Abstract
Humans, and many other species, exploit small differences in the timing of sounds at the two ears (interaural time difference, ITD) to locate their source and to enhance their detection in background noise. Despite their importance in everyday listening tasks, however, the neural representation of ITDs in human listeners remains poorly understood, and few studies have assessed ITD sensitivity to a similar resolution to that reported perceptually. Here, we report an objective measure of ITD sensitivity in electroencephalography (EEG) signals to abrupt modulations in the interaural phase of amplitude-modulated low-frequency tones. Specifically, we measured following responses to amplitude-modulated sinusoidal signals (520-Hz carrier) in which the stimulus phase at each ear was manipulated to produce discrete interaural phase modulations at minima in the modulation cycle—interaural phase modulation following responses (IPM-FRs). The depth of the interaural phase modulation (IPM) was defined by the sign and the magnitude of the interaural phase difference (IPD) transition which was symmetric around zero. Seven IPM depths were assessed over the range of ±22 ° to ±157 °, corresponding to ITDs largely within the range experienced by human listeners under natural listening conditions (120 to 841 μs). The magnitude of the IPM-FR was maximal for IPM depths in the range of ±67.6 ° to ±112.6 ° and correlated well with performance in a behavioural experiment in which listeners were required to discriminate sounds containing IPMs from those with only static IPDs. The IPM-FR provides a sensitive measure of binaural processing in the human brain and has a potential to assess temporal binaural processing.
Highlights
Binaural hearing confers considerable advantages in everyday listening environments
We recently demonstrated that periodic modulations in the interaural phase difference (IPD) of a low-frequency, amplitude modulated (AM) tone where most IPDs were restricted to the physiological range can evoke a steady-state response in human listeners (Haywood et al 2015; McAlpine et al 2016)
In contrast to the dichotic condition, in which phase transitions in the signals of each ear were of identical magnitude, but opposite sign, no interaural phase modulation following responses (IPM-following response (FR)) was observed for the diotic control condition, in which the IPD remained zero because the phase transitions were of the same magnitude, but of identical sign in both ears (Fig. 2C, D)
Summary
Binaural hearing confers considerable advantages in everyday listening environments. Comparing the timing and intensity of a sound at each ear allows listeners to locate a sound source on the horizontal plane and to hear out signals in background noise—an important component of ‘cocktail party listening’ (Bronkhorst 2000; Hawley et al 2004). ITDs contribute to ‘spatial release from masking’—sounds are more heard, and speech is more intelligible, when talker and interferer originate from different locations (Licklider 1948); detection thresholds may improve by up to 15 dB for UNDURRAGA ET AL.: Neural Representation of Interaural Time Differences in Humans two binaural signals with opposing interaural phase differences (IPDs). Sensitivity to ITDs generally decreases with age (Babkoff et al 2002) and is typically impaired with hearing loss (Moore et al 1991), impacting negatively on performance and increasing required listening effort in complex acoustic environments. To this end, measures of binaural function have obvious clinical relevance for the hearing impaired. Given the importance of ITDs to auditory perception, and their obvious clinical relevance, there is considerable benefit to be gained from developing objective measures of ITD sensitivity
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