Although such adaptation is thought to improve coding of relevant stimulus features, the relationship between adaptation at the neural and behavioral levels remains to be established. Here we describe improved discrimination performance for an auditory spatial cue (interaural time differences, ITDs) following adaptation to stimulus statistics. Physiological recordings in the midbrain of anesthetized guinea pigs and measurement of discrimination performance in humans both demonstrate improved coding of the most prevalent ITDs in a distribution, but with highest accuracy maintained for ITDs corresponding to frontal locations, suggesting the existence of a fovea for auditory space. A biologically plausible model accounting for the physiological data suggests that neural tuning is stabilized by inhibition to maintain high discriminability for frontal locations. The data support the notion that adaptive coding in the midbrain is a key element of behaviorally efficient sound localization in dynamic acoustic environments.adaptation; interaural time difference; midbrain; neural model; psychophysics MOST SPECIES EVOLVE in complex environments containing diverse sources of sensory information over a very wide range of intensities, frequencies, and locations. Sensory systems must efficiently encode over this wide range of possible stimulus values, given a limited availability of coding resources. One means by which neural systems can overcome this challenge is to adapt on a behavioral timescale in order to represent with particular efficiency the subset of natural stimuli in the current environment. Such adaptive coding is observed across a wide range of species and stimulus modalities and is apparent in the responses of single neurons (Dean et al. 2005;Fairhall et al. 2001;Ohzawa et al. 1982) and across populations of neurons (Dean et al. 2005(Dean et al. , 2008Watkins and Barbour 2008). Fisher information (FI) represents one possible measure of coding quality that can be used to evaluate adaptive coding (Dean et al. 2005).Nevertheless, despite the apparent advantage adaptive coding would confer on sensory processing, the link between adaptive coding at the neural level and performance in sensory tasks is difficult to establish. Psychophysical assessment of adapted neural systems is often considered with respect to perceptual illusions such as afterimages (McCollough 1965) At the cognitive level, the term "cuing" is employed to describe the influence of prior stimulation on sensory performance, for example, in reducing reaction times required to localize a sound source in a given spatial hemifield (Spence and Driver 1994).However, none of these concepts-aftereffects, mislocalization, or cuing-is easily reconciled with the concept of adaptive coding at the neural level. This normally describes the rapid adjustment of neural tuning properties to better represent the prevailing stimulus environment (Garcia-Lazaro et al. 2007;Maravall et al. 2007; Nagel and Doupe 2006) rather than neural fatigue or higher, perhaps attentiona...

Minimum resolvable angles (MRAs) for sound localization in azimuth in the gerbil were determined in a behavioral study using tones, 300-Hz bands of noise centered at frequencies between 500 Hz and 8 kHz and broad-band noise of on average 60 dB SPL overall level. Using the method of constant stimuli, seven gerbils were trained in a two-alternative-forced-choice procedure to indicate if sounds were presented to them from the left or from the right by choosing the left or right arm of a Y-shaped cage. The MRA is the minimum angle between two loudspeaker locations that the gerbils discriminated. Animals were either stimulated from the front (N=4) or from the back (N=3). The MRA for broad-band noise randomly varying in level by +/- 6 dB was 23 degrees and 45 degrees for gerbils stimulated from the front or back, respectively. Generally a gerbil's MRA for tones declined up to 2 kHz reaching 20 degrees and 31 degrees for gerbils stimulated from the front or back, respectively, and the MRA was generally increased above this frequency. Results for narrow-band noise stimuli were similar. Results are discussed with respect to the available interaural cues and physiological mechanisms of sound localization in the gerbil.