Visual processing in the leech central nervous system.

In animals with complex visual systems spatial contrast is enhanced by mutual inhibition between retinal neurones monitoring different fields. By analogy, if animals like leeches and some caterpillars, that have several simple (non-image-forming) eyes aimed in different directions, are capable of rudimentary form detection, one might predict mutual antagonism between eyes monitoring different fields. In support of this prediction, I report here a paired interneurone in the central nervous system (CNS) of the leech which is stimulated by eyes on one side of the animal and inhibited by eyes on the other. There are striking parallels between these neurones and other integrating neurones, in particular those processing bilateral auditory input in crickets, suggesting that the visual system of the leech may be representative of a general class of sensory processing systems.