Human sound localization at near-threshold levels.
ABSTRACT Physiological studies of spatial hearing show that the spatial receptive fields of cortical neurons typically are narrow at near-threshold levels, broadening at moderate levels. The apparent loss of neuronal spatial selectivity at increasing sound levels conflicts with the accurate performance of human subjects localizing at moderate sound levels. In the present study, human sound localization was evaluated across a wide range of sensation levels, extending down to the detection threshold. Listeners reported whether they heard each target sound and, if the target was audible, turned their heads to face the apparent source direction. Head orientation was tracked electromagnetically. At near-threshold levels, the lateral (left/right) components of responses were highly variable and slightly biased towards the midline, and front vertical components consistently exhibited a strong bias towards the horizontal plane. Stimulus levels were specified relative to the detection threshold for a front-positioned source, so low-level rear targets often were inaudible. As the sound level increased, first lateral and then vertical localization neared asymptotic levels. The improvement of localization over a range of increasing levels, in which neural spatial receptive fields presumably are broadening, indicates that sound localization does not depend on narrow spatial receptive fields of cortical neurons.
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ABSTRACT: Auditory spatial acuity was measured in mice using prepulse inhibition (PPI) of the acoustic startle reflex as the indicator response for stimulus detection. The prepulse was a "speaker swap" (SSwap), shifting a noise between two speakers located along the azimuth. Their angular separation, and the spectral composition and sound level of the noise were varied, as was the interstimulus interval (ISI) between SSwap and acoustic startle reflex elicitation. In Experiment 1 a 180 degrees SSwap of wide band noise (WBN) was compared with WBN Onset and Offset. SSwap and WBN Onset had near equal effects, but less than Offset. In Experiment 2 WBN SSwap was measured with speaker separations of 15, 22.5, 45, and 90 degrees . Asymptotic level and the growth rate of PPI increased with increased separation from 15 to 90 degrees , but even the 15 degrees SSwap provided significant PPI for the mean performance of the group. SSwap in Experiment 3 used octave band noise (2-4, 4-8, 8-16, or 16-32 kHz) and separations of 7.5 to 180 degrees . SSwap was most effective for the highest frequencies, with no significant PPI for SSwap below 8-16 kHz, or for separations of 7.5 degrees . In Experiment 4 SSwap had WBN sound levels from 40 to 78 dB SPL, and separations of 22.5, 45, 90, and 180 degrees : PPI increased with level, this effect varying with ISI and angular separation. These experiments extend the prior findings on sound localization in mice, and the dependence of PPI on ISI adds a reaction time-like dimension to this behavioral analysis.Behavioral Neuroscience 04/2010; 124(2):265-77. · 2.63 Impact Factor
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ABSTRACT: The auditory cortex is known to be a necessary neural structure for the perception of acoustic signals, particularly the spatial location and the temporal features of complex auditory stimuli. Previous studies have indicated that there is no topographic map of acoustic space in the auditory cortex and it has been proposed that spatial locations are represented by some sort of population code. Additionally, in spite of temporal processing deficits being one of the hallmark consequences of normal aging, the temporal coding of acoustic stimuli remains poorly understood. This report will address these two issues by discussing the results from several studies describing responses of single auditory cortical neurons in the non-human primate. First, we will review studies that have addressed potential spike-rate population codes of acoustic space in the caudal belt of auditory cortex. Second, we will present new data on the neuronal responses to gap stimuli in aged monkeys and compare them to published reports of gap detection thresholds. Together these studies indicate that the alert macaque monkey is an excellent model system to study both spatial and temporal processing in the auditory cortex at the single neuron level.Hearing research 01/2011; 271(1-2):115-22. · 2.18 Impact Factor
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ABSTRACT: The objective of the study was to examine the effects of stimulus duration on vowel perception in normal-hearing and hearing-impaired children. For this purpose, 80 semisynthetic vowel stimuli consisting of eight different Turkish vowels with ten different durations were presented to 14 normal-hearing and 15 hearing impaired children, and they were asked to identify the vowel they heard. Thirteen normal-hearing adults served as speaker subjects to get normative data on mean durations of the Turkish vowels. While there was no significant effect of duration on perception in normal-hearing children, perception errors for very short and very long vowels were observed in hearing-impaired children. The most frequent responses as a function of duration showed four different patterns: (1) three vowels were perceived correctly in all durations; (2) two were perceived correctly in middle and longer durations; (3) two were perceived correctly in middle duration; and (4) only one was perceived correctly in short duration. It was concluded that the effects of stimulus duration on vowel perception were determined by natural duration of the vowel in a given language, and unnaturally short and long vowels were misperceived by hearing impaired subjects.International Journal of Audiology 12/2006; 45(11):675-80. · 1.63 Impact Factor