Visual Spatial and Temporal Summation

THE interpretation of the laws of spatial and temporal summation, found to be valid at the threshold of peripheral vision, is of importance for the understanding of the mechanism of retinal excitation. Van der Velden1 was the first to give a rational interpretation of this problem. He showed that if the absorption of two light quanta elicits a threshold sensation of light, the only laws of spatial and temporal summation compatible with such a mechanism are those which are actually observed. Square root laws, namely Piper's Law for spatial and Pieron's Law for temporal summation, are of the form: where X represents the area or the exposure time of the test, and I the threshold stimulus.