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Eye torsion and the apparent horizon under head tilt and visual field rotation
0042-bYX9i8l/040543.05802.00/0 Pergmxm Press Lid
Yi.,wn Ruwurth Vol. 21. pp. 543 to 547 Prmted m Great Britain
EYE TORSION AND THE APPARENT HORIZON UNDER HEAD TILT AND VISUAL FIELD ROTATION BJORN H. MERKER and RICHARD HELD Department of Psychology (EIO-139), Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A. (Received 14 April
1980; in revised form 27 August 1980)
Abstract-Two different experimental manipulations, namely head tilt and the viewing of a visual display rotating around the line of sight, induce torsional displacements of the eyes and a tilting of the apparent horizon. The present study examines the routes by which visual (field rotation) and otolithproprioceptive (head tilt) sources of afference influence horizon judgements. In particular, the relationship between torsional eye movements and horizon estimates is addressed. The results indicate that visual and otolith-proprioceptive information sum directly in their influence on eye torsion, but interact more complexly in horizon estimates, indicating a dissociation of their central determinants.
INTRODUCl’ION
An observer viewing a large visual display rotating around his line of sight experiences a number of perceptual effects, including tiiting of contours displayed on a stationary target concentric with the surrounding moving field (Dichgans et al., 1972; Held et a/., 1975). If the observer has control of the rotary alignment of the central target and adjusts it to the apparent horizon before and then again during field rotation, the amount of target displacement provides a measure of the influence of field rotation on the observer’s horizon estimate. If the surrounding field is kept stationary, the same procedure can be used to estimate another influence on horizon judgements, namely head tilt. That is, our subjective estimate of
the horizon is influenced by at least two sources of afferent information: a visual source, represented by field rotation, and a proprioceptive-vestibular source, represented by head tilt. The question addressed by the present report concerns the routes by which visual and vestibular influences affect horizon judgements. For example, it has been noted that visual field rotation induces torsional eye movements in the same direction as that in which the field rotates. This is consistent with eye torsion serving as the source of the apparent disptacement of the horizon in the opposite direction, since images on the retina are tilted in a direction opposite to the torsion (Hughes et al., 1972). This conception has been illustrated in hypothesis 1, Fig. 1. It predicts an apparent displacement of the horizon equal in
Fig. 1. Schematic illustration of three hypotheses attempting to account for the routes (circled paths exclusively) by which visual and “vestibular” influences affect horizontal estimates. The comparator junction on the right in the figure compares the retinal position of a target with an ‘*internal gravitational reference” resulting in an error signal “e” used to align the target with respect to the gravity reference. The left part of the figure is concerned with the genesis of the gravitational reference as it bears on the empirical findings discussed in the text. According to hypothesis 1, the gravity reference is a veridical reflection of otolith and the proprioceptive information about objective horizontal. Estimates of the latter are nevertheless systematically displaced from objective horizontal when visual field rotation induces torsional eye movements that are not taken into account in estimating target position in space. According to hypothesis 2, visual and vestibular information sum directly to influence both eye torsion and the gravity reference. Only the gain (k, and k,) of the signal influencing the two dependent measures differs according to this model. According to hypothesis 3, finally, only the influence on eye torsion results from direct visual-vestibular summation. The two arerent channels interact more complexly in determining the gravity reference. This is the conception supported by the present results. 543
BJOKN H. MERKLK and RIWAKI)
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magnitude to the size of torsional eye movements, However. when measured by Dichgans rr trl. (1972). apparent tilt far exceeded the amount of eye torsion, a result confirmed by the present experiment (see results section). The conception of hypothesis I therefore has to be amended at least to the extent illustrated in hypothesis 2. Fig. 1. Here the interaction of visual and vestibular signals influences not only eye torsion. but also the observer’s internal gravitational reference. Because the two signals originate from the same summing junction. they differ at most in their relative gains (k, f k,). This model would therefore predict a constant fractional contribution of eye torsion to toal induced tilt. This in turn means that the function relating horizon estimates to the interaction of visual field rotation and head tilt should be identical, except for a scale factor. to the function relating eye torsion to the interaction of these variables. That is the prediction the experiment reported here was designed to test. The results obtained (see results and discussion sections) did not support this prediction. but rather gave evidence of non-linear interaction between visual and vestibular information in the determination of horizon estimates. Regarding the three hypotheses of Fig. 1. it may therefore be concluded that hypothesis 1, attributing horizon displacements to uncompensated torsional eye movements is clearly false. Hypothesis 2. predicting linear summation of visual and vestibular signals is disproved by the present experiment. This leaves hypothesis 3. which embodies the non-linear interaction of visual and vestibular signals observed in the present experiment. It will be considered further in the discussion section of this paper.
Ht LII
horizontal strobe hash attached to the brtc!loard. Head tilts at 15 . 30 and 45’ off vcrtrcal in ;I (‘W i)r CCW direction, with center of gaze unchanged. wer\: produced by radial displacements of the biteboard attachment along the calibrated perimeter- !I( the apparatus. All measurements from S were obtamcd m ‘I single session. divided into eight blocks of trials. one foreach of the seven tilt conditions (no tilt. and (‘W KL CCW tilt at 15-, 30^ and 45-). with “no tilt” alua:~s run first and then replicated at the end of the session as a check on baseline drifts. Within each block. trli was combined with three field rotation conditions: no rotation. CW. and CCW rotation of the visual field In the first block (no tilt). “no rotation” :IIw;I!~ occurred first and was then replicated once wrthrn that block. With these exceptions. the serial order o( tilt conditions was counterbalanced across subject\ and the serial order of rotation conditions countcrbalancedacrosstiltconditionsand subjects. Within each of the resulting 25 tilt-rotation conditions. S gave ri series of alternating horizontal estimates and eye torsion measures, five of each measure in rapid alternution. resulting in a total of 250 data points for each subject. On the basis of previous estimates of the time required for tilt and rotation effects to build up and dissipate. a period of 35 set of exposure to a rotation condition preceded such a series of IO measurements. and 20 set of rest followed each condition, A longer rest period intervened between each tilt block. The afterimage was given at the beginning of each tilt block. and was maintained throughout a block hq flickering the illumination of the target disc under s’s control.
METHODS
Six male undergraduate students served as paid subjects. The apparatus was very similar to that described in Held et ctl. (1975). with exceptions noted. S. head fixed by biteboard and using binocular vision sat facing a large circular disc of static “visual noise” (irregular white speckles on a black background) subtending 130’ of visual angle. This field could either be rotated at 2O’isec. in a clockwise (CW) or a counterclockwise (CCW) direction around the s’s line of sight. or be kept stationary. At its center was a white circular target disc subtending 32’ of visual angle. bisected by a black stripe. This target disc could be rotated by S via a control knob independently of the surrounding field. Target disc settings were continuously recorded on a strip chart. as well as read out on a digital display calibrated in degrees of angular displacement of the target stripe from objective horizontal. Resolution of measurement was better than 0.5”. with negligible drift as assessed by calibration readings. S indicated perceived horizon by target stripe settings under visual control. Eye torsion was measured by having S align the target stripe in parallel with a bar-shaped afterimage received with the head always in upright position from a slit-shaped
The averaged data for all Ss are plotted separately for torsional measures and horizontal estimates in qualitative difference Fig. 2. The most striking between the two graphs is the smooth variation 01 torsional measures with head tilt. a pattern not CI’Ident in the graph of horizon estimates. Note also that in the “no rotation”‘condition, the magnitude of eye torsion exceeds that of horizon displacements at every head tilt, whereas field rotation at every tilt has :I larger effect on horizontal estimates than on eye torsion Analysis of variance disclosed main effects of each treatment condition; that is, of head tilt. rotation, and eye torsion vs horizontal settings. interactions between each pair of the three. as well as multiple interaction between tilt, rotation. and torsion vs horizontal, all statistically significant (P < 0.001 I. Analysis of variance of the same data, but defining the rotation effect as the difference between CW and CCW rotation (the rotation “envelope”). showed a significant main effect of tilt. of torsion vs horizontal. and showed an interaction of tilt with torsion vs horizontal. The rotation “envelope” is plotted in Fig. 3. which also shows an overall increase in the variance
343
Eye torsion and apparent horizon
Fig. 2. A. Mean extent of ocular torsion in degrees plotted as a function of head tilt for three rotation conditions. B. Mean extent of target udjmfnmf off objective horizontal (in degrees) as a function of head tilt, plotted for three rotation conditions. of the effect of field rotation with increasing head tilt for horizontal settings, but not for eye torsion.
DISCUSSION
In order to determine further the nature of the difference between torsional measures and horizon estimates indicated by the analysis of variance, additional statistical tests were performed, guided by trends in the graphs of Fig. 2. For example, there appears to be a trend towards an expansion of the rotation “envelope” with increasing head tilt in the case of horizontal estimates, but not for eye torsion. The difference between the effect of CW and CCW rotation at 45” head tilt (averaged for both directions of tilt) was therefore compared by r-test with the same rotation effect at head upright, for both torsional and horizontal measures. The difference is significant (P = 0.001) for horizontal settings, but not for eye torsion measures, and this difference between the two measures is itself significant (P < 0.001). The graphs in Fig. 2 also suggest that the effect of rotation on horizontal settings is larger when the direction of field rotation is opposite to the direction of head tilt, an effect not evident for eye torsion. A t-test for mean difference between the sum of all rotation effects at head tilts in the same vs opposite directions relative to the direction of rotation gave a significant difference for horizon estimates (P < 0.05) but not for eye torsion. This difference between torsion and horizontal settings is again itself significant (P < 0.05).
Mean horizon displacements at every head tilt were of lesser extent than torsional eye movements (‘ino rotation” condition in Fig. 2), and in several cases (45” CW and CCW and 15” CCW tilt) were in opposite directions. This result would tend to indicate that torsional eye movements are largely compensated for in making horizon estimates under head tilt. Furthermore, the addition of visual field rotation to head produced a constant increment or decrement (depending on the direction of rotation) in eye torsion across all tilt conditions. That is, visual field rotation at a constant speed appears to interact in a purely additive manner with tilt-induced eye torsion, resulting in the smooth and constant rotation “envelope” of Fig. 2a and 3a. For horizon settings, on the other hand, the interaction is more complex, showing an effect of visual field rotation that is both consistently larger than the effect on torsion, and increases with increasing head tilt, a result in agreement with other studies (Dichgans rt al., 1974; Young et al., 1975). The variance data plotted in Fig. 3b also show a difference between torsional and horizon measures, with only the latter showing an overall increase in variance with head tilt. Finally, demonstration that the effect of visual field rotation in the case of horizon settings differs depending on the direction of rotation relative to the
546
BJORN H. MERKER and RICHARD HELD
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Fig. 3. A. Mean size. B. Variance of the difference between CCW and CW rotation conditions (the “rotation envelope”) plotted for horizon estimates and eye torsion as a function of head tilt. Data for pairs of opposite head tilt have been averaged. direction of head tilt, further sets the horizon estimates apart from the eye torsion results. That is, by several qualitative and statistical measures the eye torsion and horizon setting data of the present study behaved differently under the interaction of head tilt and visual field rotation. In particular, there is no support in these results for an identity (except for a scale factor) between the function relating eye torsion, on the one hand and horizon estimates, on the other, to the interaction of visual field rotation and head tilt. It would therefore seem that the conception illustrated in hypothesis 2 (Fig. 1) is not tenable. A schematic summary of the behavior of eye torsion and horizon estimates under the interaction of visual and vestibular influences in the present experiment is given in hypothesis 3, Fig. 1. The additive effects of field rotation and head tilt on eye torsion are represented by direct summation of the two sources of afference into the signal driving eye torsion. A different and more complex interaction (not specified in the diagram) between the two sources of afference appears to determine the subject’s internal gravitational reference. Not only does the effect of field rotation grow with increasing head tilt (Fig. 3a), but the trend of that influence a&ears to difrer for small and large head tilts, as evidenced by the inflections in the rotation envelope between 15” and 30” in Fig. 2b. It is over this range of small tilts that the curve describing eye-torsion as a function of head tilt has its steepest slope (Fig. 2a), presumably reflecting the
effect of otolith and proprioceptive tnformatio~~ The inflections in the rotation envelope may thereforc indicate a range of small head tilts where otolith\ and proprioceptors exert a relatively greater influeucc on horizon estimates than beyond that range (see also Dichgans et ul., 1972). The finding is further rrminihcent of the different ranges of tilt resulting III tht: Muller (1916) and Aubert (1861) phenomena, and may point to a common genesis. The fact that the Muller and Aubert phenomena are not apparent in the “no rotation” condition in this plot might be attributable to the considerable individual differences character-t+ ing these effects (see Sandstrom, 1954. 1956). Parenthetically, the apparent compensation for eye torsion due to head tilt in the no rotation condition of horizontal estimates ocould most easily be achieved by making the sum of gains k2 and k, equal to ! in the scheme of hypothesis 3. The finding that eye torsion and horizon estimates have independent central determinants drawing on visual and vestibular information is not unexpected in view of the very different response categories represented by the two dependent variables. Eye torsion is a reflexive adjustment to visual field rotation and head tilt that is outside S’s conscious control and awareness. Horizon estimates in the absence of visual cues to the objective horizon and under the confounding influences of field rotation and head tilt demand of the subject a quite deliberate and conscious judgement based on an internal gravitational reference. It was only the periphalistic proposal of hypothesis I that raised the possibility of a direct functional hnkage between the two. The present results would seem to lay this possibility to rest. Further evidence for the difference in central processing has been provided by Finke and Held (1978) and by Wolfe and Held (1979). Future studies might profitably be directed at elucidating the nature of visual and vestibular interactions in their influence on the internal gravitational reference. and sources of variance in horizon estimates based on the latter. Acknowledgemenrs--This research was supported tn part by NASA Grant NGL 22-009-308 and Grants NIH 5-ROI-EY 01191 and I-ROl-EY 02649 REFERENCES
Aubert H. (1861) Eine scheinbare bedeutende Drehung von Objekten bei Neigung des Kopfes nach rechts oder hnks. Virchows Arch. oath. Anat. Phvsiol. 20. 381-393. Dichgans J., Held- R., Young L: R. and Brandt T. (1972) Moving visual scenes influence the apparent direction of gravity. Science 178, 1217-1219. Dichgans J., Diener H. C. and Brandt T. (1974) Optokinetic-araviceotive interaction in different head positions. Act: 0tola;yng. 78, 391-398. Finke R. and Held R. (1978) State reversals of optically induced tilt and torsional eye movements. Percept. Ps.vchophys. 24(d), 337-340. Held R., Dichgans J. and Batter J. (1975) Characteristics of moving visual scenes influencing spatial orientation. Vision Rex lS, 357-365.
Eye torsion and apparent horizon Hughes P. C., Brecher G. A. and Fishkin S. M. (1972) Effects of rotating backgrounds upon the perception of verticality. Percept. Psye~oF~ys. 11, 135-138. Muller G. E. (1976) Uber das Aubertsche Phenomenon. A. Psychol. Physiol. Sinnesorg. 49. 109-246. Sandstrom C. I. (1954) A note on the Aubert phenomenon. J. exp. Psychol. 48, 209-210. Sandstrom C. I. (1956) Sex differences in tactile-kinesthetic
541
and visual perception of verticality. Q. JI. r.up. Psycho/. 8. 1-7. Wolfe J. and Held R. (1979) Eye torsion and visual tilt are mediated by different binocular processes. Vision Res. 19.917-920. Young L. R.. Oman C. M. and Dichgans J. (1975) Influence of head orientation on visually induced pitch and roll sensations. Aviar. Space Envir. Med. 46. 264-268.
Yi.,wn Ruwurth Vol. 21. pp. 543 to 547 Prmted m Great Britain
EYE TORSION AND THE APPARENT HORIZON UNDER HEAD TILT AND VISUAL FIELD ROTATION BJORN H. MERKER and RICHARD HELD Department of Psychology (EIO-139), Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A. (Received 14 April
1980; in revised form 27 August 1980)
Abstract-Two different experimental manipulations, namely head tilt and the viewing of a visual display rotating around the line of sight, induce torsional displacements of the eyes and a tilting of the apparent horizon. The present study examines the routes by which visual (field rotation) and otolithproprioceptive (head tilt) sources of afference influence horizon judgements. In particular, the relationship between torsional eye movements and horizon estimates is addressed. The results indicate that visual and otolith-proprioceptive information sum directly in their influence on eye torsion, but interact more complexly in horizon estimates, indicating a dissociation of their central determinants.
INTRODUCl’ION
An observer viewing a large visual display rotating around his line of sight experiences a number of perceptual effects, including tiiting of contours displayed on a stationary target concentric with the surrounding moving field (Dichgans et al., 1972; Held et a/., 1975). If the observer has control of the rotary alignment of the central target and adjusts it to the apparent horizon before and then again during field rotation, the amount of target displacement provides a measure of the influence of field rotation on the observer’s horizon estimate. If the surrounding field is kept stationary, the same procedure can be used to estimate another influence on horizon judgements, namely head tilt. That is, our subjective estimate of
the horizon is influenced by at least two sources of afferent information: a visual source, represented by field rotation, and a proprioceptive-vestibular source, represented by head tilt. The question addressed by the present report concerns the routes by which visual and vestibular influences affect horizon judgements. For example, it has been noted that visual field rotation induces torsional eye movements in the same direction as that in which the field rotates. This is consistent with eye torsion serving as the source of the apparent disptacement of the horizon in the opposite direction, since images on the retina are tilted in a direction opposite to the torsion (Hughes et al., 1972). This conception has been illustrated in hypothesis 1, Fig. 1. It predicts an apparent displacement of the horizon equal in
Fig. 1. Schematic illustration of three hypotheses attempting to account for the routes (circled paths exclusively) by which visual and “vestibular” influences affect horizontal estimates. The comparator junction on the right in the figure compares the retinal position of a target with an ‘*internal gravitational reference” resulting in an error signal “e” used to align the target with respect to the gravity reference. The left part of the figure is concerned with the genesis of the gravitational reference as it bears on the empirical findings discussed in the text. According to hypothesis 1, the gravity reference is a veridical reflection of otolith and the proprioceptive information about objective horizontal. Estimates of the latter are nevertheless systematically displaced from objective horizontal when visual field rotation induces torsional eye movements that are not taken into account in estimating target position in space. According to hypothesis 2, visual and vestibular information sum directly to influence both eye torsion and the gravity reference. Only the gain (k, and k,) of the signal influencing the two dependent measures differs according to this model. According to hypothesis 3, finally, only the influence on eye torsion results from direct visual-vestibular summation. The two arerent channels interact more complexly in determining the gravity reference. This is the conception supported by the present results. 543
BJOKN H. MERKLK and RIWAKI)
544
magnitude to the size of torsional eye movements, However. when measured by Dichgans rr trl. (1972). apparent tilt far exceeded the amount of eye torsion, a result confirmed by the present experiment (see results section). The conception of hypothesis I therefore has to be amended at least to the extent illustrated in hypothesis 2. Fig. 1. Here the interaction of visual and vestibular signals influences not only eye torsion. but also the observer’s internal gravitational reference. Because the two signals originate from the same summing junction. they differ at most in their relative gains (k, f k,). This model would therefore predict a constant fractional contribution of eye torsion to toal induced tilt. This in turn means that the function relating horizon estimates to the interaction of visual field rotation and head tilt should be identical, except for a scale factor. to the function relating eye torsion to the interaction of these variables. That is the prediction the experiment reported here was designed to test. The results obtained (see results and discussion sections) did not support this prediction. but rather gave evidence of non-linear interaction between visual and vestibular information in the determination of horizon estimates. Regarding the three hypotheses of Fig. 1. it may therefore be concluded that hypothesis 1, attributing horizon displacements to uncompensated torsional eye movements is clearly false. Hypothesis 2. predicting linear summation of visual and vestibular signals is disproved by the present experiment. This leaves hypothesis 3. which embodies the non-linear interaction of visual and vestibular signals observed in the present experiment. It will be considered further in the discussion section of this paper.
Ht LII
horizontal strobe hash attached to the brtc!loard. Head tilts at 15 . 30 and 45’ off vcrtrcal in ;I (‘W i)r CCW direction, with center of gaze unchanged. wer\: produced by radial displacements of the biteboard attachment along the calibrated perimeter- !I( the apparatus. All measurements from S were obtamcd m ‘I single session. divided into eight blocks of trials. one foreach of the seven tilt conditions (no tilt. and (‘W KL CCW tilt at 15-, 30^ and 45-). with “no tilt” alua:~s run first and then replicated at the end of the session as a check on baseline drifts. Within each block. trli was combined with three field rotation conditions: no rotation. CW. and CCW rotation of the visual field In the first block (no tilt). “no rotation” :IIw;I!~ occurred first and was then replicated once wrthrn that block. With these exceptions. the serial order o( tilt conditions was counterbalanced across subject\ and the serial order of rotation conditions countcrbalancedacrosstiltconditionsand subjects. Within each of the resulting 25 tilt-rotation conditions. S gave ri series of alternating horizontal estimates and eye torsion measures, five of each measure in rapid alternution. resulting in a total of 250 data points for each subject. On the basis of previous estimates of the time required for tilt and rotation effects to build up and dissipate. a period of 35 set of exposure to a rotation condition preceded such a series of IO measurements. and 20 set of rest followed each condition, A longer rest period intervened between each tilt block. The afterimage was given at the beginning of each tilt block. and was maintained throughout a block hq flickering the illumination of the target disc under s’s control.
METHODS
Six male undergraduate students served as paid subjects. The apparatus was very similar to that described in Held et ctl. (1975). with exceptions noted. S. head fixed by biteboard and using binocular vision sat facing a large circular disc of static “visual noise” (irregular white speckles on a black background) subtending 130’ of visual angle. This field could either be rotated at 2O’isec. in a clockwise (CW) or a counterclockwise (CCW) direction around the s’s line of sight. or be kept stationary. At its center was a white circular target disc subtending 32’ of visual angle. bisected by a black stripe. This target disc could be rotated by S via a control knob independently of the surrounding field. Target disc settings were continuously recorded on a strip chart. as well as read out on a digital display calibrated in degrees of angular displacement of the target stripe from objective horizontal. Resolution of measurement was better than 0.5”. with negligible drift as assessed by calibration readings. S indicated perceived horizon by target stripe settings under visual control. Eye torsion was measured by having S align the target stripe in parallel with a bar-shaped afterimage received with the head always in upright position from a slit-shaped
The averaged data for all Ss are plotted separately for torsional measures and horizontal estimates in qualitative difference Fig. 2. The most striking between the two graphs is the smooth variation 01 torsional measures with head tilt. a pattern not CI’Ident in the graph of horizon estimates. Note also that in the “no rotation”‘condition, the magnitude of eye torsion exceeds that of horizon displacements at every head tilt, whereas field rotation at every tilt has :I larger effect on horizontal estimates than on eye torsion Analysis of variance disclosed main effects of each treatment condition; that is, of head tilt. rotation, and eye torsion vs horizontal settings. interactions between each pair of the three. as well as multiple interaction between tilt, rotation. and torsion vs horizontal, all statistically significant (P < 0.001 I. Analysis of variance of the same data, but defining the rotation effect as the difference between CW and CCW rotation (the rotation “envelope”). showed a significant main effect of tilt. of torsion vs horizontal. and showed an interaction of tilt with torsion vs horizontal. The rotation “envelope” is plotted in Fig. 3. which also shows an overall increase in the variance
343
Eye torsion and apparent horizon
Fig. 2. A. Mean extent of ocular torsion in degrees plotted as a function of head tilt for three rotation conditions. B. Mean extent of target udjmfnmf off objective horizontal (in degrees) as a function of head tilt, plotted for three rotation conditions. of the effect of field rotation with increasing head tilt for horizontal settings, but not for eye torsion.
DISCUSSION
In order to determine further the nature of the difference between torsional measures and horizon estimates indicated by the analysis of variance, additional statistical tests were performed, guided by trends in the graphs of Fig. 2. For example, there appears to be a trend towards an expansion of the rotation “envelope” with increasing head tilt in the case of horizontal estimates, but not for eye torsion. The difference between the effect of CW and CCW rotation at 45” head tilt (averaged for both directions of tilt) was therefore compared by r-test with the same rotation effect at head upright, for both torsional and horizontal measures. The difference is significant (P = 0.001) for horizontal settings, but not for eye torsion measures, and this difference between the two measures is itself significant (P < 0.001). The graphs in Fig. 2 also suggest that the effect of rotation on horizontal settings is larger when the direction of field rotation is opposite to the direction of head tilt, an effect not evident for eye torsion. A t-test for mean difference between the sum of all rotation effects at head tilts in the same vs opposite directions relative to the direction of rotation gave a significant difference for horizon estimates (P < 0.05) but not for eye torsion. This difference between torsion and horizontal settings is again itself significant (P < 0.05).
Mean horizon displacements at every head tilt were of lesser extent than torsional eye movements (‘ino rotation” condition in Fig. 2), and in several cases (45” CW and CCW and 15” CCW tilt) were in opposite directions. This result would tend to indicate that torsional eye movements are largely compensated for in making horizon estimates under head tilt. Furthermore, the addition of visual field rotation to head produced a constant increment or decrement (depending on the direction of rotation) in eye torsion across all tilt conditions. That is, visual field rotation at a constant speed appears to interact in a purely additive manner with tilt-induced eye torsion, resulting in the smooth and constant rotation “envelope” of Fig. 2a and 3a. For horizon settings, on the other hand, the interaction is more complex, showing an effect of visual field rotation that is both consistently larger than the effect on torsion, and increases with increasing head tilt, a result in agreement with other studies (Dichgans rt al., 1974; Young et al., 1975). The variance data plotted in Fig. 3b also show a difference between torsional and horizon measures, with only the latter showing an overall increase in variance with head tilt. Finally, demonstration that the effect of visual field rotation in the case of horizon settings differs depending on the direction of rotation relative to the
546
BJORN H. MERKER and RICHARD HELD
L
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Is
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Fig. 3. A. Mean size. B. Variance of the difference between CCW and CW rotation conditions (the “rotation envelope”) plotted for horizon estimates and eye torsion as a function of head tilt. Data for pairs of opposite head tilt have been averaged. direction of head tilt, further sets the horizon estimates apart from the eye torsion results. That is, by several qualitative and statistical measures the eye torsion and horizon setting data of the present study behaved differently under the interaction of head tilt and visual field rotation. In particular, there is no support in these results for an identity (except for a scale factor) between the function relating eye torsion, on the one hand and horizon estimates, on the other, to the interaction of visual field rotation and head tilt. It would therefore seem that the conception illustrated in hypothesis 2 (Fig. 1) is not tenable. A schematic summary of the behavior of eye torsion and horizon estimates under the interaction of visual and vestibular influences in the present experiment is given in hypothesis 3, Fig. 1. The additive effects of field rotation and head tilt on eye torsion are represented by direct summation of the two sources of afference into the signal driving eye torsion. A different and more complex interaction (not specified in the diagram) between the two sources of afference appears to determine the subject’s internal gravitational reference. Not only does the effect of field rotation grow with increasing head tilt (Fig. 3a), but the trend of that influence a&ears to difrer for small and large head tilts, as evidenced by the inflections in the rotation envelope between 15” and 30” in Fig. 2b. It is over this range of small tilts that the curve describing eye-torsion as a function of head tilt has its steepest slope (Fig. 2a), presumably reflecting the
effect of otolith and proprioceptive tnformatio~~ The inflections in the rotation envelope may thereforc indicate a range of small head tilts where otolith\ and proprioceptors exert a relatively greater influeucc on horizon estimates than beyond that range (see also Dichgans et ul., 1972). The finding is further rrminihcent of the different ranges of tilt resulting III tht: Muller (1916) and Aubert (1861) phenomena, and may point to a common genesis. The fact that the Muller and Aubert phenomena are not apparent in the “no rotation” condition in this plot might be attributable to the considerable individual differences character-t+ ing these effects (see Sandstrom, 1954. 1956). Parenthetically, the apparent compensation for eye torsion due to head tilt in the no rotation condition of horizontal estimates ocould most easily be achieved by making the sum of gains k2 and k, equal to ! in the scheme of hypothesis 3. The finding that eye torsion and horizon estimates have independent central determinants drawing on visual and vestibular information is not unexpected in view of the very different response categories represented by the two dependent variables. Eye torsion is a reflexive adjustment to visual field rotation and head tilt that is outside S’s conscious control and awareness. Horizon estimates in the absence of visual cues to the objective horizon and under the confounding influences of field rotation and head tilt demand of the subject a quite deliberate and conscious judgement based on an internal gravitational reference. It was only the periphalistic proposal of hypothesis I that raised the possibility of a direct functional hnkage between the two. The present results would seem to lay this possibility to rest. Further evidence for the difference in central processing has been provided by Finke and Held (1978) and by Wolfe and Held (1979). Future studies might profitably be directed at elucidating the nature of visual and vestibular interactions in their influence on the internal gravitational reference. and sources of variance in horizon estimates based on the latter. Acknowledgemenrs--This research was supported tn part by NASA Grant NGL 22-009-308 and Grants NIH 5-ROI-EY 01191 and I-ROl-EY 02649 REFERENCES
Aubert H. (1861) Eine scheinbare bedeutende Drehung von Objekten bei Neigung des Kopfes nach rechts oder hnks. Virchows Arch. oath. Anat. Phvsiol. 20. 381-393. Dichgans J., Held- R., Young L: R. and Brandt T. (1972) Moving visual scenes influence the apparent direction of gravity. Science 178, 1217-1219. Dichgans J., Diener H. C. and Brandt T. (1974) Optokinetic-araviceotive interaction in different head positions. Act: 0tola;yng. 78, 391-398. Finke R. and Held R. (1978) State reversals of optically induced tilt and torsional eye movements. Percept. Ps.vchophys. 24(d), 337-340. Held R., Dichgans J. and Batter J. (1975) Characteristics of moving visual scenes influencing spatial orientation. Vision Rex lS, 357-365.
Eye torsion and apparent horizon Hughes P. C., Brecher G. A. and Fishkin S. M. (1972) Effects of rotating backgrounds upon the perception of verticality. Percept. Psye~oF~ys. 11, 135-138. Muller G. E. (1976) Uber das Aubertsche Phenomenon. A. Psychol. Physiol. Sinnesorg. 49. 109-246. Sandstrom C. I. (1954) A note on the Aubert phenomenon. J. exp. Psychol. 48, 209-210. Sandstrom C. I. (1956) Sex differences in tactile-kinesthetic
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