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The Journal of General Psychology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/vgen20
The Effect of Visual Angle Orientation on Perceived Oscillatory Motion Raymond F. Martinetti
a
a
Marywood College , USA Published online: 06 Jul 2010.
To cite this article: Raymond F. Martinetti (1976) The Effect of Visual Angle Orientation on Perceived Oscillatory Motion, The Journal of General Psychology, 94:1, 59-64, DOI: 10.1080/00221309.1976.9711590 To link to this article: http://dx.doi.org/10.1080/00221309.1976.9711590
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content.
Downloaded by [UZH Hauptbibliothek / Zentralbibliothek Zürich] at 16:44 02 January 2015
This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions
The Journal of General Psychology, 1976, 94, 39-64.
T H E EFFECT OF VISUAL ANGLE ORIENTATION ON PERCEIVED OSCILLATORY MOTION* Marywood College
Downloaded by [UZH Hauptbibliothek / Zentralbibliothek Zürich] at 16:44 02 January 2015
RAYMONDF. MARTINETTI’ SUMMARY The present experiment examined the effect of independently varying central, vertical, and horizontal visual angles (CVVA, CHVA) on proportion of perceived oscillations with rotating trapezoids. Twenty-five female undergraduates served as Ss. Diminution of the CHVA proved most effective in increasing oscillatory perception (p < .Ol), although perceived oscillation was found to be inversely proportional to C W A and CHVA (p < .001). The data indicate that 0 s primarily rely on perceptual cues determined by the horizontal dimension of a trapezoid. When cue discriminability is reduced, the results suggest that 0’s perceptual response may be a function of individual differences in cue thresholds, a n d o r response criteria.
A. INTRODUCTION I t is well known that an illusion of oscillatory motion can be produced if a rotating, isosceles trapezoid is aligned perpendicularly to am 0’s principal axis of vision. The oscillatory illusion was originally investigated by Ames ( l ) , who attributed it to past experience. The 0 perceives the trapezoid as a rectangle tipped back a t the maximally subtended visual angle. Therefore the target never appears to be in a perpendicular plane to the visual axis, and the constantly changing reference points produce an apparent oscillatory motion. Most subsequent investigations of the illusion have dealt with stimulus parameters and their enhancement or inhibition of relevant perceptual cues. Graham ( 5 ) held that the oscillatory illusion is a direct function of the lack of cue discriminability. When veridical, rotation cues are attenuated,
* Received in the Editorial Office, Provincetown, Massachusetts, on September 2 7 , 1975. Copyright, 1976, by The Journal Press. Requests for reprints should be sent to the author at the address shown at the end of this article.
’
59
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JOURNAL OF GENERAL PSYCHOLOGY
the 0 relies on linear perspective and judges the longer end of the trapezoid to be less distant than the shorter end for the entire rotation period. The result is perceived oscillatory motion for the segment during which linear perspective is giving nonveridical information. The ambiguity of the linear perspective cue has also been demonstrated by Cappone (3) who found a positive relationship between trapezoid slope and perceived oscillation. Freeman and Pasnak (4) discovered that linear perspective played a crucial role in determining the plane of apparent oscillation, but that the trapezoid’s horizontal dimension was the primary determinant of frequency of reported oscillations and plane of apparent oscillation. Martinetti (6) studied the effect of linear perspective determined by trapezoid slope and binocular disparity produced by the trapezoid’s horizontal visual subtense. The data were supportive of Cappone (3) and Freeman and Pasnak (4). When the angle of disparity (Eta) was increased, rotary perception was favored. However, when the binocular disparity cue was impoverished, the 0 relied on trapezoid slope for perceptual information, and reported more oscillations as slope (i.e., linear perspective) was increased. It is clear that the preponderance of research on the trapezoid oscillatory illusion has reinforced the notion that perceptual cues produced by stimulus conditions determine the nature and extent of illusory experience. The next logical step is an analysis of the bases of these relevant cues, from the 0 ’ s viewpoint. A basic, quantifiable determinant of cues available to the 0 is the visual angle. The purpose of the present experiment was to determine the effect on rotary information processing of independently varying the central vertical and horizontal visual angles ( C W A and CHVA) subtended by the trapezoid. The plan was implemented by using a standard square target subtending equal C W A and CHVAs. Eight additional trapezoids were used, four of which were decreased proportionately (2’ steps) in CHVA and four in C W A with slope and corresponding vertical and horizontal dimensions held constant. It was hypothesized that (a) the standard square would produce fewest oscillation perceptions; (b) decrements in CHVA or C W A would produce an increment in perceived oscillation; and (c) trapezoids reduced according to CHVA would produce more oscillation perception than those reduced according to CVVA.
61
RAYMOND F. MARTINETTI
B. METHOD
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1. Subjects
Twenty-five female undergraduates at Marywood College served as Ss in the experiment. The Ss were all volunteers naive to the trapezoid oscillatory illusion. All Ss were pretested for relevant visual competencies and only right eye dominant individuals with 20/20 vision without corrective lenses were employed. 2. Apparatus
a. Stimuli. Nine targets were used in the experiment. The standard target was a square (8.46 in), and the remainder consisted of trapezoids with a constant slope (.40 in). All targets were constructed from % in mat, board according to the specifications listed in Table 1. The targets were TABLE 1 DIMENSIONS OF TARGETSIN VISUALANGLEAND INCHES Target set no.
CHVA
1 1 1 2 1 3
8" ' 6 4"
1 4 Standard
20
square
loo
2 2 2 2
5 6 7 8
loD 10"
10'
loo
CWA
Central horizontal dimension
Central vertical dimension
Longer vertical side
Shorter vertical side
loo 10" 1oo loo
6.74 5.04 3.35 1.67
8.46 8.4b 8.46 8.46
12.86 11.50 10.14 8.80
7.46 7.46 7.46 7.46
lo0 8O ' 6 4O
8.46 8.46 8.46 8.46 8.46
8.46 6.74 5.04 3.35 1.67
8.46 12.50 10.80 9.11 7.43
8.46 5.74 4.04 2.35 .67
2O
spray painted with lemon-yellow IGylon Luminous Paint and centrally mounted on cylindrical, aluminum supports. A fixation point (rubber button) was located at the geometric center of each target. A Spectroline Ultraviolet light source was used to illuminate the stimuli (115 volts, AC, 60 cycles, .25 amp, and .29 watts). b. Rotation and recoyding instruments. The targets were rotated at a constant speed (25 RPM) by means of a silent, rotary apparatus known as "The Illusionater Set" (a0 Hz, 120 volts, Lafeyette Instrument Company). Perceived oscillations were recorded by a single impulse counter (lOS/l25 volts AC, 50/60 Hz, count ratio 20 counts/second, Lafeyette Instrument Company).
62
JOURNAL OF GENERAL PSYCHOLOGY
c. Viewing apparatus. Ss viewed targets through a rectangular tunnel 15 x 15 x 48 in. The tunnel was equipped with an adjustable headlchin rest which allowed for direct visual alignment with the target’s center.
Downloaded by [UZH Hauptbibliothek / Zentralbibliothek Zürich] at 16:44 02 January 2015
3.
Procedure
The experiment was performed in a light shielded room. Every S viewed each target binocularly for a total of 15 minutes (three minutes per trial for five trials). Individuals were tested on five consecutive days. The viewing distance measured from the bridge of S’s nose to target center was 96 in. The order of target presentation was randomized (2) for each trial. After a 15 minute dark adaptation period all Ss were told that they would observe a series of targets which may appear to rotate or oscillate. During the observation period the Ss were instructed to “keep both eyes focused on the fixation point at each target’s center, and press the response button if and when a target appears to oscillate (reverse direction).” The dependent variable, perceived oscillation, was expressed in terms of the proportion of responses per target for the 15 minute viewing period. Since a target rotating at 2 5 RPM can potentially produce 50 apparent oscillations in one minute, 100% illusory experience in the present study was 7.50 reported oscillations.
C.
RESULTS
A repeated-measures, single factor analysis of variance design was used to assess the effect of proportionately reducing visual angle in a given dimension. Reduction of the CHVA resulted in a significant increment in perceived oscillations, F(4, 96) = 37.74 p < .001. Decrements in the C W A also proved to be highly significant, F(4, 96) = 2 2 . 8 5 p < .001. Since the standard square figure was hypothesized to produce fewer oscillation illusions than the trapezoid targets, orthogonal comparisons were carried out to compare the effect of the square with that of the trapezoids within each dimension ( 7 ) . The results of these analyses indicate a significant difference for the CHVA, F ( 1 , 96) = 6.84 p < .01, and for the C W A , F(1, 96) = 4.86, p < .01. In order to determine if the significant analyses of variance were due to a large difference between the square and trapezoids as opposed to a gradual but true difference between all targets, appropriate Newman-Keuls tests were conducted for stimuli within each dimension. All relevant pairwise comparisons in each analysis were significant at p < .01 with the exception of the standard square and trapezoid six in the CVVA condition, which was significant a t p < .05. It is evident from these results, as well as
63
RAYMOND F. MARTINETTI
inspection of Table 2 , that as CHVA or C W A was increased, there was a gradual, significant increase in reported oscillations. It is also noteworthy that decrements in CHVA and C W A were accompanied by a consistent increase in standard deviation.
Downloaded by [UZH Hauptbibliothek / Zentralbibliothek Zürich] at 16:44 02 January 2015
TABLE 2 MEANSAND STANDARD DEVIATIONS O F PROWJRTION PERCEIVED OSCILLATION (N = 2 5 ) Target set no. 1 1 1 1
1 2 3 4
Standard square 2 2 2 2
5 6 7 8
OF
M
SD
43 52 67 81
5.60 6.32 7.71 8.50
19 23 36 51 64
4.45 5.13 5.89 6.42 1.37
In order to compare the relative effects of reducing CHVA vs. CVVA, four correlated t tests were used. Each trapezoid in set one (1-4) was tested against the trapezoid in set two (5-8) with opposite CHVA and C W A . The results clearly indicate that decreasing CHVA produced significantly more reported oscillations than decreasing C W A . All t tests (df = 24) were significant tl,5 = 11.57, t2,6 = 6.55, t3,, = 6.93, t4,8= 7.24, (p < .01).
D.
DISCUSSION
The results of the present experiment permit the general conclusion that perceptual cues produced by the horizontal dimension of a rotating trapezoid are more important for veridical perception than those associated with the vertical dimension. When the horizontal dimension of a trapezoid is decreased by one-half while the vertical dimension is held constant, the result is a 100% reduction of the total circular path described by the trapezoid. Therefore, although the target is rotated at a constant speed, the amount of horizontal displacement relative to the principal axis of vision is significantly less for the diminished trapezoid. The perceptual situation created decreases the 0 ’ s ability to make use of binocular disparity produced by the horizontal visual subtense of the trapezoid. If the trapezoid’s vertical dimension is decreased while the horizontal width is kept constant, the availability of binocular disparity remains the same, although linear perspective produced by the discrepancy between the trapezoid’s parallel sides is emphasized. The results of the present study
Downloaded by [UZH Hauptbibliothek / Zentralbibliothek Zürich] at 16:44 02 January 2015
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JOURNAL OF GENERAL PSYCHOLOGY
taken together with the findings of Graham (S), Freeman and Pasnak (4) and Martinetti (6) indicate that when binocular depth cues exist with linear perspective, the 0 relies on the former for perceptual information. It is only when depth cue discriminability is decreased that 0 relies on the confusing linear perspective cue. The data further suggest that regardless of the dimension of reduction, perceived oscillation is inversely related to visual angle. The analyses reveal that within each set of targets the square produced fewest oscillation perceptions, and the smallest trapezoid produced most reported illusions. It is evident from pairwise comparisons that the trend is stable for all targets within the range of visual angles sampled. Visual angle reduction in a general sense decreases the visibility of the trapezoid, as well as the discriminability of attendant perceptual cues. The stimulus impoverishment may account for the increase in reported oscillations. However, another possible explanation would be that with decreased visibility of the target, the 0 reports an oscillation when in doubt about the presence of rotation. The latter interpretation is supported by the finding that decrements in visual angle not only produce more oscillation reports but also greater intersubject variability. The finding that subjective response becomes more variable as perceptual cues are attenuated, may therefore be a function of differential cue thresholds and/or response criteria. Either alternative suggests the need for definitive study of the trapezoid oscillatory illusion in a signal detection format. REFERENCES AMES,A. Visual perception and the rotating trapezoid window. Psychol. Monog., 1951, 65 (7, Whole No. 324). 2. ANDREAS,B. G . Experimental Psychology. New York Wiley, 1972. 3. CAPPONE,M. The effect of trapezoid slant on the Ames trapezoid illusion. Unpublished doctoral dissertation, Fordham University, New York, New York, 1967. 4. FREEMAN,R. E., Jr., & Pasnak, R. Determinants of the rotating trapezoid illusion. 1. Exper. Psychol., 1968, 76, 94-101. 5 . GRAHAM,C. H. On some aspects of real and apparent visual movement. J . Opt. Soc. Amer., 1963, 63, 1019-1025. 6. MARTINETTI, R. F. The effect of varying binocular disparity and linear perspective on perceived oscillation of the Ames trapezoid. (Doctoral dissertation, Fordham University) Ann Arbor, Mich.: University Microfilms, 1974, No. 19-673. 7. WINER,B. J. Statistical Principles in Experimental Design. New York McGraw-Hill, 1971. 1.
Department of Psychology, Box 867 Marywood College Scranton, Pennsylvania 18509
The Journal of General Psychology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/vgen20
The Effect of Visual Angle Orientation on Perceived Oscillatory Motion Raymond F. Martinetti
a
a
Marywood College , USA Published online: 06 Jul 2010.
To cite this article: Raymond F. Martinetti (1976) The Effect of Visual Angle Orientation on Perceived Oscillatory Motion, The Journal of General Psychology, 94:1, 59-64, DOI: 10.1080/00221309.1976.9711590 To link to this article: http://dx.doi.org/10.1080/00221309.1976.9711590
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content.
Downloaded by [UZH Hauptbibliothek / Zentralbibliothek Zürich] at 16:44 02 January 2015
This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions
The Journal of General Psychology, 1976, 94, 39-64.
T H E EFFECT OF VISUAL ANGLE ORIENTATION ON PERCEIVED OSCILLATORY MOTION* Marywood College
Downloaded by [UZH Hauptbibliothek / Zentralbibliothek Zürich] at 16:44 02 January 2015
RAYMONDF. MARTINETTI’ SUMMARY The present experiment examined the effect of independently varying central, vertical, and horizontal visual angles (CVVA, CHVA) on proportion of perceived oscillations with rotating trapezoids. Twenty-five female undergraduates served as Ss. Diminution of the CHVA proved most effective in increasing oscillatory perception (p < .Ol), although perceived oscillation was found to be inversely proportional to C W A and CHVA (p < .001). The data indicate that 0 s primarily rely on perceptual cues determined by the horizontal dimension of a trapezoid. When cue discriminability is reduced, the results suggest that 0’s perceptual response may be a function of individual differences in cue thresholds, a n d o r response criteria.
A. INTRODUCTION I t is well known that an illusion of oscillatory motion can be produced if a rotating, isosceles trapezoid is aligned perpendicularly to am 0’s principal axis of vision. The oscillatory illusion was originally investigated by Ames ( l ) , who attributed it to past experience. The 0 perceives the trapezoid as a rectangle tipped back a t the maximally subtended visual angle. Therefore the target never appears to be in a perpendicular plane to the visual axis, and the constantly changing reference points produce an apparent oscillatory motion. Most subsequent investigations of the illusion have dealt with stimulus parameters and their enhancement or inhibition of relevant perceptual cues. Graham ( 5 ) held that the oscillatory illusion is a direct function of the lack of cue discriminability. When veridical, rotation cues are attenuated,
* Received in the Editorial Office, Provincetown, Massachusetts, on September 2 7 , 1975. Copyright, 1976, by The Journal Press. Requests for reprints should be sent to the author at the address shown at the end of this article.
’
59
Downloaded by [UZH Hauptbibliothek / Zentralbibliothek Zürich] at 16:44 02 January 2015
60
JOURNAL OF GENERAL PSYCHOLOGY
the 0 relies on linear perspective and judges the longer end of the trapezoid to be less distant than the shorter end for the entire rotation period. The result is perceived oscillatory motion for the segment during which linear perspective is giving nonveridical information. The ambiguity of the linear perspective cue has also been demonstrated by Cappone (3) who found a positive relationship between trapezoid slope and perceived oscillation. Freeman and Pasnak (4) discovered that linear perspective played a crucial role in determining the plane of apparent oscillation, but that the trapezoid’s horizontal dimension was the primary determinant of frequency of reported oscillations and plane of apparent oscillation. Martinetti (6) studied the effect of linear perspective determined by trapezoid slope and binocular disparity produced by the trapezoid’s horizontal visual subtense. The data were supportive of Cappone (3) and Freeman and Pasnak (4). When the angle of disparity (Eta) was increased, rotary perception was favored. However, when the binocular disparity cue was impoverished, the 0 relied on trapezoid slope for perceptual information, and reported more oscillations as slope (i.e., linear perspective) was increased. It is clear that the preponderance of research on the trapezoid oscillatory illusion has reinforced the notion that perceptual cues produced by stimulus conditions determine the nature and extent of illusory experience. The next logical step is an analysis of the bases of these relevant cues, from the 0 ’ s viewpoint. A basic, quantifiable determinant of cues available to the 0 is the visual angle. The purpose of the present experiment was to determine the effect on rotary information processing of independently varying the central vertical and horizontal visual angles ( C W A and CHVA) subtended by the trapezoid. The plan was implemented by using a standard square target subtending equal C W A and CHVAs. Eight additional trapezoids were used, four of which were decreased proportionately (2’ steps) in CHVA and four in C W A with slope and corresponding vertical and horizontal dimensions held constant. It was hypothesized that (a) the standard square would produce fewest oscillation perceptions; (b) decrements in CHVA or C W A would produce an increment in perceived oscillation; and (c) trapezoids reduced according to CHVA would produce more oscillation perception than those reduced according to CVVA.
61
RAYMOND F. MARTINETTI
B. METHOD
Downloaded by [UZH Hauptbibliothek / Zentralbibliothek Zürich] at 16:44 02 January 2015
1. Subjects
Twenty-five female undergraduates at Marywood College served as Ss in the experiment. The Ss were all volunteers naive to the trapezoid oscillatory illusion. All Ss were pretested for relevant visual competencies and only right eye dominant individuals with 20/20 vision without corrective lenses were employed. 2. Apparatus
a. Stimuli. Nine targets were used in the experiment. The standard target was a square (8.46 in), and the remainder consisted of trapezoids with a constant slope (.40 in). All targets were constructed from % in mat, board according to the specifications listed in Table 1. The targets were TABLE 1 DIMENSIONS OF TARGETSIN VISUALANGLEAND INCHES Target set no.
CHVA
1 1 1 2 1 3
8" ' 6 4"
1 4 Standard
20
square
loo
2 2 2 2
5 6 7 8
loD 10"
10'
loo
CWA
Central horizontal dimension
Central vertical dimension
Longer vertical side
Shorter vertical side
loo 10" 1oo loo
6.74 5.04 3.35 1.67
8.46 8.4b 8.46 8.46
12.86 11.50 10.14 8.80
7.46 7.46 7.46 7.46
lo0 8O ' 6 4O
8.46 8.46 8.46 8.46 8.46
8.46 6.74 5.04 3.35 1.67
8.46 12.50 10.80 9.11 7.43
8.46 5.74 4.04 2.35 .67
2O
spray painted with lemon-yellow IGylon Luminous Paint and centrally mounted on cylindrical, aluminum supports. A fixation point (rubber button) was located at the geometric center of each target. A Spectroline Ultraviolet light source was used to illuminate the stimuli (115 volts, AC, 60 cycles, .25 amp, and .29 watts). b. Rotation and recoyding instruments. The targets were rotated at a constant speed (25 RPM) by means of a silent, rotary apparatus known as "The Illusionater Set" (a0 Hz, 120 volts, Lafeyette Instrument Company). Perceived oscillations were recorded by a single impulse counter (lOS/l25 volts AC, 50/60 Hz, count ratio 20 counts/second, Lafeyette Instrument Company).
62
JOURNAL OF GENERAL PSYCHOLOGY
c. Viewing apparatus. Ss viewed targets through a rectangular tunnel 15 x 15 x 48 in. The tunnel was equipped with an adjustable headlchin rest which allowed for direct visual alignment with the target’s center.
Downloaded by [UZH Hauptbibliothek / Zentralbibliothek Zürich] at 16:44 02 January 2015
3.
Procedure
The experiment was performed in a light shielded room. Every S viewed each target binocularly for a total of 15 minutes (three minutes per trial for five trials). Individuals were tested on five consecutive days. The viewing distance measured from the bridge of S’s nose to target center was 96 in. The order of target presentation was randomized (2) for each trial. After a 15 minute dark adaptation period all Ss were told that they would observe a series of targets which may appear to rotate or oscillate. During the observation period the Ss were instructed to “keep both eyes focused on the fixation point at each target’s center, and press the response button if and when a target appears to oscillate (reverse direction).” The dependent variable, perceived oscillation, was expressed in terms of the proportion of responses per target for the 15 minute viewing period. Since a target rotating at 2 5 RPM can potentially produce 50 apparent oscillations in one minute, 100% illusory experience in the present study was 7.50 reported oscillations.
C.
RESULTS
A repeated-measures, single factor analysis of variance design was used to assess the effect of proportionately reducing visual angle in a given dimension. Reduction of the CHVA resulted in a significant increment in perceived oscillations, F(4, 96) = 37.74 p < .001. Decrements in the C W A also proved to be highly significant, F(4, 96) = 2 2 . 8 5 p < .001. Since the standard square figure was hypothesized to produce fewer oscillation illusions than the trapezoid targets, orthogonal comparisons were carried out to compare the effect of the square with that of the trapezoids within each dimension ( 7 ) . The results of these analyses indicate a significant difference for the CHVA, F ( 1 , 96) = 6.84 p < .01, and for the C W A , F(1, 96) = 4.86, p < .01. In order to determine if the significant analyses of variance were due to a large difference between the square and trapezoids as opposed to a gradual but true difference between all targets, appropriate Newman-Keuls tests were conducted for stimuli within each dimension. All relevant pairwise comparisons in each analysis were significant at p < .01 with the exception of the standard square and trapezoid six in the CVVA condition, which was significant a t p < .05. It is evident from these results, as well as
63
RAYMOND F. MARTINETTI
inspection of Table 2 , that as CHVA or C W A was increased, there was a gradual, significant increase in reported oscillations. It is also noteworthy that decrements in CHVA and C W A were accompanied by a consistent increase in standard deviation.
Downloaded by [UZH Hauptbibliothek / Zentralbibliothek Zürich] at 16:44 02 January 2015
TABLE 2 MEANSAND STANDARD DEVIATIONS O F PROWJRTION PERCEIVED OSCILLATION (N = 2 5 ) Target set no. 1 1 1 1
1 2 3 4
Standard square 2 2 2 2
5 6 7 8
OF
M
SD
43 52 67 81
5.60 6.32 7.71 8.50
19 23 36 51 64
4.45 5.13 5.89 6.42 1.37
In order to compare the relative effects of reducing CHVA vs. CVVA, four correlated t tests were used. Each trapezoid in set one (1-4) was tested against the trapezoid in set two (5-8) with opposite CHVA and C W A . The results clearly indicate that decreasing CHVA produced significantly more reported oscillations than decreasing C W A . All t tests (df = 24) were significant tl,5 = 11.57, t2,6 = 6.55, t3,, = 6.93, t4,8= 7.24, (p < .01).
D.
DISCUSSION
The results of the present experiment permit the general conclusion that perceptual cues produced by the horizontal dimension of a rotating trapezoid are more important for veridical perception than those associated with the vertical dimension. When the horizontal dimension of a trapezoid is decreased by one-half while the vertical dimension is held constant, the result is a 100% reduction of the total circular path described by the trapezoid. Therefore, although the target is rotated at a constant speed, the amount of horizontal displacement relative to the principal axis of vision is significantly less for the diminished trapezoid. The perceptual situation created decreases the 0 ’ s ability to make use of binocular disparity produced by the horizontal visual subtense of the trapezoid. If the trapezoid’s vertical dimension is decreased while the horizontal width is kept constant, the availability of binocular disparity remains the same, although linear perspective produced by the discrepancy between the trapezoid’s parallel sides is emphasized. The results of the present study
Downloaded by [UZH Hauptbibliothek / Zentralbibliothek Zürich] at 16:44 02 January 2015
64
JOURNAL OF GENERAL PSYCHOLOGY
taken together with the findings of Graham (S), Freeman and Pasnak (4) and Martinetti (6) indicate that when binocular depth cues exist with linear perspective, the 0 relies on the former for perceptual information. It is only when depth cue discriminability is decreased that 0 relies on the confusing linear perspective cue. The data further suggest that regardless of the dimension of reduction, perceived oscillation is inversely related to visual angle. The analyses reveal that within each set of targets the square produced fewest oscillation perceptions, and the smallest trapezoid produced most reported illusions. It is evident from pairwise comparisons that the trend is stable for all targets within the range of visual angles sampled. Visual angle reduction in a general sense decreases the visibility of the trapezoid, as well as the discriminability of attendant perceptual cues. The stimulus impoverishment may account for the increase in reported oscillations. However, another possible explanation would be that with decreased visibility of the target, the 0 reports an oscillation when in doubt about the presence of rotation. The latter interpretation is supported by the finding that decrements in visual angle not only produce more oscillation reports but also greater intersubject variability. The finding that subjective response becomes more variable as perceptual cues are attenuated, may therefore be a function of differential cue thresholds and/or response criteria. Either alternative suggests the need for definitive study of the trapezoid oscillatory illusion in a signal detection format. REFERENCES AMES,A. Visual perception and the rotating trapezoid window. Psychol. Monog., 1951, 65 (7, Whole No. 324). 2. ANDREAS,B. G . Experimental Psychology. New York Wiley, 1972. 3. CAPPONE,M. The effect of trapezoid slant on the Ames trapezoid illusion. Unpublished doctoral dissertation, Fordham University, New York, New York, 1967. 4. FREEMAN,R. E., Jr., & Pasnak, R. Determinants of the rotating trapezoid illusion. 1. Exper. Psychol., 1968, 76, 94-101. 5 . GRAHAM,C. H. On some aspects of real and apparent visual movement. J . Opt. Soc. Amer., 1963, 63, 1019-1025. 6. MARTINETTI, R. F. The effect of varying binocular disparity and linear perspective on perceived oscillation of the Ames trapezoid. (Doctoral dissertation, Fordham University) Ann Arbor, Mich.: University Microfilms, 1974, No. 19-673. 7. WINER,B. J. Statistical Principles in Experimental Design. New York McGraw-Hill, 1971. 1.
Department of Psychology, Box 867 Marywood College Scranton, Pennsylvania 18509
