Assessment of visual field dependence: Comparison ... - IOS Press

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Journal of Vestibular Research 18 (2008) 239–247 IOS Press

Assessment of visual field dependence: Comparison between the mechanical 3D rod-and-frame test developed by Oltman in 1968 with a 2D computer-based version Brice Isableua,b,∗, Marc Gueguen b, Benoˆıt Fourréb,c , Guillaume Giraudetc and Michel-Ange Amorimb,d a

Laboratoire des Techniques de l’Ing e´ nierie Médicale et de la Complexité – Informatique, Math e´ matiques et Applications de Grenoble (TIMC-IMAG, UMR UJF CNRS 5525), Grenoble, France b Laboratoire Controˆ le Moteur et Perception, Université Paris-Sud, UPRES EA 4042, Orsay F-91405, France c Essilor International, R&D, Visual System and Design, Saint Maur, France d Institut Universitaire de France, Paris F-75005, France

Received 12 August 2008 Accepted 29 December 2008

Abstract. The identification of subject’s perceptual style regarding multisensory integration is a central issue for spatial perception and sensorimotricity. In spatial orientation studies, the weighting of visual frame of reference (visual field dependence) is classically assessed by using verticality perception tasks, and especially the mechanical 3D rod-and-frame test (3D RFT). The validation of a 2D computer-based version of the RFT by virtue of its portability would facilitate the identification of modes of spatial referencing for the design and evaluation of sensory and motor rehabilitation programs. We question here whether the computerized 2D RFT yields frame effects similar (in amplitude, direction) and correlated to those induced by the mechanical 3D RFT. In both devices, 35 young and healthy males’ subjects were seated and tasked with aligning a rod to the gravity vertical within a square frame that was tilted at 18◦ . The results showed significantly larger rod deviations from the verticality in the 3D RFT. 3D and 2D RFT errors significantly correlated but shared a small amount of common variance (r2 = 0.35). In addition, left-right tilt asymmetry changes from one device to another. These results suggest that the mechanical 3D RFT for verticality perception remains a more robust test for identifying the subject’s perceptual style. Keywords: Computers, rod-and-frame test, spatial perception, verticality, visual field dependence-independence, frames of reference

1. Introduction Accurate perception of spatial orientation is of greatest importance when it is imperative to maintain balance or to make judgment about object orientation in a gravitoinertial field. The ability to integrate visual, ∗ Corresponding author: Brice Isableu, Laboratoire Contrˆ ole Moteur et Perception, UPRES EA 4042, Université Paris-Sud, Bt 335 UFRSTAPS, France. E-mail: [email protected].

vestibular and somatosensory cues is a key factor which allows the brain to construct an optimal spatial model of the orientation of the body in space [18]. However, evidence for inter-individual differences (IDs) have been well-identified in spatial perception tasks [1,35, 45] and more recently have been the focus of interest for sensorimotor control in young healthy subjects [7,29], in healthy senior subjects [31], in patients with sensory impairments [30], brain damages [47], for exploiting cues providing by sensory substitution devices (like the

ISSN 0957-4271/08/$17.00  2008 – IOS Press and the authors. All rights reserved

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tongue-display-unit for balance control, see Vuillerme et al. [43]). Close co-variations have been observed between perceptual styles (dependence vs. independence to visual spatial frame of reference) and spatial referencing modes adopted for stabilizing and orienting posture [29] and even for controlling multijoint reaching movements [3]. In order to design and evaluate sensory and motor rehabilitation programs, the identification of subject’s perceptual style of multisensory integration has become a central question for both basic and clinical research [4, 30]. Weight of visual spatial frame of reference (visual field dependence) in verticality perception tasks is classically assessed by using the popular portable mechanical 3D rod-and-frame test [3,9,19,22,27,32,33, 35] originally designed by [36]. Other common set up also exist where the subject is seated in a completely darkened room and attempts to adjust a luminous rod to a vertical (or horizontal) position within a tilted 2D luminous frame projected on a wall [21,24,25,42], or within a tilted 2D luminous plywood frame [7,13,16, 20,23,30,39,40,48,49]. The rod-and-frame paradigm, originally developed by [46] provides a quantitative measure of errors in the perception of verticality and a mean to identify the subject’s reliance to either visual frame or to non visual frame of references from a continuum of visual dependency [27,30]. Although many studies have used the portable mechanical 3D rod-and-frame systems, this system requires specialized facilities and experienced staff for both handling the different rod and frame’s conditions and gathering the data. Recently, Bagust [2] suggested that the computer-based rod-and-frame test may provide a more convenient alternative to the mechanical rod-and-frame test for verticality perception. Nevertheless, the mean frame effect reported in studies using the 2D rod-and-frame test seems smaller (see Bagust [2], 1.35◦ ± 0.31◦; Guerraz [23], 3.49 ◦ ± 3.8◦ ; Vibert [42], 2.54◦ ± 2.32◦ ; Sholl [39], 2.59 ◦; Lopez et al. [30], 2.5◦ ± 1.3◦ ; see also [7]) than those reported in studies using the mechanical 3D RFT (see Golomer [19], 4.9 ◦ ± 0.58; Bernardin [3], 6.1 ◦ ± 2◦ ; Isableu [27], 7.4 ◦ ± 1.3◦ ; Rousseu et Crémieux [39], between 3.5 ◦ and 5.5◦ in athletic males]. Such differences may be due to variation in the size of the rod-and-frame display [14,16, 41,49] and by a larger impingement of the 3D structure of the mechanical 3D RFT on the peripheral retina. In addition, some studies let the subject unrestrained [2, 30], a postural condition known to provide verticality cues from vestibular and proprioceptive inputs, which in turn may to limit the effect of the visual frame. The

present report aimed at comparing perceptual style detection by the mechanical 3D rod-and-frame systems vs a 2D computer-based version of the rod-and-frame test which can run on a standard office computer or displayed a TV screen. The question addressed here is whether the 2D computer-based version of the mechanical 3D rod-and-frame test is a sufficiently powerful test to discriminate subjects’ reliance to visual frame of reference (FoR) over non visual FoR.

2. Materials and methods A total of thirty five healthy men (22 y ± 2), university students, with normal or corrected-to normal visual acuity volunteered to participate in the experiment. They gave oral informed consent to the experimental procedure as required by the Helsinki declaration and the EA 4042 local Ethics Committee. None of the subjects presented any history of motor problem, neurological disease or known sensory deficit. All participants were right-handed. The subject’s task in both versions of the rod-andframe tests was to rotate the rod to what he perceived as vertical relative to gravity, while confronted to large tilted visual frame. The computerised rod-and-frame program was developed using OpenGL software and displayed in a high-resolution graphic mode (1024*740 dpi) on a TV screen. During this test, subjects were confronted to a 2D square black frame on a white background (Fig. 1). Within the frame, a single black line could be rotated around its center in clockwise or counterclockwise directions using the keyboard’s left and right arrows, the adjustment precision being 0.2 ◦ . When satisfied with the position of the rod, the subject pressed the space bar on the keyboard, clearing the screen for 1 second before the next rod and frame were displayed. A single key touch increased or decreased the rod’s tilt-angle by 0.2◦ , and a continuous pressure increased or decreased the rod’s tilt-angle by 3 ◦ /s. Before each rod display and setting, a fixation point appeared during 500 ms in the centre of the screen followed by a white screen. The rod was anti-aliased to smooth the outline. The edges of the monitor screen were masked by means of a black optical tunnel (0.6 m long and 50 ◦ Ø) to withdraw peripheral visual cues. The subject sat in darkness looking to the screen through an optical tunnel with the head kept still in a headrest to prevent using vestibular cues.


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Fig. 1. Insert 1 and 2: Side and front views of the computerised 2D rod-and-frame test displayed on a TV monitor screen. This system was used in complete darkness. Peripheral visual cues were withdrawn using an optical tunnel. Insert 3 and 4: Side and front views of the portable mechanical 3D Rod-and-frame test. This system was used in an illuminated environment.

The portable mechanical 3D rod-and-frame test apparatus (see Fig. 1) originally designed by Oltman [36] was used. This table-top device consists of a rectangular tunnel (0.6 m long, 30 × 30 cm section) made of translucent white plastic (3 mm), which acts as a visual frame of reference and can be tilted to the right or the left by specific amount. At the closed end of the frame is positioned a black rod which can be tilted separately from the frame. As for the optical tunnel, subject sat 0.6 m from the screen at the open end of the optical rectangular box with his head kept still in a headrest to prevent using vestibular cues and motion parallax. A curtain was lowered after each setting to prevent the subject from seeing rod and frame orientation changes. Subjects were asked to rotate the rod using a lever with their right hand, to a setting that looks vertical. Rod setting error from verticality was read on a graduated disc fixed back to the device. The test was performed in an illuminated room. In both devices, angular size of the square frame was equated to 28 ◦. Rods and frames were 1 cm thick. Rods were 20 cm long. The frames were tilted 18 ◦ to the left (L) or to the right (R), tilting value for which the frame was reported to produce its maximal effect [2,8]. The verticalness and horizontalness of the TV screen and mechanical 3D RFT were checked to guarantee rod verticality at 0◦ . Frame tilts followed the sequence: LLRRLLRR. Rods were initially tilted by −18 ◦ clockwise or +18◦ counterclockwise from the vertical, thereby either paralleling the frame lean or tilted in the opposite direction (following the sequence: LRRLLRRL). In both tests, the rod-and-frame procedure and instructions described by Oltman [36] were followed. Subjects were given no feedback about their performances during both tests. Binocular viewing was employed in both tests. Subjects were instructed to keep their eyes directed to the centre of the rod during the set-

ting. Proprioceptive cues stemming from plantar sole contacts with the ground were limited to the heels by instructing subjects to extend their legs (see Fig. 1). A total of eight trials per test (four trials per frame) were administrated to each subject. A five minutes rest period was given between each rod-and-frame test. The RFT sequence, including instructions to subjects, rod and frame handlings, and effective settings lasted about 10 to 15 minutes. Order of presentation and tasks were counterbalanced. Performance was scored in degrees of deviation from vertical. Different indices of subject field dependenceindependence (frame effect, constant error, rod starting position effect and frame error for each tilting side) were calculated according to Nyborg and Isaksen [34] procedure. Individual score was the mean of the 4 settings per frame (L and R). The degree to which judgments of the position of the rod are influenced by the tilt of the frame is an indication of degree of visual field dependence.

3. Results Individual scores of visual field dependency for both version of rod-and-frame tests are shown in Fig. 3 panels A (frame tilted to the left) & B (frame tilted to the right) and ranged in ascending order (based on the mechanical 3D RFT data). Large interindividual differences in rod-and-frame score were evidenced. Min and max deviations from the gravitational vertical in both frame (left and right) and in both version of RFT (2D and 3D RFT) are shown in Table 1. In both rod-and-frame tests, rod settings in the direction of the frame rotation were most common (direct effect, see [40]), but negative frame effects (or indirect effects) were observed (i.e., rod setting in the opposite

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B. Isableu et al. / Assessment of visual field dependence Table 1 Min and max deviations from the gravitational vertical in both frame (left and right) and in both version of RFT (2D and 3D RFT) Computerised 2D RFT version Left frame Right frame Min Max Min Max 2.8◦ to −3.51◦ −1◦ to 5.19◦

Table 2 Percentage of negative frame effects (FE) in the 2D and 3D RFT in both frame orientation (left and right) Computerised Left frame Negative FE 34.29%

2D RFT version Right frame Negative FE 11.43%

3D RFT version Left frame Right frame Negative FE Negative FE 14.28% 0%

direction of the frame lean, see [40]). 2D and 3D negative frame effects (in %) in both frame orientation (left and right) are given in Table 2. Negative frame effects were significantly more frequently observed in the computerised 2D RFT than in the mechanical 3D RFT version (see Table 2) in the right tilted frame (p < 0.04) and approached significance for the left frame (p < 0.054). Interestingly, a large proportion of the visual field independent subjects (small frame effect) in the 3D RFT committed negative errors in the 2D RFT, frame to the left, (see Fig. 3, panel A). Data were then subjected to repeated-measures ANOVA combining RFT versions (2D vs. 3D) x tilting frame side (Left vs. Right). Main effect of RFT versions (2D vs. 3D) on rod settings was significant with Frame effect calculated by removing the constant error F(1, 34) = 68.49, p > 0.000, η p2 = 0.67 (see Nyborg and Isaksen [34], 1978). Main effect of tilting frame direction (Left vs. Right) was significant F(1, 34) = 117.73, p < 0.001, η p2 = 0.78. indicating that the effect of the right frame was of larger amplitude. This result is confirmed using constant error F(1, 34) = 3.92, p = 0.056, ηp2 = 0.10 which also indicated side frames asymmetry. The RFT versions (2D vs. 3D) x tilting frame side (Left vs. Right) interaction was significant F(1, 34) = 68.45, p < 0.001, η p2 = 0.67 (see Fig. 2). Post-hoc Tukey HSD test showed that both tests significantly differed for left frame (p < 0.05) and right frame (p < 0.05). For both frame tilts, the size of the 2D frame effect was on average almost half the 3D one. Individual asymmetries observed between left vs. right frame effects on subjective vertical require checking whether distributions of frame effects induced by both RFT versions remained correlated across subjects (see Fig. 3, panel A). Correlational analyses confirmed that 2D and 3D frame effects remain significantly cor-

3D RFT version Left frame Right frame Min Max Min Max 3◦ to −7.38◦ 0.88◦ to 8.88◦

related in the left (r = 0.49, p < 0.002) and in right frame tilt (r = 0.40, p < 0.05) conditions. 2D and 3D Frame effects (calculated by removing the constant error, see [34]) significantly correlated (r = 0.66, p < 0.001). 2D and 3D constant errors (which give a measure of frame effect asymmetry between left and right frame tilts, or personal bias, see [34]) also significantly correlated (r = 0.38, p < 0.02), indicating that large asymmetry in one test holds in the other. Frame effects may contain effect of rod starting position (either paralleling the frame lean or tilted in the opposite direction). This is particularly awkward whether the amplitude of the frame effect is small as it is the case in the 2D RFT. A one-way ANOVA showed a significant rod starting position difference between 2D and 3D displays F(1, 34) = 27.88, p > 0.000, indicating that the effect of the rod starting position was larger in the 3D RFT. Bravais-Pearson correlation analyses showed that the 2D and 3D rod starting position significantly positively correlated (r = 0.49, p = 0.003). Effect of rod starting position significantly positively correlated with the 2D frame effect only (r = 0.39, p = 0.02), (3D RFT: r = 0.04, p = ns). This result indicated that 15% of 2D frame effect was in part corrupted by the effect of rod starting position errors. Finally, we found that the 2D and 3D frame effects remains significantly correlated (r = 0.59, p < 0.001) after removing their respective mean rod starting position error.

4. Discussion The computer-based rod-and-frame test is easy to handle by subjects and requires minimum intervention from the evaluator. This study was aimed at addressing the question whether such a computerized 2D rod-andframe test induces frame effects of comparable amplitude, in the same direction and systematically correlated across subjects as compared to those induced by the mechanical 3D RFT. The results showed that the computer-based RFT produced deviations of the subjective vertical in the direction of the frame tilt (i.e., direct effect, see [40, 44]), similarly to what was reported in this study and


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Fig. 2. Rod settings errors averaged across subjects for both version of RFT in both frame orientation.

previous ones using the mechanical 3D rod-and-frame system [2,28]. Interestingly, we observed a larger proportion of negative frame effects in the computer-based version [7] than in the 3D version. In addition and more importantly, our findings showed that the mechanical 3D RFT yielded significantly larger errors (frame effect) than did the 2D RFT. Indeed, individual values of frame effect induced by the 2D computer-based system rarely exceeded 5 ◦ , whereas 28% of subjects exceeded this value with the mechanical 3D RFT system (until 9◦ of deviation from the vertical). Although correlations were significant, the amount of common variance shared by both versions of RFT remains weak (r 2 = 0.25 for left frame; r 2 = 0.16, for right frame, see Fig. 3, panels C & D; r 2 = 0.44 for frame effect minus constant error or r 2 = 0.35 for frame effect minus both constant error and rod starting position effect). It is also important to emphasize that 15% of the 2D frame effect was due to an effect of the rod starting position. These findings suggest that the use of the 2D computer-based RFT to determine individual value along the field dependence-independence continuum may lead to a relatively large number of misclassifications with respect to the mechanical 3D RFT device. It is likely that the 3D structure of the mechanical RFT system (i.e., the rectangular optical tunnel, which

is the perpendicular extension of the plain frame), in providing perspective cues and gradient texture (though weak), plays an important role in producing a larger frame effect. In addition, although the size of the visual angle subtended by the 2D and 3D background frame was equated, the 3D structure of the mechanical RFT impinged largely more on the peripheral visual field than did the 2D RFT. Earlier studies have provided evidences that rod settings errors increased linearly with increasing angular retinal size of the frame, rather than with the apparent size of the frame [14,15, 41,49]. It was suggested that the mechanisms responsible of the frame effect under large frame condition (>20◦ of angular size) would mainly depend on visualvestibular [14,16,20,49] or visual-proprioception interactions [26]. Interestingly, it is surprising that the correlation observed in this study between the 2D and 3D visual frame effects was less than the one reported between the 2D visual frame effect on postural orientation (postural frame effect visually induced by a 2D visual frame) and the mechanical 3D RFT scores [28]. Zoccolotti et al. [49] showed that visual-vestibular interactions also plays a significant role for small frame (10.5◦). Some authors [9] have proposed that the distinction between the small and large frame might be

Fig. 3. Panel A: Distribution of individual rod settings errors for both version of RFT in the left frame condition. Scores were ranged in ascending order on the basis of the mechanical 3D RFT. Panel B: Distribution of individual rod settings errors for both version of RFT in the right frame condition. Scores were ranged in ascending order on the basis of the mechanical 3D RFT. Panel C: Distribution of individual rod settings errors for both version of RFT in the left and right frames conditions. Scores were ranged in ascending order on the basis of the mechanical 3D RFT, left frame orientation.

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related to a more quantitative (effect magnitude) than a qualitative effect (absence vs presence of the phenomenon). The larger is the frame in peripheral vision, the more likely and with more certainty it would serve as “world surrogate” [13] that defines the main axes of the Cartesian space. Given that visual inputs are capable of modulating nervous activity in the vestibular nuclei [5], it seems likely that the frame effect is the by-product of the activities of central mechanisms related to functional inter-sensory reciprocal inhibitory interactions (IRII) underlying sensory cortical brain activation and deactivation observed during competing visual and vestibular inputs [5,6,11,12]. These IRII are likely to work whatever the sensory channels involved. Nevertheless, the strength of these intersensory IRII for reweighting cues and hence to reduce sensory mismatch are likely to depend on prior experience inducing well-defined and well-structured somesthetic maps [10]. Long years of practice and training by professional musicians are associated with enlarged cortical representations in the somatosensory areas [17,37]. Thus, the more the egocentric somato-proprioceptive maps would be well defined and well structured the more exact and accurately the proprioceptive directions of body segments should be theoretically perceived and resistance to misleading visual signals (through) enhanced. Following this view, it could be that visual field independency results from stronger visual deactivation, (i.e., stronger IRII), thanks to larger prior experience in using proprioceptive cues. Our findings have also shown rod settings in the direction opposite that of the frame (indirect effects), which are reminiscent to earlier observations [7,50]. Zoccolotti et al. [50] showed that such ‘indirect effect’ depended on the gap size between the ends of a rod and the frame. Increasing gap size would produce a tendency toward negativity (away from frame tilt) but only for frames of small retinal angular size. With a large frame, Zoccolotti et al. [50] showed that rod settings were always in the direction of frame tilt (direct effects) and varied inversely with gap size. Interestingly, by using a large 2D frame (40 ◦ of visual angle) Bray et al. [7] observed as in our experiment such ‘indirect or negative frame effect’ in 25% of the tested subjects in the classical sitting condition. The frequency of observation of these negative frame effects increased in more demanding equilibrium postures to reach 50% of the tested subjects. This trend also appeared correlatively to a decreased visual dependency. The likelihood to observe such negative or indirect frame effects is enhanced in visual field independent subjects, given

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the proximity of rods settings with verticality. In our experiment gap size in both devices were identical. Further studies are needed to determine which optimal angular size a 2D frame should have to induce similar frame effects (and perceptual style measures) than those obtained from the mechanical 3D RFT. Binocular disparity is greatly lowered in the 2D RFT and may have played a significant role in the reduction of the frame effect. The respective role of binocular disparity vs. angular size should be addressed in further experiments.

5. Conclusion The mechanical 3D RFT produced larger and more significant rod setting errors than those induced by the 2D computer-based RFT. It therefore appears that the use of the 2D computer-based RFT to determine individual value along the field dependence-independence continuum may leads to a relatively large number of misclassifications with respect to the mechanical 3D RFT device. Both laboratories and healthy institutions working in the field of rehabilitation medicine require robust tests to evaluate the effects of sensory and motor rehabilitation programs or to obtain a predictive measure of sensory tactics in motor and cognitive abilities. The readily portable, easy-to-use system makes computerized RFT seducing but does not warrant accurate identification of dominant modes of spatial referencing among individuals.

Acknowledgements The authors would like to thank the students at the University Paris-Sud, UFR STAPS who volunteered to participate in this study. This work was supported by grants from the Centre National de la Recherche Scientifique, Ile de France (UMR 5525).

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