Receptive field sizes of direction-selective units in the fish tectum

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.