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Induced rotary motion and ocular torsion
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Vision Res. Vol. 31, No. 11, pp. 1979-1983. 1991 Printed in Great Britain. All rights reserved
Copyright
© 1991 Pergamon Press pic
INDUCED ROTARY MOTION AND OCULAR TORSION N. J.
WADE,l*
M. T.
SWANSTON,2
I. P. HOWARD,3 H. ON0 3 and X. SHEN 3
1 Department of Psychology, University of Dundee, Dundee DDI 4HN, 2Dundee Institute of Technology, Dundee, Scotland and 3Institute for Space and Terrestrial Science, York University, North York, Ontario, Canada
(Received 9 November 1990) Abstract-When a large patterned annulus rotates around a stationary sectored disc the latter appears to rotate in the opposite direction. Such induced rotary motion was examined with central discs subtending 5, 20 and 40 deg at the eye, with the surround filling the remainder of the visual field. The annular surround or the central disc could be oscillated sinusoidally around the fixation point through 20 deg at 0.2 Hz. In each case, subjects estimated the angles through which the moving and stationary parts of the display appeared to rotate on one half-cycle. Subjects also estimated the angle of rotation of an oscillating display that filled the visual field. Induced rotation of the centre was around 100% of the inducing amplitude for all disc sizes, but there was no induced motion of the surround when the centre rotated. Ocular torsion was measured under the same conditions, using the scleral search-coil technique. The amplitude of ocular torsion was a function of the size of the stationary or rotating field. Thus, variations in stimulus conditions affected induced rotary motion and ocular torsion in different ways. The implications of the results for theories of induced motion in terms of underregistered eye movements are discussed. Induced motion
Ocular torsion
Optokinetic nystagmus
Induced motion occurs when there is misallocation of the motion of one visual stimulus with respect to another, and it has been studied most intensively with lateral motion. For example, Duncker (1929) described the motion perceived in a stationary dot seen within a moving rectangular outline. The induced motion is in the opposite direction to the inducing motion. Duncker also described the rotation induced in a stationary radial pattern by rotation of a patterned annulus. Relational movement in a variety of other displays can result in induced motion (see Howard, 1982; Mack, 1986; Reinhardt-Rutland, 1988; Wade & Swanston, 1987). The role of eye movements in induced motion has attracted a great deal of attention. In the simplest account it is proposed that when people make judgements about the motion of objects with respect to the head they do not properly assign movements of the retinal image in terms of movements of the eyes (Kaufman, 1974). If an observer visually pursues the inducing stimulus, wholly or partially, the image of the test stimulus moves, and if the retinal motion is interpreted as movement of the stimulus in space this would account for induced motion.
*To whom reprint requests should be addressed.
Motion perception
There is considerable evidence that such a simple scheme is inadequate. Eye movements do not correlate significantly with induced movement, in either direction or extent (Bassili & Farber, 1977; Brosgo1e, Cristal & Carpenter, 1968; Levi & Schor, 1984; Schulman, 1979). Furthermore, induced movement can occur simultaneously in different directions (Anstis & Reinhardt-Rutland, 1976; Gogel, 1977; Nakayama & Tyler, 1978), which obviously could not be a consequence of any eye movements (see Wade & Swanston, 1987). A more subtle influence of eye movement control on induced motion was proposed by Roelofs and van der Waals (1939) and elaborated by Howard (1982, p. 303) and Post and Leibowitz (1985). According to this theory, when one fixates a stationary spot superimposed on a moving background, involuntary optokinetic nystagmus (OKN) is held in check by an opposing voluntary efferent signal. Full perceptual registration of the voluntary signal, but incomplete registration of the involuntary signal, creates the mistaken impression that the eyes, and hence the fixated object, are moving in a direction opposite to the motion of the background. In support of this theory it has been shown that factors which increase the gain of OKN in the absence of the stationary target also
1979
N. J.
1980
WADE
increase the vigor of induced motion when the target is present (Heckmann & Post, 1988; Post & Lott, 1990). Furthermore, it has been shown that moving displays out of the plane of convergence do not evoke OKN (Howard & Simpson, 1989), and that they also do not evoke induced visual motion attributable to eye movements (Heckmann & Howard, 1991). However, OKN is generally associated with visual fields that are relatively extensive (Howard, 1982), whereas induced movement can be produced by as few as two moving points (Day, Millar & Dickinson, 1979; Gogel, 1982). Evidence provided by Heckmann and Howard (1991) suggests that the misregistration of eye movement signals is one of a number of mechanisms which can give rise to induced movement. Other mechanisms include contrast between visual motion detectors, and the illusory motion of a stationary object that accompanies illusory motion of the self (vection) induced by a large moving display. Induced rotary motion is of particular interest in the context of the misregistered eye-movement theory, since torsional OKN is not subject to voluntary suppression. That is, induced rotation occurs during torsional OKN. However, a voluntary inhibitory signal is not essential to the misregistered eye-movement theory of induced visual motion, because involuntary eye-movement signals may be underestimated even when not inhibited. In either case, if underregistration of torsional eye movements is the cause of induced rotary movement, the magnitude of induced movement should be related to the magnitude of ocular torsion. The present experiment was designed to test this prediction. METHOD
Subjects Induced rotary motion was measured in six adult subjects, three males and three females. All had normal or corrected-to-normal vision. Eye movements were recorded from four male subjects, none of whom participated in the induced motion trials.
Apparatus and procedure The stimulus display consisted of a horizontal cylinder which could be rotated around its mid-horizontal axis, as shown in Fig. 1. The inside of the cylinder was lined with 30 equally spaced black and white sectors which radiated
et
at.
Fig. 1. Diagrammatic representation of the stimulus display. The subject fixated the centre of the inner disc, which could be stationary relative to the surround (the outer disc and cylindrical collar) that oscillated through 20 deg, or vice versa.
from the centre of rotation and extended as stripes on the sides of the cylinder. The subject's head was anchored on a bite bar just inside the open end of the cylinder, 40 cm from its base which subtended 75 deg. The striped sides of the cylinder filled the remainder of the visual field, and the display was illuminated by two tungsten lights mounted behind the subject's head. A circular disc subtending 5, 20 or 40 deg, with the same pattern of radiating sectors, could be mounted on a rod which projected about I mm from the hollow axis which supported the cylinder. The cylinder (surround) could be oscillated about the subject's line of sight with the disc stationary, or the disc could be oscillated with the surround stationary. Rotation was sinusoidal, with a frequency of 0.2 Hz and a peakto-peak amplitude of 20 deg; Howard and Cheung (1990) have shown that these values are optimal for optokinetic torsion. Initially, the cylinder with no disc was presented (full-field oscillation) and subjects estimated the angle through which it rotated. The amplitude of perceived rotary motion was measured by magnitude estimation; subjects reported the angle in degrees through which the display appeared to rotate on one half-cycle of oscillation. Subjects then made similar estimates for the central disc and for the surround under the following conditions: stationary surround and rotating 5, 20 and 40 deg central discs; rotating surround and stationary 5, 20 and 40 deg central discs. Three subjects first made judgements with the surround rotating and the other three subjects first made judgements with the centre rotating. The discs were presented in
Induced rotation and torsion
1981
random order. Finally, the full-field was presented again. Torsional movements of the eyes were recorded by the scleral search-coil technique. under the same conditions but with a different group of four subjects. In this method the subject sits inside a magnetic field and a voltage proportional to the torsional position of the eye is induced in a coil of wire embedded in a scleral contact lens. The method provides a virtually noise-free, linear signal sensitive to a few min of arc of eye rotation. The voltage per unit rotation was derived by rotating the scleral coil on an artificial eye. Absolute calibration of eye torsional position was not required because we measured only the peak-to-peak displacement of the sinusoidal response. This measure would not be severely perturbed by slipping of the coil caused by eye blinks. Measurements of torsion were conducted in the following sequence: fullfield rotation; stationary surround and rotating 5, 20 and 40 deg central discs; rotating surround and stationary 40, 20 and 5 deg central discs; finally, the full-field rotation was presented again. In all conditions records were taken over about a 40 sec period, so that responses during at least 6 cycles of oscillation were obtained.
surround rotated, estimates of its rotation were very similar to those obtained for the full-field. However, estimates of apparent rotation of the stationary centre when the surround rotated were very similar to judgments of full-field rotation. That is, the induced rotation was about 100% of the judged rotation of the inducing stimulus. On the other hand, the stationary surround appeared stationary when the centre rotated. In all cases the values were similar for the three sizes of central discs. No subject ever reported rotation of the surround when the 5 deg centre rotated, and so there was no variance in that condition. Accordingly the analysis of variance was confined to log transformations of the data from conditions in which the centre was judged, and the factors were centre/surround motion and the angular subtense of the central disc. Judgement of the rotation of the centre when it was physically rotating was significantly greater than when its rotation was induced by the surround [F(l, 5) = 16.0, P < 0.01]. However, the effect of disc size was not significant [F(2, 10) = 1.5, P > 0.05], nor was the interaction between these two factors [F(2, 10) = 1.6, P > 0.05].
RESULTS
Torsion was driven in phase with rotation of the full-field, or with that of the central or surround areas, and its amplitude was taken as the mean of 6 cycles. A sample of the torsion records is shown in Fig. 2. Following calibration with an artificial eye, torsion with full-field rotation had a gain of 0.1 (2 deg torsion for 20 deg stimulus rotation), corresponding to the values obtained by Howard and Cheung (1990). For each individual. amplitudes of torsion were expressed as a proportion of torsion with the full-field display. The means of the individual ratios are given in Table 2 for each condition. The amplitude of torsion varied as a function of the size of the rotating area: as the size of the central rotating disc decreased, or the size of the stationary central disc increased, the amplitude of torsion decreased. However, there was still
Psychophysical results
Since the angular estimates of rotation in the initial and final full-field conditions did not differ significantly they were combined. The overall mean estimate of the peak-to-peak rotation of the full-field of 20 deg was 28.8 deg. For each individual, all estimates of rotation were expressed as a proportion of that subject's mean estimate for the full-field condition. Thus. a value of 1.0 indicates that the rotation of the centre or surround appeared to be as great as that of the full-field display. The means of the individual ratios are given in Table 1 for each condition. When the centre rotated, it appeared to rotate through a greater angle than that through which the full-field rotated. When the
Torsion results
Table I. Mean estimates of angular rotation taken as a ratio of the full-field estimate (6 subjects) Centre judged
Angular subtense of central disc (deg)
5 20 40
Surround judged
Centre moving
Surround moving
Centre moving
Surround moving
1.31 1.02 1.56
1.01 1.02 0.89
0.00 0.07 0.02
0.98 0.97 1.26
N. J.
1982 cw
WADE el
c:
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0
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10
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Time-
0
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5 sec
Fig. 2. A sample record of stimulus oscillation and torsional eye movements from one subject. The stimulus in this case consisted of a rotating surround and a 5 deg stationary centre. Torsion was in phase with the stimulus oscillation, and the amplitude of torsion was about 10% of that of the stimulus.
some torsion with rotation of a 5 deg central disc with the rest of the visual field stationary. An analysis of variance on the log-transformed ratio data indicated that the amplitude of torsion was not significantly influenced by either the angular subtense of the central disc or by which component rotated [F(2, 6) = 0.2, P > 0.05 and F(1, 3) = 1.85, P > 0.05, respectively] but the interaction between these two [F(2, 6) = 28.0, factors was significant P < 0.005]. That is, the amplitude of torsion increased as the size of the central rotating disc increased and as the size of the rotating surround increased. DISCUSSION
Previous studies of induced rotary motion differed from the present one in two important respects. Firstly, the inducing stimulus typically rotated at a constant angular velocity rather than sinusoidally. Secondly, previous displays did not occupy the whole visual field. For example, Day (1981) rotated an annulus subtending 20 deg around a 6 deg stationary disc, both radially lined, and measured induced rotation by a nulling method (Le. the central disc was rotated in the same direction as the annulus, so that it appeared to be stationary). The velocity of induced rotation was about one third of the inducing velocity. Similar values were found by Day and Wade (1988), using magnitude Table 2. Mean measures of ocular torsion taken as a ratio of the full-field measure (4 subjects)
Angular subtense of central disc (deg)
5 20 40
Centre moving
Surround moving
0.30 0.42 0.57
0.78 0.55 0.43
al.
estimation, with concentric displays having the surround and centre subtending about 12 and 6 deg, respectively. In both cases, the angular velocity of the inducing annulus was 9 deg/sec. As in our study, Day (1981) found that rotation of the central disc did not induce motion in the surrounding annulus. In the context of induced linear motion, there has been much debate concerning the assignment of perceived motions within the displays. It has been argued that the total amount of motion perceived in the induced and inducing components remains constant (see Wade & Swanston, 1987). However, we found that for rotary movement the induced and inducing components are both perceived to rotate by about the same amount as the full-field alone, that is, with a relative amplitude of about twice that of the full-field. The differences between our procedure and those of previous experimenters may account for the larger magnitude of induced motion in our experiment. Induced rotary motion does not occur with rotation of a central disc relative to a stationary surround, but this condition does evoke ocular torsion. The torsion would have resulted in the stationary surround rotating with respect to the retina, but this was not an adequate stimulus for perceived rotation of the surround. Rotation of the surround with respect to a stationary centre produced both induced rotary motion and ocular torsion. The amplitude of ocular torsion increased with the increasing area of the rotating surround. It is clear that the amplitude of induced rotary motion does not vary systematically with the dimensions of the stationary centre, but the amplitude of torsion does. Induced rotary motion due to illusory motion of the self (vection) would be expected to produce the asymmetry between centre and surround induction that we found, since vection is induced more effectively by a large surround display than by a smaller central display especially when, as in the present experiment, the central display is in front of the surround display (Howard & Heckmann, 1991). However, none of the subjects reported vection and further experiments are needed to throw light on this issue. The results of this study indicate that induced rotary motion is not a consequence of eye movement (torsion), in agreement with studies of induced linear motion. Moreover, it is un-
Induced rotation and torsion
likely that induced rotation evoked dichoptically is due to slight ocular torsion, as was suggested by Day and Wade (1988). Whatever the contribution of underregistered eye movements may be to horizontal and vertical induced visual motion, the results of the present experiment cast doubt on interpretations of induced rotary motion in terms of underregistration of eye movements since variations in the amplitude of torsional OKN are not accompanied by corresponding variations in the amplitude of induced rotary motion. Acknowledgements-This work was supported in part by NATO Grant 0067/89 to M. T. Swanston, N. J. Wade, 1. P. Howard and H. Ono; by Science and Engineering Research Council Grant GR/E 87618 to N. J. Wade and M. T. Swanston; by National Science and Engineering Research Council of Canada Grant OGP 0000195 to I. P. Howard. The data on eye movements are part of a larger study that will be reported separately.
REFERENCES Anstis, S. M. & Reinhardt-Rutland. A. H. (1976). Interactions between motion aftereffects and induced movement. Vision Research, 16. 1391-1394. Bassili, J. N. & Farber, J. M. (1977). Experiments on the locus of induced motion. PerceptIOn and Psychophysics, 21, 157-161. Brosgole, L., Cristal, R. M. & Carpenter, O. (1968). The role of eye movements in the perception of visually induced motion. Perception and Psychophysics, 3. 166-168. Day, R. H. (1981). Induced rotation with concentric patterns. Perception and Psychophysics, 29, 493-499. Day, R. H. & Wade, N. J. (1988). Binocular interaction 10 induced rotary motion. Australian Journal of Psychology, 40, 159-164. Day, R. H., Millar, J. & Dickinson, R. G. (1979). Induced movement as nonveridical resolutIOn of displacement ambiguity: Effects of enclosure and number of field elements. PerceptIOn and PsychophysIcs, 25, 23-28. Duncker, K. (1929). Uber induzierte Bewegung (Ein Beitrag zur Theorie optisch wahrgenommener Bewegung). Psychologische Forschung. 12, 180-259. Gogel, W. C. (1977). Independent motion induced in separate portions of the visual field. Bulletin of the Psychonomlc Society, /0, 408-410. Gogel, W. C. (1982). Analysis of the perception of motion concomitant with a lateral motion of the head. Perception and Psychophysics, 32, 241-250.
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1983
Heckmann, T. & Howard, 1. P. (1991). Induced visual motion: Dissociation of exocentric and egocentric components. Perception. In press. Ht"ckmann, T. & Post, R. B. (1988). Induced motion and optokinetic afternystagmus: Parallel response dynamics with prolonged stimulation. Vision Research, 28, 681-{j94. Howard, 1. P. (1982). Human spatial orientation. Chichester: Wiley. Howard, 1. P. & Cheung, B. (1990). Optokinetic torsion in response to sinusoidal stimulus rotation. Investigative Ophthalmology and Visual Science, 31, 84. Howard, 1. P. & Heckmann, T. (1989). Circular vection as a function of the relative sizes, distances and positions of two competing visual displays. Perception, 18, 657-667. Howard, 1. P. & Simpson, W. S. (1989). Human optokinetic nystagmus is linked to the stereoscopic system. Experimental Brain Research, 78, 309-314. Kaufman, L. (1974). Sight and mind. New York: Oxford University Press. Levi, D. M. & Schor, C. M. (1984). Spatial and velocity tuning of processes underlying induced motion. Vision Research, 24, 1189-1196. Mack, A. (1986). Perceptual aspects of motion in the frontal plane. In Boff, K. R., Kaufman, L. & Thomas, J. P. (Eds), Handbook of perception and human performance (Chap. 17, pp. 1-38). Nakayama, K. & Tyler, C. W. (1978). Relative motion induced between stationary lines. Vision Research, 18, 1663-1668. Post, R. B. (1986). Induced motion considered as a visually induced oculo-gyral illusion. Perception, 15, 131-138. Post, R. B. & Heckmann, T. (1986). Induced motion and apparent straight ahead during prolonged stimulation. PerceptIOn and Psychophysics, 40, 263-270. Post, R. B. & Leibowitz, H. W. (1985). A revised analysis of the role of efference in motion perception. Perception, 14, 631-{j43. Post, R. B. & Lott, L. A. (1990). Relationship of induced motion and apparent straight-ahead shifts to optokinetic stimulus velocity. Perception and Psychophysics, 48, 401-406. Reinhardt-Rutland, A. H. (1988). Induced movement in the visual modality. Psychological Bulletin, /03, 57-71. Rock, 1., Auster, M., Schiffman, M. & Wheeler, D. (1982). Induced movement based on subtraction of motion from the moving object. Journal of Experimental Psychology: Human Perception and Performance, 6, 391-403. Roelofs, C. & van der Waals, H. G. (1939). Veriinderung der haptischen Lokalisation bei optokinetischer Reizung. Zeitschrift fiir Psychologie, 136, 5-49. Schulman, P. H. (1979). Eye movements do not cause induced motion. Perception and Psychophysics, 26, 381-383. Wade, N. J. & Swanston, M. T. (1987). The representation of nonuniform motion: Induced movement. Perception, 16, 555-571.
Vision Res. Vol. 31, No. 11, pp. 1979-1983. 1991 Printed in Great Britain. All rights reserved
Copyright
© 1991 Pergamon Press pic
INDUCED ROTARY MOTION AND OCULAR TORSION N. J.
WADE,l*
M. T.
SWANSTON,2
I. P. HOWARD,3 H. ON0 3 and X. SHEN 3
1 Department of Psychology, University of Dundee, Dundee DDI 4HN, 2Dundee Institute of Technology, Dundee, Scotland and 3Institute for Space and Terrestrial Science, York University, North York, Ontario, Canada
(Received 9 November 1990) Abstract-When a large patterned annulus rotates around a stationary sectored disc the latter appears to rotate in the opposite direction. Such induced rotary motion was examined with central discs subtending 5, 20 and 40 deg at the eye, with the surround filling the remainder of the visual field. The annular surround or the central disc could be oscillated sinusoidally around the fixation point through 20 deg at 0.2 Hz. In each case, subjects estimated the angles through which the moving and stationary parts of the display appeared to rotate on one half-cycle. Subjects also estimated the angle of rotation of an oscillating display that filled the visual field. Induced rotation of the centre was around 100% of the inducing amplitude for all disc sizes, but there was no induced motion of the surround when the centre rotated. Ocular torsion was measured under the same conditions, using the scleral search-coil technique. The amplitude of ocular torsion was a function of the size of the stationary or rotating field. Thus, variations in stimulus conditions affected induced rotary motion and ocular torsion in different ways. The implications of the results for theories of induced motion in terms of underregistered eye movements are discussed. Induced motion
Ocular torsion
Optokinetic nystagmus
Induced motion occurs when there is misallocation of the motion of one visual stimulus with respect to another, and it has been studied most intensively with lateral motion. For example, Duncker (1929) described the motion perceived in a stationary dot seen within a moving rectangular outline. The induced motion is in the opposite direction to the inducing motion. Duncker also described the rotation induced in a stationary radial pattern by rotation of a patterned annulus. Relational movement in a variety of other displays can result in induced motion (see Howard, 1982; Mack, 1986; Reinhardt-Rutland, 1988; Wade & Swanston, 1987). The role of eye movements in induced motion has attracted a great deal of attention. In the simplest account it is proposed that when people make judgements about the motion of objects with respect to the head they do not properly assign movements of the retinal image in terms of movements of the eyes (Kaufman, 1974). If an observer visually pursues the inducing stimulus, wholly or partially, the image of the test stimulus moves, and if the retinal motion is interpreted as movement of the stimulus in space this would account for induced motion.
*To whom reprint requests should be addressed.
Motion perception
There is considerable evidence that such a simple scheme is inadequate. Eye movements do not correlate significantly with induced movement, in either direction or extent (Bassili & Farber, 1977; Brosgo1e, Cristal & Carpenter, 1968; Levi & Schor, 1984; Schulman, 1979). Furthermore, induced movement can occur simultaneously in different directions (Anstis & Reinhardt-Rutland, 1976; Gogel, 1977; Nakayama & Tyler, 1978), which obviously could not be a consequence of any eye movements (see Wade & Swanston, 1987). A more subtle influence of eye movement control on induced motion was proposed by Roelofs and van der Waals (1939) and elaborated by Howard (1982, p. 303) and Post and Leibowitz (1985). According to this theory, when one fixates a stationary spot superimposed on a moving background, involuntary optokinetic nystagmus (OKN) is held in check by an opposing voluntary efferent signal. Full perceptual registration of the voluntary signal, but incomplete registration of the involuntary signal, creates the mistaken impression that the eyes, and hence the fixated object, are moving in a direction opposite to the motion of the background. In support of this theory it has been shown that factors which increase the gain of OKN in the absence of the stationary target also
1979
N. J.
1980
WADE
increase the vigor of induced motion when the target is present (Heckmann & Post, 1988; Post & Lott, 1990). Furthermore, it has been shown that moving displays out of the plane of convergence do not evoke OKN (Howard & Simpson, 1989), and that they also do not evoke induced visual motion attributable to eye movements (Heckmann & Howard, 1991). However, OKN is generally associated with visual fields that are relatively extensive (Howard, 1982), whereas induced movement can be produced by as few as two moving points (Day, Millar & Dickinson, 1979; Gogel, 1982). Evidence provided by Heckmann and Howard (1991) suggests that the misregistration of eye movement signals is one of a number of mechanisms which can give rise to induced movement. Other mechanisms include contrast between visual motion detectors, and the illusory motion of a stationary object that accompanies illusory motion of the self (vection) induced by a large moving display. Induced rotary motion is of particular interest in the context of the misregistered eye-movement theory, since torsional OKN is not subject to voluntary suppression. That is, induced rotation occurs during torsional OKN. However, a voluntary inhibitory signal is not essential to the misregistered eye-movement theory of induced visual motion, because involuntary eye-movement signals may be underestimated even when not inhibited. In either case, if underregistration of torsional eye movements is the cause of induced rotary movement, the magnitude of induced movement should be related to the magnitude of ocular torsion. The present experiment was designed to test this prediction. METHOD
Subjects Induced rotary motion was measured in six adult subjects, three males and three females. All had normal or corrected-to-normal vision. Eye movements were recorded from four male subjects, none of whom participated in the induced motion trials.
Apparatus and procedure The stimulus display consisted of a horizontal cylinder which could be rotated around its mid-horizontal axis, as shown in Fig. 1. The inside of the cylinder was lined with 30 equally spaced black and white sectors which radiated
et
at.
Fig. 1. Diagrammatic representation of the stimulus display. The subject fixated the centre of the inner disc, which could be stationary relative to the surround (the outer disc and cylindrical collar) that oscillated through 20 deg, or vice versa.
from the centre of rotation and extended as stripes on the sides of the cylinder. The subject's head was anchored on a bite bar just inside the open end of the cylinder, 40 cm from its base which subtended 75 deg. The striped sides of the cylinder filled the remainder of the visual field, and the display was illuminated by two tungsten lights mounted behind the subject's head. A circular disc subtending 5, 20 or 40 deg, with the same pattern of radiating sectors, could be mounted on a rod which projected about I mm from the hollow axis which supported the cylinder. The cylinder (surround) could be oscillated about the subject's line of sight with the disc stationary, or the disc could be oscillated with the surround stationary. Rotation was sinusoidal, with a frequency of 0.2 Hz and a peakto-peak amplitude of 20 deg; Howard and Cheung (1990) have shown that these values are optimal for optokinetic torsion. Initially, the cylinder with no disc was presented (full-field oscillation) and subjects estimated the angle through which it rotated. The amplitude of perceived rotary motion was measured by magnitude estimation; subjects reported the angle in degrees through which the display appeared to rotate on one half-cycle of oscillation. Subjects then made similar estimates for the central disc and for the surround under the following conditions: stationary surround and rotating 5, 20 and 40 deg central discs; rotating surround and stationary 5, 20 and 40 deg central discs. Three subjects first made judgements with the surround rotating and the other three subjects first made judgements with the centre rotating. The discs were presented in
Induced rotation and torsion
1981
random order. Finally, the full-field was presented again. Torsional movements of the eyes were recorded by the scleral search-coil technique. under the same conditions but with a different group of four subjects. In this method the subject sits inside a magnetic field and a voltage proportional to the torsional position of the eye is induced in a coil of wire embedded in a scleral contact lens. The method provides a virtually noise-free, linear signal sensitive to a few min of arc of eye rotation. The voltage per unit rotation was derived by rotating the scleral coil on an artificial eye. Absolute calibration of eye torsional position was not required because we measured only the peak-to-peak displacement of the sinusoidal response. This measure would not be severely perturbed by slipping of the coil caused by eye blinks. Measurements of torsion were conducted in the following sequence: fullfield rotation; stationary surround and rotating 5, 20 and 40 deg central discs; rotating surround and stationary 40, 20 and 5 deg central discs; finally, the full-field rotation was presented again. In all conditions records were taken over about a 40 sec period, so that responses during at least 6 cycles of oscillation were obtained.
surround rotated, estimates of its rotation were very similar to those obtained for the full-field. However, estimates of apparent rotation of the stationary centre when the surround rotated were very similar to judgments of full-field rotation. That is, the induced rotation was about 100% of the judged rotation of the inducing stimulus. On the other hand, the stationary surround appeared stationary when the centre rotated. In all cases the values were similar for the three sizes of central discs. No subject ever reported rotation of the surround when the 5 deg centre rotated, and so there was no variance in that condition. Accordingly the analysis of variance was confined to log transformations of the data from conditions in which the centre was judged, and the factors were centre/surround motion and the angular subtense of the central disc. Judgement of the rotation of the centre when it was physically rotating was significantly greater than when its rotation was induced by the surround [F(l, 5) = 16.0, P < 0.01]. However, the effect of disc size was not significant [F(2, 10) = 1.5, P > 0.05], nor was the interaction between these two factors [F(2, 10) = 1.6, P > 0.05].
RESULTS
Torsion was driven in phase with rotation of the full-field, or with that of the central or surround areas, and its amplitude was taken as the mean of 6 cycles. A sample of the torsion records is shown in Fig. 2. Following calibration with an artificial eye, torsion with full-field rotation had a gain of 0.1 (2 deg torsion for 20 deg stimulus rotation), corresponding to the values obtained by Howard and Cheung (1990). For each individual. amplitudes of torsion were expressed as a proportion of torsion with the full-field display. The means of the individual ratios are given in Table 2 for each condition. The amplitude of torsion varied as a function of the size of the rotating area: as the size of the central rotating disc decreased, or the size of the stationary central disc increased, the amplitude of torsion decreased. However, there was still
Psychophysical results
Since the angular estimates of rotation in the initial and final full-field conditions did not differ significantly they were combined. The overall mean estimate of the peak-to-peak rotation of the full-field of 20 deg was 28.8 deg. For each individual, all estimates of rotation were expressed as a proportion of that subject's mean estimate for the full-field condition. Thus. a value of 1.0 indicates that the rotation of the centre or surround appeared to be as great as that of the full-field display. The means of the individual ratios are given in Table 1 for each condition. When the centre rotated, it appeared to rotate through a greater angle than that through which the full-field rotated. When the
Torsion results
Table I. Mean estimates of angular rotation taken as a ratio of the full-field estimate (6 subjects) Centre judged
Angular subtense of central disc (deg)
5 20 40
Surround judged
Centre moving
Surround moving
Centre moving
Surround moving
1.31 1.02 1.56
1.01 1.02 0.89
0.00 0.07 0.02
0.98 0.97 1.26
N. J.
1982 cw
WADE el
c:
j ~o
0
.~
~
10
~
ccw
U.J
cw
j ~o
10 CCW
Time-
0
'"
.2 :::J
.~
Ci5
L----J
5 sec
Fig. 2. A sample record of stimulus oscillation and torsional eye movements from one subject. The stimulus in this case consisted of a rotating surround and a 5 deg stationary centre. Torsion was in phase with the stimulus oscillation, and the amplitude of torsion was about 10% of that of the stimulus.
some torsion with rotation of a 5 deg central disc with the rest of the visual field stationary. An analysis of variance on the log-transformed ratio data indicated that the amplitude of torsion was not significantly influenced by either the angular subtense of the central disc or by which component rotated [F(2, 6) = 0.2, P > 0.05 and F(1, 3) = 1.85, P > 0.05, respectively] but the interaction between these two [F(2, 6) = 28.0, factors was significant P < 0.005]. That is, the amplitude of torsion increased as the size of the central rotating disc increased and as the size of the rotating surround increased. DISCUSSION
Previous studies of induced rotary motion differed from the present one in two important respects. Firstly, the inducing stimulus typically rotated at a constant angular velocity rather than sinusoidally. Secondly, previous displays did not occupy the whole visual field. For example, Day (1981) rotated an annulus subtending 20 deg around a 6 deg stationary disc, both radially lined, and measured induced rotation by a nulling method (Le. the central disc was rotated in the same direction as the annulus, so that it appeared to be stationary). The velocity of induced rotation was about one third of the inducing velocity. Similar values were found by Day and Wade (1988), using magnitude Table 2. Mean measures of ocular torsion taken as a ratio of the full-field measure (4 subjects)
Angular subtense of central disc (deg)
5 20 40
Centre moving
Surround moving
0.30 0.42 0.57
0.78 0.55 0.43
al.
estimation, with concentric displays having the surround and centre subtending about 12 and 6 deg, respectively. In both cases, the angular velocity of the inducing annulus was 9 deg/sec. As in our study, Day (1981) found that rotation of the central disc did not induce motion in the surrounding annulus. In the context of induced linear motion, there has been much debate concerning the assignment of perceived motions within the displays. It has been argued that the total amount of motion perceived in the induced and inducing components remains constant (see Wade & Swanston, 1987). However, we found that for rotary movement the induced and inducing components are both perceived to rotate by about the same amount as the full-field alone, that is, with a relative amplitude of about twice that of the full-field. The differences between our procedure and those of previous experimenters may account for the larger magnitude of induced motion in our experiment. Induced rotary motion does not occur with rotation of a central disc relative to a stationary surround, but this condition does evoke ocular torsion. The torsion would have resulted in the stationary surround rotating with respect to the retina, but this was not an adequate stimulus for perceived rotation of the surround. Rotation of the surround with respect to a stationary centre produced both induced rotary motion and ocular torsion. The amplitude of ocular torsion increased with the increasing area of the rotating surround. It is clear that the amplitude of induced rotary motion does not vary systematically with the dimensions of the stationary centre, but the amplitude of torsion does. Induced rotary motion due to illusory motion of the self (vection) would be expected to produce the asymmetry between centre and surround induction that we found, since vection is induced more effectively by a large surround display than by a smaller central display especially when, as in the present experiment, the central display is in front of the surround display (Howard & Heckmann, 1991). However, none of the subjects reported vection and further experiments are needed to throw light on this issue. The results of this study indicate that induced rotary motion is not a consequence of eye movement (torsion), in agreement with studies of induced linear motion. Moreover, it is un-
Induced rotation and torsion
likely that induced rotation evoked dichoptically is due to slight ocular torsion, as was suggested by Day and Wade (1988). Whatever the contribution of underregistered eye movements may be to horizontal and vertical induced visual motion, the results of the present experiment cast doubt on interpretations of induced rotary motion in terms of underregistration of eye movements since variations in the amplitude of torsional OKN are not accompanied by corresponding variations in the amplitude of induced rotary motion. Acknowledgements-This work was supported in part by NATO Grant 0067/89 to M. T. Swanston, N. J. Wade, 1. P. Howard and H. Ono; by Science and Engineering Research Council Grant GR/E 87618 to N. J. Wade and M. T. Swanston; by National Science and Engineering Research Council of Canada Grant OGP 0000195 to I. P. Howard. The data on eye movements are part of a larger study that will be reported separately.
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