Responses of direction-selective (DS) ganglion cells (GCs) were recorded extracellularly from their axon terminals in the superficial layer of tectum opticum (TO) of immobilized cyprinid fish Carassius gibelio (Bloch, 1782). Excitatory receptive field (ERF) sizes of six types of DS GCs (ON and OFF cells, each of three distinct preferred directions) were evaluated on the basis of four different methods. In Method 1, the ERF width was calculated as a product of duration of spike train, generated in response to contrast edge moving across the ERF in preferred direction, and the velocity of the stimulus movement. The duration of spike train was estimated either as an interval between the first and the last spikes, or on the basis of the width of bell-shaped post-stimulus histogram of spike response according to its standard deviation. More precise size and position of the ERF can be outlined with edges moving in many different directions. So, in Method 2 diameter of the ERF was calculated on the basis of a mean distance of position of spike appearance from the center of ERF. Method 3 - ERF tracing by small contrast spot moving on several parallel tracks allowed estimation of the ERF width by number of spikes along each track and the ERF length by the duration of spike train. When tracing in two mutually orthogonal projections, the method also permitted calculation of the value of the temporal delay in the network from the same experiment. Canonical method (Method 4) used the ERF mapping with contrast spots flickering sequentially in different places of stimulation area. The length, width and orientation of the ERF were evaluated according to the two-dimensional equivalent of the standard deviation for this data set. All applied methods gave consistent estimates of ERF sizes - mean values of ERF sizes for all four procedures ranged between 4 degrees and 4.8 degrees . These angle values corresponded to retinal area of approximately 300 mum. Small ERFs of the fish DS GCs measured in the current study, indicate that the fish DS units should be classified as "fast" DS units, and are most likely involved in the detection of small objects moving in the surrounding environment.

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.

Responses from two types of orientation-selective units of retinal origin (detectors of horizontal lines and detectors of vertical lines) were recorded extracellularly from their axon terminals in the medial sublamina of tectal retinorecipient layer of immobilized cyprinid fish Carassius gibelio. Excitatory and inhibitory influences across receptive fields of orientation-selective units were evaluated. Positions, sizes and forms of the responsive parts of the receptive field were estimated by moving edges and flashing narrow light and dark stripes. It was shown that the orientation-selective units in fish are characterized by small responsive receptive fields with mean width of 4.8 +/- 1.6 degrees (n = 176). The comparison of different types of orientation-selective units revealed that the responsive receptive fields of detectors of vertical lines are significantly wider (13%) than those of detectors of horizontal lines. Statistically significant difference was also found between sizes of responsive receptive fields evaluated by light and dark edges. Mean responsive receptive field width, estimated for light edges (ON responses) were wider than those evaluated for dark edges (OFF responses). Inhibition in the receptive field of orientation-selective units was evaluated on the basis of two experimental methods. Evidence that signals are not linearly summed across the receptive field was derived from experimental results. Inhibitory influences, recorded in the receptive field of orientation-selective units, always initiated inside the responsive receptive field area and spread towards the periphery. Results of the study indicate that receptive fields cannot be defined as homogeneous sensory zone driven by a linear mechanism of response generation. The receptive fields of orientation-selective units, in fish appear to be composed of subunits sensitive to the appropriately oriented stimuli.

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The action spectrum of the electroretinographic (ERG) b-wave and the unmasked late receptor potential (LRP) were measured under a variety of conditions (isolated eyecup, detached retina, b-wave recording in fresh preparations, LRP measurements after low-temperature aging, dark and light adaptation). It was shown that in the dogfish, Scyliorhinus canicula (L.), eyecup spectral sensitivity matched closely the pigment 502 spectral curve like other rhodopsin-possessing marine species. The ERG b-wave is as good an indicator of spectral sensitivity as the unmasked LRP that directly reflects the responsiveness of photoreceptors. Differences in spectral sensitivity were not revealed between dogfish preparations studied under a variety of conditions (isolated eyecup vs. detached retina; b-wave recording in fresh preparations vs. LRP measurements after low-temperature aging; dark vs. light adaptation). We developed a new model for spectral sensitivity data.