Encoding of amplitude modulation in the cochlear nucleus of the cat

Abstract

1. Amplitude modulation (AM) is a pervasive property of acoustic communication systems. In the present study we investigate neural temporal mechanisms in the auditory nerve and cochlear nuclei of the pentobarbital sodium-anesthesized cat associated with the neural coding of 100% AM tones, both in quiet and in the presence of wideband, quasi-flat-spectrum noise. The AM carrier frequency was set to the neuron's characteristic frequency (CF) and the sound pressure level (SPL) of acoustic stimuli was varied over a wide dynamic range of intensities (< or = 40 dB). The temporal AM-encoding capability of auditory neurons was measured by computing the synchronization coefficient (SC) of the neural response to the signal's modulation and carrier frequency. The temporal modulation transfer function (tMTF) of a neuron was then computed by measuring the SC of the response to signals of variable fmod (50-2550 Hz). 2. Neurons in the cochlear nuclei synchronize on average more highly to the modulation frequency than fibers of comparable CF, threshold, and spontaneous rate in the auditory nerve. The disparity in performance is greatest at high SPLs and low signal-to-noise ratios. However, there is a significant degree of diversity in AM-encoding capability among neurons in both the cochlear nuclei and auditory nerve. Among auditory nerve fibers (ANFs), low- and medium-spontaneous-rate (SR) units (SR < 18 spike/s) phase-lock with greater precision than comparable high-SR units at any given frequency, particularly at moderate to high SPLs, consistent with previous studies. 3. The phase-locking capabilities of neurons in the cochlear nucleus are considerably more variable than in the auditory nerve. Moreover, the variability itself depends on two distinct measures of phase-locking performance. Most ANFs are capable of phase-locking to frequencies as high as 3-4 kHz. In the cochlear nucleus many unit types do not phase-lock to modulation frequencies > 1 kHz. As a result, phase-locking performance is measured on the basis of two parameters, maximum synchronization, irrespective of stimulus frequency, and the upper frequency limit for significant phase-locking. 4. Cochlear nucleus neurons may be divided into three distinct groups on the basis of maximum synchronization capability. In group 1 are the primary-like (PL) units of the anteroventral division, whose phase-locking capabilities are comparable with those of high-SR ANFs.(ABSTRACT TRUNCATED AT 400 WORDS)

Aged-related loss of temporal processing: Altered responses to amplitude modulated tones in rat dorsal cochlear nucleus

“…At all temporal modulation depths, and at either age, buildup and pauser-buildup units have higher peak modulation sync functions than units showing widechopper temporal responses at all temporal modulation depths, and at either age. Thus the agerelated shift toward wide-chopper response types, a temporal response type with higher firing rates and a decreased ability to synchronize to a SAM envelop could explain the present findings and is consistent with a loss of glycinergic inhibition.Inhibitory circuitry may play a role in maintaining/enhancing synchronization to SAM tones (Frisina et al, 1994;Rhode and Greenberg, 1994;Zhao and Liang, 1995;Young, 1998;Backoff et al, 1999;Krishna and Semple, 2000;Hancock and Voigt, 2002b;Joris et al, 2004;Kanold and Manis, 2005;Street and Manis, 2007). The present findings from aged DCN output neurons resemble what is seen when inhibitory neurotransmission is blocked during temporal processing paradigms in CN and IC (Koch and Grothe, 1998;Backoff et al, 1999;.…”

Response patterns to sound associated with labeled globular/bushy cells in cat

“…Results also show GBCs can have significant VSs at high intensities, even better than ANFs, where VS decreases at high intensities (cf. Rhode and Greenberg, 1994). Overall, these data suggest envelope coding is preserved by the MNTB neurons that project to LSO and may be used in localization of sounds.…”

“…Most TMTFs of PLn units are bandpass in shape, which differs from the low pass TMTFs of ANFs (Joris and Yin, 1992). A second indication of selective feature enhancement is that VS decreases less with increasing intensity (indicated by increasing line thickness) than seen in ANFs (Rhode and Greenberg, 1994). This is an example of improved coding of temporal signal features in the CN unit population that may play an important role in signal processing in higher auditory centers.…”

“…An attempt was made to characterize each unit with the following paradigms: (a) the receptive field was determined using 50 ms tone pips with the intensity varied from 0 to 90 dB SPL in 10 dB increments. Frequency was varied in CF/10 to CF/20 Hz steps; (b) the PSTH at the CF was generated using 250 repetitions of a 50 ms tone presented every 105 ms, typically at 60 dB SPL; (c) an FM sweep response at a single intensity (usually 70 dB SPL) was collected; and (d) an amplitude modulated (AM) signal was presented to determine the temporal modulation transfer function (Rhode and Greenberg, 1994). The AM carrier was set to CF and the modulation depth was 100%.…”

Spectral integration plasticity in cat auditory cortex induced by perceptual training

“…Studies of general features of human and animal vocalizations, such as fundamental frequency of the sound source (e.g. Langner, 1992;Langner and Schreiner, 1988;Rhode and Greenberg, 1994) and vocal tract resonances (e.g., Klein et al, 2000;Schreiner and Calhoun, 1994;Shamma et al, 1995;Wang and Sachs, 1995) provide useful insights into auditory neuronal coding.…”