Synaptic mechanisms of sensory perception
Carl Petersen, Ecole Polytechnique Federale de Lausanne (EPFL), Lausanne Switzerland
Sensorimotor loops are amongst the most fundamental aspects of brain function. Simple reflex behaviors can be accomplished by neural circuits in the spinal cord, but more complex behaviors require the participation of the neocortex. A causal and mechanistic description of the sensorimotor transformations underlying goal-directed behavior is currently lacking. Using the mouse whisker system to provide a well-defined sensory stimulus and licking a reward spout as the motor output, we have begun to explore the neural circuits underlying simple forms of sensory perception associated with behavioral report. Head-restrained, water-restricted mice learn to associate a single 1 ms deflection of the C2 whisker with water availability within a week of daily training sessions. We quantified hit rates as the fraction of trials in which the whisker stimulus led to licking within a 0.5 - 1 s reward window leading to a drop of water being delivered to the mouse. Inactivation of primary somatosensory neocortex (S1) impaired performance. Optogenetic stimulation of S1 evoked licking responses in mice trained to lick in response to the whisker stimulus. S1 therefore plays an essential role in this behavior. Whole-cell recordings from neurons in layer 2/3 of the C2 barrel column in S1 revealed interesting correlations between membrane potential and behavior. The whisker stimulus evoked a biphasic response. The early component reliably drove membrane potential towards a reversal potential, which for most neurons was hyperpolarized relative to action potential threshold. A late depolarization was present on hit trials but was significantly smaller on miss trials. The late depolarization might result from feedback inputs from motor cortex. Interestingly we found that this simple behavior is compatible with a wide range of brain states, including ones with highly-correlated, large-amplitude, slow membrane potential fluctuations. These results begin to shed light on the membrane potential correlates of sensory perception.
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