The equivalent background and retinal eccentricity

THE EQUIVALENT BACKGROUND ECCENTRICITY 1,2

and KENNETH FULD~

LOTHAR SPILLMANN Ncurologische Universitiitsklinik. (Received

5 December

AND RETINAL

Freiburg i. Br., West Germany

1977; in revised form

I9 June 1978)

Abstract-The validity of the equivalent background concept was tested for 8 retinal loci ranging from 3” to 70” on the nasal half of the horizontal meridian. Dark adaptation and increment lhreshold curves were obtained for two different test field conditions: 2*/l 75 msec and 0.5”/40 msec. both monochromatic at 55Onm. Results show that the equivalent background follows its own time course at each location. In photopic vision, the equivalent background for the inner periphery is lower than for the outer periphery. In scotopic vision, this relationship reverses. The findings suggest that the equivaknt background concept may obtain only for loci having comparable spatial and temporal properties. Key Words-equivalent

background; dark adaptation; increment threshold.

1NTRODUCTION

The course of dark adaptation in human vision depends not only on the characteristics of the preceding light adapting field, but also on the properties of the test field used for determining the dark threshold Differences of size. duration and wavelength of the test field may considerably affect the shape and overall level of the dark adaptation curve. These different test field properties also affect increment threshold curves, and Stiles and Crawford (1932) were the first to show how, by a simple transformation, the resulting discrepant dark adaptation and increment threshold curves could be united into one equivalent background vs time curve that was independent of the test field parameters. The Crawford transformation is based on the concept that each luminance threshold obtained at a given time during dark adaptation can be expressed in terms of the steady background that raises the increment threshold to the same value. The intensity of this background (real light) is thought to have an effect equivalent to the process (dark light) underlying the dark threshold, and it is by substituting one for the other

that

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background

vs time

curve is derived. Using this transformation, Crawford has shown equivalence to hold for size (1937, 1947) as well as, to some extent, for duration (1937) of the test field, and comparable results were obtained by Blakemore and Rushton (1965a). using both uniform test fields and gratings. Experiments using pupillary diameter (Alpern and Campbell, 1%2) and brightness of the afterimage (Barlow and Sparrock, 1964) as response

criteria lent further support to the equivalent background concept. In addition, Blakemore and Rushton (1%5b), DuCroz and Rushton (1966), Spillmann, Wolf and Nowlan (1971), and Spillmann, Nowlan and Bernholz (1972) have demonstrated that the effects of a bleaching background and a real-light background are not only interchangeable but also linearly additive. Other studies, however. of summation pools (Rushton, 1%5), CFF (Ernst, 1%8), spatial sensitization (Westheimer, 1968), photopic spatial summation (Rinalducci, 1968; Rinalducci, Higgins and Cramer, 1970). photopic temporal summation (Stewart, 1972; Rinalducci, Lowenhaupt and Martin= 1973) and border contrast (Tachibana, 1977) showed that the effects of dark light and real light are not equivalent. Non-equivalence was also found in records of the ERG (Cone, 1964; Maffei and Poppele, 1968), S-potentials (Naka and Rushton, 1968) and single unit activity (Berger, 1972). These studies, then, suggest that the usefulness of the Crawford transformation may be limited. Barlow (1972) has reviewed the evidence both for and against the equivalent background concept. In our experiment, we asked whether a single equivalent background curve could be obtained if retinal eccentricity were used as a parameter. Such a curve would describe the time course of dark a&ptation from the fovea to the periphery regardless of local threshold differences, and would thus be quite important. METHOD

The twc+channel Maxwellian view optical system de’ This work was presented at the Spring Meeting of the Association for Research in Vision and Ophthalmology in Sarasota, Florida, 1976. * Supported by the Deutsche Forschungsgemeinschaft (SFB 70. Teilprojekt A6). ’ Present address: Walter S. Hunter Laboratory of Psychology. Brown University. Providence. RI 02912. U.S.A. 117

scribed by Wooten, Fuld and Spillmann (1975) was used. Dark thresholds and increment thresholds were determined in two subjects at a total of eight retinal loci. For KF. measurements were made at 5”. lo”, 30’ and 50’; for LS, measurements were made at 3”. 8”. 40” and 70”. This was done for two ditTerent test field conditions: 2”/175 msec and 0.5”/40 msec. Both stimuli were mono-

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Fig. 1. Top-Dark adaptation curves obtained at retinal eccentricities of 5’. lO_, 30’ and 50- for KF (1eR)and 3’. 8’. 40’ and 70’ for LS (right). The test field had a diameter of 2: and lasted 175 msec. For both subjects, curves are based on two separate runs. Middle-Increment threshold curves obtained under the same conditions as above. Bottom-Equivalent background curves from the same data as above after performing the Crawford transformation. The cone “plateaux” have been omitted.

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chromatic at a wavelength of 550nm. Flashes were repeated once every 2 sec. Prior to each session, the subject dark-adapted for 30min. The eye was then exposed to 300,CUtOphotopic td of white (6740” K) light for 3 min. Upon termination of this light, thresholds were continually measured in the dark for 40min. at which time the rod plateau was nearly reached. About 50 thresholds were determined. Following a brief rest, increment thresholds were measured, first in ‘The number of data points was so large and the daytoday variability so small_ that any error introduced by fitting the curves by eye was considered to be minimal.

the dark and then at increasingly higher iuminances in steps of 0.3 log units up to 3 log td. Three thresholds were obtained at each luminance level and averaged. Both types of measurement were repeated at a later date and combined with the previous results. Curves were then fitted to the data by eye and the Crawford transformation applied. All thresholds were obtained using the method of ascending limits. The criterion for threshold was 3 consecutive responses to the same test field luminance. The eye was precisely aiigned by means of a pupil viewer, and only in the far periphery was it necessary to dilate the pupil (0.5% Mydriacil).

120

LOII+AR SPILLMANN and KENNETH FULD Testfield :27175 msec KF

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Fig 3. Equivulent background as a function of retinal eccentricity. The parameter is time during dark adaptation as measured from the moment of offset of the light adapting field. All curves refer to a test field of 2“ in diameter and 175 msec duration. The results on the left are from KF. and those on the right are from LS. Data were re-plotted from Fig. I.

RESULTS The top portion of Fig. I shows dark adaptation curves obtained with the Y/l75 msec test field and refers to retinal eccentricities of 5”. I&‘. 30” and 50” for KF (left) and to eccentricities of 3”. 8”. 40” and 70” for LS (right). At the outset. the dark adaptation curves appear to lie closely together. However. it can be seen, by taking into account the steepness of the slopes. that the curves are actually quite disparate. They remain separated at the cone plateau, but shortly thereafter begin to converge. In fact, the curves for 5” and 3” cross over the others. The middle panel of Fig. 1 shows the increment threshold curves for the same conditions. The bottom part of Fig. 1 gives the equivalent background curves derived from the dark adaptation curves and their corresponding increment threshold curves. The fist section, between 0 and 3 min. refers to cone adaptation. The second section, between 14 and 4Omin. refers to rod adaptation. The period representing the cone plateau has been neglected.5 In a way similar to the dark adaptation curves. the equivalent background curves initially appear to

5 If plotted. the cone plateau from the equivalent background curve would also show the pronounced *humps” seen in the dark adaptation curves. This threshold rise along the cone “plateau” has been observed by others (Spillmann, Hendershot and Nowlan. 1971: Wooten, Fuld and Spillmann, 1975; Wooten and Butler. 1976) and has been interpreted in terms of possible rod-cone interaction.

lie closely together, but in fact are discrepant. In the scotopic range, they diverge considerably and also cross over one another. It is clear that at the end, the total span is much larger than that for the dark adaptation curves (on top). Similar results are obtained when a test field of 0.5” and 4Omsec duration is used (Fig. 2). Here, as in the previous figure. the Crawford transformation does not reduce the differences between the curves but rather enhances it. This applies even when the tail ends of the curves are corrected for the Eigengrau. by extrapolating from the linear portion of the increment threshold curve toward the absolute threshold (Biakemore and Rushton, I965a). There is no indication that the four dark adaptation curves could be united into only one equivalent background curve. From Figs I and 2, one can derive functions that show how the equivalent background varies with retinal eccentricity at discrete times during dark adaptation. This is done by re-plotting the equivalent background values lying on any of an arbitrary number of hypothetical vertical lines intersecting the time axis in these two figures. Figure 3 shows the resulting functions for the 2”/175 msec test field. If the values of the equivalent background were independent of eccentricity. all functions would run parallel to the abscissa. This is clearly not the case. Instead. two types of curves are observed that deviate from the horizontal: one with a generally positive slope and another with a generally negative slope. From 0.5 to about 18 min. the values representing

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The equivalent background and retinal eccentricity Testfield

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the equivalent background for the inner periphery are about 0.5 log units smaller than those for the outer periphery. Thereafter, the relationship reverses. Beginning with 20min, the values of the equivalent background for the outer periphery fall by about an average of 0.5 log units below the values for the inner periphery. Similar results are obtained for the OS”/40 msec test field (Fig. 4). For this condition, the transition between the two types of curves occurs between I6 and 18 min. DISCUSSION

Our data show that there is no common equivalent background governing dark adaptation all across the retina. Instead, they suggest that- the time course of the equivalent background is specific for each Iocation tested. This agrees with earlier results obtained by Crawford (1937). who tested at 5” and 14” with a 27.6’ test spot presented for 50msec. His data, if replotted on an arithmetic time scale (Fig. 2, lower left) compare reasonably well with KF’s curves at 5 and 10”. Crawford’s equivalent background values descend faster, due to his weaker pre-exposure (79 c/ft’). However, his curves also reverse their relative pos ition after the change from cone to rod vision. This reversal reflects the various time constants for cone and rod dark adaptation associated with particular retinal eccentricities. In the photopic range, adap tation is more rapid in the near periphery, whereas in the scotopic range adaptation proceeds faster in the far periphery. In Fig, 1, the time constants increase with retinal eccentricity from about 1.5 to

1.8 min (KF) and 1.0 to I.Smin (LS) in the case of cones, but decrease from about 11.5 to 8.9 min (KF) and 9.2 to 6.2min (LS) in the case of rods. In Fig. f time constants similarly increase from about 1.0 to 1.3 min (KF) and 1.0 to 1.8min (LS) for cone adap tation and decrease from about 16.0 to 9.1 min (KF) and 13.3 to 6.1 min (LS) for rod adaptation. Although in our experiment there is considerable similarity between the quivalent background curves for some of the “adjacent” loci, such as 3” and 8” or lo” and 30” (Fig. I) and 10” and 30” or 40” and 70” (Fig. 2), one must also realize that the dark adap tation curves for these loci are quite similar even before the transformation is made. On the other hand, there are about as many examples where the spread between pairs of curves becomes /urger after the data are transformed. This enhanced separation between quivalent background curves results from changes of the dark adaptation curves that are not proportional to changes of the increment threshold curves (!%c Fig. 2). A much closer fit to a unified equivalent background curve is obtained when the results from the two test field conditions are compared at each retinal locus. In this case, the differences in the respective dark adaptation curves are greatly reduced when the Crawford transformation is made. This might be seen by superimposing any pair of dark adaptation curves (from Figs I and 2) for a given retinal locus and comparing it with the corresponding pair of superimposed equivalent background curves. However, if this comparison is made, there appear to be some systematic deviations. For locations from 3” to lo”, the equivalent background curve for the

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larger and longer-lasting test spot tends to fall slightly below the curve for the smaller and shorter test spot. At 30” and 40”. the curves match- fairly well, and at 50” and 70”. the relationship reverses. These trends are similar for both subjects and agree with Crawford’s (1937) results with different conditions of test field size and duration at 14” retinal eccentricity. Rinalducci et al. (1973) have pointed out that in fovea1 dark adaptation. test spot size and test spot duration interact with each other. This is also to be expected for extrafoveal regions and may be the reason why the Crawford transformation fails to produce fully coalescent curves when stimulus size and duration are covaried as in our experiment. From studies on spatial and temporal summation across the retina (Halkt. Marriott and Roda;er. 1962; Ronchi and NovPkov& 1971: Scholtes and Bouman. 1977). we may assume that there is also an interaction between either of these variables and retinal eccentricity. Differences in spat&temporal integration would thus enter if the equivalent background is compared at different retinal locations. For rod vision, our results suggest that summation on a dark-light background is more complete than on a real-light background. particularIy as eccentricity increases and time in the dark proceeds. This interpretation is compatible with Wcsthcimcr’s (1968) and Tachibana’s (1977) conclusions that under certain conditions inhibitory interactions tiectiag thresholds on real light do not occur in the presence of dark light. The present results are also cortoborative with those of Drum ipersonal communication) and support his interpretation that dark light produces less lateral inhibition than real light for small to moderate threshotd ekvations. It appears, then, that the Crawford transformation obtains only for retinal loci having comparable spatial and temporal propertics or for stimuli that are chosen to compensate for the difference of spatiotemporal summation at each eccentricity. This latter akemative remains to be tested. Ackno~ledgernenrs-We thank Harold Bedell. Bruce Drum and Charles Sternheim for their helpful comments on the manuscript.

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