Interaural Time Difference Processing in Wistar Rats: Sensitivity and Temporal Weighting

Wistar大鼠對雙耳時間差的處理︰靈敏度及時間權重

Student thesis: Doctoral Thesis

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Award date2 Nov 2020

Abstract

Mammals use binaural cues, interaural time differences (ITDs) and interaural level differences (ILDs), to localize sound. Bilateral cochlear implants help people suffering from severe to profound hearing loss in both ears to restore binaural hearing to a certain extent, but not as good as normal hearing listeners. The restored binaural hearing benefits significantly from ILDs. But a lack of ITD sensitivity has been observed in bilateral cochlear implant users, and it hinders their ability to perceive speech in a noisy environment. A novel cochlear implant stimulation strategy may be needed to facilitate ITD sensitivity as well as speech perception. Before a novel strategy can be tested and applied on deaf children in clinics, tests must be done on animals. Hence, a suitable animal model is needed.

Rats are widely used in science, but not commonly seen in binaural hearing research. One concern about using laboratory rats in binaural hearing research was that they were suspected of not to be sensitive to ITDs. Previous studies conducted with rats trained to localize in the free field had concluded that rats are insensitive to the interaural phase of pure tones. However, we suspected that rats may nevertheless be sensitive to envelope ITDs.

Here, we designed and used a behavioural training setup to train rats in a near-field 2-alternative forced choice (2-AFC) sound localization task, presenting them with more natural “pulse-resonance” sounds to test their perception of ecologically more relevant stimuli. A wide range of pulse rates were tested: 50 Hz, 300 Hz, 900 Hz, 1800 Hz, 2400Hz, 4800 Hz. Those click trains were enveloped with rectangular window which provided onset ITD cue or Hanning window which only ongoing ITD was conserved to test the sensitivities to onset and ongoing ITD, respectively. The results supported our suspicions and revealed that rats are highly sensitive to both onset and ongoing envelope ITDs down to a microsecond level. The ITD sensitivity to rectangular windowed click trains was higher than to the Hanning ones of the same click rate. The sensitivity dropped as the pulse rate increased for both window types, and dramatically declined at 900 Hz for Hanning windowed, and at 1800 Hz for rectangular windowed click trains.

I also performed electrophysiological recordings from inferior colliculus of rats, and observed that the envelope type and click rate, but not training, are the factors that govern the extent to which neuronal responses distinguish left ear leading from right ear leading ITDs. The trends in neural sensitivity as a function of pulse rate and inflection point were consistent with our behavioural results. Our findings confirmed that the rat is a highly suitable model for both psychoacoustic and physiological binaural hearing research.

In order to investigate the importance of onset, ongoing and offset parts of a stimulus carrying ITD information, behavioural “temporal weighting functions” were measured to quantify the weights of each click in an 8-click click train with different ITD values embedded in each click. Rats performed 2-AFC sound localization task while listening to the target stimuli in 20 Hz, 50 Hz, 300 Hz and 900 Hz click rates, respectively. A Probit regression was conducted to generate the coefficients which represent the weight of the corresponding click. The results demonstrated clear onset dominance, with the ITD of the first click in the train dominating the perceptual decisions. Rats weighted the first click increasingly highly as click rate increased. The neural decoding for electrocorticographic signal recorded at 300 Hz and 900 Hz from the auditory cortex also revealed profound precedence effect. Our results are strikingly similar to those previously reported for humans, further illustrating that rats are a highly suitable model for the study of mammalian ITD processing.

    Research areas

  • interaural time difference, ITD sensitivity, temporal weighting, precedence effect, onset dominance, rat, inferior colliculus, primary auditory cortex