LINEAR AND NONLINEAR CONTRAST CODING IN LIGHT-ADAPTED BLOWFLY PHOTORECEPTORS

The response properties of R1-6 type photoreceptors of the blowfly (Ca lliphora 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 photorecept ors were treated with ionophoretically injected tetraethylammonium. Th e linearity and time-course of the photoresponses depended on the adap ting background and the duration of the contrast stimulus. At low back grounds photoresponses were approximately linear regardless of the con trast step duration. However, at higher backgrounds photoreceptors pro duced, with long contrast steps of opposite polarities, larger and slo wer hyperpolarizing responses than depolarizing ones, but linear respo nses with equal time-courses to transient (less-than-or-equal-to 2 ms) contrast changes. The early rising phases of photoresponses were inde pendent of the contrast signal duration, deviated to the same extent f rom the steady-state potential and steepened as the value of contrast was increased. The photoreceptor contrast gain increased with the back ground, but its value depended on the duration, the magnitude, and the polarity of the contrast step. Tetraethylammonium linearised the phot oreceptor membrane, increased and delayed photoresponses probably by b locking the shunting of the outwardly rectifying potassium channels. T hese effects did not strongly influence on the photoreceptors' charact eristic non-linear contrast dependence on light adaptation.