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.

Fish have highly developed vision that plays an important role in detecting and recognizing objects in different forms of visually guided behavior. All of these behaviors require high spatial resolution. The theoretical limit of spatial resolution is determined by the optics of the eye and the density of photoreceptors. However, further in the fish retina, each bipolar cell may collect signals from tens of photoreceptors, and each ganglion cell may collect signals from tens to hundreds of bipolar cells. If we assume that the input signals in this physiological funnel are simply summed, then fine gratings that are still distinguishable at the level of cones should not differ from the homogeneous surface for the ganglion cells. It is therefore generally considered that the resolution of the eye is determined not by the density of cones, but by the density of ganglion cells. Given the size of the receptive field of ganglion cells, one can conclude that the resolving power at the output of the fish retina should be ten times worse than at its input. But this contradicts the results of behavioral studies, for, as it is known, fish are able to distinguish periodic gratings at the limit of resolution of the cones. Our electrophysiological studies with extracellular recording of responses of individual ganglion cells to the motion of contrast gratings of different periods showed that the acuity of ganglion cells themselves is much higher and is close to the limit determined by the density of cones. The contradiction is explained by the fact that ganglion cells are not linear integrators of the input signals, their receptive fields being composed of subunits with significantly smaller zones of signal summation where nonlinear retinal processing takes place.

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A variety of visually evoked responses are recorded in the fish optic tectum using single-cell recording technique. Based on indirect criteria (frequency power spectrum of spikes, spike waveform, receptive field size), they may be divided into two groups: those presumably recorded from axon terminals of retinal ganglion cells projecting to the tectum (precynaptic recording), and those recorded from tectal neurons (postsynaptic recording). In the present study, we used cobalt, a reversible blocker of synaptic transmission, as a more crucial criterion to identify the source of these responses. After cobalt application, some units (such as ON- and OFF-types of direction-selective units, orientation-selective and spontaneously active units) were visually responsive, while others (including ON-OFF direction-selective units with large receptive fields) ceased firing. Discrimination of the units by the use of cobalt has been found to coincide with that by the indirect physiological criteria. Thus, the differences in frequency power spectrum of spikes, spike waveform, and receptive field size may be used for efficient and reliable discrimination between pre- and post-synaptic recordings in the fish tectum.

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.