Visual sensitivity in the region of chromatic borders

V&m

Rrr. vol.

12,pp.1715-1724.

Pergamon Press 1972.

VISUAL SENSITIVITY

Printed inGreat Britain

IN THE REGION BORDERS

OF CHROMATIC

CHARLESE. STERNHEIM, ROBERTA. GLASSand JOHNV. KELLER

Department of Psychology, University of Maryland, College Park, Maryland 20742 (Receiued 11 October

1971; in reoised form 17 February 1972)

INTRODUCTION DIFFERENCES are sometimes

found between the light distribution on the retina and the variation of brightness of corresponding portions of the visual field, e.g. Mach bands. Another example is simultaneous brightness contrast produced by adjacent fields of differing luminance. Electrophysiological studies have demonstrated the existence of summative and inhibitory processes which may in part be responsible for these phenomena. Eye movements and the resultant modulation of receptor illumination may also be involved. The increment threshold has been determined for a small spot of light superimposed upon a gradient of illumination that produces Mach bands. FIORENTINI and TORALDODI FRANCIA (1955) and BURKHARDT(1966) found an increase in threshold in the region corresponding to the position of the bright band. The increment threshold is also found to increase in the region of a luminance step (BURKHARDT, 1966; MATHEWS, 1966, 1967; NOVAK and SPERLING,1963; TELLER,1968), although there is disagreement over whether Mach bands are seen in this region (BEKESY,1968; MATTHEWS, 1966; THOMAS, 1965). In the present experiment visual sensitivity was measured using the increment threshold technique in retinal areas where there was a step-wise change from one monochromatic light to another. Both lights were matched in luminance. Chromatic Mach bands have been observed in the region of step-wise borders that separated fields differing in hue (DAw, 1964) but have not been observed consistently in a region where there was a gradual change from

one hue to another (DAw, 1964; VAN DER HORSTand BOUMAN, 1967). It was the purpose of the present experiment to compare spatial interactions in the chromatic and brightness channels of the visual system by determining the manner in which sensitivity to a small test field varies in the region of a chromatic as well as a luminance border. METHOD Apparatus

The three channel Maxwellian-view optical system used in the present experiment is shown in Fig. 1. The adaptation field was provided by the left and center channels, the test field by the right channel. The source (S) was a Sylvania Tungsten Halogen lamp Model EHR (400 W, 120 V). A selenium photocell was placed near the lamp in order to monitor its output. The output from a constant voltage transformer (SOLA Co.) was adjusted by means of a Variac (Model 2) so that the light output was constant from session to session. The light from the left and center channels were combined by a front surface mirror (MM) mounted on a moveable stage. The edge of the mirror had no obvious imperfections in silvering and appeared in sharp focus as the border between the two components of the adaptation field. With the position of the test field held constant. the location of the border was varied in small steps by moving the stage upon which the mirror was mounted. The scale attached to the stage was read to the nearest O-5 mm. 1715

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CHARLES E. STERNHEIM, ROBERTA. GLASSANDJOHNV.

KELLER

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Visual Sensitivity in the Region

of Chromatic Borders

1717

Procedure The right and left components of the adapting field were equated in luminance based upon a heterochromatic step-by-step brightness matching procedure. Threshold measurements followed a ten-minute adaptation period during which time a subject fixated the center of the adapting field. ft has been shown that fixation within the center of a small field of light can be accurately maintained (Rnrr~~. 1969). The 2’ test field was presented in the center once every second for 80 msec. The subject adjusted the neutral density wedge in the test channel until the t&t field was just visible. The position of the border within the adaoting field-was varied after each threshold determination. The diameter-of the adapting field remained fixed. Nine border positions were sampled in an experimental session. Threshold values were found to be independent of the order in which these positions were selected. They were typically chosen in a haphazard manner. The scale setting which positioned the border in the center of the adaptation field was found at the end of the experimental session. This determined the relative position of the other settings. Although the same scale values were selected in different sessionsthese did not alwayscorrespondto the same retinal positionsdue to small changes in alignment. The border, therefore, was not necessarily moved the same d&an* from the center of the adaptation field in both the right and left directions. One to four threshold determinations were made for each border position depending upon the phase of the experiment. The method of adjustment was se&ted since a large number of determinations had to be made in one experimental session. A double random staircase procedure OXZNSWEET. 1962) was used to confirm threshold values obtained under sekcted conditions. RESULTS Experiment

1

The dominant wavelength of the left aad right component of the adapting field was ,540 nm. A wavelength in the middfe part of the spectrum was chosen since it could be paired in the following experiments with both shorter and longer wavelengths which were considerably different but clearly within the visible range. The left half had a luminance of 2.26 log

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FIG. 2. Log relative threshold (mean of four dete~inations)

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for a white test field as a function of thedisplacement of the border from the centerof the adaptation field. Thedominant wavelength of the left and right components of the adaptation field was 540 nm. The left component contained 2.26 log ft-L. the right component l-36 log ft-L. Data for S1 and Sz. Curves in this and in the following figures are arbitrarily positioned along the ordinate for comparison purposes.

t7is

CHARLES

E.

STERNHEIM,ROBERT A. GLASS AND JOHN V. KELLER

ft-L; the right half was O-9 log unit lower being l-36 log f&L. Figure 2 shows the log relative threshold values for a 2’ white test field as a function of the displacement of the border from the center of the adapting field. There is an elevation in ~esho~d in the region of the luminance border. This result is similar to that reported by other investigators (e.g. MATTHEWS, 1967; NOVAK and SPERLING, 1963). Experiment 2 The dominant wavelength (540 nm) and luminance (2.26 Iog f&L) of the left component of the adapting field were held constant. The dominant wavelength of the right component was varied. The energy was adjusted in each case so that the luminance of the right component would remain equal to that of the left component. Log relative threshold values for a 2’ white test field as a function of border position are shown in Fig. 3. The parameter on the curve is the dominant wavelength of the right component of the adapting field. There is an elevation in threshold in the region of the chromatic border. The size of the elevation is dependent upon the wavelength difference across the border. When the two components are matched in wavelength as wet1 as luminance (middle curve) there is a small elevation in threshold in the region of the border. The elevation in threshold increases as the wavelength difference between the two components increases. 25

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FIG. 3. Log relative thresholdtmean of four determinations)for a white test field as a fwtion of the displacement of a border from the cpnter of the adaptation field, The don&wat wavelength of the left compoflent was 540 nm. The parameter on the curves is the dominant wavelength of the right component. Both components contained 2.26 log ft-L. Data for Sr.

The data for a ae~ond subject obtained under identical conditions are shown in Fig. 4. Again, it is seen that sensitivity in the region of the chromatic border is indent upon the wavelength difference across that border. When the fields were matched in lumiinzmce and wavelength the threshold kurve is relatively flat. With one exception (590-640 nm) the

Visual Sensitivity in the Region of Chromatic Borders

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elevation in threshold increased as the wavelength difference between the components increased. The marked asymmetry which is seen in some of the threshold functions for this subject is probably not due to a mismatch in luminance. Luminance levels were checked before each session. The difference in thresholds obtained in the extreme spatial positions may reflect selective adaptation. One component of the adaptation field may desensitize the mechanism detecting the test field to a greater extent than the other component. For example, in the lowest curve where asymmetry is most noticeable, the 540 nm component seems to have a greater effect than the 480 nm component. Experiment 3 The luminance of both components of the adapting field was constant in Experiment 2. Experiment 3 was designed to determine whether the variation in threshold in the region of the border was dependent upon the (2.26 log ft-L) Ievet chosen. The adapting wavelengths were 540 nm (left component) and 640 nm (right component). The Iuminance levels of both components were varied together, in an ascending series, so that the match between them was maintained. The entire threshold function was not determined at each level. Rather, thresholds were deter~ned for 2’ and 17’ border displacements to the right of center. Figure 5 shows an elevation in threshoId in the 2’ compared to the 17’ position at the 2.26 log ft-L luminance level used throughout Experiment 2. The curves for the two positions begin to diverge at approximately 0.2 log ft-L, indicating that threshold is not elevated below this value. Experiment 4 The main effect shown in Experiments 2 and 3 was an increase in threshold in the region of a boundary separating two fields of light of differing wavelength. One hypothesis expiaining this effect is that the distribution of retinal illuminance is such that the amount

1720

CHARLES E. STERNHEM.ROBERTA. GLASSANDJOHN V. KELLER

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Fro. 5. Log dative t&mkM (ona dctamierrtion) for a white test &Id as a fuacth of the l~ofthe~&d&Ths~wrHssreplthrwcwe5+DmllodtcomI~SGhWtc& field 17’from border. ponent) and 640 nm (right

of light is gmter at the boundary than fufthcr away. When the ccmpommts were metA8d in wavekngth and lurninancc a vague dark band of less than 1’ visual angle epgbarcd the corner which may have thereWCl.8fli&BRt~V~

thtacbronMWgItns.Inatenindication of itu&ance sumnation in the region of the border which is evidcsx agbst this hypothesis. Experimant 4 tested the same hypothesis in a different manner. The opthe leftcompanentoftheadagti~ElaMwas540nmandthatoftht~t~t660am. The border evenly divided the fkld. The luminance of the left side remain& fbmd (l-13 log ft-L) while that of the right side was varied. The test field was premed on the left side

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FIO. 6. Log mhtive threshold (mean of two detwnbtbm)

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VisualSensitivityin the Region of Chromatic Borders

2’ away from the border. Figure 6 shows the log relative threshold values of a 2’ white test field as a function of the luminance of the right component. It is seen that threshold remains relatively constant until the matching level of luminance is approached, when it decreases. Threshold increases in value at luminance levels above the matching level. This result shows that this component of the adapting field has to be considerably more intense than the other before it may begin to increase threshold. Since the components were matched in luminance in Experiment 2, it is unlikely that an increase in luminance was responsible for the increase in threshold in the region of the chromatic border. Experiment 5 The dominant wavelengths of the adapting field components were fixed at 540 nm (left side) and 640 nm (right side). The test field consisted of either one of these wavelengths or white light. In Fig. 7 it is seen that for both subjects the variation of threshold in the region 3.0

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FIG. 7. Log relative threshold(mean of 2 determinations) as a function of the displacement of a border from the center of the adaptation fieId. The dominant wavelength of the left component was 540 nm and that of the right component 640 nm. The luminance of both components was 1.13 log ft-L for S1 and 0.94 log ft-L for SJ. The test field consisted of monochromatic light of 540 nm or 640 nm dominant wavelength, or white light (indicated by the parameter on the curves). FIG. 8. Log relative threshold (mean of two determinations) for a white test field as a function of the displacement of the border from the center of the adaptation field. The dominant wavelength of the left component was 480 nm. The parameter on the curves indicates the dominant wavelength of the right component. The luminance of both components was 1.13 log f&L for S, and 0.94 log ft-L for SJ.

17’2

CHARLESE. STEP.NHEIM. ROBERT A.

GLAND AND JOHN V. KELLER

of the chromatic border varies with the chromaticity of the test field. The data for the two subjects do not agree well enough to permit a statement as to which threshold function was elevated the most, Both subjects do show evidence of selective adaptation when the dominant wavelength of the test field was 541) nm. Com~ring values obtained in the extreme spatial positions, thresholds were relatively high when the test field was presented against a component of similar wavelength. S3 shows selective adaptation with a test wavelength of 640 nm, although S1 does not. Experiment 6

The results of Experiment 2 indicated that threshold variation in the region of a chromatic border depended upon the wavelength difference across the border. Assuming that visual ~nsitivity is dependent upon the interaction among neigh~ring elements in the visual pathway, we decided, within the framework of opponent-colors theory, to determine whether the pairing of complementary wavelengths yielded maximal elevation in threshold. Complementary wavelengths (480 nm and 580 nm) were selected (JUDD, 1951) for the left and right components of the adapting field, respectively. Thresholds were determined in the usual manner with a 2’ white test field. In Fig. 8 the results are compared to those obtained with adapting fields having wavelength separations of smaller (480-560 nm) and larger (480-600 nm) extents. Subjects show individual differences in the shapes of their threshold functions. It can be seen, however, that for each subject spectral complementaries yield the typical pattern of sensitivity change in the region of the border. Threshold elevation is not markedly different for this combination of wavelengths than for others which were separated by slightly different amounts. Essentially the same results were found when this experiment was repeated using a test wavelength of 480 nm. DISCUSSION The results clearly show that visual sensitivity is reduced in the region of a chromatic border. The extent of the reduction is dependent upon the wavelength di&rence across the border, the luminance of the two fields, and the wavelength of the test field. Control experiments show that this effect cannot be accounted for on the basis of variation in the luminance distribution in the region of the border. Visual sensitivity has also been found to increase in the region of a luminance border (BURKHARDT,1966; MATTHEWS,1966, 1967; NOVAKand SP~LING, 1963; TELLIZR,1968). MAX-THEWS (1967) selected wavelengths and intensities of chromatic light which enabled him to investigate the luminance border effect within isolated cone mechanisms. By aeIecting stimuli on this basis in future studies of the chromatic border effect it will be possible to specify the contrast ratio within the cone mechanism that is detecting the test fieId. If threshold is determined solely by this contrast it would indicate that this mechanism is independent of other cone mechanisms that are also stimulated. This would be in agreement with the results of MCKEE and WESTHEMER (1970) who have shown that cone meobanirms act independently in spatial interactions involving circular fields of light. DAW (I 967) has recorded activity from ganglion cells in the retina of gold&& in response to red-green boundaries placed within their receptive fields. Many cells showed maximum response when the boundary was near the center of their field. Some cells showed very little response when the field was illuminated with only one light. In the present experiment we have used a similar stimulus pattern. The reduction in sensitivity in the region of the chromatic border may be due to an increase in the rate of activity in neural elements in

Visual Sensitivi!y in the Region of Chromatic Borders

1723

this region resulting from lateral neural interaction. One would expect the threshold luminance of a test spot of light to be greater in the presence of this increase in background activity than it would be when detected upon a homogeneous field. The electrophysiological data collected from goldfish is important to consider in this regard since their color vision has been shown to have similar properties to that of humans (YAGER, 1967). An alternate hypothesis that must be conside!ed involves the effect of involuntary eye movements and the resultant modulation of illumination upon receptors in the region of the border. TELLER(1968) suggests that transient changes in sensitivity that may occur as a result of eye movements complicate studies of spatial interaction. She has shown that this factor is significant when luminance borders consisting of different levels of white light are used as adapting stimuli. A recently completed study in our laboratory (GLASS, 1971) suggests that there are changes in visual sensitivity at the time of transition between monochromatic fields of light which have been matched in luminance which are comparable to those obtained when one luminance level is substituted for another (BOYNTON,1961). It is possible that modulation of receptor illumination as a result of eye movements and/or lateral neural interaction is responsible for the findings of the present study. A stabilized image experiment is presently being planned to assess the contribution of each of these factors. Acknowledgemenrs-Thisresearch was conducted under Grant EYOO539-01 from the National Institute of Health to the senior author. Partial support was also provided by a Biomedical Sciences Research Roard Grant from the University of Maryland. The authors would like to thank Miss FRAN FROME, Mrs. SANDIE WEISFELD,and Mr. THOMASPFAU for their help in running experimental sessions. REFERENCES BEKESY,G. (1968). Mach- and Hering-type lateral inhibition in vision. Vision Res. 8, 1483-1499. BOYNTON,R. M. (1961). Some temporal factors in vision. In Sensory Communic~rion (edited by W. A. ROSENBL~), pp. 739-756. The M.I.T. Press, Cambridge, Mass. BOYNTON,R. M. (1966). Vision. In Experimental Methods and Instrumentation in Psychology. (edited by J. B. SIDOWSKI),pp. 273-330. McGraw-Hill, New York. BURKHARDT,D. W. (1966). Brightness and the increment threshold. J. opt. Sot. Am. 56, 979-981. CORNSWEET,T. N. (1962). The staircase method in psychophysics. Am. J. Psychol. X,485-491. DAW, N. W. (1964). Visual response to gradients of varying color and unequal luminance. Nature, Land. 203, 215-216.

DAW, N. W. (1967). Goldfish retina: Organization for simultaneous color contrast. Science, N. Y. 158,942944. FIORENTINI,A. M. and TORALDODI FRANCIA(1955). Measures photometriques visuellea sur un champ a gradient d’eclairement variable. Optico Acta. 1,192-193. GLASS, R. A. (1971). Visual sensitivity in the presence of alternating monochromatic fields of light. Unpublished Masters thesis, University of Maryland, College Park. HORST,VAN DERG. J. C. and BOWMAN,M. A. (1967). On searching for “Mach band type? phenomena in vision. Vision Res. 7, 1027-1029. JUDD, D. B. (1951). Basic correlates of the visual stimulus. In Handbook of ExperimentalPsychology (edited by S. S. STEVENS),pp. 811-867. Wiley, New York. MATTHEWS,M. L. (1966). Appearance of Mach bands for short durations and at sharply focused contours. J. opt. Sot. Am. 56, 1401-1402. MA~EWS, M. L. (1967). Mach-band increment threshold and the mechanisms of color vision. J. opt. SOC. Am. 57. 1033-1036. MCKEE, S.P. and WESTHEIMER, G. (1970). Specificity of cone mechanisms in lateral interaction. J. Physiol., Land. 206, 117-128. NOVAK, S. and SPEIUING, G. (1963). Visual thresholds near a continuously visible or a briefly presented light-dark boundary. Optica Acta. 10,187-191. RATTLE,J. D. (1969). Effect of target size on monocular fixation. Optica Actu. 16, 183-192. TELLER,D. (1968). Increment thresholds on black bars. Vision Res. 8, 713-718. THOMAS,J. P. (1965). Threshold measurements of Mach bands. J. opt. Sot. Am. 55, 521-524. YAGER,D. (1967). Behavioral measures and theoretical analysis of spectral sensitivity and spectral saturation in the goldfish Carassius uuratus. Vision Res. 7, 707-727.

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CHARLES E. STERNHEM,ROBERT A.GLASS AND JOHN V. KELLER

Abstract-Visual sensitivity to a small briefly presented test field was measured in retinal areas where them was an abrupt change in the background from one monochromatic light to another. The components of the background were matched in htminance. The results showed visual sensitivity is reduced in the reqion of the chromatic border. The magnitude of the effect was dependent upon the wavelength difference across the border, the luminance of the background components, and the tiavelength of the test field. Control experiments showed that this effect cannot be punted for on the basis of the luminance distribution in the region of the border. Underlying mechanisms, involving lateral neural interaction and modulation of receptor illumination as a result of eye movements, are discussed in relation to recent electrophysiological and psychophysical research.

R&sum&On mesure la sensibiliti visuelle a la presentation b&e de petits tests dans des aims rttiniennes oh le fond change brusquement dune lumi&re monochromatique a une autre. Les composantes du fond sent de m&meluminance. On constate que la sensibilitC est r&h&e dans la r&ion du bard chromatique. La grandeur de cet e&t d&end de la dim an longueur d’onde a travers le bard, de la luminance du fond et de la longueur d’onde du test. Des exp& riences de contr& montrent que cet effet ne peut pas&e expliquC par la distribution de huninance p&s du bard. On discute les m&canismes sowjacents, en particuliar f’interaction nerveuse la&ale et la modulation de l%clairement des *pteurs a la suite de mouvements des yeux, en relation avec les recherches r&entes en Ckctrophysiologie et psychophysique.

m--Die Erkennbarkeit eines kleinen, kurzzeitig dargebotenen Testreixes wurde in einern RetitmJ&eicit pnarsen, in dun der Hintergrund durch xwei monoehromatische Lichter acharf getrennt Weuchtet wurde. Die K~mponentetr der iiin~euchtdichte wurden dahei baatimmt. B ergah sich, da6 die En&&&keit des CSe&htssinus im Bereich der Farbkontm@mnra fiaabg##at ist. Die St&rke des Pffekts hing van der Wellenl&ngendifferenx an der Kante, den Leuchtdichtekomponenten des Hintergrunds turd der Wellem des Testreixes ab. Kontroilexperimente r&ten, dab dieser Effekt nicht erklitrt werden kann, indent man die LeuchWitevs&ilung im Kantenbueich heran&ht. MQghche Mecha&nen unter Bu(klpichriauyl latemler Neuronenwechselwirkung und Moduhttion der Kexeptorbekuchtung info@ Auganheweftungen werden in Verbindtmg mit neuerhchen elektrophysidogh&en und psychophysischen Untersuchungen diskutiert.