In auditory cortex, several functional maps have been reported recently in addition to the well-established tonotopic organization. However it is still far from clear how the brain processes complex sounds, because only a few groups are working on the auditory cortex and also because pure tones were the only stimuli used in most studies. Moreover, since most studies have concentrated on the primary auditory cortex (AI), the functional specialization of different auditory cortical areas remains completely obscure. In the past ten years, optical imaging of intrinsic signals has been successfully applied to visualize several functionally specialized maps in Imaging techniques is still in its early stage. This project proposes to combine the strength of optical imaging of intrinsic signals has been successfully applied to visualize several functionally specialized maps in imaging techniques is still in its early stage. This project proposes to combine the strength of optical imaging of intrinsic signals with auditory field (AAF), and the posterior (PAF) auditory fields, in response to complex sounds. We have developed intermediately complex sounds with defined physical dimensions, such as band-passed noise (BPN) bursts and frequency-modulated (FM) sounds. Noise pulses and FM components are ubiquitous in animal vocalizations and human speech. FM sweeps are time- varying stimuli and also resemble moving bars in the visual system as they both excite the sensory epithelium in a consecutive fashion. We will test the hypothesis that the auditory cortex is organized into functional domains, in which a number of independent functions are superimposed, e.g., bandwidth, FM rate and direction, etc.
We aim to visualize the functional modules processing direction preference of FM sweeps since electrophysiological recordings have shown that the direction preference of auditory cortical neurons in response to FM rate changes along thee isofrequency domain in a non-random fashion. If complex sound processing units (modules) are organized in local domain, we will be able to see segregated patches along the isofrequency domain when different FM directions are applied. Optical imaging is a promising tool to visualize such representations as evidenced by the orientation preference map in the visual cortex. However, since optical imaging can only reveal averaged global activity, single-unit recording with guidance of optical maps will be used to examine local functional organization. Using the same set of stimuli, penetrations will be made into the active patches revealed by optical imaging, and results of single-unit recording will be compared with that of optical imaging to verify the findings in optical imaging, using surface blood vessel patterns to align the maps. Through proposed experiments, we anticipate to gain deeper insight into how the rain processes complex sounds.
| Tian, B; Reser, D; Durham, A et al. (2001) Functional specialization in rhesus monkey auditory cortex. Science 292:290-3 |
