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)