LINEAR AND NONLINEAR CONTRAST CODING IN LIGHT-ADAPTED BLOWFLY PHOTORECEPTORS

The response properties of R1-6 type photoreceptors of the blowfly (Calliphora vicina) were investigated using 1- to 600-ms light steps with contrasts from - 1.00 to + 1.12 at different adapting backgrounds. To prevent activation of voltage-dependent K+-channels, some photoreceptors were treated with ionophoretically injected tetraethylammonium. The linearity and time-course of the photoresponses depended on the adapting background and the duration of the contrast stimulus. At low backgrounds photoresponses were approximately linear regardless of the contrast step duration. However, at higher backgrounds photoreceptors produced, with long contrast steps of opposite polarities, larger and slower hyperpolarizing responses than depolarizing ones, but linear responses with equal time-courses to transient (less-than-or-equal-to 2 ms) contrast changes. The early rising phases of photoresponses were independent of the contrast signal duration, deviated to the same extent from the steady-state potential and steepened as the value of contrast was increased. The photoreceptor contrast gain increased with the background, but its value depended on the duration, the magnitude, and the polarity of the contrast step. Tetraethylammonium linearised the photoreceptor membrane, increased and delayed photoresponses probably by blocking the shunting of the outwardly rectifying potassium channels. These effects did not strongly influence on the photoreceptors' characteristic non-linear contrast dependence on light adaptation.