Spatial Sensitization and Adaptation in a Long-Wavelength Cone Pathway

Flashed‐field sensitivities measured with a long‐wavelength probe have a red/green shape when stimuli are presented on a steady background but resemble the L‐cone absorption spectrum in the dark [M. A. Finkelstein, M. Harrison, and D. C. Hood, Sites of sensitivity control within a long‐wavelength cone pathway, Vision Res. 30, 1145–1158 (1990)]. We previously speculated that this could be due to concealed color opponency; receptive‐field surrounds, fed by M cones, are eliminated under dark adaptation. The current study tested this hypothesis with a sensitization paradigm. Thresholds were measured for a 640‐nm probe presented on flashes of two sizes. Data were obtained in the dark and on a 4.0‐log10 td steady background. The results suggest a model in which the L cones comprise a center mechanism and both M and L cones a weaker inhibitory surround. Dark adaptation eliminates only the M component of the surround