The superior olivary complex sharpens phase locking to enable left-right sound localization

Evidence

This is a very small delay but a variation from this to zero determines the direction of sounds at low frequencies. Figure 2.25 shows how this delay varies as a function of angle, where positive delay corresponds to a source at the right of the median plane and negative delay corresponds to a source on the left. Note that there is no difference in the delay between front and back positions at the same angle. This means that we must use different mechanisms and strategies to differentiate between front and back sounds. There is also a frequency limit to the way in which sound direc­ tion can be resolved by the ear in this way. This is due to the fact that the ear appears to use the phase shift in the wave caused by the interaural time difference to resolve the direction; that is, the ear measures the phase shift given by: where the phase difference between the ears in radia ITD Φ  ( ns and the frequency in Hz When this phase shift is greater than  radians (180°) there will be an unre­ solvable ambiguity in the direction because there are two possible angles- one to the left and one to the right-that could cause such a phase shift. — David Howard, Jamie Angus, coustics and Psychoacoustics, 712 (#expands, via linked claim)

The low-frequency stimulus was a spectrally shaped multicomponent complex (SSMC), which was a harmonic complex with a fundamental frequency (F 0 ) of 100 Hz. The spectral envelope had a flat passband and sloping edges (5×F 0 centered at 1100 Hz).The overall level of the complex was 54 dB SPL. Threshold equalizing noise, extending from 125 to 15,000 Hz, was added. The high-frequency stimulus was a ˵transposed stimulus,˶ which was a 4-kHz tone carrier amplitude-modulated with a half-wave rectified 125-Hz sinusoid (Bernstein and Trahiotis 2002 ). It is considered that the auditory-nerve firing is phase locked to the modulator waveform, which provides the cue for judging the ITD and modulation rate of the stimulus. — Dijk, Pim van, Physiology, Psychoacoustics and…, 1031 (#corroborates, via linked claim)

Categorization

• XREF: connects to: Phase-based sound localization becomes ambiguous beyond 180 degrees phase shift; High-frequency transposed stimuli make auditory-nerve firing phase-lock to a low-frequency modulator — Dijk, Pim van, Physiology, Psychoacoustics and…, 1031; The ear resolves low-frequency direction by measuring phase shift — David Howard, Jamie Angus, coustics and Psychoacoustics, 712 (+2 more)

Broader Connections

• Phase-based sound localization becomes ambiguous beyond 180 degrees phase shift #qualifies (0.80)
• The ear resolves low-frequency direction by measuring phase shift #expands (0.80) — David Howard, Jamie Angus, coustics and Psychoacoustics, 712
• High-frequency transposed stimuli make auditory-nerve firing phase-lock to a low-frequency modulator #corroborates (0.75) — Dijk, Pim van, Physiology, Psychoacoustics and…, 1031
• Phase transitions hidden at envelope minima decrease monaural detection of interaural cues #complicates (0.75) — Dijk, Pim van, Physiology, Psychoacoustics and…, 2955
• Phase locking fails above 5 kHz, so high-pitch perception relies on place theory #qualifies (0.75)