Noninvasive imaging of the thirteen-lined ground squirrel photoreceptor mosaic.
The spectral sensitivity of the European ground squirrel was investigated by electroretinography (ERG) and visual-evoked cortical potentials. Using chromatic adaptation, two spectral systems were found, one peaking between 520 and 530 nm, the other at 465 nm. The blue system was characterized by a slower course of the potential, a lower threshold and the lack of a d wave in the ERG. However, its threshold is higher than the rod threshold and it was shown to be more resistant to adapting light stimuli than rod systems are. It is concluded that the second spectral system is a blue cone system which in its rod-like properties is comparable to the blue cones found in cat, monkey and man.