Neuroethology, sensory information processing in vertebrate brains.
A principal function of sensory information processing in the brains of vertebrates is the detection of weak biologically-relevant sensory signals in a “noisy” environment. One source of sensory interference or noise is an animal’s own movements. For example, the brain must distinguish between salient movements in our visual world and the apparent movements created when we move our own eyes. Understanding the brain’s mechanisms of detecting signals in self-generated noise is one of the goals of our research on the electrosensory system of elasmobranch fishes. For most of our studies we use the little skate, Raja erinacea, an elasmobranch fish, which has a well-developed electrosense and is abundant in Long Island Sound.
The sharks, skates, and rays possess an exquisitely sensitive electrosense, which includes specialized skin receptor cells (Fig. A) and a series of nuclei within the brain for processing the electrosensory information. The sense is used to detect very weak electric fields that occur naturally in the sea. These fields may be biological, electrochemical or geophysical in origin, and they are useful in directing elasmobranch predators to their prey or potential mates, and in orientation and navigation.
The great sensitivity of elasmobranch electroreceptors makes them subject to interference from weak electric fields created by the fish’s own swimming and other movements. The electroreceptors in skates are strongly activated by the fish’s breathing movements. However, we find that electrosensory neurons within the brain are selectively insensitive to these self-generated electric fields while retaining their responsiveness to important environmental electric fields (Fig. B). Using extracellular and intracellular electrophysiological recordings and neuroanatomical methods, we are studying the neuronal mechanisms that are responsible for rejecting self-generated noise. We find that at least two different mechanisms are responsible for the noise rejection. One of these depends on neuronal plasticity mechanisms. Specifically, electrosensory neurons within the hindbrain learn to recognize and cancel any stimuli that are consistently associated with the fish’s own movements. The structural and functional organization of the system suggests that the plasticity mechanisms involved here may be the same as those thought to be involved in motor learning within the cerebellum (long term depression, LTD). Studies are underway to elucidate these mechanisms further and to explore their generalizability to other sensory systems and functions of the vertebrate hindbrain.
Funding: National Science Foundation