PROCESSING OF MODULATION FREQUENCY IN THE DORSAL COCHLEAR NUCLEUS OF THE GUINEA-PIG - SINUSOIDAL FREQUENCY-MODULATED TONES

Frequency- and amplitude-modulated (FM and AM, respectively) tones are important information-bearing elements in voice sounds and can also b e produced by the spatial movement of sound sources. Zhao and Liang (1 995) recently reported the response features of dorsal cochlear nucleu s (DCN) neurons to AM tones. In the present study, the responses of th e guinea pig DCN neurons to sinusoidal FM (SFM) tones were examined. D ischarges of the DCN units to the SFM tones phase-locked to the stimul us modulation frequencies (f(m)). The phase-locked discharge patterns existed over broad ranges of modulation parameters and at stimulus lev els as high as 95 dB SPL or modulation depths (d(m)) as low as 2%. Rob ust phase-locking to the f(m) was observed in samples of all DCN unit types studied. The means of best f(m) (B-fm) and upper limit f(m) (ULf (m)) of all recorded units were 510 Hz and 940 Hz, respectively. Pause r/Buildup (P/B) units had mean maximum synchronization index (SImax) o f 0.57. ON units had the highest Bf(m) with the mean of 646 Hz and sub type ON-S showed the highest mean of SImax at 0.63. Phase-locking to t he f(m) was independent of discharge rates and existed even when the d ischarge rates were reduced to the background spontaneous rate (SR). A few units showed stronger synchronous responses to the square and tri angular FM stimuli instead of the SFM tones. The relationship between the modulated responses and the unit's response area were further exam ined. The f(m) phase-locking occurred to modulation bands (or frequenc y ranges) within the response area, with the modulation bands as narro w as +/-160 Hz in the central inhibitory areas of the type IV units, A s the width of the modulation band changed within a unit's response ar ea, the phases of the f(m) phase-locked responses of P/B units linearl y changed while for Onset units, the change was lesser. The P/B and On set units had a pi phase shift and a pi/2 phase change, respectively, as carrier frequencies (f(c)s) passed through characteristic frequenci es (CF) and the excitatory/inhibitory response boundaries. The phase-l ocked responses to the f(m)s were dependent on the SR but were indepen dent of the CF. Low-SR (less than or equal to 2 spikes/s) units had hi gher synchronization of responses to the f(m) than the high-SR(> 2 spi kes/s) units (SImax = 0.64 and 0.42, respectively). These results sugg est that the temporal characteristics:of the f(m) is effectively repre sented in the responses of DCN units to the SFM tones. Such temporal e ncoding behavior can play an important role in the processing of the c omplex sounds in the auditory system. These results also have implicat ions for a possible role for the DCN is in identifying the spatial mov ement of a sound source.