Temporal Measures and Neural Strategies for Detection of Tones in Noise Based on Responses in Anteroventral Cochlear Nucleus

Abstract

To examine possible neural strategies for the detection of tones in broadband noise, single-neuron extracellular recordings were obtained from the anteroventral cochlear nucleus (AVCN) in anesthetized gerbils. Detection thresholds determined by average discharge rate and several temporal metrics were compared with previously reported psychophysical detection thresholds in cats ( Costalupes 1985 ). Because of their limited dynamic range, the average discharge rates of single neurons failed to predict psychophysical detection thresholds for relatively high-level noise at all measured characteristic frequencies (CFs). However, temporal responses changed significantly when a tone was added to a noise, even for neurons with flat masked rate-level functions. Three specific temporal analyses were applied to neural responses to tones in noise. First, temporal reliability, a measure of discharge time consistency across stimulus repetitions, decreased with increasing tone level for most AVCN neurons at all measured CFs. Second, synchronization to the tone frequency, a measure of phase-locking to the tone, increased with tone level for low-CF neurons. Third, rapid fluctuations in the poststimulus time histograms (PSTHs) decreased with tone level for a number of neurons at all CFs. For each of the three temporal measures, some neurons had detection thresholds at or below psychophysical thresholds. A physiological model of a higher-stage auditory neuron that received simple excitatory and inhibitory inputs from AVCN neurons was able to extract the PSTH fluctuation information in a form of decreased rate with tone level.

Statistical Analyses of Temporal Information in Auditory Brainstem Responses to Tones in Noise: Correlation Index and Spike-distance Metric

“…The present study extends Gai and Carney (2006) by applying two additional temporal approaches, the correlation index and the spike-distance metric, to the same set of tone-in-noise responses recorded in that study. The correlation index (Louage et al 2005;Joris et al 2006) measures the occurrence of simultaneous discharges across stimulus repetitions.…”

“…This study involved further analysis of responses collected for Gai and Carney (2006) in which the experimental procedures are detailed. Briefly, gerbils were anesthetized with ketamine and xylazine.…”

“…Window sizes of 0.20 and 0.57 ms were used in this study. These are the two smallest temporal resolutions used in Gai and Carney (2006); larger temporal windows were not used in this study because they could result in multiple spikes falling within one window, which is not appropriate for these analyses.…”

“…Previous studies have examined average discharge rate and synchronization to fine structure at various stages of the auditory pathway in response to tones in noise (Rhode et al 1978;Geisler and Sinex 1980;Gibson et al 1985;Costalupes 1985;Young and Barta 1986;Miller et al 1987;Rees and Palmer 1988;May and Sachs 1992). Gai and Carney (2006) describe several temporal analyses, based on information related to stimulus fine structure, envelope, and discharge reliability, of tone-in-noise responses in the anteroventral cochlear nucleus (AVCN) in gerbil. Comparisons made between these analyses and with psychophysical detection performance suggest that some representations of temporal information can be more robust for detection than average discharge rate.…”

“…For responses to sinusoidally amplitudemodulated (SAM) tones, the correlation index measures synchronization to modulation frequency, and for noise responses, synchronization to the noise envelope (Louage et al 2005;Joris et al 2006). The correlation index has not previously been applied to tone-in-noise responses to address the question of how the index is influenced by the combination of neural synchronization to tone frequency and to envelope, which can be counteracting cues in the presence of noise (Gai and Carney 2006).…”