Retroactive visual masking: Effects of test flash duration on the masking interval

Vision Res. Vol. 7, pp. 1947.

RETROACTIVE FLASH

Fbrgamon Press 1967. Printed in Gnat Britain.

VISUAL

DURATION

MASKING:

EFFECTS

ON THE MASKING

OF TEST

INTERVAL

EMANUEL DONCHIN Divisionof Neurology,StanfordUniversitySchoolof Medicine,Palo Alto, C&for&+ (Received 24 March 1966)

THE PERCEP~ONof a brief test flash (TF) can be interfered with if it is followed by a brighter flash (BF) within a critical interval (CRAWFORD, 1947;see reviews by RAAB, 1963; SPERLING, 1965). The critical interval at which the BF masks the TF depends on the luminance difference between the BF and the TF (ALPERN, 1965; BATTERSBYand WAGMAN, 1959; DONCHXN and LINDSLEY, 1965). The higher the luminance of the TF, the shorter the masking interval. As two brief flashes of equal luminance are not necessarily of equal brightness, statements about the dependence of some performance measure on luminance cannot be generalized to statements about the de~&n~ of that measure on stimulus brightness. To determine the degree to which the masking interval depends on the brightness of the TF, brightness must be varied independently of TF luminance. This can be achieved by varying the duration of the TF at different levels of TF luminance. It has been the purpose of the present study to determine the degree to which the masking interval depends on TF duration for various levels of TF luminance. This question has been briefly touched upon in a study by KAHNEMAN(1966); using a Landolt C, an attempt was made to determine the degree to which luminance-duration reciprocity (Bloch’s law) holds in an acuity task under various conditions. Kahneman concludes that for retroactive masking reciprocity does not hold, and found that increases in TF duration resulted in reduced masking in spite of reciprocal reductions in the luminance of the TF. Kahneman used a constant interflash interval of 0.1 msec, measured from the @set of the TF to the onset of the BF. Increases in flash duration, thus, necessarily involved increases in the interval between the onsets of the two flashes. It is quite possible that the failure to find reciprocity under these conditions is due to the dependence of the masking effect on onset-to-onset, rather than on offset-to-onset intervals. To avoid this dif%ulty, the interval between the two flashes has to be varied independently of TF duration. The present study was designed accordingly as a three-factor experiment in which TF luminance, TF duration, and onset-to-onset intervals were varied. The percentage of correct identiftcations of the position of the target was used as the dependent variable. METHOD Subjects. Two male subjects participated. Both had vision corrected to 20/20 and both had practice in psychophysi~ experiments. Equipment. Two light beams generated by Sylvania R-l 131C glow modulator tubes were superimposed concentrically, and presented in Maxwellian view to the subject. A dim, red, broken cross fixation pattern converging on the target area guided the subject in 79

80

EMANUEL DONCHIN

maintaining head position in the lateral axis, and a chin and head rest helped fix the subject’s head in the anterior-posterior axis. The BF was presented through a circular stop subtending 2” 6’ visual angle along its diameter, the TF was presented through a semicircular stop subtend~g 1” 22’ along its diameter. Both beams were of 6000 mL. However, due to the area1 differences between the two flashes, their brightness expressed in iog units above threshold for 10 msec flashes was 4.3 for the TF and 48 for the BF. The TF semicircular stop could be rotated randomly into one of eight different positions. Wratten neutral density filters were used to determine the luminance of the TF. All temporal parameters were determined by two Grass S4 stimulators the controls of which were superseded by decade capacitance boxes to assure the resettability of the values; interhash intervals were monitored with an Electronic Counter (Hewlett Packard 522B). The duration of the BF throughout the experiment was 10 msec and its luminance was kept at the maximum of 6000 mL. Procedure. Critical masking intervals were determined for subject MR using the method of constant stimuli. The t~eshol~ for subject ED were determined using a staircase method. The subject’s task in both cases involved the identification of the position of the TF semicircle. The subject triggered the stimuli at his own rate. He was instructed to report the position of the TF on all trials, including those in which he “did not see” the TF. The subject was asked to state whether or not he was guessing on every trial. For subject MR the interflash interval was changed once every five trials. Eight to twelve ditTerent interflash intervals for each luminance-duration value were used ranging from an interval at which he gave 100 per cent correct responses to an interval at which he gave O-20 per cent correct responses. The range was covered in steps of 2 msec and 10 The sequence of intervals was determined judgments were obtained at each interval. randomly. For subject ED the interflash interval was changed on every trial, in accord with the requirements of the staircase method (CORNSWEET,1962). Two msec steps were used and each descending series was continued until two incorrect responses were given, while ascending series continued until two correct responses were given. On each session 3-4 dif%erent combinations of TF luminance and duration were used thus giving 3-4 determinations of a masking interval per session. Sessions were continued until two determinations were made for each subject at each of thirty combinations of six luminance values (log mL--ls8, l-5, 1.1, 0.8, 0.5) and five duration values (log msec-1.3, 1.2, 1.0, 0.7, 05). RESULTS The critical masking interval is detined in this study as that interflash interval at which the probability of correct identification of TF position is 0.50. It has been the purpose of the study to determine the functional relationships between this masking interval and the duration of the TF. The raw data obtained from subject MR for three luminance levels is presented in Fig. 1. The per cent correct responses obtained on different sessions are plotted against TF ofiet to BF onset intervals for different values of TF luminance and duration. The data suggest a close agreement between sessions, and in subsequent analyses session differences will be ignored. (No significant sessions effects were found when sessions were included as a variable in the regression analysis described below.)

Retroactive Visual Masking

81

The data as plotted here, indicate quite clearly that increases in TF duration at constant IF luminance leads to improved performance. However, as pointed out above, the magni:ude of the effect of duration is exaggerated in Fig. 1, due to the use of offset to onset ntervals. This point is illustrated in Fig. 2, where the same data are plotted once in terms of offset intervals (top) and once in terms of onset intervals (bottom). The effects of TF duration are less pronounced when stated in terms of the onset to onset interval. This reduction suggests that in the data as presented in Fig. 1, two effects are confounded, the Sect of TF duration with that of the increasing onset-to-onset intervals. In the following, the results will be stated in terms of onset intervals. The estimation of the mashing interval for the various TF luminance and duration combinations was done by multiple regression. Inspection of the raw data suggested that

FIG. 1. Per cent correct identifications of TF position as a function of interval between offset of TF and onset of BF, TF luminance, and TF duration. Abscissa in all three figures: offsetto-onset intervals in msec. Each of the three subfigures represents data for one level of TF luminance (log mL 1.8, I.1 and 0.5). Each of the lines represents data obtained with TF duration in msec indicated by the lines, obtained on one session. BF luminance at 6000 mL, BF duration 10 msec. Percentages are not corrected for chance; chance level would be 0.125, all percentages based on N=lO. P

a2

EMANUELDONCHIN

the per cent of correct responses would be a linear function in the logarithms of TF luminance, duration, and the inter-hash interval. The set of data could thus be summarized by an equation of the form 2arcsinz/P=

a log L-j-b log T-l-c log D-f-d log L log T+K

(1)

where P is the per cent of correct responses, L and T are TF luminance and duration respectively (in msec and mL), D is the onset to onset interflash interval in msec and a, b, c, d and K are constants. An interaction term (log L log 7’) is added to test for interactions

FIO. 2. Average per cent correct identifications of TF position for TF luminance of log mL 1.8, at four different durations, plotted as a function of offset-to-onset intervals (top), and onset-to-onset intervals (bottom). Each point represents average of 2 sessions. TF durations in msec indicated by each line.

between luminance and duration. The arc sine transformation of the proportions has been used to satisfy the requirement of equal variances for the conditional distributions of the dependent variable (EISENHART, 1947; WILKS, 1962). In fitting this function to the data, a stepwise multiple regression technique was used (EFROYMSON, 1960; DIXON, 1964). In this procedure the dependent variables are added one by one to the regression equation if their addition provides a significant improvement over the preceding stages. A set of intermediate regression equations is obtained for subsects of the independent variables. Table 1 presents the results of such an analysis for MR’s data. The variables are listed in the order in which they were introduced into the equation; the coefficients of the successive equations and the corresponding squared multiple correlation coefficients are also given. The squared multiple correlation is equal to the proportion of the total variance accounted for by the regression. The final equation can be solved to find an expression for the inter&h interval at which P=O-50 (or 2 arc sin dP= 157). After collecting terms, the following equation was obtained Log D,,.so = 2.07-0.30

(Log T+1.2 log L)+O*O8 log L log T

(2)

In other words, the masking interval is a power function of a weighted combination of the luminance and the duration of the TF adjusted by an interaction term. It will be noted that for the range of values studied here, strict luminance-duration

8902

11511

0.564

3

4

S.E.*

0.383

3.450

1.285

1.420

log L log T

0.344

4.160

1.385

log L

0.273

-0.984

log T

-0.037

-0.230

-0.060

S.E.

OQ36

-0.178

3

-0.111

-0.257

1

2

log L

log T

Equation

Coefficients for :

-0G43

-0.153

log T

TABLE 2. REGRESSIONANALYSIS FOR SUBJECTED.

8.250

0923

o-213 (201)

0.297 (202)

0.397 (203)

0.589 (204)

Regression

(3)

(2)

22.802 (4)

27.67

26.69

13.712 (1)

Residual

1.952

1.795

1.749

K

Meau squares with

1.387 (3)

2.045 (2)

3.804 (1)

Regression

oxlO (246)

OGO6 (247)

0.007 (248)

Residual

degrees of freedom

Square of

0.747

0.735

0.683

multiple correlation

Square of

0.6808

05525

0.3986

0.1024

multiple correlation

INDEPENDENTVARIABLE: LOQARITHMOF MASKING INTERVAL

-21.931

- 14.050

-

-

K

degrees of freedom

Mean squares with

INDEPENDENTVARIABLE:ARC SINE TRANSFORMATIONOF PROPORTIONCORRECTRESPONSES

* Standard errors of regressioncoefficients for last equation.

I.825

5869

1

2

log D

Equation

Coefficients for :

TABLE 1. REGRESSIONANALYSIS FOR SUBJECTMR.

E

84

EMANUEL DONCHIN

reciprocity has not been observed. The deviation from reciprocity is indicated by the difference between the coefficients of log L and log Tin the equation. The data suggest that increases in luminance are somewhat more effective than increases in duration in decreasing the masking interval. For subject ED, the dependent variable was the onset-to-onset interval at which the subject made the transition from a correct to an incorrect response. These data permitted a direct evaluation of an equation describing the relation between log D, log T,and log L. The advantage of this approach is that it permits the construction of confidence regions around the obtained equation. The analysis of these data is given in Table 2, and the obtained equation was Log Domjo= 1.95-0.15

(log Tf 1.6 log L)-0.06log L logT

(3)

The equations obtained for ED and MR differ both in the values of the parameters and in the sign of the interaction term. However, the two equations are similar in the two characteristics which are most important for the present discussion-both show the masking interval to be a power function of luminance and duration, with the coefficients of both quite similar; furthermore, in both cases, luminance is more effective than duration in determining the interval. The different procedures used in obtaining the two equations preclude a direct comparison between the two equations. The coefficients obtained. for MR are, in fact, outside the 90 per cent confidence band for the coefficients obtained for ED (see standard errors in Table 2). DISCUSSION The data presented above indicate quite clearly that TF duration has a definite effect on the masking interval. This interval, when measured from the onset of the TF to the onset of the BF, is shown to be a power function of a weighted combination of the luminance and duration of the TF. The weight assigned to luminance is somewhat greater than the weight assigned to duration. These data bear upon the interpretation of the retroactive masking effect. It has been proposed by Eriksen and his co-workers (ERIKSEN and HOFFMAN, 1963; ERIKSEN and COLLINS, 1964) that retroactive masking is an instance of brightness summation according to Bloch’s law. They suggest that the fact that the TF is not perceived is due to the presentation of the TF and the BF within the critical interval, over which brightness is integrated by the visual system. The perception of the TF in this case would depend on the contrast between the two flashes. Test flashes which are not sufficiently bright to be detected when presented simultaneously with the BF will be masked by the BF whenever the interval between the two flashes is sufliciently short. The data presented above are in apparent agreement with this hypothesis in that they point to a dependence of the masking effect on the brightness rather than on the luminance of the TF. However, a certain difficulty for the hypothesis is apparent. In the form in which it is stated above, it cannot explain why different TFs will be masked at different interflash intervals. If we assume that any two flashes presented within the summation interval are “effectively simultaneous”, and if we assume that the summation interval is tixed, then for any TF and BF presented within that summation interval the degree of masking is determined by the brightness contrast for these two flashes when presented simultaneously. If masking is obtained within the summation interval, then it should be obtained at any other interflash interval shorter than or equal to the summation interval. By the same token, if

Retroactive Visual Masking

85

is not obtained at some IF1 shorter than the summation interval, it should not be obtained at any other interval as “no masking” implies “adequate contrast” at simultaneous presentation. As Eriksen points out (ERIKSEN and STEFFY, 1964), the summation interval is not fixed. ln fact, with increasing TF luminance the summation interval decreases (GRAHAM and KEMP, 1938), and to obtain “effective simultaneity” the inter-flash interval must be decreased. However, for any given luminance of the TF, the summation interval is tied. Thus, if a 10 msec TF, 18 log mL, is masked at 30 msec, then the summation interval is at least 30 msec long. If flash duration is increased to 16 msec, the TF is no longer masked at an IF1 of 30 msec. It follows from the brightness summation hypothesis that a TF l-8 log mL and 16 msec in duration is sufficiently bright to be perceived on a background of the BF when both are presented simultaneously. This, however, is not the case, as can be seen from the fact that this flash can be masked by the BF if the interflash interval is decreased to 20 msec. Thus, the data on the effects of TF duration on the masking interval are not consonant with the brightness summation hypothesis in that they indicate that masking does not depend on summation within a tied critical interval. The above discussion assumes that the summation interval is measured from the onset of the TF. The above contradiction is removed if the assumption is made that perception is organized in frames of time, each frame equivalent to one summation interval, and that the beginning and termination of each frame is determined by a process independentof the specific inputs. For such a system, increasing TF duration would lead to an increased probability that the TF and the BF will not be wholly included within a single frame. While there is no evidence contradicting such a hypothesis there is also no evidence in its support. A more appropriate model has been proposed by RATLIFF, HARTLINE and MILLER (1963). In a sense, their model is a restatement of the latency hypothesis presented by CRAWFORD(1947), BAKER(1963), and others. The latency hypothesis accounts for masking in terms of the well-established relationship between the intensity of the stimulus and the latency of its physiological effects (HARTLINE, 1938). The BF’s effects, having a shorter latency, are able to “overtake” and “mask” the TF. The nature of this overtaking activity and its locus have not been specified. Ratliff restates the latency hypothesis in terms of the interaction between excitatory influences generated by the TF, and inhibitory activity generated by the BF. The latency difference between the two stimuli is such that the inhibitory influences of the BF arrive in time to counteract the excitatory influences of the TF. While the latency of the onset of physiological processes is independent of stimulus duration within the range used here (KOELLA, 1959; LENNOX, 1959), it is reasonable to assume that both inhibitory and excitatory influences accumulate over some summation interval. The balance between the two effects would thus depend on two factors, on the latency of the onset of the accumulation process and on the rate at which the accumulation proceeds. The rate of accumulation will depend on both luminance and duration of the stimulus, the latency on luminance only. The time it takes the TF-generated excitatory influence to reach the critical amplitude that precludes any inhibition by the BF would thus depend on both luminance and duration of the TF, with luminance having a stronger effect, in accord with the data presented above. masking

Acknowledgemenls-This work was completed while the author held a post-doctoral appointment in the laboratories of Professor DONALD B. LINDSLEYat the University of California, Los Angeles; it was supported by NSF grant GE1844 and NASA contract NsG-623. Computer time was made available by the Health Sciences Computing Facility at UCLA. Additional support was provided by NASA grant NsG 215-62 Sl.

86

EMANUELDONCHIN

REFERENCES ALPERN, M. (1965). Rod-cone independence in the after-gash effect. J. Physiol., Land. 176,462472. BAKER,H. D. (1963). Initial stages of dark- and light-adaptation. J. opt. Sot. Am. 53.98-103. BATTHRSBY, W. S. and WAOMAN,I. H. (1959). Neural limitations of visual excitability. I. The time course of monocular light-adaptation. J. opt. Sot. Am. 49.752-759. CRAWFORD,B. H. (1947). Visual adaptation in relation to brief conditioning stimuli. Proc. R. Sot. B134, 282302. CORNSWEET,T. N. (1962). The staircase-method in psychophysics. Am. J. Psychol. 75,485-491. DIXON, W. J. (1964). BMD-Biomedical Computer Programs. University of California, Los Angeles. DONCHIN,E. and LINDSLEY,D. B. (1965). Visually evoked response correlates of perceptual masking and enhancement. Electroenceph. din. Neurophysioi. 19, 325-335. EISHNHART,C. (1947). Inverse sine transformation of proportions. In: Techniques of Statistical Analysis, pp. 39746. McGraw-Hill, New York. EPROYMSON, M. A. (1960). Multiple regression analysis. In: Mathematical Methodsfor Digital Computers, edited by A. RALSTONand H. S. WILP. John Wiley, New York. ERIKSBN,C. W. and COLLINS,J. F. (1964). Backward maskhrg in vision. Psychon. Sci. 1, 101-102. ERIKSEN,C. W. and HOFFMAN,M. (1963). Form recognition at brief durations as a function of adapting field and interval between stimulations. J. exp. Psychol. 66,4SW99. ERIKSEN,C. W. and STBFFY,R. A. (1964). Short term memory and retroactive interference in visual perception. J. exp. Psychol. 6&423-434. GRAHAM,C. H. and KEMP,E. H. (1938). Brightness discrimination as a function of the duration of the increment in intensity. J. gen. Physiof. 21, 635-650. HAMXINE,H. K. (1938). The response of singIe optic nerve fibers of the vertebrate eye to illumination of the retina. Am. 1. PhysioL l21,400-415. KAHNEMAN,D. (1966). Timaintensity reciprocity under various conditions of adaptation and backwardmasking. J. exp. Psychol. 71,543-549. KOBLLA,W. P. (1959). Some functional properties of optically evoked potentials in cerebellar cortex of cat. J. Neurophysiol. 22,61-77. LENNOX,M. A. (1959). Single units of responses to brief flashes in cat’s optic tract. J. Neurophysiol. 22, 88-97. RAAB, D. H. (1%3). Backwardmasking. Psychok Bull. 60, 118-129. RATLIPF,F., HARTLINE,H. K. and MILLER,W. (1963). Spatial and temporal aspects of retinal inhibitory interaction. J. opt. Sot. Am. 53, 110420. SPBRLINO, G. (1965). Temporal and spatial masking. I. Masking by impulse hashes. J. opt. Sot. Am. 55, 541-559. WILKII,S. S. (1962). Mathematicd Statistics. John Wiley, New York.

Abatmct-The interval at which a dim test flash (IF) is masked by a succeeding bright flash (BF) was determined for two subjects at 6 levels of TF luminnnac (logmlam-1~8,1~5,1~1,0*8, 0.5) and 5 levels of TF duration (log msec-1.3 ,1.2, 1.0, O-7, 0.5). BF luminance was 3.8 log mlam, and I.0 log msec. The masking interval when measured from the onset of the Tf to the onset of the BF was found to be a power function of a weighted combination of TF luminance and duration. Luminance was somewhat more effective than duration in determining the masking interval. The implications of these results for the bright summation model are discussed. R&mm&On mesure sur deux sujets l’intervalle tel qu’un eclair lumineux faible (TF) soit masque par un &lair brillant (BF) qui lui su&de, pour 6 niveaux de luminance de TF (log mlam-1,8 1.5 1,l 0.8 0.5) et 5 dur&es de TF (log msec-1.3 1,2 LO, 0.7 0,5). La luminance de BF &it 3.8 log mlam, et sa dur& 1,Olog msec. L’intervalle de masquage, mesure du debut de TF au debut de BF, est une fonction de puissance dune combinaison de la luminance et de la d&e de TF. La luminance est un peu plus efiicace que la durte pour determiner cet intervahe. On discute ces r&mats et leurs consequences pour le modele de sommation de luminosit& Zv-Das Intervall, in dem ein dunkler Testblitz (TF) von einem nachfolgenden hellen Blitz (BF) unterdrtickt wird, wurde bei 2 Versuchspersonen bei 6 Werten der TF Leuchtdichte (log mlam-1.8, 1.5, 1.1,0*8,0~5) und 5 Werten der TF Biitzdauer (log msec1.3, 1.2, 1*0,0.7,0*5) gemessen. Die BF Leuchtdichte war 3.8 long mhnn. die Dauer 1.0 log msec. Das Unterdrtickungsintervall, gemessen vom Beginn des Testblitzes bis zum Anfang de-s hellen Blitzes, ergab sich als Potenzfunktion der TF Leuchtdichte und Dauer. Die Leuchtdichte hatte einen etwas gr&seren eintluss auf die L8nge des Verdeckungsintervalls als die Dauer. Die Bedeutung dieser Ergebnisse auf ein Helligkeitssummationsmodell werden diskutiert.

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