National Academies Press: OpenBook

Recent Developments in Vision Research (1966)


Suggested Citation:"CONCLUSIONS." National Research Council. 1966. Recent Developments in Vision Research. Washington, DC: The National Academies Press. doi: 10.17226/18667.
Page 77
Suggested Citation:"CONCLUSIONS." National Research Council. 1966. Recent Developments in Vision Research. Washington, DC: The National Academies Press. doi: 10.17226/18667.
Page 78
Suggested Citation:"CONCLUSIONS." National Research Council. 1966. Recent Developments in Vision Research. Washington, DC: The National Academies Press. doi: 10.17226/18667.
Page 79

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77 target slowly approached his eyes. The importance of these mechanisms for work in which the pupil is used as an indicator, especially when the experiments are time-consuming, cannot be stressed too much. As a diagnostic tool for the neurologist and ophthalmologist, the pupillary near-vision response is less reliable than the light reflex be- cause of its subjective features. When the pupillary near-vision contrac- tion is poor or absent it is often not possible to be sure that the patient actually made a satisfactory effort of near vision. The reaction is important diagnostically when it is more extensive than the contraction to light, that is, in the Argyll Robertson syndrome and in "Adie's" syndrome. Conclusions The controversies that have clouded a clearer understanding of the pupillary receptor and near-vision mechanisms have had their origin mainly in (a) the difficulties in observing an organ as small and as mobile as the iris, especially in dim light; (b) differences in experimental pro- cedure, that is, the stimuli and state of adaptation used in different in- vestigations; and (c) differences in criteria used to determine pupillary responsiveness. Despite all these divergencies, an impressive number of experimental facts now available furnish convincing support for the assumption that, at the level of the retinal receptors, pupillary and visual afferent impulses arise from the same structures. The following are the most important findings: (1) In the normal, dark-adapted human eye (a) small pupillary contractions can be obtained with stimulus intensities below the photopic range; (b) the pupillary threshold is lower at the parafovea and the retinal periphery than at the fovea; (c) the pupillary threshold is lowered, and the contractions en- hanced, when a small stimulus field is enlarged, and also when an already large field is further increased in extent; (d) the pupillomotor effectiveness of colored stimuli is related to their apparent brightness, with the peripheral retina far more sensitive to white,- green, or blue light than to red light; in other words, the pupillary spectral sensitivity curve for large fields resembles closely the human scotopic visibility curve. (2) Totally color-blind patients, and patients who have lost all their visual field except for a small, peripheral area may have extensive pupillary reflexes.

78 (3) Animals with predominantly rod retinae, such as the owl and the rat, have sensitive pupillary responses, whereby the owl's pupillary spectral sensitivity curve is very similar to that of the human dark- adapted eye. (4) In the normal, light-adapted human eye (a) the pupillary threshold is much higher than after dark adaptation; (b) the retinal periphery is no longer more sensitive than the fovea; (c) compared to the conditions after dark adaptation, red light has gained in effectiveness; in other words, the Purkinje shift exists for the pupil; and for small, centrally fixated fields, the pupillary spectral sensitivity curve agrees with the CIE photopic visibility curve and with the spectral sensitivity curve obtained by flicker fusion photometry under the same experimental conditions.1l (5) Animals with predominantly cone retinae, such as the pigeon or chicken, have vigorous pupillary reflexes, and these animals are more sensitive to red light and less sensitive to blue light than is the owl. (6) When the stimulus intensity is increased above threshold values, the pupillary reflexes increase in amplitude. The reflex increments show behavior very similar to that of the increments in critical flicker fusion frequency: the pupillary and flicker fusion increment curves run parallel for stimuli of the same color and retinal location. In view of all these many kinds of functional parallelism between pupillary reflexes and subjective visual phenomena, it appears impossible, in the author's opinion, to uphold the theory of the existence of separate, specialized, pupillary receptors. This assumption would require the existence of a complex arrangement of at least two pairs of separate but closely related retinal elememts: a set of "visual rods" and "visual cones", and a second set of "pupillary rods" and "pupillary cones" that would mirror accurately the foveal and peripheral thresholds, the color sensi- tivity, and the incremental behavior of the visual cells. In addition, there is no reason why one should assume a double set of sensory elements in the eye, any more than one assumes a double set of auditory receptors for the appreciation of sound and for reflex movements elicited by sound, or a double set of temperature-or pain-receptors for sensory perception and for reflex adjustments. 11 In such experiments, the effect of stray light on the sensitive peripheral retina must be reduced by adaptation or by pre-adaptation to a back- ground field.

79 Pupillary reactions to light are very sensitive in the fovea as well as in the retinal periphery, even when small stimulus areas are used. In response to bright light, distinctive kinds of pupillary behavior are found, according to the conditions of adaptation, and the intensity, dura- tion, wave form, and frequency of the stimuli. Pupillary movements can be measured accurately in intact, conscious animals or man, without operative procedure of any kind. Be- cause of their objective and functionally unequivocal nature, it is hoped that they may become increasingly useful indicators of retinal activity, as more refined and reliable instrumentation becomes available. As to the pupillary contraction to near vision, the persistent ques- tions about its dependence on accommodation or on convergence appear outdated, since it is clear that it can take place in the absence of either of these functions. Normally, convergence, accommodation, and pupil- lary constriction are associated movements, brought into play by supra- nuclear mechanisms. Details of their normal interrelation, and of devia- tions from this normal correlation in pathological cases, might be clari- fied by simultaneous recording with the new electronic devices now available.

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