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Many mammals, including humans, are exquisitely sensitive to tiny time differences between sounds at the two ears. These interaural time differences are an important source of information for sound detection, for sound localization in space, and for environmental awareness. Two brainstem circuits are involved in the initial temporal comparisons between the ears, centered on the medial and lateral superior olive. Cells in these nuclei, as well as their afferents, display a large number of striking physiological and anatomical specializations to enable submillisecond sensitivity. As such, they provide an important model system to study temporal processing in the central nervous system. We review the progress that has been made in characterizing these primary binaural circuits as well as the variety of mechanisms that have been proposed to underlie their function.
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Supplemental Video 1. Enhanced phase-locking in a monaural axon (Figure 6)
A binaural beat (two tones of slightly different frequency) is played to the two ears. The left ear is at 501 Hz, the right ear at 500 Hz. This causes a running phase difference at 1 Hz. The neuron is insensitive to this phase difference and fires spikes throughout the duration of the stimulus. Note that the spikes are stationary with respect to the tone in the left ear, and occur over a restricted phase range of the stimulus. Moreover, a spike is triggered at almost every stimulus cycle ("entrainment"), every 2 ms. The sound of the movie is that of the neural signal: because of the enhanced phase-locking, the sound has a pitch at 500 Hz.
Supplemental Video 2. Sensitivity to interaural phase in an MSO axon (Figure 9)
A binaural beat (two tones of slightly different frequency) is played to the two ears. The left ear is at 501 Hz, the right ear at 500 Hz. This causes a running phase difference at 1 Hz. The neuron is clearly sensitive to this phase difference: the spike rate fluctuates at a rate of 1 Hz. This neuron prefers a phase difference near 0 cycle (i.e. when the sinewaves are approximately aligned in phase across the two ears. Note that the spikes are phase-locked to the waveform of the stimuli and occur at about every 2 ms. The sound of the movie is that of the neural signal: it has a pitch at 500 Hz due to the phase-locking of the spikes.
Supplemental Audio 1. Low-frequency binaural beat (listen over headphones). Two tones of slightly different frequency are played to the two ears, causing a running phase difference that is perceived as a "spatial roughness." This requires that phase information is preserved in the neural coding of the sounds, which is the case for low frequencies of this demo: 500 and 508 Hz. [Note: computer sound reproduction is typically affected by many soft- and hardware components. It is essential for this demo to have clean separation between the electrical signals going to each earphone. This is easily checked by lifting one earphone and letting it rest e.g. on the temple of the head, while listening to the other earphone: the binaural effect, here the "roughness," should then disappear. If it doesn't, this is an indication of (electrical) interaction somewhere in the computer's reproduction of the signals to the two earphones.]
Supplemental Audio 2. High-frequency binaural beat (listen over headphones). The two tones are now 2000 and 2008 Hz. When listening over headphones the spatial roughness is no longer heard, as the phase is no longer preserved above 1500 Hz. Note: when placing both speakers of the headphone next to one ear, the interaction of the acoustic waveforms cause a regular (acoustical) beat, which is easily heard.
Supplemental Audio 3. Binaural unmasking at low frequencies, version 1 (listen over headphones). A noise is played. Listen carefully for faint pulsed tones mixed in with the noise. Toward the end, the noise is faded out and the tones become clearly audible. Two tones having slightly different pitch (two semitones apart) are alternated. Which of these two was more audible when the noise was still playing, the lower-pitched one or the higher-pitched one? They differ not only in their pitch, but also in their interaural phase.
In this instance the low-pitched tone is presented antiphasically, which improves its audibility for most listeners. The higher-pitched tone is less audible because it is presented in-phase, preventing the use of binaural information for its detection.
It is also instructive to listen to this demo monaurally by lifting one earphone and letting it rest e.g. on the temple of the head, while listening to the other earphone: the binaural effect, here the unmasking of the antiphasic tone, should then disappear. When listening monaurally over the other ear, the tones should still be inaudible. This shows that binaural detection is really "listening between the ears." That is, with both ears you can hear things by either ear alone.
Supplemental Audio 4. Binaural unmasking at low frequencies, version 2 (listen over headphones). The demo is identical to audio 3, except that the roles of the tones is reversed. Now it is the higher-pitched tone which is played anti-phasically, making it more audible against the noise for most listeners.