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Saturation and Adaptation in the Rod System

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Saturation and Adaptation in the Rod System Vision Res. Vol. 22, pp. 1299 to 1312, 1982 0042-6989/82/101299-14$03.00/0 Printed in Great Britain Pergamon Press Ltd SATURATION AND ADAPTATION IN THE ROD SYSTEM Edward H. Adelson* Vision Research Laboratory, University of Michigan, Department of Psychology, Ann Arbor, MI 48109, U.S.A. (Received 16 July 1881; in revised form 5 March 1982) Abstract—A background that is briefly flashed to a dark-adapted eye saturates the rod system. This transient saturation occurs with backgrounds that are as much as 2 log units dimmer than those producing saturation under steady viewing. Rod threshold is highest when the background is first turned on, and falls as adaptation proceeds. The nature of the adaptive processes are studied by presenting flashed backgrounds on pre-adapting fields. The data can be interpreted in terms of two adaptive processes: the first is multiplicative, and occurs rapidly; the second is subtractive, and occurs more slowly. INTRODUCTION have traced out the time-course of the effects of light adaptation. Crawford (1947) measured foveal incre- The retina is required to function under light levels ment thresholds against a flashed field, and found a that span an enormous range, and to do this effec- large transient threshold elevation during the first tively it must adapt to whatever ambient light level is 100 msec of background presentation (indeed, the present. In the cone system an important source of flashed background paradigm employed in the adaptation at high light levels is photopigment present study is sometimes known as "Crawford bleaching: as the pigment is depleted, the cones catch masking"). Boynton and Kandel (1957) measured fewer quanta, and so give smaller responses. Indeed, thresholds with flashed backgrounds, and found that pigment depletion seems to prevent cone signals from if the eye was adapted to a "pre-adapting" level before saturating under steady adaptation, as was shown by the flash appeared, threshold elevation could be Alpern et al (1970b). Those investigators also pointed reduced (but see Sperling, 1965). Although Boynton out that, since the bleaching process takes time to and Kandel did not reach saturating conditions, the come to equilibrium, one might expect that cone adaptive effects they observed are no doubt related to saturation could occur if thresholds were measured those that Hood and Geisler found would reduce the in a background that was flashed, rather than being tendency to saturation in cones steadily present. This effect had actually been ob- The present study is concerned with rods, rather served by Brindley, who hinted at it in Brindley than cones, and so the situation is somewhat different. (1959), without showing the relevant data points, and The rod system does show saturation in steady state then showed it clearly in Brindley (1963), where he (Aguilar and Stiles, 1954) and so one must conclude replotted some of his 1959 data. Brindley found cone that whatever adaptive mechanisms are present in the saturation to occur with flash intensities of about rod pathway are insufficient to prevent the signals 200 cd/m2, with a 4 mm pupil, which converts to from saturating. Photopigment depletion will not about 3.4 log td-sec. This value is similar to that later affect rod sensitivity via reduced quantum catch, since measured by Shevell (1977). (See also King-Smith and only in significant amounts of pigment are bleached Webb, 1974: Geisler, 1978.) below saturation. There is some evidence that verte- Hood and his co-workers (Hood, 1978; Hood et al, brate rods show no adaptation at all (Dowling, 1967; 1978), and Geisler (1978a, 1980) have found evidence Penn and Hagitts, 1972), although the evidence on that other adaptive mechanisms, besides pigment de- this is uncertain (see appendix 11), and in any case pletion, can also help prevent cone saturation they leaves open the question of adaptation beyond the have related these findings to the large body of elec- receptors trophysiological evidence on adaptation in photo- One way of studying adaptation in the rod system receptors (e g Kleinschmidt and Dowling, 1975; Nor- is to ask whether rod saturation is any different when man and Werblin, 1974) the backgrounds are flashed rather than steady. As Earlier psychophysical workers have also inves- was first reported in a preliminary description of the tigated the initial effects of a background flash, and present work (Adelson, 1977), the rod system can saturate with flashed backgrounds that are some 100 *Present address: RCA, David Sarnoff Research Center, times dimmer than those that cause saturation in Princeton, NJ 08540, U. S. A. steady state in the experiments that will be described, 1299 1300 Edward H. Adelson rod saturation was studied at various times after the The second beam followed a similar path, except onset of a back-ground, in an attempt to understand that it emerged from the lower pinhole at P, and was ° some of the adaptive processes that occur between the immediately reflected at 90 by mirror M0; it then onset of the background and the ultimate state of continued its route through S2, A2, W2, M2, FH2, FS adaptational equilibrium. (the field stop, defining the test patch), BS, FH4 and ML. METHODS When a third channel was needed, the optional beam splitter BS' was inserted, and the extra tungsten All experiments were done on the Maxwellian view lamp T' was turned on. This lamp (Mazda F-58) had system shown in Fig. 1. The system normally had 2 its filament arranged in a compact square, and the channels, but a third channel could be added by the filament itself was imaged in the plane of the subject's insertion of the beam splitter BS', shown in dashed entrance pupil (without the lens-pinhole system that lines. was used with the other channels). A tungsten ribbon filament, T, was run from a con- The observer stabilized his head by biting on a bite stant voltage power supply. Lens L imaged the fila- bar, which had an impression of his teeth in dental ment onto pinhole P, which actually consisted of two compound. With his right eye, he fixed on a fixation pinholes, one above the other, each selecting one por- point (FP) which was generated by a tiny light emit- tion of the elongated ribbon image. The beam from ting diode, imaged at optical infinity by a small lens, the top pinhole continued in a straight line, and and encased in a metal tube. encountered shutter S1 (Uniblitz; Vincent Associates), For most of the experiments, the author was the aperture A1, and neutral density wedge W, (Inconel; subject; one other subject was sometimes used. Both ° Bausch & Lomb). Mirror M1 introduced a 90 turn, subjects were well-corrected myopes, and wore their and the beam proceeded through the filters in filter spectacles during the experiments. holder FH1, the optional beam splitter BS' (if used), the combining beam splitter BS, and then to the Calibration of filters and light levels observer's eye (O) through filter holder FH4 and Four types of filters were used: neutral density Maxwellian lens ML. ML imaged the pinhole P in the inconels (Bausch & Lomb); neutral density Wratten plane of the eye's entrance pupil, and imaged the field No. 96 gelatines (Eastman Kodak, Rochester, NY); stop FS at optical infinity. colored gelatines (Eastman Kodak and Ilford Ltd., Es- Fig 1. (a) The Maxwellian view system. See text for details. (b) The observer's view in the apparatus. The test patch was 4.5° square, and was centered on the 11° background field. The fixation point (F P.) was 12°·to the right of the field. Saturation and adaptation 1301 sex, U. K.); narrow band inference filters (Baird The subject triggered all trials with a hand-opera- Atomic). The spectral transmittances of all filters were ted switch. He could control wedge W2 with a servo calibrated using a Beckman Acta II spectrophoto- motor, and wedge W1 with a mechanical linkage. meter. In all the experiments to follow, an attempt was The scotopic densities of the colored gelatins were made to isolate the rod system, and to prevent the measured in the apparatus by the method of Flamant cones from detecting the test flash. The conditions and Stiles (1948). The scotopic effectiveness of the adopted were similar to those of Aguilar and Stiles filtered light was compared with that of white light, (1954). by determining the amount of light needed to raise the As shown in Fig. 1b, the background subtended 11° scotopic threshold by l.0 log unit. The relative scoto- and appeared in the nasal half-field of the retina, cen- pic densities of the interference filters were measured tered 12° from the point of fixation. This background in the apparatus with a PIN-10 photodiode, using the was red (Wratten No. 26 gelatin; dom. wave. 621 nm). quantized CIE scotopic luminosity function. The ab- To adapt the cones maximally, the background light solute scotopic densities of two interference filters entered the subject's eye through the center of the were then measured by the method of Flamant and pupil. The background could either be steadily Stiles, and all the other interference filter densities present (as in Aguilar and Stiles' experiment), or were normalized against these. flashed. Absolute light levels, in scotopic trolands, were The test was a square, 4.5° on a side, and fell in the measured in two ways. First, the absolute calibration center of the background. To excite the rods preferen- of the PIN-10, supplied by the manufacturer, was tially, rather than the cones, it entered through the used to derive quanta sec-1 deg-1 with a given inter- nasal edge of the subject's pupil.
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