Motor commands sent from the pacemaker nucleus in the brain of an electric fish (green region in inset) cause the electric organ in the tail (red region) to emit electric pulses (blue arrows) that travel through the surrounding water. These pulses are shaped by nearby objects and then detected by active electroreceptors (not shown) in the skin. Electric fish also have passive electroreceptors that detect the weak electric signals caused by the muscle movements of other creatures, including the crustaceans that electric fish feed upon: the signals from these receptors are sent to principal cells in a region of the hindbrain called the passive electrosensory lobe (ELL). Electric fish have evolved a sophisticated method to prevent the electric pulses produced by their active sense from interfering with the passive detection of, for example, signals due to body movements made by prey. For each electric pulse it sends, the pacemaker nucleus (top right) also sends a 'corollary discharge' signal along mossy fibers to granule cells in the cerebellum. This signal is processed by the granule cells, and an array of precisely timed signals is sent along parallel fibers to the principal cells in the ELL. These signals alter the strength of the synapses between the parallel fibres and the principal cells in a way that creates a 'negative image' of the original active pulse: the production of the negative image relies on a process known as anti-Hebbian spike time plasticity (STDP). The cells in the ELL use the negative image to subtract the effect of the active pulses from the signals they are receiving from the passive electroreceptors. Any useful information that remains in the signal is forwarded to other regions of the brain. Electric fish tend to emit a few pulses per second when they are at rest, and up to about 60 pulses per second when they are hunting. Dempsey et al. show that electric fish of the species Gnathonemus petersii (aka Peter's elephant-nose fish) learn to cancel images at low pulse rates, and are able to generalize this learning to automatically cancel images at the higher pulse rates that they use during hunting.
IMAGE: G. petersii: J Jury derivative work: Jmvgpartner [CC BY-SA 3.0].