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Inhibitory influences in receptive fields (RFs) of the fish retinal direction-selective ganglion cells (DS GCs) were investigated. Responses of the fast retinal DS GCs were recorded extracellularly from their axon terminals in the superficial layer of tectum opticum of immobilized fish. The data were collected from two cyprinid species - Carassius gibelio, a wild form of the goldfish, and the barbel fish Labeobarbus intermedius. Visual stimuli were presented to the fish on the monitor screen within a square area of stimulation occupying approximately 11 × 11° of the visual field. DS GCs were stimulated by pairs of narrow stripes moving in opposing directions. One of them entered central (responsive) area of cell receptive field (RRF) from the preferred, and the other one from the null side. Stimuli merged at center of stimulation area, and subsequently moved away from each other. It was shown that the cell response evoked by the stripe coming from the preferred side of RF was inhibited by the stimulus coming from the opposite direction. In the majority of units recorded inhibitory effect induced by the null-side stimulus was initiated in the RF periphery. As a rule, inhibitory influences sent from the RF periphery were spread across the entire central area of RF. Modifications of the inhibitory influences were investigated throughout the whole motion of paired stimuli. Evident inhibitory effects mediated from the null direction were recorded during the approach of stimuli. When stripes crossed each other and moved apart inhibition was terminated, and cell response appeared again. Null-side inhibition observed in fish DS GCs is most likely induced by starburst-like amacrine cells described in morphological studies of different fish species. Possible mechanisms underlying direction selectivity in fish DS GCs are discussed.

Responses of ON‐ and OFF‐ganglion cells (GCs) were recorded extracellularly from their axon terminals in the medial sublamina of tectal retino‐recipient layer of immobilized cyprinid fish (goldfish and carp). These units were recorded deeper than direction selective (DS) ones and at the same depth where responses of orientation selective (OS) GCs were recorded. Prominent responses of these units are evoked by small contrast spots flickering within or moving across their visual field. They are not selective either to the direction of motion or to the orientation of stimuli and are not characterized by any spontaneous spike activity. We refer to these fish GCs as spot detectors (SDs) by analogy with the frog SD. Receptive fields (RFs) of SDs are organized concentrically: the excitatory center (about 4.5°) is surrounded by opponent periphery. Study of interactions in the RF has shown that inhibitory influences are generated already inside the central RF area. This fact suggests that RFs of SDs cannot be defined as homogeneous sensory zone driven by a linear mechanism of response generation. Physiological properties of fish SDs are compared with the properties of frog SDs and analogous mammalian retinal GCs—local edge detectors (LEDs). The potential role of the SDs in visually guided fish behavior is discussed.

In all animals the optic tectum (OT) (or superior colliculi in mammals) provides guidance for external attention; this is not the only function of the tectum but is critically important for the development of behavioral visual reactions. In fi sh, the OT is the main primary visual center. It receives signals from most ganglion cells (GC) of different (known) types in the retina. Knowledge of the properties -both structural and physiological -of the neurons in the OT is important for understanding the mechanisms organizing behavior. We recorded extracellular electrical activity in the OT in living adult fi sh (Carassius auratus gibelio). Simultaneous recordings were made of the responses of retinal GC (from their axon terminals) and tectal neurons (TN), probably from cell bodies. Four types of TN are described with directional selectivity (DS) (henceforth these neurons are termed DS TN) at different (defi ned) depths of the OT. In addition to these, rare sporadic spikes lacking DS and arising on stimulation at any locus in a large area were consistently recorded simultaneously (superfi cially) with the responses of caudorostral DS GC with the electrode in one position. These are presumptively the responses of superfi cial tectal neurons (superfi cial inhibitory neurons, SIN). Various different types of stimulation were applied with the aim of obtaining clear SIN responses. Comparison of the results of our electrophysiological studies with published data (most studies in this direction have used calcium imaging in transparent Danio rerio fry) showed that DS TN were identical to glutamatergic periventricular interneurons in the OT, while SIN were identical to GABAergic inhibitory interneurons (SIN). These latter presumptively mediate detection of the main object (pop-out) in the fi eld of vision.