Nobuo Suga - US grants

For echolocation, the mustached bat Pteronotus parnellii emits complex orientation sounds and listens to echoes. We have demonstrated that certain types of biosonar information are extracted by neurons examining different combinations of signal elements and are systematically represented in separate areas of the auditory cortex. We now want to explore how other types of biosonar information are represented in other areas, how biosonar information flows through these areas, whether the brain has areas for integration of the different types of biosonar information to recognize an overall target image and how high is the upper limit in specialization (complexity of response properties) of single neurons. To explore these important problems in neural mechanisms for processing complex acoustic signals, we will combine electrophysiology with "tracer anatomy". We will examine response properties of single neurons in the individual areas and inject radioactive amino acids or horseradish peroxidase or fluorescent dyes into the recording sites to identify the destination or origin of auditory information through these areas. We will then insert microelectrodes into "target areas" to study response properties of single neurons and functional organization (spatial distribution of response properties of single neurons) of the target areas. Tracers are injected into the target areas for further exploration of the information flow. The auditory system of the mustached bat is specialized for processing biosonar information for echolocation (communication with environment). The left cerebral hemisphere of a man contains Wernicke's area which is specialized for processing speech. A thorough understanding of the specialization in speech processing will not be obtained without direct physiological studies on the Wernicke's area. However, the insight to it will be obtained from research on animals specialized for processing complex acoustic signals, as recently demonstrated. The acoustic signals used by the mustached bat are high in frequency but share basic acoustic patterns with those used by many other species of mammals, including man. Our proposed research is to explore neural mechanisms for processing complex acoustic signals in the bat, but it will significantly contribute to an understanding of the basic mechanisms for processing speech sounds.

Our proposed research is to explore the neural mechanisms of complex-sound processing for specialized (biosonar) and common (communication) auditory functions. There are now a number of data indicating that the recognition of speech sounds in humans is based upon auditory perceptual mechanisms common to mammals. Our proposed research is very important for understanding auditory mechanisms in higher vertebrates, including humans, as well as in bats. Our ultimate goal is the complete understanding of both species-specific and common neural mechanisms for the processing of complex sounds. The mustached bat emits complex sounds for biosonar and communication. Its communication calls are quite different film its biosonar sounds, but are similar in spectral patterns to calls of other mammals, except for being high in frequency and relatively short in duration. We have demonstrated: (i) the processing of different types of biosonar information is parallel- hierarchical, (ii) the central auditory system creates neurons tuned to different types of information-bearing parameters (IBP's) characterizing biosonar signals, (iii) certain types of IBP's are extracted by neurons sensitive to different combinations of signal elements, (iv) different types of IBP's are systematically represented in separate cortical areas, (v) certain response properties of combination-sensitive neurons in the cortex are created by prethalamic auditory nuclei and art shaped by both thalamo-cortical ascending and cortico-thalamic descending systems, and (vi) the call-processing system overlaps with the biosonar-signal- processing system even in the auditory cortex. These findings are very important for understanding neural mechanisms for processing complex sounds in general. Our project I is to study further whether different cortical areas have neurons tuned to different types of IBP's found in the biosonar and communication sounds, whether these cortical areas are organized for a systematic representation of a particular IBP, and how tuning to a particular IBP is created. We will deliver acoustic stimuli mimicking natural sounds and information bearing elements (IBE's) in them and will study responses of single neurons in different cortical areas. In Project II, we will focus on the response properties of collicular, thalamic or cortical neurons which were well characterized with acoustic stimuli mimicking biosonar signals (pulse-echo pairs), and we will study how the response properties of thalamic or collicular neurons are influenced by electrical micro-stimulations of or micro-drug (local anesthetic or inhibitory-synaptic-transmitter agonist) injections into isotopic or anisotopic portions of cortical areas. Our proposed research will further contribute to the understanding of neural mechanisms for processing complex sounds, including speech sounds.

DESCRIPTION (provided by applicant): Our ultimate goal is the complete understanding of neural mechanisms for both species-specific (bisonar) and common (communication) auditory functions. Unlike the auditory periphery, the central auditory system contains many different types of neurons. Some neurons are tuned to particular acoustic parameters characterizing sounds other than frequency or to specific parameters characterizing combinations of signal elements in complex sounds. Different types of neurons are clustered in different cortical areas and form maps for systematic representations of acoustic parameters. It has been explained that all these neurons and maps resulted from the neural interactions within the ascending auditory system. However, we had found that, according to auditory experience, the descending (corticofugal) system changes response properties of subcortical neurons and subcortical maps, accordingly those of cortical neurons and cortical maps. Our aim of the proposed research is to explore the functions of the corticofugal system and to test our hypotheses (1) that the auditory cortex, through the corticofugal system, adjusts and improves auditory signal processing in the frequency, amplitude and time domain according to auditory experience, (2) that this mechanism is altered by the modulatory systems (cholinergic, serotonergic, dopaminergic and noradrenergic systems) if the acoustic become behaviorally relevant to the animal, and (3) that in addition to corticofugal modulation, colliculofugal modulation, improves and adjusts subcolliclar auditory signal processing. Our proposed research will contribute further toward our understanding of the neural mechanisms for processing behaviorally relevant sounds and the functional organization of the central auditory system. We will be able to propose possible neural mechanisms for the reorganization of the auditory system caused by hearing disorders. We will study the response properties of cortical and subcortical auditory neurons, and then study how their response properties are changed by focal electric stimulation of the auditory cortex, conditioning, sensitization and focal applications of different types of drugs to cortical and subcortical auditory neurons. All the data obtained will be evaluated in relation to our hypotheses.