Afferent connections to the dorsal nucleus of the lateral lemniscus of the mustache bat: evidence for two functional subdivisions.

The dorsal nucleus of the lateral lemniscus (DNLL) of the mustache bat, Pteronotus parnellii, was found to consist of two divisions. The neurons in each division were distinguished by their temporal discharge patterns evoked both by tone bursts and sinusoidal amplitude-modulated (SAM) signals. Neurons in the anterior one-third of the DNLL responded to tone bursts with an onset discharge pattern and only phase-locked to SAM signals with low modulation frequencies (< 300 Hz). Neurons in the posterior two-thirds of the DNLL responded to tone bursts with a sustained discharge pattern and phase-locked to SAM signals with much higher modulation frequencies (400-800 Hz). In addition, there was a different frequency representation in the two divisions. The frequency representation in the posterior division was from about 15 to 120 kHz, whereas in the anterior division it was only up to 62 kHz. The physiological differences were further supported by data from experiments that revealed the sources of afferent projections to the two DNLL divisions. Retrograde labeling showed that the afferent projections to the two divisions were from different neuronal populations. Input differences were of two types. Some nuclei projected to one or the other DNLL division, but not to both. For instance, the ventral nucleus of the lateral lemniscus projected predominately to the anterior DNLL and provided little or no inputs to the posterior DNLL, whereas the medial superior olive innervated the posterior but not the anterior DNLL. Other lower nuclei projected to both DNLL divisions. These include the contralateral cochlear nucleus, the ipsi- and contralateral lateral superior olives, the intermediate nucleus of the lateral lemniscus, and the contralateral DNLL. However, the projections to each division of the DNLL originate from different neuronal subpopulations in each lower nucleus. The functional implications of these findings are discussed in the context of the possible impacts that the two DNLL divisions exert on their postsynaptic targets in the inferior colliculus.