Submitted on April 16, 2007
Accepted on July 14, 2007
Response Properties and Location of Neurons Selective for Sinusoidal Frequency Modulations in the Inferior Colliculus of the Big Brown Bat
QI YUE1, John H Casseday2, and Ellen Covey2*
1 SEATTLE, Washington, United States; Department of Psychology, University of Washington, Seattle, Washington, United States
2 Department of Psychology, University of Washington, Seattle, Washington, United States
* To whom correspondence should be addressed. E-mail: ecovey{at}u.washington.edu.
Most animal vocalizations, including echolocation signals used by bats, contain frequency-modulated components. Previous studies have described a class of neurons in the inferior colliculus (IC) of the big brown bat that respond exclusively to SFM signals and fail to respond to pure tones, noise, amplitude modulated tones or single FM sweeps. The aims of this study were to further characterize these neurons' response properties, and determine whether they are localized within a specific area of the IC. We recorded extracellularly from 214 neurons throughout the IC. Of these, 47 (22%) responded exclusively to SFM. SFM-selective cells were tuned to relatively low carrier frequencies (9 to 50 kHz), low modulation rates (20 to 210 Hz) and shallow modulation depths (3 to 10 kHz). Most had extremely low thresholds, with an average of 16.5±7.6 dB SPL, and 89% had upper thresholds and closed response areas. For SFM-selective cells with spontaneous activity, the spontaneous activity was eliminated when sound amplitude exceeded their upper threshold and resumed after the stimulus was over. These findings suggest that SFM-selective cells receive low threshold excitatory inputs and high threshold inhibitory inputs. SFM-selective cells were clustered in the rostro-dorsal part of the IC. Within this area, best modulation rate appeared to be correlated with best carrier frequency and depth within the IC.
