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A photoreceptor class important for low-light vision. Greater sensitivity and downstream pooling support detection under different conditions from cone-dominated vision. When only one spectral receptor class supplies usable information, the principle of univariance prevents independent wavelength–intensity identification.

Selig Hecht was a researcher in the physiology of vision who measured how sensitivity changes during light adaptation and dark adaptation. In a 1937 study, Selig Hecht and colleagues varied prior illumination and then measured recovery of the human eye's ability to detect faint test lights.^[ref]

The recovery curve depended on the preceding exposure. Under sufficiently bright preadaptation, an early phase associated with cone cells was followed by a slower phase associated with rod cells; other exposure conditions did not reveal both phases in the same way.^[ref]

This makes prior conditions part of the experiment. Two observers tested after the same time in darkness need not be in comparable states if their preceding illumination differed. A curve measured over time reveals changes that a single final sensitivity measurement would hide. The historical illumination unit called photons in this paper should not be interpreted as a count of individually detected photons.

A sensitive period is an interval of development during which a particular kind of experience has an unusually strong effect on subsequent behaviour or nervous-system organization. It is specific to a function and experimental outcome: susceptibility of visual acuity and susceptibility of a recorded neuron's response need not have identical schedules. Such timing is one aspect of neuroplasticity.

A revealing experiment varied both the age at which a kitten's eyelids were sutured shut on one side and the duration of closure. The investigators later compared vision through the two eyes using graded behavioural tasks. Starting deprivation at different ages produced different lasting effects, and longer early deprivation affected more demanding tasks. This separates the timing of experience from merely its total absence.^[ref]

The same paper reported some improvement when subsequent experience forced use of the previously deprived eye, including in one older animal. A sensitive period can therefore coexist with later improvement. Its experimental meaning depends on the measured behaviour, the timing and duration of deprivation, and the opportunity for recovery.^[ref]

A category of sensory input and processing, such as vision, hearing or gustation. Receptor type, pathway and experienced quality provide related classifications, but should not be assumed to define identical boundaries.

Corrective lenses worn before the eyes to modify incident light. Lens power and orientation can compensate for aspects of refractive error, while ocular disease can still limit the best achievable vision.

The region of space available to vision under specified viewing conditions. Field testing and attention testing assess different potential reasons for missing a target.