SENSITIVITY LOSS AND ADAPTATION WITHIN THE SHORT-WAVELENGTH CONE PATHWAY

Two-color increment threshold data are used to develop a model of sens itivity control in short-wavelength detection. The model includes two sites of compressive nonlinearity. The first site has the characterist ics of the S cones. Their output combines antagonistically with signal s from the L and M cones at a second (blue/yellow opponent) stage. The response nonlinearities were examined by measuring thresholds for a 4 20-nm probe presented at the onset of flashed fields of 420-nm, 520-nm , and 580-nm light. Adaptive mechanisms of multiplication and subtract ion were identified by presenting probe and flash on steady background s of varying intensity. Adaptation partially restores the sensitivity lost to response compression.