Detection of bimodal stimuli in the frog retina

The present report addresses the electrical activity of the frog retina assessed using electroretinogram (ERG) recordings of responses to instantaneous changes from a reference line stimulus with fixed luminance and orientation to a test stimulus consisting of a line of different luminance and orientation. The analysis revealed two types of retinal responses. Responses to onset and offset of the line stimulus were analogous to responses obtained by homogeneous illumination of the retina and were characterized by a high-amplitude b wave (hundreds of microvolts) and significant asymmetry between the b and d waves. At the same time, interaction of these two stimuli yielded more symmetrical b and d waves, with amplitudes an order of magnitude lower, such that this frog retinal activity approached the ERG pattern recorded in response to formed stimuli in higher vertebrates. Analysis of the interaction of the mechanisms detecting the luminance and configurative characteristics of the stimuli, based on construction of V-shaped discrimination functions, showed that when the luminance of the stimulus line was only slightly different from the luminance of the reference line (i.e., discrimination occurred in the retinal stimulus adaptation zone), the activities of the luminance and orientation channels were summed in the ERG. This indicates that these mechanisms function independently and in parallel. From the moment at which the test stimulus luminance became twice that of the reference stimulus, the increase in ERG amplitude grew in a non-linear manner. This two-stage change in amplitude is explained by the existence of two luminance encoding mechanisms in the frog retina, acting simultaneously with the orientation encoding mechanism. One luminance mechanism encodes the action of the stimulus as the discharge power, generating information encoding the absolute level of environmental illumination. The activity of this mechanism is determined mainly by receptors and cells in the outer plexiform layer of the retina. The other mechanism is based on vector encoding of stimulus actions, generating information encoding the spatial and temporal differentiation of light in the visual field and is mainly associated with cells in the inner plexiform layer of the retina.