Discrimination of Direction in Fast Frequency-Modulated Tones by Rats

Abstract

Fast frequency modulations (FM) are an essential part of species-specific auditory signals in animals as well as in human speech. Major parameters characterizing non-periodic frequency modulations are the direction of frequency change in the FM sweep (upward/downward) and the sweep speed, i.e., the speed of frequency change. While it is well established that both parameters are represented in the mammalian central auditory pathway, their importance at the perceptual level in animals is unclear. We determined the ability of rats to discriminate between upward and downward modulated FM-tones as a function of sweep speed in a two-alternative-forced-choice-paradigm. Directional discrimination in logarithmic FM-sweeps was reduced with increasing sweep speed between 20 and 1,000 octaves/s following a psychometric function. Average threshold sweep speed for FM directional discrimination was 96 octaves/s. This upper limit of perceptual FM discrimination fits well the upper limit of preferred sweep speeds in auditory neurons and the upper limit of neuronal direction selectivity in the rat auditory cortex and midbrain, as it is found in the literature. Influences of additional stimulus parameters on FM discrimination were determined using an adaptive testing-procedure for efficient threshold estimation based on a maximum likelihood approach. Directional discrimination improved with extended FM sweep range between two and five octaves. Discrimination performance declined with increasing lower frequency boundary of FM sweeps, showing an especially strong deterioration when the boundary was raised from 2 to 4 kHz. This deterioration corresponds to a frequency-dependent decline in direction selectivity of FM-encoding neurons in the rat auditory cortex, as described in the literature. Taken together, by investigating directional discrimination of FM sweeps in the rat we found characteristics at the perceptual level that can be related to several aspects of FM encoding in the central auditory pathway.

Feature-Dependent Sensitive Periods in the Development of Complex Sound Representation

“…Auditory cortical neurons are selective for FM directions at 80 octaves per second, which corresponds to the highest sweep rate at which rats can discriminate sweep directions (Zhang et al, 2003; Gaese et al, 2006). To probe plasticity, we exposed rat pups to downward logarithmic FM sweeps (50-0.5 kHz, 60 dB SPL, - 80 octaves per second, 5 sweeps in a train at 5 Hz, with 500 ms of silence between trains) in one of four time windows (Window 1 through Window 4, see Methods).…”

Fear Conditioned Discrimination of Frequency Modulated Sweeps within Species-Specific Calls of Mustached Bats

“…Most behavioral studies of FMs use logarithmic FMs also referred to as “fast FMs” that cover a frequency range greater than 10% of the central frequency on a relatively short time scale [16] . One major rationale for the use of logarithmic FMs is that they more closely conform to cochleotopic organization and thus, unlike linear FMs, ensure equivalent acoustic stimulation across audible frequencies.…”

“…Other features, in particular FM direction, rate (or “slope”), bandwidth (or “depth”) and duration of an FM embedded within a call likely convey meaningful information per Morton's motivation-structure hypothesis [1] , [2] . Accordingly, Mongolian gerbils ( Meriones unguiculatus ) can discriminate between different directions of FM sweeps [16] and rats ( Rattus norvegicus ) can categorize FM sweeps based on either the direction or rate of modulation [17] .…”

Dependence of the Startle Response on Temporal and Spectral Characteristics of Acoustic Modulatory Influences in Rats and Gerbils

“…Additional applications use inhibition of the ASR as a test paradigm in psychoacoustics. This has simplified studies in that field, as long periods of training are not necessary using this approach (Hoffman and Searle, 1965 ; Ison and Bowen, 2000 ) other than in classical approaches based on conditioned responses (e.g., Gaese and Wagner, 2002 ; Gaese et al, 2006 ). Lately, PPI was even used in complex acoustic discrimination experiments (Fitch et al, 2008 ).…”