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A static perimetry technique is described, which was first introduced by Enoch. The test-target is flashing and the increment threshold is recorded continuously for five minutes. In this way, using a commercially available Goldmann perimeter, it is possible to find a time-dependent decrease in sensitivity in patients with lesions of the optic pathways. No significant loss in sensitivity is found in patients with pre-disc pathologies, who show visual field deficits. This drop in sensitivity is at least of one log unit. The phenomenon is considered to be a "fatigue" in transmission of visual information and is presumably due to lesions in the conduction system (myelin). The test, which is easily performable, can be used follow-up the lesion: when this resolves, the decrease in sensitivity also disappears. Two patients with retro-bulbar neuritis with the typical characteristics of the above mention phenomenon are presented.