Threshold Vision and Light Quanta: A Thread through the History of Visual Science

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Abstract
For almost a century quantitative determinations of the ‘minimum perceptible’ have intrigued scientists in various disciplines in their search for knowledge and insight in the processes of visual perception. On several occasions the instability of vision near threshold has been attributed to the statistical variation of the light stimulus as is inherent in its quantal nature. However, variations of a neuro-physiological and psychological nature also contribute to this instability. Their respective contributions are not precisely determinable. For further analysis it was in most instances assumed that receptive units in the retina produce a signal for the cortex by the coincidental absorption of at least a critical and small number of quanta. Values proposed, and still occurring in the literature, for the lowest possible critical number range from one to seven. The elicitation itself of such a signal, as well as of its perceptual attributes such as hue and brightness, seems to depend on the spatial spread of the quanta over the receptors of distinct receptive units. Indications for this are found in the configuration-dependent summation of the incident light energy at threshold, including the failure of Ricco's law for the smallest circular test objects at far eccentric retinal locations. These facts lead to the supposition that a potentially visible signal for the brain is not obtained from the coincidental absorption of at least a critical number of quanta, no matter their spatial spread over the receptive unit concerned, but from the coincidental excitation of at least a critical number of sub-units by the absorption of at least one quantum in each of these sub-units. The latter may be individual receptors.