MODULATION OF SYNAPTIC TRANSFER BETWEEN RETINAL CONES AND HORIZONTAL CELLS BY SPATIAL CONTRAST

We studied the influence of steady annular light on the kinetics and s ensitivity of horizontal cell (HC) responses to modulation of the inte nsity of small concentric spots in the turtle retina. As shown by prev ious investigators, when the intensity of the annulus was equal to the mean spot intensity, spot response kinetics were the same as those fo r the modulation of spatially uniform light. Turning off the annulus a ttenuated dramatically high-frequency flicker sensitivity and enhanced somewhat low-frequency sensitivity. This phenomenon reflects a modula tion of synaptic transfer between cones and second-order neurons that is mediated by cones, and it will be referred to as cone-mediated surr ound enhancement (CMSE). Our main results are as follows: (a) The chan ge in test-spot response sensitivity and kinetics upon dimming a stead y surrounding annulus is a consequence of the change in spatial contra st rather than change in overall light level. (b) Introduction of mode rate contrast between the mean spot intensity and steady surrounding l ight intensity causes a marked change in spot response kinetics. (c) T he dependence of spot response kinetics on surrounding light can be de scribed by a phenomenological model in which the steady state gain and the time constant of one or two single-stage, low-pass filters increa se with decreasing annular light intensity (d) The effect of surroundi ng light on spot responses of a given HC is not determined by change i n the steady component of the membrane potential of that cell. (e) Lig ht outside the receptive field of an HC can affect that cell's spot re sponse kinetics. (f) In an expanding annulus experiment, the distance over which steady annular light affects spot response kinetics varies among HCs and can be quite different even between two cells with close ly matched receptive field sizes. (g) The degree of CMSE is correlated with HC receptive field size. This correlation suggests that part of the enhancement mechanism is located in the HC. Taken together, our re sults suggest the involvement of the inner retina in CMSE.