ABSTRACT
The ability to perceive a certain number of visual stimuli per unit of time has been found to differ significantly between hypothyroid patients and normals, 1 between older and younger persons, 2 ’ 3 between fatigued and rested truck drivers, 4 etc. In all such cases, the number of light-dark cycles per second at which a physically intermittent light was just perceived as a steady light was found to be lower for the more fatigued cases or those with lower metabolic rates than for normals or rested persons.
