Temporal modulation transfer functions
Jos Eggerrmont · Oxford University Press eBooks · 2015
A temporal modulation transfer function (tMTF) reflects how well a person or a neuron can follow amplitude modulations of a sound. The neural responses to the first modulations of a noise burst decrease to a steady state value after about 5 modulations in the auditory nerve, but more abruptly in auditory cortex. These steady state values as a function of modulation frequency form the tMTF. Under a variety of conditions, and for most species that have been studied, the tMTF for continuous carriers resembles a low-pass filter function. This is taken as evidence that the auditory system integrates incoming information over time. There is a decrease of the maximum modulation frequency that can be represented more downstream in the auditory system, Auditory nerve fibers can easily follow modulations up to 2 kHz, in the auditory midbrain the highest frequencies are around 800 Hz, and in the awake auditory cortex about 200 Hz. Electrophysiological recordings of local field potentials from the surface of auditory cortex in humans also are limited to ~200 Hz. Behaviorally, in humans, depending on the carrier the upper limit for discriminating modulated noise from unmodulated noise is around 1 kHz. Discrimination may be more sensitive than the demonstration of phase-locking to the envelope as done in electrophysiology. A comparison suggests that average electrophysiological recordings underestimate behavioral results in humans by about a factor 3–4. Using average data may not be appropriate and if the lower envelope of the single unit data (in awake animals) are used the comparison may be more favorable.