Midbrain local circuits shape sound intensity codes

Hierarchical processing of sensory information requires interaction at multiple levels along the peripheral to central pathway. Recent evidence suggests that interaction between driving and modulating components can shape both top down and bottom up processing of sensory information. Here we show that a component inherited from extrinsic sources combines with local components to code sound intensity. By applying high concentrations of divalent cations to neurons in the nucleus of the inferior colliculus in the auditory midbrain, we show that as sound intensity increases, the source of synaptic efficacy changes from inherited inputs to local circuits. In neurons with a wide dynamic range response to intensity, inherited inputs increase firing rates at low sound intensities but saturate at mid-to-high intensities. Local circuits activate at high sound intensities and widen dynamic range by continuously increasing their output gain with intensity. Inherited inputs are necessary and sufficient to evoke tuned responses, however local circuits change peak output. Push-pull driving inhibition and excitation create net excitatory drive to intensity-variant neurons and tune neurons to intensity. Our results reveal that dynamic range and tuning re-emerge in the auditory midbrain through local circuits that are themselves variable or tuned

Contextual modulation of vocal behavior in mouse: newly identified 12 kHz ‘mid-frequency’ vocalization emitted during restraint.

While several studies have investigated mouse ultrasonic vocalizations (USVs) emitted by isolated pups or by males in mating contexts, studies of behavioral contexts other than mating and vocalization categories other than USVs have been limited. By improving our understanding of the vocalizations emitted by mice across behavioral contexts, we will better understand the natural vocal behavior of mice and better interpret vocalizations from mouse models of disease. Hypothesizing that mouse vocal behavior would differ depending on behavioral context, we recorded vocalizations from male CBA/CaJ mice across three behavioral contexts including mating, isolation, and restraint. We found that brief restraint elevated blood corticosterone levels of mice, indicating increased stress relative to isolation. Further, after three days of brief restraint, mice displayed behavioral changes indicative of stress. These persisted for at least two days after restraint. Contextual differences in mouse vocal behavior were striking and robust across animals. Thus, while USVs were the most common vocalization type across contexts, the spectrotemporal features of USVs were context-dependent. Compared to the mating context, vocalizations during isolation and restraint displayed a broader frequency range, with a greater emphasis on frequencies below 50 kHz. These contexts also included more non-USV vocal categories and different vocal patterns. We identified a new Mid-Frequency Vocalization, a tonal vocalization with fundamental frequencies below 18 kHz, which was almost exclusively emitted by mice undergoing restraint stress. These differences combine to form vocal behavior that is grossly different among behavioral contexts and may reflect the level of anxiety in these contexts