Visual thresholds in albino and pigmented rats

Albino rats have recently been reported to have increment thresholds against dim backgrounds that are two log units higher than those of pigmented rats. We, on the other hand, have failed to confirm these differences using electroretinogram b-waves and pupillary light reflexes. This paper reports on experiments using evoked potentials from cortex and colliculus and single-unit recordings from colliculus. We recorded visual-evoked potentials from cortex and superior colliculus in the strains of albino (CD) and pigmented (Long-Evans) rats used in the earlier studies. Thresholds were determined on eight fully dark-adapted animals by extrapolating intensity-response curves to the point at which there was zero evoked potential. The average dark-adapted threshold for the visual-evoked cortical potential was -5.6 log cd/m2 in pigmented and -5.80 log cd/m2 in albino animals. The average dark-adapted threshold for the superior colliculus evoked response was -5.54 log cd/m2 in pigmented and -5.84 log cd/m2 in albinos. The differences were not statistically significant. On the same apparatus, the average absolute threshold for three human observers was -5.3 log cd/m2, a value close to the rat dark-adapted thresholds. Thus, visual-evoked cortical potentials and superior collicular evoked potentials failed to confirm the report of higher dark-adapted thresholds for albinos. In addition, we find that single units in superior colliculus in the albino rat respond to very dim flashes.