Toward understanding neural mechanisms of active sensing in weakly electric fish.

Objective: Sensory and motor systems are linked: motor actions alter the information that animals receive from their sensory receptors, which in turn is used to control subsequent motor actions. In active sensing, animals execute ancillary motor movements to obtain and/or modulate sensory information. The neural mechanisms for the control of active sensing are not known. Our goal is to describe the neurophysiological interactions between the sensory and motor circuits in the CNS toward understanding the neural mechanisms of the active sensing. Methods: Weakly electric fishes such as Apteronotus lep-torhynchus robustly perform an image stabilization task in which animals track the movement of a refuge in a single linear dimension. During this behavior, these animals produce large fore-aft movements for active sensing that modulate and maintain a robust level of sensory slip, the main form of feedback used in the control of refuge tracking. We performed chronic tetrode recordings in midbrain circuits of (n=17) Apteronotus during their free refuge tracking behavior to reveal the sensorimotor activity related to the control of these active sensing movements. We implanted electrodes in the Torus semicircularis, a midbrain electrosensory area where direction-selective responses first emerge. Results: We found direction-selective neurons, which respond to a range of velocities for the sensory slip, the error between the refuge and fish movements. Our analysis revealed that these direction-selective neurons are also selective for ranges of acceleration of the sensory slip. Midbrain electrosensory neurons responded best to high-frequency features, behaving as a high-pass filter on the sensory slip. Conclusion: The behavioral studies suggest high-pass sensorimotor transformations in the CNS. The spatiotemporal filtering properties of the midbrain neurons we obtained matched the predictions of the behavioral experiments. These preliminary findings are a first step toward understanding neural mechanisms for control of active sensing in a freely behaving animal. [ABSTRACT FROM AUTHOR]