The hemeralopic aspect of achromatopsia can be diagnosed non-invasively using electroretinography. The response at low (scotopic) and median (mesotopic) light levels will be normal but the response under high light level (photopic) conditions will be absent. The mesotopic level is approximately a hundred times lower than the clinical level used for the typical high level electroretinogram. When as described, the condition is due to a saturation in the neural portion of the retina and not due to the absence of the photoreceptors per se.