LIGHT ADAPTATION AND LATERAL INHIBITION IN THE VERTEBRATE RETINA

Light adaptation denotes the ability of the visual system to function properly in a wide range of luminance levels. Lateral inhibition designates a property of organization of the retina that is important for enhancement of spatial gradients of luminance. The mechanisms underlying these properties reside at an early stage of retinal processing and they both involve interactions among retinal cells.; The intention of this work is to first characterize light adaptation and lateral inhibition in the vertebrate retina in a concise way so as to make these properties amenable to modeling, and then model light adaptation and lateral inhibition simultaneously in the distal retina using known electrical properties of receptors and horizontal cells. The main purpose of the model is to examine possible mechanisms of light adaptation and lateral inhibition that would result from electrical properties of receptors and horizontal cells.; To that end I have used cellular and psychophysical measures of retinal sensitivity published by other authors to define a set of "response functions". These functions represent the size of the response of the retina to transient or steady light of different intensities that would be obtained in various conditions of light adaptation.; Then I have developed an electrical analog of an array of receptors and horizontal cells using the electrical representation of actual structures found in vertebrate retinas. The main features of each stage of the model were demonstrated by simulating the electrical responses of the array to simple patterns of light. Only spatial and intensity-dependent aspects of the model were simulated.; The response functions derived from psychological measurements in humans agree well with those obtained from cellular recordings in non-human vertebrates. Sensitivity to light transients within Weber's law is shown to be important for the preservation of the subjective appearance of visual scenes under different conditions of illumination.; Computer simulation of the electrical response of the model shows important properties that are also found in cell recordings: saturation, spatial summation and antagonistic center-surround receptive field organization. The simulations also show light-adaptation behavior that closely follows the response functions developed earlier. In particular the steady-state response of the receptors in the model is graded over nine decades of irradiance.; In the model lateral inhibition is a consequence of the resistance of the extracellular space in the receptor terminal invagination and the lateral inhibition effect is present in the potential of the invagination and of the horizontal terminals. Light adaptation on the other hand is a consequence of a slowly changing voltage-dependent membrane resistance in the receptor.; In conclusion one can say that light adaptation and lateral inhibition in the vertebrate retina may result from simple electrical events in receptors and horizontal cells.; Negative feedback on receptors can result from the lateral inhibition signal present at the invagination, without the need of an inhibitory synapse. Inhibitory surrounds in bipolar and ganglion cell receptive fields can be produced by the direct transmission of the lateral inhibition effect from horizontal cell terminals to bipolar cell terminals without the intervention of the negative feedback signal from receptor terminals.; Also the extension from two to nine decades in the light range over which receptors give graded responses may be a consequence of a voltage-sensitive conductance that partially counteracts the effect of the photosensitive conductance on the receptor potential. Finally it can be concluded from the simulations that lateral inhibition helps maintain the sensitivity of the light-adapted retina to light transients within Weber's law.