Direction selectivity in the goldfish tectum revisited.

Responses of direction-selective (DS) ganglion cells (GCs) were recorded extracellularly from their axon terminals in the superficial layer of the tectum opticum (TO) of immobilized goldfish, Carassius auratus gibelio (Bloch). Directional tuning curves were measured with contrast edges moving in 12 or more different directions across the receptive field (RF). All directional tuning curves had cardioid-like appearance, their acceptance angles amounted to somewhat more than 180 degrees . According to their preferred directions DS GCs proved to comprise three distinct groups, each group containing DS GCs of ON and OFF subtypes approximately in equal quantity. Thus, this gives six physiological types of DS GCs in total. The preferred direction of a DS GC does not depend to some extent on a value of contrast, speed, size, and form of the stimuli. Coincidence in number of preferred directions with number of semicircular canals implies that DS GCs projecting to tectum are involved in some multimodal sensory integration in postural, locomotor, and oculomotor control in the three-dimensional aquatic world. DS neurons of the TO itself respond independently of the sign of stimulus contrast, have enormous receptive fields, and seem likely to collect signals from the retinal DS units of both ON and OFF subtypes with the same preferred direction.