"Binaural hearing" broadly defines those aspects of audition which rely on the interaction between the two ears. Many important components of normal auditory perception are driven by binaural neural processing. Small differences in the timing and intensity of sounds at the two ears are the major physical cues that brainstem binaural circuits exploit to improve perceptual signal-to-noise ratio in complex listening environments and to localize sound-sources.
Inter-aural Time Differences
Timing differences between the two ears are a strong cue to sound-source location in the horizontal plane. Moreover, these timing differences are important for coding inter-aural correlation which forms the basis of a noise-cancellation strategy the brain exploits for listening in noisy environments. We study the neural mechanisms of sound-source localization using electrophysiological techniques to characterize the response properties of binaural neurons in the superior olivary complex, part of the ventral brainstem. In particular, we are interested in the coding of inter-aural time differences by the population of neurons in the medial superior olive (MSO). These cells receive excitatory synaptic inputs from both cochlear nuclei. Each MSO neuron is "tuned" to a preferred ITD, known as its best delay (BD).
Video 1: Inter-aural time delays as a cue to sound-source location
This short movie illustrates how, for lateralized sound sources, there is a difference in arrival time of sound at the two ears. Sounds arriving at the ear farthest from (i.e., contra-lateral to) the sound source are delayed relative to the nearest (ipsi-lateral) ear. It is important to realize that since the auditory system codes temporal features of on-going sounds, these timing differences are not simply at sound onset, but on-going inter-aural time (phase) differences exist for the entire duration of a sound.
Video 2: The axonal delay-line hypothesis
Binaurally sensitive neurons in the superior olivary complex each have a best delay. To create non-zero BDs requires a source of internal delay, such that an external acoustic inter-aural delay is perfectly balanced by an equal (but opposite) internal delay. The classic hypothesis for how this might be achieved is known as the axonal delay-line hypothesis. This short movie illustrates how a series of axonal delay lines could achieve neural tuning with a range of different BDs. A simple interpretation is that the most active neuron in the population at any instant would signal the sound-source position in the horizontal plane.
Video 3: ITD sensitivity in the medial superior olive
This short movie shows action potentials recorded from the axon of a single MSO neuron. The neuron is responding to a binaural sound known as a "binaural beat." This consists of two pure tones, one presented to each ear, with a small frequency difference between them. In this case, the frequency difference is 1 Hz. Therefore the inter-aural phase changes periodically, once per second. This simulates the basic aspects of a moving sound source. The changing inter-aural phase corresponds to motion of a sound source in the horizontal plane from left to right, repetitively. The sound you hear in this movie is the sound of the action potentials from the neuron. Notice that there are bursts of action potentials occurring once per second. This indicates the neuron has a preferred inter-aural phase, such that when the binaural sound has that phase relationship the neuron receives coincident input from both ears and fires a burst of action potentials. At all other phases of the binaural beat, the neuron fires only a few random spontaneous action potentials. That is, instantaneous firing probability is a function of inter-aural phase.
(Some of) our (broad) questions in binaural hearing
- How are internal delays generated? (Axonal delay lines are not the only possibility)
- What is the role of neural inhibition in coding of ITDs? (MSO neurons also receive inhibitory synaptic input)
- What does the population code in the MSO tell us about binaural signal processing? (We use electrophysiology and computational modelling approaches to address this question)
- How do complex binaural phenomena (such as dichotic pitch) emerge from neural processing in binaural circuits?
- How does cochlear damage impact neural processing in binaural circuits? (We try to understand the relationship between neural processing deficits and binaural perceptual deficits in hearing-impaired humans)
- Joris PX, Yin TC (2007). A matter of time: internal delays in binaural processing. Trends Neurosci. 2007 Feb;30(2):70-8.
- Benichoux V, Fontaine B, Franken TP, Karino S, Joris PX, Brette R (2015). Neural tuning matches frequency-dependent time differences between the ears. Elife. 2015 Apr 27;4.