Submitted on April 14, 2009
Revised on June 16, 2009
Accepted on June 19, 2009
GABA shapes selectivity for the rate and direction of frequency modulated sweeps in the auditory cortex
Khaleel A Razak1 and Zoltan M Fuzessery2*
1 University of California Riverside
2 University of Wyoming
* To whom correspondence should be addressed. E-mail: zmf{at}uwyo.edu.
In the pallid bat auditory cortex and inferior colliculus (IC), the majority of neurons tuned in the echolocation range is selective for the direction and rate of frequency modulated (FM) sweeps used in echolocation. Such selectivity is shaped mainly by spectrotemporal asymmetries in sideband inhibition. An early-arriving, low-frequency inhibition (LFI) shapes direction selectivity. A delayed, high-frequency inhibition (HFI) shapes rate selectivity for downward sweeps. Using iontophoretic blockade of GABAa receptors, we show that cortical FM sweep selectivity is at least partially shaped locally. GABAa receptor antagonists, bicuculline or gabazine, reduced or eliminated direction and rate selectivity in ~50% of neurons. Intracortical GABA shapes FM sweep selectivity by either creating the underlying sideband inhibition or by advancing the arrival time of inhibition relative to excitation. Given that FM sweep selectivity and asymmetries in sideband inhibition are already present in the IC, these data suggest a refinement or recreation of similar response properties at the cortical level.
