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Visual Consequences of the Foveal Pit

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Visual Consequences of the Foveal Pit Visual consequences of the foveal pit David R. Williams When viewing a uniform field through a small aperture moved circularly before the pupil of the eye, many observers report a diffuse dim spot subtending roughly 1° visual angle, which rolls around the fixation point in synchrony with the motion of the aperture. Measurements of the Stiles-Crawford maximum for different locations within the fovea confirm that small but systematic shifts in foveal directional sensitivity are responsible for this entoptic effect. A quantitative model is developed which accounts for the directional sensitivity shifts on the basis of refraction of light at the sloping sides of the foveal pit. Supporting evidence for the model is provided by a second entoptic phenomenon: fundal scatter from the image of a glare source on the eccentric retina is reflected by the foveal pit in such a way as to render it visible. Estimates of the slope of the foveal pit from this effect agree well with that predicted from the observed shifts in directional sensitivity. Key words: Stiles-Crawford effect, fovea, entoptic effects, directional sensitivity, receptor orientation, fundal scatter <J ince the first report of Stiles and Craw- patches to be a result of small differences in ford,' a large body of histological and psy- photoreceptor orientation in different retinal chophysical evidence has accumulated to locations. show that cones in different retinal regions The subject of this investigation is an en- are directionally sensitive, having peak sen- toptic effect which implies a systematic vari- sitivities clustered about a common point in ation in the directional sensitivity of cones the exit pupil of the eye.2' 3 In addition to this within the fovea. If a small aperture is moved general directional tendency, psychophysical in a circular fashion close to the eye while the evidence shows that small local variations in observer views a dim uniform surface, the the directional sensitivity of the normal ret- retinal vasculature can be seen as a fine ina do exist. O'Brien and Miller4 (see also branching pattern surrounding the foveal Enoch5) demonstrated these local variations avascular area. Within this avascular area, by rapidly changing the entry point of light in many observers report a gray patch roughly 1° the pupil while continuously adjusting its in- in diameter which moves within the central 2° tensity to compensate for the Stiles-Crawford of the fovea, depending on the entry point of effect. Observers reported light and dark light in the pupil. Circular motion of the aper- patches in the visual field which did not ture causes this dim spot to revolve around the change with the entry point of light in the line of sight in phase with the aperture, ap- pupil. O'Brien and Miller interpreted these pearing on the same side of fixation as the light is entering in the pupil. There is often a From the Department of Psychology, University of less conspicuous brightening which revolves California, San Diego.- in diametrical opposition to the dark spot. Supported in part by NSF Graduate Fellowship SMI Like the entoptic retinal blood vessels, this 7622813 and NIH Grant EY 10711. phenomenon disappears quickly when the Submitted for publication Feb. 28, 1979. Reprint requests: David R. Williams, 2C-548, Bell Tele- aperture motion is stopped. phone Laboratories, Murray Hill, N. J. 07974. The appearance of the dim patch on the 0146-0404/80/060653+15$01.50/0 © 1980 Assoc. for Res. in Vis. and Ophthal., Inc. 653 Downloaded from iovs.arvojournals.org on 05/05/2020 Invest. Ophthalmol. Vis. Sci. 654 Williams June 1980 Koppenberg* has described the entoptic ef- fect, attributing it to the hillock formed by the elongated cones in the central fovea.8 He assumes that rays entering through one side of the pupil are less effective in stimulating cones on the far side of the hillock than those on the near side. Unfortunately, there is no obvious physical justification for this as- sumption. Heath and Walraven9 briefly re- port a similar effect, i.e., uniform fields briefly imaged within the fovea have a gra- dient of brightness across the field, depen- dent on the peripheral entry point of light in the pupil. On the basis of Stiles-Crawford measurements for 1° fields within the fovea, Heath and Walraven9 conclude that foveal cones are oriented nearly parallel to each other rather than pointing toward a common Fig. 1. Hypothesized directional sensitivity of location in the pupil. cones on opposite sides of fixation which would This paper argues that small local varia- account for the entoptic dim patch seen in the tions in directional sensitivity may not always fovea when the pupillary entry point is changed. reflect differences in photoreceptor orienta- tion; a quantitative model is developed which same side of fixation as the light is entering in shows that the entoptic effect can be ex- the pupil suggests that cones at slightly ec- plained by refraction at the sloping sides of centric positions within the fovea are less the foveal pit rather than by some perturba- sensitive to light entering from the opposite tion in the orientation of the cones them- side of the pupil than they are to light enter- selves. ing from the same side. In Fig. 1 the lines within the eye connect cones in various reti- Local variation in directional sensitivity nal locations with the respective points in the within the fovea pupil at which they are most sensitive. Ex- In order to find out about small variations trafoveal cones (labeled P) obey the anterior in the Stiles-Crawford effect across the fovea, 6 pointing hypothesis ; they are maximally a technique was developed which provided sensitive to rays passing through the center of an accurate measure of directional sensitivity the exit pupil, D. However, suppose that for in the pupil for small retinal locations. some reason cones in the nasal fovea (N) are maximally sensitive to light entering in the Apparatus. The Maxwellian view system shown in Fig. 2 consisted of a test channel, a fixation nasal pupil (N') whereas temporal cones (T) prefer light from the temporal pupil (T") (and similarly for cones in the superior and in- *The phenomenon exhibits significant individual differ- ferior retina). Then a light entering, say, in ences, but Koppenberg7 reported that with his apparatus the temporal pupil will be relatively inef- nearly everyone he examined could eventually see the effect. Relatively crude observations were made in this fective in stimulating nasal cones; the di- laboratory by having observers view a dim white surface minished signal from these cones could be through a 2 mm perature jiggled by hand in front of the responsible for the entoptic gray patch. pupil. The effect was visible in only about 40% of the 62 Although it seems reasonably clear that the eyes tested. The remaining observations consisted of a bewildering variety of foveal percepts including flicker- entoptic effect implies local variations in di- ing but motionless gray patches of various shapes and rectional sensitivity within the fovea, the ac- sizes. Two observers even reported a light spot instead of tual cause of these variations is less certain. a dark spot which moved in phase with the aperture. Downloaded from iovs.arvojournals.org on 05/05/2020 Volume 19 Number 6 Visual consequences of foveal pit 655 TUNGSTEN SOURCE OIFFUSER 500 r.m FILTER ARTIFICIAL PUPIL POLARIZER FIXATION HOLES TEST APERTURE B, o ALIGNMENT EXPERIMENTER CHANNEL Fig. 2. Apparatus used to measure foveal directional sensitivity. channel, and an alignment channel. The test and test channel at 4 to 5 Hz, sinusoidally modulating fixation channels were combined by beamsplitter light from each hole in the artificial pupil aperture B,, which was in turn combined with the align- at 8 to 10 Hz. ment channel by beamsplitter B2. The source was This arrangement had the effect of changing the a tungsten lamp (GE 200 watt, 120 V Quartzline entry point of light in the observer's pupil in a lamp). A piece of frosted glass between the lamp circular fashion at 8 to 10 Hz while keeping the and the artificial pupil in the test channel provided total flux entering the eye constant over time. This uniform illumination of the aperture defining the was verified by placing a photocell in the image artificial pupil. This aperture was imaged at plane and noting that the output did not change by slightly less than unit magnification by lens L{ iii more than 2.8% with the orientation of the circular the plane of the observer's pupil. A test aperture polarizer. The photocell output was also indepen- at roughly one focal length from L, formed a small dent of the position of the test aperture (which was circular test field on the observer's retina. The moved to different locations in a plane orthogonal retinal illuminance of the 500 nm test field was 460 to the optical axis during the experiment), estab- trolands. lishing that the test aperture did not selectively The artificial pupil in the test channel consisted vignette any of the four spots in the image plane at of a metal plate in which four small holes were the pupil. drilled, forming a diamond-shaped array as shown The position of the artificial pupil aperture and in Fig. 2. Thus light entered the eye through four hence of the image in the pupil could be con- spots in the plane of the pupil, each spot measur- trolled by the observer with horizontal and verti- ing 0.75 mm in diameter, with spots opposite each cal micrometer adjustments. His task was to find a other spaced 4 mm apart from outer edge to outer position of the array in the pupil for which flicker edge.
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