuous rattle sound but not toward 14- and 500-msec rattle sounds. Signal energy as a possible explanation for the infants' difficulty orienting toward brief sounds was explored in Experiment 2. Twenty neonates did not turn toward a single 90 dB, 14-msec rattle sound, although a longer duration (10 sec) sound containing less energy (70 dB) did elicit reliable head orientation. In Experiment 3, 16 neonates heard trains of repeated 14-msec rattle sounds (2/sec, 1.3/sec, and 1/sec) lasting 10 sec as well as a 10-sec continuous rattle sound. They turned toward the most rapidly repeating brief sound and the continuous one, while the slowly repeating sounds elicited little head movement in any direction. These results suggest that newborns' head orientation is selectively deficient for brief sounds, that the difficulty does not result from lessened energy in the brief sounds, and that the efficacy of repeated brief sounds depends upon their repetition rates.