CHARACTERIZATION OF SOUND EVOKED RESPONSES IN NEURONS OF THE INLL

The intermediate nucleus of the lateral lemniscus (INLL) plays a critical role in early cross-frequency integration of auditory information, as demonstrated in both bats and gerbils. Among the nuclei of the lateral lemniscus, the dorsal nucleus is most clearly defined, whereas the ventral and intermediate nuclei are rarely distinguished physiologically, despite notable differences in neurotransmitter composition. INLL neurons display striking heterogeneity in their biophysical properties, exhibiting diverse sub- and suprathreshold voltage responses. While its involvement in cross-frequency integration is established, little is known about the sound-evoked response properties of the INLL itself.
Using in vivo single-unit recordings in anesthetized gerbils, we characterized INLL activity in response to acoustic stimulation. Two distinct frequency tuning profiles were observed: neurons exhibiting either a single, or dual best frequency. Regardless of tuning, neurons responded to pure tones with onset, primary-like, or sustained firing patterns. First-spike latency and temporal jitter were intensity-dependent and shorter in onset neurons. Analysis of modulation transfer functions revealed that INLL neurons prefer relatively low modulation frequencies, coinciding with high vector strength.
Spectrotemporal receptive field analysis using conspecific vocalizations showed that most INLL neurons exhibited combination sensitivity, integrating spectral components spanning several octaves beyond their best frequencies. Stimulus-specific information increased with acoustic complexity, and spectrotemporal receptive fields shifted accordingly, indicating dynamic adjustments in frequency tuning. Together, these findings demonstrate that complex auditory feature integration begins earlier in the brainstem than traditionally assumed and that the INLL actively shapes temporal and spectral sound representations rather than functioning as a simple relay nucleus.