S-CONE SIGNALS TO TEMPORAL OFF-CHANNELS - ASYMMETRICAL CONNECTIONS TOPOSTRECEPTORAL CHROMATIC MECHANISMS

Psychophysical tests of S-cone contributions to temporal ON- and OFF-c hannels were conducted. Detection thresholds for S-cone modulation wer e measured with two kinds of test stimuli presented on a CRT: a rapid- on sawtooth test and a rapid-off sawtooth test, assumed to be detected differentially by temporal ON- and OFF-channels, respectively. S-cone related ON- and OFF-temporal responses were separated by adapting for 5 min to 1 Hz monochromatic (420, 440, 450, 540, or 650 nm in separat e sessions) sawtooth flicker presented in Maxwellian view. Circular te st stimuli, with a sawtooth temporal profile and a Gaussian spatial ta per, were presented for I s in one of four quadrants 1.0 degrees from a central fixation point. A four-alternative forced-choice method comb ined with a double-staircase procedure was used to determine ON- and O FF-thresholds in the same session. Following adaptation, the threshold elevation was greater if the polarity of the test stimulus was the sa me as the polarity of the sawtooth adaptation flicker, consistent with separate ON- and OFF-responses from S-cones. This asymmetrical patter n was obtained, however, only when the adaptation stimuli appeared blu e with a little redness. When the adaptation flicker had a clear reddi sh hue component, the threshold elevation did not depend on the polari ty of the sawtooth test stimuli. These results are consistent with a m odel in which OFF-signals originating from S-cones are maintained by a postreceptoral mechanism signaling redness, but not by a postreceptor al chromatic mechanism signaling blueness. (C) 1998 Elsevier Science L td. All rights reserved.