Survey of hippocampal responses to sound in naïve mice reveals widespread activation by broadband noise onsets

Recent studies suggest some hippocampal (HC) neurons respond to passively presented sounds in naïve subjects, but the specificity and prevalence of these responses remain unclear. We used Neuropixels probes to record unit activity across layers in mid-ventral HC and auditory cortex (ACtx) of awake, untrained mice (male and female) while presenting diverse sounds at typical environmental levels (65–70 dB SPL). A subset of HC neurons exhibited reliable, short-latency responses to passive sounds, including tones and broadband noise. HC units showed evidence of tuning for tone frequency but not spectrotemporal features in continuous dynamic moving ripples. Across sound types, HC responses overwhelmingly occurred at stimulus onset; they quickly adapted to continuous sounds and did not respond at sound offset. Among all sounds tested, broadband noise was most effective at driving HC activity. Spectral manipulations indicated response prevalence scaled with increasing spectral bandwidth and density. Similar responses were also observed for visual flash and contrast modulated noise movies, although these were less common than for broadband noise. Sound-evoked face movements, quantified by total face motion energy (FME), correlated with population-level HC activity. However, many individual units responded regardless of FME strength, suggesting both auditory and motor-correlated inputs. Together, our results show that abrupt sound onsets are sufficient to activate many HC neurons in the absence of learning or behavioral engagement. This supports a role for HC in detecting salient environmental changes and supports the idea that auditory inputs contribute directly to HC function. Significance statement Hippocampus is critical for learning and memory, but its role in sensory processing is less understood. Here, we show many hippocampal neurons in awake, untrained mice respond to passive sounds, especially broadband noise. Sound onsets – transitions from silence to sound – are critical for these responses, suggesting a role in detecting abrupt, salient environmental changes. Consistent with this possibility, some units also responded to visual events, though fewer than responded to noise. In contrast to auditory cortex, hippocampal units were not reliably tuned for spectrotemporal modulation features, suggesting independent functional roles. The prevalence of passive auditory processing in hippocampus builds on previous work suggesting hearing may interact with general cognitive health.