Abstract 975, Date 1:00 pm Tuesday, February 25, 2003 (24 hours)
Session Q10: Auditory Midbrain: Directional Hearing and Physiology in Bats
Sonar Signal Temporal Patterning Shapes Echo-Delay Tuning in the Bat Midbrain
Cynthia F. Moss, Shiva Ranjan Sinha
Echolocating bats transmit ultrasonic vocalizations and use information in returning echoes for spatial orientation and insect capture.   Analyses of signals produced by the FM bat, Eptesicus fuscus, during the approach phase of insect pursuit reveal plateaus at repetition rates of 50-60 Hz for time periods as long as 700 ms. Stable periods of sound repetition rate (sonar “strobe groups”) occur when the bat is selecting a target, changing the direction of its flight path and in the presence of obstacles (Moss & Surlykke, 2001, JASA, 110: 2207-2226).    We hypothesize that sonar “strobe groups” shape the spatial-temporal response profiles of neurons in the bat auditory system.   Our current experiments directly test this hypothesis.
      Here, we report on single unit extracellular recording experiments in the bat midbrain superior colliculus (SC), a brain region where we have previously identified a class of combination-sensitive auditory neurons that exhibits spatial selectivity in azimuth, elevation, and range (Valentine & Moss, 1997, J. Neurosci., 17: 1720-1733).   Range tuning is quantified using simulated pulse-echo pairs with variable delays, and the spatial response profiles of these neurons are broad when measured with slow presentation rates (5 Hz) of stimulus pairs.   In this study, we measured echo-delay tuning across a range of signal repetition rates that approximate those used by the bat during the search and approach phase of insect pursuit (5-50 Hz).    Echo-delay tuning curves sharpened when sound stimuli simulated the sonar “strobe groups” observed during insect capture.   In addition, responses at a neuron’s “best echo delay” could be attenuated by stimulation with changing intervals between pulse-echo pairs. These data suggest that sonar signal temporal patterning directly impacts the spatial response profiles of SC neurons in the bat.  
      Supported by NIMH, NIDCD Institutional Training Grant & Whitehall Foundation.