Mathew Alpern

Variability of Accommodation during Steady Fixation at Various Levels of Illuminance

Using the stigmatoscope, the variability of the dioptric power of the eye during steady fixation was measured for four observers over a range of intensities from 2¯.5 to 3.0 log trolands in one-half logarithmic steps. Two different viewing conditions, (constant and variable size test letters), were studied but no significant difference between them was obtained. There was almost a fourfold decrease in the variability of the settings as the intensity was increased over this range. There was a sharp transition at one troland, suggestive of a discontinuity between scotopic and photopic vision. These changes could be eliminated by cycloplegia of the fixating eye. Essentially the same results were obtained when the refraction of one eye was measured objectively (with a concidence optometer), while the other eye fixated the chart. This latter technique is, however, less valid at the lower intensities because of the tendency to fixate the measuring light of the optometer. The data can be quantitatively described by a theory which postulates that the accommodation is continually fluctuating and that the limits of fluctuation are proportional to threshold ΔI/I.

Luminance-Duration Relationship in the Electric Response of the Human Retina*

Measurements of the magnitude of the a and b waves of the human electroretinogram on the right eye of one dark adapted observer have been carried on over a range of durations [5 to 143 msec] and retinal illuminances [2.61 (10)6 to 2.61 (10)2 trolands] of the stimulating flash. The results have been analyzed in terms of their implications for photochemical theory of the voltage generation in the electroretinogram as well as for the theory of metacontrast.

The kinetics of cone visual pigments in man

Abstract Rushtons general kinetic equation for human cone pigments is tested by estimating the photolysis rate at equilibrium from the initial photolysis rate and comparing it to the equilibrium rate of regeneration measured immediately after the bleaching light is turned off. Thirty-seven experiments carried out at a variety of intermediate bleaches confirm the validity of this equation. The hypothesis that the regeneration rate depends upon the store of 11- cis retinal leads to the expectation that recovery from a prolonged weak bleach will proceed with a shorter time constant than recovery from a long intense bleach, and this is not found. This could happen if even the prolonged weak bleach depleted the 11- cis store but this possibility is excluded by an experiment in which recovery is measured following 5 sec full bleaching which follows straight on a prolonged weak one. Why recovery from short full bleaches proceed twice as fast as recovery from long full ones remains unexplained.

Pupil responses to foveal exchange of monochromatic lights

Temporal exchanges of equiluminant monochromatic lights of different spectral distributions produced a momentary constriction of the pupil in man. This is not a stimulus artifact because exchanges of two lights with identical distributions in the same apparatus produced no response. Responses evoked by rod signals were successfully obviated by presenting the foveal stimulus inside a large rod saturating annulus. The amplitude of the response varied systematically with stimulus wavelength. The exchange of a standard light to either shorter or longer wavelength lights produced a momentary constriction of the pupil; the greater the wavelength difference (between them); the larger the constriction. This ability to respond to exchanges of one spectral distribution for another is not a consequence of chromatic aberration or chromatic differences in magnification. Chromatic exchanges between lights of equal chromatic aberration do not produce identical pupillary response in deuteranopes: The exchange 560 nm→650 nm produced no pupil response, while the 560 nm→498 nm exchange produced a sizable response. Exchange of equally luminant heterochromatic lights evoked a response with 50 ms longer latency than the same amplitude constriction evoked by a step increase in luminance of a homochromatic light. The homochromatic contrast needed to evoke the same constriction as a given equal luminance heterochromatic exchange closely follows the homochromatic contrast which matched the residual flicker in flicker photometry of that same wavelength pair.

Durations of the after-images of brief light flashes and the theory of the Broca and Sulzer phenomenon.

Measurements were made of the time interval between the primary stimulus and the moment of disappearance of every trace of the after-image for flashes of very intense lights of variable duration. The results can be explained on the assumption that the duration of the after-image is determined by the time that it takes for the product of a photochemical reaction to fall to some threshold value. Certain predictions from this hypothesis are experimentally verified.

The effect of bleaching and backgrounds on pupil size

Abstract Measurements of pupil size and retinal rhodopsin in the dark after a full bleach confirm the suggestion of Alpern and Campbell that pupil dia. is linearly related to the fraction of unregenerated rhodopsin. This provides a transform [equation (1)] which can be used in conjunction with current ideas of visual adaptation to infer the pupil diameter at any luminance level ( log I ≤ 5.0 scotopic td) in equilibrium viewing in a uniformly illuminated ganzfeld. Experimental determinations of such values on three normal subjects are in reasonable accord with expectation.

Photic Driving of the Critical Flicker Frequency

A light pulsing above the critical flicker frequency (CFF) significantly elevates, and one pulsing below CFF significantly depresses, subsequent CFF measurements. The characteristics of these phenomena, their dependence upon the duration of fixation, the luminance of the measuring and the adaptation lights, and the duration of the after-effect are described in the present series of experiments. The effects are virtually as pronounced if the pulsing adaptation light is viewed with one eye and the measuring light with the other, as when both adapting and measuring lights are seen by the same eye. It is proposed that the driving is a direct consequence of a change in the frequency characteristics of the responses of cells in the visual system to photic stimulation. Measurements of the brightness of lights, pulsing at various rates immediately after viewing lights pulsing above and below the CFF, confirm certain predictions of this hypothesis.

Response of the Pupil to Steady-State Retinal Illumination: Contribution by Cones

Response of the pupil to steady-state retinal illumination was measured in an observer who lacked functioning rods. At high intensities, this response was as great as that of a normal eye. These results cannot be explained by the hypothesis that only rods are receptors for the steady-state response.

Spectral Transmittance of Visible Light by the Living Human Eye

The ocular media transmit different amounts of visible light depending on wavelength. The magnitude of this transmission can be estimated by reflecting a monochromatic beam of light off the inside of the sclera of a living eye which has an anomalous absence of choroid and retina. Measurements of this kind on three living human eyes are in good agreement with previous transmittance estimates based on in vitro spectrophotometry of enucleated eyes.

Adaptation of the pupil light reflex

Abstract The consensual photopupil responses to light flashes presented in uniform illumination to the entire visual field were recorded in full dark adaptation, over a gamut of intensities of ganzfeld backgrounds and in the dark after full bleach of all visual pigment in the retina. The equivalent background of such bleaching was found to be valid not only at the photopupillomotor threshold but also for flashes several log units suprathreshold (as judged by matching of the waveform of the pupillomotor response). The pupillomotor equivalent background is in reasonable agreement with its psychophysical counterpart, suggesting that the neural pathways for vision and photopupillary motion are common, at least to the level that retinal sensitivity adjustment in dark and light adaptation has been achieved.