In dynamically changing environments, it is critical to identify the boundaries of current knowledge, in order to initiate new learning whenever necessary. Animals are capable of rapidly discriminating novel from familiar stimuli or situations. In fact, novel stimuli trigger distinct orienting and exploratory behaviors, which habituate after only a few exposures, suggesting a very rapid form of memory formation. How the brain detects and processes different forms of novelty and familiarity, is currently poorly understood.
To study the neural circuits underlying novelty processing, we have developed a spontaneous novelty detection task which involves bottom-up, stimulus-driven attention, but is non-declarative. In the task, we present common familiar, rare familiar, contextually novel and entirely novel olfactory stimuli to head-restrained mice, while measuring sniffing, a well-established behavioral response to novelty. Mice respond to novel stimuli, rare familiar and contextually novel stimuli with a graded increase in respiration rate. No response was observed for standard familiar stimuli.
Given previous work has implicated the medial temporal lobe (MTL) in novelty processing, we next performed extracellular recordings in the lateral entorhinal cortex (LEC), the entry point of olfactory information in the MTL, during the spontaneous novelty detection task. Among 199 units recorded across the MTL, we identified 123 odor-responsive cells. Most cells responded to a single odorant indicating a high degree of stimulus selectivity. The response magnitude of odor-responsive cells was graded, concomitant with graded sniffing response. We did not find indication of modulation by familiarity or novelty. Taken together our results suggest, olfactory sensory responses in the LEC are gated by an attentional process.