Spectral Sensitivity in the Pigeon

Measurements of light- and dark-adapted absolute thresholds were obtained from three pigeons at 15 wavelengths ranging from 380 mμ to 700 mμ. Pecking responses caused a stimulus patch to fluctuate in intensity up and down across the pigeon’s threshold, and a record of the intensity provided the sensitivity data. The sensitivity of the birds was followed throughout a period of 80 min following a standard pre-exposure to white light. Four complete dark adaptation curves were obtained from each bird at each wavelength. Spectral sensitivity functions derived from these curves place the photopic maximum at 560–580 mμ, and the scotopic maximum at about 500 mμ. The scotopic function is fitted closely by aphakic human data. The photopic function shows inflections that may be related to similar inflections in corresponding human curves. The functions are quite similar to those found in electrophysiological studies of the pigeon eye. They also correspond rather well to the absorption spectra of chicken rhodopsin and iodopsin.