The neural basis of active echo imaging

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Echolocating bats can localize and identify three dimensional objects through the auditory analysis of the echoes of their sonar emissions. The spectro-temporal structure of an echo depends not only on the acoustical properties of the reflecting object, but also on the properties of the emission. Echo imaging requires the detailed comparison of the emission and echo. Previous physiological studies investigated the neural basis of echo imaging only in anaesthetized, passively listening animals. This contrasts to the psychophysical work where behavioral responses were obtained in vocalizing and actively listening animals.This proposal aims to combine these approaches to investigate neural processing in active echo imaging. Two different approaches will be used: in one set of experiments echolocation calls will be evoked by electrical microstimulation in an area of the descending vocalization system, the paralemniscal area, in awake bats. Using the techniques from behavioral phantom-target work, the acoustic images of 3D objects are imprinted on the calls in real-time and played back as synthetic echoes. This technique allows to systematically manipulating object-related acoustic information. The responses from neurons in the auditory cortex to both the call and the synthetic echo will be recorded. By comparing the response of a neuron with and without previous vocalization, the effect of the bat’s vocalization on neural processing can be revealed. In a second approach the responses from neurons in auditory cortex will be recorded with chronically implanted electrodes in freely moving bats. The bats will be trained in a psychophysical discrimination task and the neural responses will be transmitted with a small telemetric system while the bats are performing the task. With this approach insight in the neural activity underlying the bats performance will be gained.