Sensitivity of single units in the LSO of decerebrate cat to sinusoidally amplitude modulated tones

Abstract

1Fluctuations in amplitude are a common component of behaviorally important sound 2 stimuli. Amplitude modulation (AM) is encoded by the peripheral auditory system in the timing 3 of discharge spikes, and, more centrally, in the discharge rate. The mechanism producing this 4 transformation from a time-to rate-based code is not known, but recent modeling efforts have 5 suggested a role for neurons with response characteristics consistent with cells in the lateral 6 superior olive (LSO). The responses of single units in the LSO of unanesthetized decerebrate cat 7 were recorded to monaural sinusoidally amplitude modulated (SAM) tones by systematically 8 varying sound level and modulation frequency (f m ), and are described in terms of 9 synchronization to the envelope and average discharge rate as a function of f m . LSO units 10 typically synchronize strongly to low f m , and discharge preferentially (i.e. more strongly) over a 11small range of f m in response to low level SAM tones. At higher sound levels synchronization 12 decreases and response rate increases until most or all modulation in the response is lost. These 13 results contrast with responses recorded in the barbiturate-anesthetized cat, which tend to 14 respond to most low-frequency modulations, and are consistent with LSO as an intermediate 15 processing stage between the peripheral temporal-and central rate-based code for AM sounds. 16 17 KEYWORDS 18 Lateral superior olive, envelope coding, ipsilateral inhibition, same-frequency excitation and 19 inhibition (SFIE), feed-forward inhibition olive. 20 21 1998), can produce a discharge rate response tuned to a particular range of f m without requiring 43 specialized discharge characteristics. Bushy cell outputs are relayed to the ICC through cells in 44 the superior olivary complex (SOC) and ventral nucleus of the lateral lemniscus (VNLL) to ICC 45 (Cant and Benson 2003; Cant and Casseday 1986), suggesting that SOC cells could contribute to 46 AM processing. Responses of VNLL cells to AM stimuli, which provides a major source of 47 inhibitory input to ICC (Saint Marie et al. 1997; Whitley and Henkel 1984; Winer et al. 1995), 48 have received considerable attention recently, which has revealed tuned rate functions (Batra 49 2006; Zhang and Kelly 2006). In contrast, investigations of the responses of SOC cells to AM 50 stimuli have been more limited. 51 Cells in the SOC are among the first cells to receive information from both ears. In 52 particular, cells in the lateral superior olive (LSO) receive excitatory input from ipsilateral 53 spherical bushy cells, and inhibitory input from contralateral globular bushy cells, via the 54 ipsilateral medial nucleus of the trapezoid body (MNTB) (Glendenning et al. 1985; Glendenning 55 and Masterton 1983; Tolbert et al. 1982), thus are sensitive to interaural level differences (ILDs; 56 e.g. Boudreau and Tsuchitani 1968; Tollin et al. 2008). LSO cells then send excitatory 57 projections to contralateral, and inhibitory projections to ipsilateral ICC (Glendenning et...