The inferior colliculus (IC) receives input from a host of nuclei including the CN, the SOC, and the nuclei of the lateral lemniscus (NLL). The details of the physiology are still uncertain, but evidence is mounting that representations of both temporal and spectral features are present. In addition to the work on the IC proposed here, Carney is currently recording responses from single cells from the IC in awake behaving rabbits. The stimuli used in these experiments are some of the same stimuli to be used in Thrust area III for the studies of human performance in multisource environments.
Duration Encoding: The duration of sound is a biologically significant feature and the encoding of sound duration is thought to be carried out by neurons in the inferior colliculus (IC). Experiments demonstrated that there is a population of IC neurons that are tuned to signal duration. The tuning is produced by coincidence between stimulus delayed-onset and offset responses, and mediated by inhibitory and excitatory interactions among a diverse population of cell types [. Singh Mountain 1996.]. Our plan is to develop further these duration-encoding models to account for recent data, refine model parameters, and advance it to a level allowing straightforward hardware implementation.
Envelope Encoding: Even short-duration sounds generally have complex envelopes. Highlights (local maxima) in the envelope are often periodic, or quasiperiodic which has led to extensive research in many labs on periodicity encoding. It has been demonstrated that there are cells in the IC which respond best to a specific envelop periodicity and that there may be an anatomical axis corresponding to the range of best envelope frequencies. A model of these cells which relies on the enhancement of the highlights by onset cells in the CN has already been developed using essentially the same elements as those producing the delay-tuning cells above [.Deligeorges Mountain 1996.]. We plan to elaborate the envelope-encoding model to take into account new physiological data recorded in Shamma's laboratory as described in section (D) below. We also propose to carry out a test of a full network implementation of the model, followed by hardware implementation.
Echo Suppression: There is evidence that echo suppression present at this level, a process that is crucial for functioning in a reverberant environment. Recordings in the IC have demonstrated a form of echo suppression that is similar to the perceptual phenomenon called the precedence effect [.Yin Litovsky 1993.]. These data have led Colburn's group to develop a computational model involving an interplay of inhibition and excitation that reproduces this effect [.Cai Carney Colburn 1996.]. As part of this proposal this echo-suppression model will be refined and integrated with other biophysical models that are under development. The modeling effort will be enhanced by the fact that Litovsky and Delgutte are collaborating on an NIH-funded project to continue the physiological studies of echo suppression in the IC, which uses the same stimuli as those to be used in the human studies for the Thrust area III experiments on human performance in reverberant environments in this proposal.