Fig. 2. Spectral sensitivity of fish after 2 weeks of thyroxine, or control, treatment. All spectral sensitivity curves were obtained under a long-wavelength background to isolate the short-wavelength and ultraviolet cone mechanisms (see text). Both control (A,C) and thyroxine-treated (B,D) fish were tested for dorsal and ventral retinal sensitivity (Fig.1). (A,B) Results for dorsal stimulation, and (C,D) results for ventral stimulation. The values are means ± S.E.M. for all fish from each group (filled diamonds, with sample size (N) indicated on each graph). Data were first normalized to 420nm (i.e. sensitivity at 420nm was set to 0 in all fish). Pigment absorptance curves for the ultraviolet cone mechanism (purple dashed line) and the short-wavelength cone mechanism (blue dashed line) were fitted by eye to the averaged data. The resulting function of the linear-additive model for the ultraviolet and short-wavelength cone mechanisms is represented by the continuous black line. The weights determined by the linear-additive model for the ultraviolet (K_U) and short-wavelength (K_S) mechanisms (see text for further details) are presented on each graph.