Abstract:

The coding of temporal information, as reflected in the response to amplitude-modulated signals, changes dramatically between the auditory midbrain and the auditory cortex. Most neurons in the inferior colliculus are tuned to specific AM rates between ten and several hundred Hertz. This tuning is expressed either in their capacity to phaselock their response to the AM rate or by their overall response magnitude. In the primary auditory cortex, this capacity is largely reduced to AM rates below 20 Hz. Other cortical fields show even lower AM limiting rates and usually only one field (the anterior field in the cat, and the posterior field in New World monkeys) shows a slightly higher following capacity. The consequences of this overall reduction in repetition coding capacity for the representation of temporal properties of complex signals, such as fundamental frequency or details of the temporal envelope, are profound. A second property of the temporal coding in the midbrain, its systematic distribution along the isofrequency-axis of the nucleus, is also diminished or absent at the cortical level. Potential origins of this transformation in temporal coding and consequences for the perception of temporal information will be discussed. [Work supported by NIH R01-02660.]