METAC~NTRAST
MASKING DEPENDS TRANSIENTS’
ON LUMINANCE
RICHARD W. BOWEN,JOELPOKORNY* and DARLENE CACCIATO Department of Psychology, Loyola University of Chicago, Chicago, IL 60626, U.S.AA. * Eye Research Laboratories, The University of Chicago, 950 East 59th Street, Chicago, IL 60637, U.S.A. (Receined 26 January 1977) Abstract-Metacontrast (reduction in brightness of a test gash by the subsequent presentation of a spatially-adjacent masking flash) is abolished if the test and mask flashes are changed in hue with no luminance transients. To obtain metaeontrast. the masking flash need only be a luminance transient of as little as 0.05 log unit.
Metacontrast refers to the brightness reduction of a test flash that occurs if a spatially-adjacent masking flash is presented 5&125msec later (AIpern, 1953; Kahneman, 1968; Weisstein, 1972; Lefton, 1973). Previous studies of metacontrast used test and mask stimuli presented as luminance transients, either as luminance increments above a dim or dark background, or as luminance decrements presented against a higher luminance background field.’ We here report measures of metacontrast for conditions of hue substi-
tution, a paradigm in which test and mask stimuli occur as changes in the spectral composition, but not the luminance, of a background field (Weingarten, 1972). With chromatic stimuli presented in hue substitution, we find that metacontrast is abolished-no change in the brightness of the test stimulus occurs. When the chromatic masking stimulus is presented as a luminance increment above the background field,
masking occurs. METHOD Obsercers Two of the authors (RWB and DC) served as observers. Both had normat color vision and normal acuity with corrective lenses. The stimulus array The observer viewed a fixation point 2.5’ from the center of a 2.5’ x 3.5” achromatic field (the background) and initiated presentation of the test flash-mask flash sequence by pressing a button. The spatial conB~ration (Fig la} consisted of a central rectanguiar test stimulus (1.4” x 0.4”), Banked by two rectangular mask stimuli (each 1.4” x 0.7”). The separation between test and mask stimuli was 0.2”. Both the test and mask stimuli were reddish-orange (620 nm) in color. The observer viewed the field monocularly (viewing distance = 0.45 m) with his head supported by a chin rest. ’ Supported in part by NIH. USPH. NET grants EY70652. EYOO901(Pokomy) and EYOO523(F. W. Newell, Principal Investigator). The major substance of this paper was presented at the annual meeting of A.R.V.O., April, 1976. ’ If test and mask~stimuli are dark figures on a lighted
back~ound. brightness enhancement occurs in which the dark test figure appears lighter when masked
Apparatus The apparatus was a modification of an optical system described in detail by Breton (1977). The background and the test and mask stimuli were projected onto a rear projection screen by a multibeam optical system that employed a 75 W xenon arc lamp as a common source. Presentation of test or mask stimuli was accomplished as follows: the section of the achromatic background field representing either the test or the mask stimulus was illuminated, in separate channels, by white light focused at and reflected from the mirror-finish blade of a high-speed shutter (Uniblitz). Orange light (obtained with a Schott interference filter) was focused at the back of the shutter blade. As the shutter blade wiped (in less than Zmsec) the orange hue was substituted for the achromatic section of the background field.
The stimulus array (Fig. la) was created by superimposing two separate background fields, one containing the test rectangle and the other containing the masking reetangIes. Thus, for all of the experimental conditions described below, the orange (620nm) test and mask stimuli had values of calorimetric purity less than 1.0, since, for example, the test stimulus location was covered by achromatic tight from the background geld associated with the masking stimulus, and vice versa for the masking stimulus location. The orange test or mask stimulus was matched in luminance to the achromatic background field using the method of heterochromatic flicker photometry (shutter operated in repetitive mode at 10 Hz alternation rate). With such a luminance match, the test or mask stimulus is said to be in a hue substitution made, a condition where the chromatic stimulus replaces a portion of the background field without a luminance transient. When the luminance of the background field is reduced, the test or mask stimulus flash is presented as a huniuana increment. Experimental conditions We used four experimental conditions (Fig. lb). In condition I (typical metacontrast paradigm) test and mask gashes were equal iii Iuminance (0.145 mL) and were presented as increments above the background fiefd (0.032 mL). The calorimetric purities of test and mask were 0.9. In condition II, we presented test and mask stimuli in the hue substitution mode. The luminance of the background field and the test and mask stimuli were 0.20 mL; their calorimetric purities were 0.5. In condition III, the test rectangle was in the hue substitution mode, at the same luminance (0.12 mL) as the background (cotorimetric purity 0.83X and the tlanking mask rectangtes were presented as luminance increments at 0.20mL (calorimetric 971
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RICHGUJ W. Bows,
JOELPOKORNY and DARLE?;ECACCIATO
purity 0.3). In condition IV, the mask rectangIes were in the hue substitution mode (at 0.12 mL, calorimetric purity 0.53) and the test stimulus occurred as a luminance increment (at 0.20 mL, calorimetric purity 0.5). For all four conditions, we manipulated the stimulus onset asynchrony (SOA). the delay from test onset to mask onset (Fig. Ic). SOA values ranged from 0 (simultaneous) to 100 msec (test leads mask) at 25 msec intervals. For observer RWB, the test stimulus duration was 25 msec and the mask duration was 50 msec. For observer DC, both the test and mask stimuli had a duration of 50 msec.
observer was presented first with the test-mask sequence at a selected SOA, and then. approx 2sec later, with the test stimulus (comparison). The observer’s task was to report whether the test presented with the mask appeared brighter or dimmer than the comparison stimulus. From trial to trial, luminance values for the comparison stimulus were varied in 0.05 log unit steps, according to a doublerandom staircase method (Cornsweet, 1962). Final values of masking effect were based on a total of 10 reversal points at asymptote for both independent staircase series. determined in a daily experimental session. For conditions I and IV, the luminance of the comparison stimulus was manipulated. For conditions II and III. we changed the luminance of the comparison stimulus and background field jointly to eliminate possible effects of brightness contrast.’ The observers closed their eyes in the 2sec period between the presentation of the two stimuli. During a daily experimental session, all SOA values were presented in random order for a given condition. During each expe;imental session, a control condition was run in which the test stimulus without the mask was matched to the comparison stimulus. This control measured any constant error in brightness judgments that might be associated with the successive brightness matching technique we employed. Experimental matches were corrected for constant error values (which were always less than 0.03 log unit) for each observer.
PROCEDURE
Metacontrast masking was measured using a method of successive brightness comparison. On each trial the 3 When the test stimulus is in a hue substitution mode, decreasing its luminance causes it to become a luminance decrement. Under these conditions, brightness contrast between the test and the background field could occur (see, for example, Onley, 1961). We also have data for RWB on conditions II and III using the matching procedure specified for conditions I and IV. There is no qualitative difference between these data and those reported in Fig. 2. The matching procedure used for conditions II and III was intended to ensure that any absence of masking was not an artifact of the testing procedure. +2.5*4
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Fig. 1. (a) The stimulus array. The white background field (2.Y x 3.5”) was centered 2.5’ to the right of a Iixation dot. located at the center of a separate 0.4” x 0.4’ illuminated field. The center rectangle (1.4’ x 0.6’) was the test stimulus. The top and bottom rectangles (each 1.4’ x 0.7’) served as the masking stimulus. The separation between test and mask stimuli was 0.2”. Test and mask stimuli were illuminated by orange (620 nm) light. (b) Schematic representation of the four experimental conditions, showing for each a vertical luminance cross section of the stimulus array. In condition I, test and mask were increments of orange light above the white background field. In condition II, the test and mask were presented at the same luminance as the background field, a hue substitution condition. In condition III, the test occurred in hue substitution, and the mask was presented as a luminance increment. In condition IV, the masking rectangles were in the hue substitution mode, with the test presented as a luminance increment. The amount of 620nm light actually present in test and mask stimuli is represented by the length of the stippled bar at each test and mask stimulus location. (c) Representation of the temporal relations between the test and mask stimuli. Masking effects were measured for values of stimulus onset asynchrony (SOA) ranging from 0 (simultaneous onset of test and mask) to 100 msec at 25 msec intervals. By convention, positive values of SOA indicate that test precedes mask.
973
Metacontrast masking depends on luminance transients
CONOITION XI
CONOlTlON
T: S”B.sTlT”TlON
T. SUSSTITUTION
T
INCREMENT
H SUBSTITUTION
M: INCREMENT
”
SUBSTITUTION
CONDITION
II
0
STIMULUS
ONSET
ASYNCHRONY
50
100
0
E
50
two
(MSEC)
Fig. 3. Masking effect (in density units) as a function of stimulus onset asynchrony (SOA) under the four experimental conditions for observer RWB (top panels) and observer DC (bottom panels). Error bars indicate the 9S% confidence interval for each measurement.
RESULTS If metacontrast masking of the test stimulus occurs, density must be added to the comparison for it to
appear equal in brightness to the test presented with the mask. The results for both observers are given in Fig. 2 where masking effect (density added to comparison stimulus, corrected for constant error) is shown as a function of test-mask SOA. For condition I, where test and mask stimuli were luminance increments, an inverted U-shaped masking function was obtained for both observers. The degree of masking is slight when test and mask have simultaneous onset, and increases to a peak value (at 75 msec for observer RWB, 25-50 msec for observer DC), thereafter declining. Similar masking can be observed for condition III. The incremental masking stimulus decreased the brightness of a test stimulus presented in hue substitution. Peak masking occurred at 25 msec for observer DC and was asymptotic at 75-1OOmsec for observer RWB. For conditions II and IV, where the masking stimulus was in the hue substitution mode, the functions are nearly flat and show no significant masking. These results indicate that metacontrast masking depends on the presence of luminance transients in the masking stimulus. There were no obvious changes in the hue or saturation of the test stirnums for any condition. In conditions I and III, the masking effect was one of apparent * We limit ourselves to a consideration of the characteristics of retinal ganglion cells of the monkey. “Phasic” and “tonic” units have also been identified at the level of the lateral geniculate nucleus (Hubel and Wiesel, 1968; DeValois and Pease, 1971).
darkening of the test stimulus, as though by a neutral density filter. Low luminnnce-conrmst conditions
In a separate procedure we investigated whether a small luminance increment is sufficient to produce masking in conditions I and III. We find that a luminance difference as small as 0.05 log unit between background field and test and mask stimuli in condition I, or background field and mask stimulus in condition III can produce statistically significant masking. Masking functions for these conditions for one observer are given in Fig. 3. DISCC’SSION
Our data demonstrate that metacontrast masking depends upon the presence of luminance transients in the masking stimulus. If the masking stimulus was a change in hue without a luminance transient (hue substitution), metacontrast did not occur. Significant masking effects on test stimuli that are presented either as luminance increments or in hue substitution can be obtained with very low contrast luminance increments as the masking stimulus. How can our data be understood? In recent neurophysiological studies of the primate visual system (Gouras, 1968, 1969), two functional classes of single units have been identified among retinal ganglion cells: “phasic” cells and “tonic” cells.’ The phasic units respond only to stimulus transients and have a center-surround organization that is activated jointly by mixed populations of cone types such that the unit is not capable of responding to wavelength changes per se. The tonic units exhibit a maintained
RICHARD W. BOMN, JOEL
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Fig. 3. Masking functions obtained with observer RWB for a 0.05 log unit luminance difference between the background field and the masking stimulus in condition I (filled circles) and condition III (open circles). The single data point (open square) at 75 msec SOA is a replication of condition II (test and mask presented in hue substitution). Error bars indicate the 95% confidence interval for each measurement. For the second observer (DC), a luminance difference of 0.1 log unit between mask and background field produced significant masking effects in conditions I and III.
discharge to stimulus presentations, and have a center-surround receptive field organization that is of the “opponent” type, such that the cell’s activity is modulated as a function of wavelength. The tonic unit shows changes in activity for lights of different wavelengths that are matched in luminance and will, like the phasic unit, respond to luminance transients falling within its receptive field. There is a substantial latency difference between the two types of units, with the phasic system exhibiting the shorter latency (Gouras, 1969). An explanation of our data and a more general account of metacontrast phenomena as a whole results if we assume that populations of phasic and tonic units, with characteristics similar to those described above, exist in the human visual system. Modern color theory suggests that perception of brightness is modeled as a vector sum of activity in chromatic and achromatic channels (Guth, Donley and Marroco, 1969; Kaiser, Henberg and Boynton, 1971; Guth and Lodge, 1973). Phasic cells have been associated with the achromatic psychophysical channel, and tonic cells have been associated with the chromatic opponent-color channels (Ingling and Drum, 1973).
5 Given the postulated latency relations between the tonic and phasic systems, inhibition of the phasic system by the tonic system might occur -if the masking stimulus preceded the test stimulus. We tested this possibility for condition IV with observer RWB by presenting the mask at SOA values from -75 msec (mask leads test) to 0 at 25 msec intervals, and, as with the positive SOA conditions, found no masking. Apparently, the tonic system does not inhibit the phasic system for any test-mask asynchrony, at least under our experimental conditions.
A stimulus presented in hue substitution should selectively activate only the population of tonic units (since no luminance transient is present); stimuli that are luminance increments would evoke a response from both phasic and tonic units. In condition I, we postulate that metacontrast masking occurring when test and mask are luminance increments results from inhibition of the tonic system by the phasic system. Peak masking at positive SOA values is predicted on the basis of latency differences between the tonic and phasic system. If test and mask are presented with simultaneous onset, the slower tonic activity due to the test stimulus would escape inhibition by the faster phasic activity elicited by the masking stimulus. Accordingly, the masking stimulus must be delayed in order for a maximum degree of brightness suppression to occur. In conditions II and IV we infer that tonic activity due to a masking stimulus does not interfere with either tonic activity (condition II) or phasic activity (condition IV) due to a test stimulus5 In condition III, the masking stimulus would activate both phasic and tonic units while the test would activate tonic units only. Masking occurs when phasic activity elicited by the mask interferes with tonic activity due to the test stimulus. In a quantitative model, we should consider response characteristics of these cell types to edges, since brightness in a structured field is at least in part determined by the luminance profile at the edge of a stimulus (Comsweet, 1970; but see Barlow and Verrillo, 1976). Both phasic and tonic cells show an enhanced response at a luminance edge (Ingling and Drum, 1973). During masking, some small change in color appearance (e.g. desaturation) might be predicted from the vector models of color and brightness. However, for our stimulus conditions, the brightness change is the overwhelming perceptual phenomenon. Walters (1970) and Simon (1974) have demonstrated wavelength-specific masking effects with chromatic test and mask stimuli presented as luminance increments (as in our condition I). These studies show that the magnitude of metacontrast is greatest with test and mask stimuli of the same wavelength, and that the masking effect is decreased if test and mask differ in wavelength. Our model of metacontrast can account for wavelength-dependent masking effects if it is assumed that interactions can take place between tonic channels. With incremental stimuli, simultaneous color contrast may occur between heterochromatic test-mask pairs. It is possible that color contrast within the tonic system works to cancel out masking effects produced by phasic-to-tonic inhibition, yielding a net decrement in peak masking between stimuli of different wavelengths. Two recent theoretical treatments of metacontrast (Weisstein, Ozog and Szoc, 1975; Breitmeyer and Ganz, 1976) have proposed that metacontrast masking is due to interaction between separate fast- and slow-responding neural channels. These models account for a variety of experimental results on metacontrast. Our approach is complementary to that of Weisstein et al. (1973) and Breitmel-er and Ganz (1976). With the hypothesis that the hue substitution technique eliminates phasic activity, we demonstrate that metacontrast depends on the interaction of phasic and tonic channels.
Metaeonrrast masking depends on luminance transients ,dnowiedgemencs--We thank V. C. Smith for numerous helpful suggestions and L. Matin and D. C. Hood for their critical reading of the manuscript.
M. (1953) Metacontrast. J. opt. Sot. Am AS, 648-657. Barlow R. 13.and Verrillo R. T. (1976) Brightness sensation in a ganzFeld. t&ion Res. 16, 1291-1297. 3reitmeyer B. and Ganz L. (1976) Imp&cations of sustained and transient channets for theories of visual pattern masking, saccadic suppression and information processing. Psychol. Rev. 33, t-36. Breton M. (1977) Hue substitution: wavelength latency effects. Vision Res. 17, 435-443. Cornsweet T. (1970) Visual Perception. Academic Press, New York. DeVafois R. and Pease P. (1971) Contours and contrast: responses of monkey lateral genie&ate nucleus cells to luminance and coIor figures. Science, Ilt, 6QG96. Goutas P. (1968) Identitication of cone mechanism in monkey ganglion cells. J. Physiol., Land. 199, 533-547. Gouras P. (1969) Antidromic responses of orthodromically identified ganglion cells in monkey retina. J. Pkysiol., Land. 204, 407-419. Gurh S L, Donley N. J. and Marroeo R. T. (1969) On Luminance additivity and related topics. Vision Res. 9, 537-576. Alpem
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Guth S. L. and Lodge H. (1973) Heterochromatic additivity, foveal spectra1 sensitivity, and a new color model. J. opt. Sot. Am. 63, 450-462. Hubel D. H. and Wiesei T. (1968) Spatial and chromatic interactions in the lateral geniculate body of the rhesus monkey. 3, ‘~~rop~ys~oi. 29, 11E-1 1%. 1ngk-g C. and Drum B. (f973) Retinal receptive Frekts: correlations between psychophysics and electrophysiology. tisiou Res. t3. I I5 l-1 163. Kahneman D. (1968) Method, findings, and theory in studies of visual masking, Psycho/. Bull. 70, 404-425. Kaiser P., Henberg P. and Boynton R. M. (1971) Chromatic border distinctness and its relation to saturation. Vision Ref. 11, 9%968. Lefton L. (1973) Metacontrast: a review. Perrepf. Pq&ophjx 13, 161-171. Onley J. W. (1961) Light adaptation and the brightness of brief fovea1 stimuli. J. opr. Sot. rim. St, 647-673. Simon L. (1974) Color specific effects in metacontrast masking. Personal communication. Walters J. fI970) Metacontrast: the eRects obtained with consecutively presented concentric disks and rings of dilifferent wavelengths. Am. $. Optom. 45, 634-639. Weingarten F. (1972) Wavelength effects on visual latency. Science t 76. 6922694. Weisstein N. (1972) Metacontmst. In Handbook oj Sensory Physiology, Vol. 7, Part 4: Visual Psycho&&s (Edited by Jameson D. and Hurvich L.). Springer, Berlin. Weisstein N., Ozog G. and Szoc R. (1975) A comparison and elaboration of two models OFmetacontrast. Ps#toI. Rev. 82, 325-343.
MASKING DEPENDS TRANSIENTS’
ON LUMINANCE
RICHARD W. BOWEN,JOELPOKORNY* and DARLENE CACCIATO Department of Psychology, Loyola University of Chicago, Chicago, IL 60626, U.S.AA. * Eye Research Laboratories, The University of Chicago, 950 East 59th Street, Chicago, IL 60637, U.S.A. (Receined 26 January 1977) Abstract-Metacontrast (reduction in brightness of a test gash by the subsequent presentation of a spatially-adjacent masking flash) is abolished if the test and mask flashes are changed in hue with no luminance transients. To obtain metaeontrast. the masking flash need only be a luminance transient of as little as 0.05 log unit.
Metacontrast refers to the brightness reduction of a test flash that occurs if a spatially-adjacent masking flash is presented 5&125msec later (AIpern, 1953; Kahneman, 1968; Weisstein, 1972; Lefton, 1973). Previous studies of metacontrast used test and mask stimuli presented as luminance transients, either as luminance increments above a dim or dark background, or as luminance decrements presented against a higher luminance background field.’ We here report measures of metacontrast for conditions of hue substi-
tution, a paradigm in which test and mask stimuli occur as changes in the spectral composition, but not the luminance, of a background field (Weingarten, 1972). With chromatic stimuli presented in hue substitution, we find that metacontrast is abolished-no change in the brightness of the test stimulus occurs. When the chromatic masking stimulus is presented as a luminance increment above the background field,
masking occurs. METHOD Obsercers Two of the authors (RWB and DC) served as observers. Both had normat color vision and normal acuity with corrective lenses. The stimulus array The observer viewed a fixation point 2.5’ from the center of a 2.5’ x 3.5” achromatic field (the background) and initiated presentation of the test flash-mask flash sequence by pressing a button. The spatial conB~ration (Fig la} consisted of a central rectanguiar test stimulus (1.4” x 0.4”), Banked by two rectangular mask stimuli (each 1.4” x 0.7”). The separation between test and mask stimuli was 0.2”. Both the test and mask stimuli were reddish-orange (620 nm) in color. The observer viewed the field monocularly (viewing distance = 0.45 m) with his head supported by a chin rest. ’ Supported in part by NIH. USPH. NET grants EY70652. EYOO901(Pokomy) and EYOO523(F. W. Newell, Principal Investigator). The major substance of this paper was presented at the annual meeting of A.R.V.O., April, 1976. ’ If test and mask~stimuli are dark figures on a lighted
back~ound. brightness enhancement occurs in which the dark test figure appears lighter when masked
Apparatus The apparatus was a modification of an optical system described in detail by Breton (1977). The background and the test and mask stimuli were projected onto a rear projection screen by a multibeam optical system that employed a 75 W xenon arc lamp as a common source. Presentation of test or mask stimuli was accomplished as follows: the section of the achromatic background field representing either the test or the mask stimulus was illuminated, in separate channels, by white light focused at and reflected from the mirror-finish blade of a high-speed shutter (Uniblitz). Orange light (obtained with a Schott interference filter) was focused at the back of the shutter blade. As the shutter blade wiped (in less than Zmsec) the orange hue was substituted for the achromatic section of the background field.
The stimulus array (Fig. la) was created by superimposing two separate background fields, one containing the test rectangle and the other containing the masking reetangIes. Thus, for all of the experimental conditions described below, the orange (620nm) test and mask stimuli had values of calorimetric purity less than 1.0, since, for example, the test stimulus location was covered by achromatic tight from the background geld associated with the masking stimulus, and vice versa for the masking stimulus location. The orange test or mask stimulus was matched in luminance to the achromatic background field using the method of heterochromatic flicker photometry (shutter operated in repetitive mode at 10 Hz alternation rate). With such a luminance match, the test or mask stimulus is said to be in a hue substitution made, a condition where the chromatic stimulus replaces a portion of the background field without a luminance transient. When the luminance of the background field is reduced, the test or mask stimulus flash is presented as a huniuana increment. Experimental conditions We used four experimental conditions (Fig. lb). In condition I (typical metacontrast paradigm) test and mask gashes were equal iii Iuminance (0.145 mL) and were presented as increments above the background fiefd (0.032 mL). The calorimetric purities of test and mask were 0.9. In condition II, we presented test and mask stimuli in the hue substitution mode. The luminance of the background field and the test and mask stimuli were 0.20 mL; their calorimetric purities were 0.5. In condition III, the test rectangle was in the hue substitution mode, at the same luminance (0.12 mL) as the background (cotorimetric purity 0.83X and the tlanking mask rectangtes were presented as luminance increments at 0.20mL (calorimetric 971
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RICHGUJ W. Bows,
JOELPOKORNY and DARLE?;ECACCIATO
purity 0.3). In condition IV, the mask rectangIes were in the hue substitution mode (at 0.12 mL, calorimetric purity 0.53) and the test stimulus occurred as a luminance increment (at 0.20 mL, calorimetric purity 0.5). For all four conditions, we manipulated the stimulus onset asynchrony (SOA). the delay from test onset to mask onset (Fig. Ic). SOA values ranged from 0 (simultaneous) to 100 msec (test leads mask) at 25 msec intervals. For observer RWB, the test stimulus duration was 25 msec and the mask duration was 50 msec. For observer DC, both the test and mask stimuli had a duration of 50 msec.
observer was presented first with the test-mask sequence at a selected SOA, and then. approx 2sec later, with the test stimulus (comparison). The observer’s task was to report whether the test presented with the mask appeared brighter or dimmer than the comparison stimulus. From trial to trial, luminance values for the comparison stimulus were varied in 0.05 log unit steps, according to a doublerandom staircase method (Cornsweet, 1962). Final values of masking effect were based on a total of 10 reversal points at asymptote for both independent staircase series. determined in a daily experimental session. For conditions I and IV, the luminance of the comparison stimulus was manipulated. For conditions II and III. we changed the luminance of the comparison stimulus and background field jointly to eliminate possible effects of brightness contrast.’ The observers closed their eyes in the 2sec period between the presentation of the two stimuli. During a daily experimental session, all SOA values were presented in random order for a given condition. During each expe;imental session, a control condition was run in which the test stimulus without the mask was matched to the comparison stimulus. This control measured any constant error in brightness judgments that might be associated with the successive brightness matching technique we employed. Experimental matches were corrected for constant error values (which were always less than 0.03 log unit) for each observer.
PROCEDURE
Metacontrast masking was measured using a method of successive brightness comparison. On each trial the 3 When the test stimulus is in a hue substitution mode, decreasing its luminance causes it to become a luminance decrement. Under these conditions, brightness contrast between the test and the background field could occur (see, for example, Onley, 1961). We also have data for RWB on conditions II and III using the matching procedure specified for conditions I and IV. There is no qualitative difference between these data and those reported in Fig. 2. The matching procedure used for conditions II and III was intended to ensure that any absence of masking was not an artifact of the testing procedure. +2.5*4
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Fig. 1. (a) The stimulus array. The white background field (2.Y x 3.5”) was centered 2.5’ to the right of a Iixation dot. located at the center of a separate 0.4” x 0.4’ illuminated field. The center rectangle (1.4’ x 0.6’) was the test stimulus. The top and bottom rectangles (each 1.4’ x 0.7’) served as the masking stimulus. The separation between test and mask stimuli was 0.2”. Test and mask stimuli were illuminated by orange (620 nm) light. (b) Schematic representation of the four experimental conditions, showing for each a vertical luminance cross section of the stimulus array. In condition I, test and mask were increments of orange light above the white background field. In condition II, the test and mask were presented at the same luminance as the background field, a hue substitution condition. In condition III, the test occurred in hue substitution, and the mask was presented as a luminance increment. In condition IV, the masking rectangles were in the hue substitution mode, with the test presented as a luminance increment. The amount of 620nm light actually present in test and mask stimuli is represented by the length of the stippled bar at each test and mask stimulus location. (c) Representation of the temporal relations between the test and mask stimuli. Masking effects were measured for values of stimulus onset asynchrony (SOA) ranging from 0 (simultaneous onset of test and mask) to 100 msec at 25 msec intervals. By convention, positive values of SOA indicate that test precedes mask.
973
Metacontrast masking depends on luminance transients
CONOITION XI
CONOlTlON
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T. SUSSTITUTION
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INCREMENT
H SUBSTITUTION
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SUBSTITUTION
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ONSET
ASYNCHRONY
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0
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50
two
(MSEC)
Fig. 3. Masking effect (in density units) as a function of stimulus onset asynchrony (SOA) under the four experimental conditions for observer RWB (top panels) and observer DC (bottom panels). Error bars indicate the 9S% confidence interval for each measurement.
RESULTS If metacontrast masking of the test stimulus occurs, density must be added to the comparison for it to
appear equal in brightness to the test presented with the mask. The results for both observers are given in Fig. 2 where masking effect (density added to comparison stimulus, corrected for constant error) is shown as a function of test-mask SOA. For condition I, where test and mask stimuli were luminance increments, an inverted U-shaped masking function was obtained for both observers. The degree of masking is slight when test and mask have simultaneous onset, and increases to a peak value (at 75 msec for observer RWB, 25-50 msec for observer DC), thereafter declining. Similar masking can be observed for condition III. The incremental masking stimulus decreased the brightness of a test stimulus presented in hue substitution. Peak masking occurred at 25 msec for observer DC and was asymptotic at 75-1OOmsec for observer RWB. For conditions II and IV, where the masking stimulus was in the hue substitution mode, the functions are nearly flat and show no significant masking. These results indicate that metacontrast masking depends on the presence of luminance transients in the masking stimulus. There were no obvious changes in the hue or saturation of the test stirnums for any condition. In conditions I and III, the masking effect was one of apparent * We limit ourselves to a consideration of the characteristics of retinal ganglion cells of the monkey. “Phasic” and “tonic” units have also been identified at the level of the lateral geniculate nucleus (Hubel and Wiesel, 1968; DeValois and Pease, 1971).
darkening of the test stimulus, as though by a neutral density filter. Low luminnnce-conrmst conditions
In a separate procedure we investigated whether a small luminance increment is sufficient to produce masking in conditions I and III. We find that a luminance difference as small as 0.05 log unit between background field and test and mask stimuli in condition I, or background field and mask stimulus in condition III can produce statistically significant masking. Masking functions for these conditions for one observer are given in Fig. 3. DISCC’SSION
Our data demonstrate that metacontrast masking depends upon the presence of luminance transients in the masking stimulus. If the masking stimulus was a change in hue without a luminance transient (hue substitution), metacontrast did not occur. Significant masking effects on test stimuli that are presented either as luminance increments or in hue substitution can be obtained with very low contrast luminance increments as the masking stimulus. How can our data be understood? In recent neurophysiological studies of the primate visual system (Gouras, 1968, 1969), two functional classes of single units have been identified among retinal ganglion cells: “phasic” cells and “tonic” cells.’ The phasic units respond only to stimulus transients and have a center-surround organization that is activated jointly by mixed populations of cone types such that the unit is not capable of responding to wavelength changes per se. The tonic units exhibit a maintained
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Fig. 3. Masking functions obtained with observer RWB for a 0.05 log unit luminance difference between the background field and the masking stimulus in condition I (filled circles) and condition III (open circles). The single data point (open square) at 75 msec SOA is a replication of condition II (test and mask presented in hue substitution). Error bars indicate the 95% confidence interval for each measurement. For the second observer (DC), a luminance difference of 0.1 log unit between mask and background field produced significant masking effects in conditions I and III.
discharge to stimulus presentations, and have a center-surround receptive field organization that is of the “opponent” type, such that the cell’s activity is modulated as a function of wavelength. The tonic unit shows changes in activity for lights of different wavelengths that are matched in luminance and will, like the phasic unit, respond to luminance transients falling within its receptive field. There is a substantial latency difference between the two types of units, with the phasic system exhibiting the shorter latency (Gouras, 1969). An explanation of our data and a more general account of metacontrast phenomena as a whole results if we assume that populations of phasic and tonic units, with characteristics similar to those described above, exist in the human visual system. Modern color theory suggests that perception of brightness is modeled as a vector sum of activity in chromatic and achromatic channels (Guth, Donley and Marroco, 1969; Kaiser, Henberg and Boynton, 1971; Guth and Lodge, 1973). Phasic cells have been associated with the achromatic psychophysical channel, and tonic cells have been associated with the chromatic opponent-color channels (Ingling and Drum, 1973).
5 Given the postulated latency relations between the tonic and phasic systems, inhibition of the phasic system by the tonic system might occur -if the masking stimulus preceded the test stimulus. We tested this possibility for condition IV with observer RWB by presenting the mask at SOA values from -75 msec (mask leads test) to 0 at 25 msec intervals, and, as with the positive SOA conditions, found no masking. Apparently, the tonic system does not inhibit the phasic system for any test-mask asynchrony, at least under our experimental conditions.
A stimulus presented in hue substitution should selectively activate only the population of tonic units (since no luminance transient is present); stimuli that are luminance increments would evoke a response from both phasic and tonic units. In condition I, we postulate that metacontrast masking occurring when test and mask are luminance increments results from inhibition of the tonic system by the phasic system. Peak masking at positive SOA values is predicted on the basis of latency differences between the tonic and phasic system. If test and mask are presented with simultaneous onset, the slower tonic activity due to the test stimulus would escape inhibition by the faster phasic activity elicited by the masking stimulus. Accordingly, the masking stimulus must be delayed in order for a maximum degree of brightness suppression to occur. In conditions II and IV we infer that tonic activity due to a masking stimulus does not interfere with either tonic activity (condition II) or phasic activity (condition IV) due to a test stimulus5 In condition III, the masking stimulus would activate both phasic and tonic units while the test would activate tonic units only. Masking occurs when phasic activity elicited by the mask interferes with tonic activity due to the test stimulus. In a quantitative model, we should consider response characteristics of these cell types to edges, since brightness in a structured field is at least in part determined by the luminance profile at the edge of a stimulus (Comsweet, 1970; but see Barlow and Verrillo, 1976). Both phasic and tonic cells show an enhanced response at a luminance edge (Ingling and Drum, 1973). During masking, some small change in color appearance (e.g. desaturation) might be predicted from the vector models of color and brightness. However, for our stimulus conditions, the brightness change is the overwhelming perceptual phenomenon. Walters (1970) and Simon (1974) have demonstrated wavelength-specific masking effects with chromatic test and mask stimuli presented as luminance increments (as in our condition I). These studies show that the magnitude of metacontrast is greatest with test and mask stimuli of the same wavelength, and that the masking effect is decreased if test and mask differ in wavelength. Our model of metacontrast can account for wavelength-dependent masking effects if it is assumed that interactions can take place between tonic channels. With incremental stimuli, simultaneous color contrast may occur between heterochromatic test-mask pairs. It is possible that color contrast within the tonic system works to cancel out masking effects produced by phasic-to-tonic inhibition, yielding a net decrement in peak masking between stimuli of different wavelengths. Two recent theoretical treatments of metacontrast (Weisstein, Ozog and Szoc, 1975; Breitmeyer and Ganz, 1976) have proposed that metacontrast masking is due to interaction between separate fast- and slow-responding neural channels. These models account for a variety of experimental results on metacontrast. Our approach is complementary to that of Weisstein et al. (1973) and Breitmel-er and Ganz (1976). With the hypothesis that the hue substitution technique eliminates phasic activity, we demonstrate that metacontrast depends on the interaction of phasic and tonic channels.
Metaeonrrast masking depends on luminance transients ,dnowiedgemencs--We thank V. C. Smith for numerous helpful suggestions and L. Matin and D. C. Hood for their critical reading of the manuscript.
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