- Email: [email protected]
Visual backward masking of a single line by a single line
Vision
Rrr.
Vol.
9. pp.
Perzamon
199-205.
Press 1969. Printed in Great Britain.
LETTER VISUAL BACKWARD
MASKING
TO THE EDITORS OF A SINGLE
LINE
BY A SINGLE
LINE’
(Received 5 May 1968; in revisedform 4 October 1968)
IN VISUALbackward masking experiments, the detectability of a tachistoscopically presented target pattern is reduced when a masking pattern follows it within an interval of approximately 50 msec (RAAB, 1963; SCHILLER, 1965). SEKULER(1965) has shown that the spatial relationship between contours of the target and masking pattern is one important determinant of the extent of masking. In his experiment, Sekuler used a single target line masked by a pattern of light and dark stripes, This allowed a precise statement of the relationship between the contours of the target and masking stimuli. In the experiments reported below, even greater control of this relationship is possible because both the target and the masking stimuli are extremely simple, each being a singk straight line. With such a configuration, the roles of both overlapping and non-overlapping contours can be studied. Neurophysiological studies in the cat and monkey suggest that certain cells in the visual system respond maximally to line stimuli at some stage in the processing of visual information (HUBELand WIESEL,1962). Using spots of light to stimulate the retina, Hubel and Wiesel have mapped out excitatory and inhibitory areas in the receptive field of these line detectors. Presumably, this organization is responsible for the fact that the line receptors are sensitive to the orientation of the stimulating line. If the human visual system is similarly organized, it is possible that activity in such a line receptor (or group of them) is related to overt behavior-viz. it could be the input to some sort of decision process which determines whether the S says “yes” he saw a line or “no” he did not. In other words, what functions is the amount of activity in the line receptor, integrated over some short interval of time. We shall assume that if a second (masking) line is presented 30 msec after the target line, it may fall on the receptive field of the target line receptor. When it does, some portion of the masking line’s effect on the activity of the target line receptor occurs during the judgment interval. Under these assumptions, since the masking line contributes the same amount of noise whether or not the target is present, the detectability of the target line reflects the relative amount of activity in the target line receptor when the target line is present, compared with when it is absent. In a backward masking experiment where the S is confronted with a yes-no detection problem, the data are well suited to analysis according to signal detection theory (SWETS, TANNERand BIRDSALL,1961; GREEN and SWETS, 1966). On each trial or observation
1 This work was supported by a National Defense Further aid was received from a National Aeronautics Hans-Lukas Teuber. 199
Education Act, Title IV Fellowship (65-1298). and Space Administration Grant NsG 496 to
200
klTER
TO THE EDITORS
interval, the 5’ tries to decide whether the input (activity in the target line receptor) arose From a signal-plus-noise event (target line followed by masking line) or From a noise event alone (nothing preceding the masking line). His decision is based on several factors, of which two important ones are the sensory capacity For detecting the signal in noise and the criterion adopted For classifying the input as either noise or signal-plus-noise. By treating the detection task as a decision process, the theory of signal detectability can provide a measure of the observer’s ability to detect the signal independent of the particular criterion he adopts. Under certain assumptions of the theory, d’ is this measure of detectability of the signal in noise; the larger d’ the greater the detectability. In the Following experiment the target line was present 50 per cent of the time (signal-plus-noise event) and absent the other 50 per cent (noise event); an empirical measure of detectability independent of criterion (d’,) could thus be obtained For each condition. To get the values of d’,, a method of confidence judgments was used (EGAN,SCHULMAN and GREENBERG,1959). Experimentally, this means that on each trial the S responded “yes” or “no” and gave an estimate of how confident he was in his answer (3 being most, 1 least). The method relies on the fact that during a series of trials the observer can adopt multiple criteria (of various degrees of strictness) For making his judgments. These criteria are reflected in his confidence judgments-“ yes most confident” being the most lax, and “no most confident” being strictest. The assumption is that intervals accepted as containing the signal under one criterion will also be accepted under less strict criteria. For each criterion the probability of a “yes” decision under that criterion includes the cumulative probability of “yes” decisions under the less strict criteria. Since this is true of yes decisions for both signal-plus-noise and noise only events, computing hits [Pr(yes/S -+ N)] and false alarms [Pr(yes/N)] over the entire range of criteria (rating scale) generates the ROC curves (see EGAN et al., for details). On normal-normal paper these curves should be straight lines provided that the distributions of noise and signal-plus-noise are normal. Chance performance would yield the straight line drawn from the origin through the point (O-50, 0.50). The “height” of the experimental curves above chance is a measure of d’,. Comparison of the dlcs from the different conditions shows the relative ease of discrimination of signal-plus-noise from noise. Since the noise contributed by the masking line varied for the different conditions, the results permit a comparison across conditions of the relative discriminability of signal against noise, although not a comparison of absolute strength of the signal. In the following three experiments, the detectability, d’,, of the target line was determined under conditions which allowed precise control of the contour relationship: the amount of overlapping and non-overlapping contours of the target and masking lines. EXPERIMENT I The independent variable in this backward masking experiment was the angle of the masking line with respeict to the vertical target line on which it was centered. The dependent variable was the detectability of the target line. Method Subjects. The 3 Ss, male, were M.I.T. undergraduateswho were paid for their time. Apparatus. Tht stimuli were tachistoscopicailypresented to the S’s r&t eye. The S’s head was held inBxed~tionbyachintaPtandf~support,and~nrrradyhoinitiUedeachtriPtbypreatinsa button. The intensitiesof the adaptation, tar@, and m&ins fleids were equated by q!c for brigbtaass. Measurements of these fields with a MaeBeth ilhuninometer after the experiment showed the lumiaance
Visual Backward Masking of a Single Line by a Single Line
201
to be @91mL, 0.97mL, and 0+94mL respectively. The interstimulus field was dark. Electronic timers controlled the durations of all the fields. MureriuZs. The target and masking patterns were “Lettraset” lines, each 1.28 cm X 0.05 cm and subtending 282” x 0.11” visual angle, mounted on white cardboard (5.08 cm X 5.08 cm). The vertical target line was centered in the field. The masking line was placed so that whatever its angle its center would coincide with the center of the target line if they were exposed simultaneously. The contrast of the line with the background was O-98 for both the target and masking lines. The adaptation field contained four faint red fixation points arranged in a square. The square was 1.0” visual angle on a side and was centered in the field. Design. The apparatus and materials were arranged so that on any given trial S saw the following sequence of events: 1. Adaptation field with fixation points (terminated when S initiated the trial). 2. Target field (duration determined for each 5’). 3. Interstimulus interval field (30 msec duration). 4. Masking field (40 rnsec duration). 5. Adaptation field. For each S the duration of the target field chosen was the longest that would give approximately 50 per cent correct recognition in the experimental situation when the masking line was at 0” (i.e. exactly superimposed on the target line). For MG this duration was 15 msec, for JS 9 msec, and 10 rnsec for RS. Procedure. The Ss were individually tested in I-hr sessions. The target field duration was determined for each S at the end of at least three practice sessions. The conditions tested in Experiment I were: masking line 2, 4, 8, 16,45 deg to the right and the same angles to the left of the target line, plus 0” and 90”. (N = 75 for target line present and N = 75 for target line absent for each angle of the masking line.) In addition to varying the angle of the masking line, the case was also examined where the masking line was absent (just the lighted masking field followed the ISI). For this condition N = 56, 64, and 55 for Ss MG, JS, and RS respectively.
Results Since Ss varied, from condition to condition, in the number of rating categories that they were willing to use, the experimental curves are based on different number of points. The ROC curves for each S for each condition are fitted (by eye) fairly well with straight lines on normal-normal paper. Measures of d’, are taken from the curves along the negative diagonal. There is a slight tendency for conditions where the top of the masking line is angled to the left of the target line to have larger d’ values than the corresponding angles on the right side. These differences, however, are not large or even entirely consistent. When the responses to the right and left are pooled for each angle before scoring, we get ROC curves from which d’, can be obtained. In Fig. 1 the d’,s from these curves are plotted against the angle, 8, of the masking line from the vertical. As shown, the detectability of the target line is not much above chance for 0 = 0, 2, and 4 deg. For angles greater than 4”, d’, increases rapidly at first, then levels off and continues to increase more slowly. Data from two of the three Ss show that even at 90” the presence of the masking line causes some decrease in detectability compared with masking by flash alone. EXPERIMENT
II
In Experiment I the contours of the target and masking lines were related in two ways : there were overlapping and non-overlapping areas. In Experiment II, the effect of overlap alone on detectability was examined further. Method The apparatus, Ss, and basic design were the same as in the tirst experiment. The ISI was 30 msec, the duration of the masking field was 60 msec, and the individual target field durations were 8, 5, and 7 msec for MG, JS, and RS respectively. The target line was the same and the masking line was the same width as previously and again centered on the target line. The difference was that the masking line was always vertical (fl = 0’) and its length was varied. The lengths of the masking line used were O-82, 1.82, 2.82, 3 82, and 4.82 degrees.
LETTERTOTHEEDITORS
202
3.20 A
i o MG
4
b JS
a RS
8 = angle (indegrees 1 of masking line from vertical target line FIG.
1.
Detectability (d;) of the vertical taqpt line as a function of the angle between the masking and target lines.
Results
The values of d’, for each condition in Experiment II are plotted in Fig. 2 against percentage of target line overlapped by masking line. As the data show, increasing amounts of overlap result in decreasing detectability of the target line. Masking lines longer than the target line give essentially the same dlcs as masking lines of equal length. EXPERIMENTIII Experiment III studied the effect on target line detectability of a non-overlapping line at different distances from it.
masking Method
Theapparptusandbasic&&nwemthesameasinEx
perimentsIandI1.
ThreenewSswereused.
They w&students recruitedthr&h the M.I.T.Fay&alo~wt; they werepaid foT their time. of the adaptation. TheISIwaraOain30mccrcandthemaianP8cld~40~duration.~lu~ targetand masking&lds [email protected], and 6 52mL rcqectively. The durations of tho tar@ &Ids (found for each S as before) were 13. 15, and 14 msec for Tf, JK, and EM. The target line was of the same
Results
In Fig. 3 the data from Expuiment III are plotted as d’e vs. distance between the parallel target and masking lines. As this figure shows, there is increasing dctecSaW&y with increasing separation between the non-overlapping contours of the tarst and masking lines.
203
Visual Backward Masking of a Single Line by a Single Line
Percent
of target line overlapped by masking lme (d’e) as a function of percentage of target line overlapped by masking line.
FIG. 2. Detectability
d’
Distance
masking
between parallel lines (in degrees
target and of visual
angle) FIG.
3. Detectability
(d’,) of target line as a function of the lateral separation target and masking lines.
between the
DISCUSSION
These results support the conclusion that the relationship between the contours of the masking and target stimuli is important in backward masking. As far as it is possible to compare data from the two experiments, our curves of this relationship from Experiment I have the same general appearance as those found by Sekuler. In both experiments the masking effect seems to decrease rapidly with increasing angle until somewhere between 15-25” when it starts to decrease more slowly and continues to taper off out to 90”.
204
L&~-ERTO-I-HE EDITORS
Experiments II and III show that different amounts and degrees of overlapping and non-overlapping contours of the target and masking stimuli result in changes in detectability. These experiments may be useful in interpreting the results of the first experiment where both overlapping and non-overlapping contours were involved. As the masking line was rotated in Experiment I, the amount of overlapping contour decreased. Figure 2 of Experiment II shows that the detectability increases as overlap decreases. In terms of underlying physiology, this might mean that as the masking line contributes a relatively smaller amount to the excitation of the target line receptor, the contribution of the target line becomes more detectable. (Weber’s law is an expression of this relationship in other psychophysical situations.) It is possible that rotating the masking line only affected activity in the target tine receptor by decreasing the excitation contributed by the masking line. If so, Experiment II suggests that if one replotted the curve of d’ vs. 0 in Experiment I as d’ vs. percentage of overlapped area, the function should be linear. Since this is not the case, there must be another factor involved if this interpretation is to account for the results. As 6 increased in the first experiment, less of the non-overlapping part of the masking line falls on the area immediately next to the target line, and more of the non-overlapping portion falls farther away. Experiment III indicates that such a change of non-overlapping contour from near the target line to away from it would be expected to result in increasing detectability. Physiologically this could be due to the fact that the non-overlapping part of the masking line falls on the inhibitory surround of the receptive field of the target line receptor. The inhibitory effect of the surround on the center is known to decrease with increasing distance from the center (HuEiEL, 1963). The increase in detectability as a function of separation (Fig. 3) is best fitted by a function that exponentially approaches a limit. The curve of the results of Experiment I, however, is not well fitted by such a function. The data of the first experiment, then, cannot be predicted by either of the two factors, overlapping and non-overlapping contours alone. Taken together, however, an additive combination of the linear and the exponential functions expressing these two factors might be expected to give the observed results. In terms of underlying physiology, the subject’s response in this masking situation could be due to the net activity of the target line receptor integrated over some short time interval. The contribution of the masking line to such activity would be the sum of the inhibitory effect of stimulating the surround of the target line’s receptive field and the excitatory effect of stimulating its center. MARYB. PARLEE* Department of Psychology, Massachusetts Institute of Technology, Cam&ridge, Massachusetts REFERENCES
A. I. and G UBNEWIO, G. Z. (1959). Gpcra~ &uy de&&m aad by rating. 1. BcDIut.Sec. Am. 31,768-73.
EOAN, J. P., -,
characteristics
dctetined by
GRBEN,D. M. aud Swm, J. A. (1966). Signal D&u&n Tkoty aitdPsy&pkysks, Wiley, New York. HUBEL,D. H. (1963). Integrative procwm in central visual pathways of the cat. J. opt. Sot. Am. 53, 58-66. 2 The author wishes to thank Wayne A. Wickeigmn of the MasanchusettsInstitute of Tbhnoiogy for his generous advice and encouragement.
Visual Backward Masking of a Single Line by a Single Line
205
HUUL, D. H. and WIESEL,T. N. (1962). Receptive fields, binocular interaction and functional architecture in the cat’s visual cortex. J. Physiof. 160,106-54. RUEI, D. H. (1963). Backward masking. Psychol. Bull. 60, 118-29. SCHILLER,P. H. (1965). Monoptic and dichoptic visual maskings by patterns and flashes. J. exp. Psychol. 69, 193-99.
SEKULER,R. W. (1965). Spatial and temporal determinants
of visual backward masking.
70,401-06. SWETS, J. A., TANNER,W. P., JR. and BIRDSALL,T. G. (1961). Decision processes Rev. 68, 30140.
J. exp. Psychol.
in perception.
Psychol.
Rrr.
Vol.
9. pp.
Perzamon
199-205.
Press 1969. Printed in Great Britain.
LETTER VISUAL BACKWARD
MASKING
TO THE EDITORS OF A SINGLE
LINE
BY A SINGLE
LINE’
(Received 5 May 1968; in revisedform 4 October 1968)
IN VISUALbackward masking experiments, the detectability of a tachistoscopically presented target pattern is reduced when a masking pattern follows it within an interval of approximately 50 msec (RAAB, 1963; SCHILLER, 1965). SEKULER(1965) has shown that the spatial relationship between contours of the target and masking pattern is one important determinant of the extent of masking. In his experiment, Sekuler used a single target line masked by a pattern of light and dark stripes, This allowed a precise statement of the relationship between the contours of the target and masking stimuli. In the experiments reported below, even greater control of this relationship is possible because both the target and the masking stimuli are extremely simple, each being a singk straight line. With such a configuration, the roles of both overlapping and non-overlapping contours can be studied. Neurophysiological studies in the cat and monkey suggest that certain cells in the visual system respond maximally to line stimuli at some stage in the processing of visual information (HUBELand WIESEL,1962). Using spots of light to stimulate the retina, Hubel and Wiesel have mapped out excitatory and inhibitory areas in the receptive field of these line detectors. Presumably, this organization is responsible for the fact that the line receptors are sensitive to the orientation of the stimulating line. If the human visual system is similarly organized, it is possible that activity in such a line receptor (or group of them) is related to overt behavior-viz. it could be the input to some sort of decision process which determines whether the S says “yes” he saw a line or “no” he did not. In other words, what functions is the amount of activity in the line receptor, integrated over some short interval of time. We shall assume that if a second (masking) line is presented 30 msec after the target line, it may fall on the receptive field of the target line receptor. When it does, some portion of the masking line’s effect on the activity of the target line receptor occurs during the judgment interval. Under these assumptions, since the masking line contributes the same amount of noise whether or not the target is present, the detectability of the target line reflects the relative amount of activity in the target line receptor when the target line is present, compared with when it is absent. In a backward masking experiment where the S is confronted with a yes-no detection problem, the data are well suited to analysis according to signal detection theory (SWETS, TANNERand BIRDSALL,1961; GREEN and SWETS, 1966). On each trial or observation
1 This work was supported by a National Defense Further aid was received from a National Aeronautics Hans-Lukas Teuber. 199
Education Act, Title IV Fellowship (65-1298). and Space Administration Grant NsG 496 to
200
klTER
TO THE EDITORS
interval, the 5’ tries to decide whether the input (activity in the target line receptor) arose From a signal-plus-noise event (target line followed by masking line) or From a noise event alone (nothing preceding the masking line). His decision is based on several factors, of which two important ones are the sensory capacity For detecting the signal in noise and the criterion adopted For classifying the input as either noise or signal-plus-noise. By treating the detection task as a decision process, the theory of signal detectability can provide a measure of the observer’s ability to detect the signal independent of the particular criterion he adopts. Under certain assumptions of the theory, d’ is this measure of detectability of the signal in noise; the larger d’ the greater the detectability. In the Following experiment the target line was present 50 per cent of the time (signal-plus-noise event) and absent the other 50 per cent (noise event); an empirical measure of detectability independent of criterion (d’,) could thus be obtained For each condition. To get the values of d’,, a method of confidence judgments was used (EGAN,SCHULMAN and GREENBERG,1959). Experimentally, this means that on each trial the S responded “yes” or “no” and gave an estimate of how confident he was in his answer (3 being most, 1 least). The method relies on the fact that during a series of trials the observer can adopt multiple criteria (of various degrees of strictness) For making his judgments. These criteria are reflected in his confidence judgments-“ yes most confident” being the most lax, and “no most confident” being strictest. The assumption is that intervals accepted as containing the signal under one criterion will also be accepted under less strict criteria. For each criterion the probability of a “yes” decision under that criterion includes the cumulative probability of “yes” decisions under the less strict criteria. Since this is true of yes decisions for both signal-plus-noise and noise only events, computing hits [Pr(yes/S -+ N)] and false alarms [Pr(yes/N)] over the entire range of criteria (rating scale) generates the ROC curves (see EGAN et al., for details). On normal-normal paper these curves should be straight lines provided that the distributions of noise and signal-plus-noise are normal. Chance performance would yield the straight line drawn from the origin through the point (O-50, 0.50). The “height” of the experimental curves above chance is a measure of d’,. Comparison of the dlcs from the different conditions shows the relative ease of discrimination of signal-plus-noise from noise. Since the noise contributed by the masking line varied for the different conditions, the results permit a comparison across conditions of the relative discriminability of signal against noise, although not a comparison of absolute strength of the signal. In the following three experiments, the detectability, d’,, of the target line was determined under conditions which allowed precise control of the contour relationship: the amount of overlapping and non-overlapping contours of the target and masking lines. EXPERIMENT I The independent variable in this backward masking experiment was the angle of the masking line with respeict to the vertical target line on which it was centered. The dependent variable was the detectability of the target line. Method Subjects. The 3 Ss, male, were M.I.T. undergraduateswho were paid for their time. Apparatus. Tht stimuli were tachistoscopicailypresented to the S’s r&t eye. The S’s head was held inBxed~tionbyachintaPtandf~support,and~nrrradyhoinitiUedeachtriPtbypreatinsa button. The intensitiesof the adaptation, tar@, and m&ins fleids were equated by q!c for brigbtaass. Measurements of these fields with a MaeBeth ilhuninometer after the experiment showed the lumiaance
Visual Backward Masking of a Single Line by a Single Line
201
to be @91mL, 0.97mL, and 0+94mL respectively. The interstimulus field was dark. Electronic timers controlled the durations of all the fields. MureriuZs. The target and masking patterns were “Lettraset” lines, each 1.28 cm X 0.05 cm and subtending 282” x 0.11” visual angle, mounted on white cardboard (5.08 cm X 5.08 cm). The vertical target line was centered in the field. The masking line was placed so that whatever its angle its center would coincide with the center of the target line if they were exposed simultaneously. The contrast of the line with the background was O-98 for both the target and masking lines. The adaptation field contained four faint red fixation points arranged in a square. The square was 1.0” visual angle on a side and was centered in the field. Design. The apparatus and materials were arranged so that on any given trial S saw the following sequence of events: 1. Adaptation field with fixation points (terminated when S initiated the trial). 2. Target field (duration determined for each 5’). 3. Interstimulus interval field (30 msec duration). 4. Masking field (40 rnsec duration). 5. Adaptation field. For each S the duration of the target field chosen was the longest that would give approximately 50 per cent correct recognition in the experimental situation when the masking line was at 0” (i.e. exactly superimposed on the target line). For MG this duration was 15 msec, for JS 9 msec, and 10 rnsec for RS. Procedure. The Ss were individually tested in I-hr sessions. The target field duration was determined for each S at the end of at least three practice sessions. The conditions tested in Experiment I were: masking line 2, 4, 8, 16,45 deg to the right and the same angles to the left of the target line, plus 0” and 90”. (N = 75 for target line present and N = 75 for target line absent for each angle of the masking line.) In addition to varying the angle of the masking line, the case was also examined where the masking line was absent (just the lighted masking field followed the ISI). For this condition N = 56, 64, and 55 for Ss MG, JS, and RS respectively.
Results Since Ss varied, from condition to condition, in the number of rating categories that they were willing to use, the experimental curves are based on different number of points. The ROC curves for each S for each condition are fitted (by eye) fairly well with straight lines on normal-normal paper. Measures of d’, are taken from the curves along the negative diagonal. There is a slight tendency for conditions where the top of the masking line is angled to the left of the target line to have larger d’ values than the corresponding angles on the right side. These differences, however, are not large or even entirely consistent. When the responses to the right and left are pooled for each angle before scoring, we get ROC curves from which d’, can be obtained. In Fig. 1 the d’,s from these curves are plotted against the angle, 8, of the masking line from the vertical. As shown, the detectability of the target line is not much above chance for 0 = 0, 2, and 4 deg. For angles greater than 4”, d’, increases rapidly at first, then levels off and continues to increase more slowly. Data from two of the three Ss show that even at 90” the presence of the masking line causes some decrease in detectability compared with masking by flash alone. EXPERIMENT
II
In Experiment I the contours of the target and masking lines were related in two ways : there were overlapping and non-overlapping areas. In Experiment II, the effect of overlap alone on detectability was examined further. Method The apparatus, Ss, and basic design were the same as in the tirst experiment. The ISI was 30 msec, the duration of the masking field was 60 msec, and the individual target field durations were 8, 5, and 7 msec for MG, JS, and RS respectively. The target line was the same and the masking line was the same width as previously and again centered on the target line. The difference was that the masking line was always vertical (fl = 0’) and its length was varied. The lengths of the masking line used were O-82, 1.82, 2.82, 3 82, and 4.82 degrees.
LETTERTOTHEEDITORS
202
3.20 A
i o MG
4
b JS
a RS
8 = angle (indegrees 1 of masking line from vertical target line FIG.
1.
Detectability (d;) of the vertical taqpt line as a function of the angle between the masking and target lines.
Results
The values of d’, for each condition in Experiment II are plotted in Fig. 2 against percentage of target line overlapped by masking line. As the data show, increasing amounts of overlap result in decreasing detectability of the target line. Masking lines longer than the target line give essentially the same dlcs as masking lines of equal length. EXPERIMENTIII Experiment III studied the effect on target line detectability of a non-overlapping line at different distances from it.
masking Method
Theapparptusandbasic&&nwemthesameasinEx
perimentsIandI1.
ThreenewSswereused.
They w&students recruitedthr&h the M.I.T.Fay&alo~wt; they werepaid foT their time. of the adaptation. TheISIwaraOain30mccrcandthemaianP8cld~40~duration.~lu~ targetand masking&lds [email protected], and 6 52mL rcqectively. The durations of tho tar@ &Ids (found for each S as before) were 13. 15, and 14 msec for Tf, JK, and EM. The target line was of the same
Results
In Fig. 3 the data from Expuiment III are plotted as d’e vs. distance between the parallel target and masking lines. As this figure shows, there is increasing dctecSaW&y with increasing separation between the non-overlapping contours of the tarst and masking lines.
203
Visual Backward Masking of a Single Line by a Single Line
Percent
of target line overlapped by masking lme (d’e) as a function of percentage of target line overlapped by masking line.
FIG. 2. Detectability
d’
Distance
masking
between parallel lines (in degrees
target and of visual
angle) FIG.
3. Detectability
(d’,) of target line as a function of the lateral separation target and masking lines.
between the
DISCUSSION
These results support the conclusion that the relationship between the contours of the masking and target stimuli is important in backward masking. As far as it is possible to compare data from the two experiments, our curves of this relationship from Experiment I have the same general appearance as those found by Sekuler. In both experiments the masking effect seems to decrease rapidly with increasing angle until somewhere between 15-25” when it starts to decrease more slowly and continues to taper off out to 90”.
204
L&~-ERTO-I-HE EDITORS
Experiments II and III show that different amounts and degrees of overlapping and non-overlapping contours of the target and masking stimuli result in changes in detectability. These experiments may be useful in interpreting the results of the first experiment where both overlapping and non-overlapping contours were involved. As the masking line was rotated in Experiment I, the amount of overlapping contour decreased. Figure 2 of Experiment II shows that the detectability increases as overlap decreases. In terms of underlying physiology, this might mean that as the masking line contributes a relatively smaller amount to the excitation of the target line receptor, the contribution of the target line becomes more detectable. (Weber’s law is an expression of this relationship in other psychophysical situations.) It is possible that rotating the masking line only affected activity in the target tine receptor by decreasing the excitation contributed by the masking line. If so, Experiment II suggests that if one replotted the curve of d’ vs. 0 in Experiment I as d’ vs. percentage of overlapped area, the function should be linear. Since this is not the case, there must be another factor involved if this interpretation is to account for the results. As 6 increased in the first experiment, less of the non-overlapping part of the masking line falls on the area immediately next to the target line, and more of the non-overlapping portion falls farther away. Experiment III indicates that such a change of non-overlapping contour from near the target line to away from it would be expected to result in increasing detectability. Physiologically this could be due to the fact that the non-overlapping part of the masking line falls on the inhibitory surround of the receptive field of the target line receptor. The inhibitory effect of the surround on the center is known to decrease with increasing distance from the center (HuEiEL, 1963). The increase in detectability as a function of separation (Fig. 3) is best fitted by a function that exponentially approaches a limit. The curve of the results of Experiment I, however, is not well fitted by such a function. The data of the first experiment, then, cannot be predicted by either of the two factors, overlapping and non-overlapping contours alone. Taken together, however, an additive combination of the linear and the exponential functions expressing these two factors might be expected to give the observed results. In terms of underlying physiology, the subject’s response in this masking situation could be due to the net activity of the target line receptor integrated over some short time interval. The contribution of the masking line to such activity would be the sum of the inhibitory effect of stimulating the surround of the target line’s receptive field and the excitatory effect of stimulating its center. MARYB. PARLEE* Department of Psychology, Massachusetts Institute of Technology, Cam&ridge, Massachusetts REFERENCES
A. I. and G UBNEWIO, G. Z. (1959). Gpcra~ &uy de&&m aad by rating. 1. BcDIut.Sec. Am. 31,768-73.
EOAN, J. P., -,
characteristics
dctetined by
GRBEN,D. M. aud Swm, J. A. (1966). Signal D&u&n Tkoty aitdPsy&pkysks, Wiley, New York. HUBEL,D. H. (1963). Integrative procwm in central visual pathways of the cat. J. opt. Sot. Am. 53, 58-66. 2 The author wishes to thank Wayne A. Wickeigmn of the MasanchusettsInstitute of Tbhnoiogy for his generous advice and encouragement.
Visual Backward Masking of a Single Line by a Single Line
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