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The influence of illusory motion on line bisection performance in normal subjects

Published online by Cambridge University Press:  16 December 2005

KYUNG MOOK CHOI ,
BON D. KU ,
YONG JEONG ,
BYUNG HWA LEE ,
HYUN-JUNG AHN ,
SUE J. KANG ,
JUHEE CHIN ,
KENNETH M. HEILMAN  and
DUK L. NA
KYUNG MOOK CHOI
Affiliation:
Department of Neurology, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, Korea
BON D. KU
Affiliation:
Department of Neurology, Kwandong University College of Medicine, Myongji Hospital, Gyeonggi, Korea
YONG JEONG
Affiliation:
Department of Neurology, University of Florida and Veterans Affairs Medical Center, Gainesville, Florida, USA
BYUNG HWA LEE
Affiliation:
Department of Neurology, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, Korea
HYUN-JUNG AHN
Affiliation:
Department of Neurology, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, Korea
SUE J. KANG
Affiliation:
Department of Neurology, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, Korea
JUHEE CHIN
Affiliation:
Department of Neurology, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, Korea
KENNETH M. HEILMAN
Affiliation:
Department of Neurology, University of Florida and Veterans Affairs Medical Center, Gainesville, Florida, USA
DUK L. NA
Affiliation:
Department of Neurology, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, Korea
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Abstract

The present study examines whether illusory movement (IM) of a horizontal line, induced by a moving background (MB), influences line-bisection performance in normal subjects. The first experiment attempted to identify the speeds of MB that induce IM. We found that when speed is increased from 1.53° to 13.3°/sec, IM increases, but that with further speed increases, IM decreases. Leftward MB induces rightward IM, and vice versa. In the second experiment, we had subjects bisect lines at MB speeds that had been shown to induce IM in the first experiment. We found that leftward MB induced a rightward bias, and vice versa. We also found that there was a relationship between the magnitude of IM and the degree of bias. In the third experiment, by making the target line larger than the MB, we made the conditions where IM was presumably absent. Unlike the results of bisection performed with IM, subjects showed a bias in the direction of the MB. Overall, these experiments demonstrated that the perception of motion induces subjects to attend in the direction of movement. (JINS, 2005, 11, 881–888.)

Keywords

AttentionInduced motionMotion aftereffectMotion perceptionSpatial neglectVisual illusion
Type
Research Article
Information
Journal of the International Neuropsychological Society , Volume 11 , Issue 7 , November 2005 , pp. 881 - 888
Copyright
© 2005 The International Neuropsychological Society

INTRODUCTION

Hemispatial neglect is the failure to report, or orient to, novel or meaningful stimuli presented to the side opposite a brain lesion, when this failure cannot be attributed to either sensory or motor defects (Heilman et al., 2003). When asked to bisect a horizontal line, for instance, patients with hemispatial neglect place the bisection mark toward the ipsilesional space. Even normal subjects tend to bisect the line slightly leftward from the true midpoint (Jewell & McCourt, 2000), a phenomenon termed “pseudoneglect” (Bowers & Heilman, 1980). Although the mechanism of pseudoneglect is not fully understood, one theory attributes it to right-hemispheric dominance for directed attention (Heilman & Van Den Abell, 1980). According to this theory, the visuospatial nature of line bisection produces relatively more cortical arousal in the right than the left hemisphere. Thus, as patients with unilateral brain injury misbisect the line toward the ipsilesional space, normal subjects with less active left hemisphere in terms of attention bisect toward the left hemispace.

Bisection biases of normal subjects are influenced by many different factors which include age, sex, handedness, hand used to perform bisection, eye for regard, direction of scanning, lateralized cues, spatial location of stimulus, line length, vertical and radial orientation of line (Jewell & McCourt, 2000). Several studies have also demonstrated that movement of the background can influence bisection biases in normal subjects (Bisiach et al., 1996; Mattingley et al., 1994; Pizzamiglio et al., 1990). In these studies subjects were asked to bisect stationary horizontal lines superimposed on a moving background (MB). The results showed that when the background moved leftward, the line bisection error (LBE) deviated leftward with respect to the LBE of the control condition in which the background was stationary or absent. Likewise, when the background moved rightward, the LBE deviated rightward from LBE of the control condition.

When normal subjects are looking at a stationary object on MB, however, the stationary object appears to move. This perception of illusory motion (IM) might bias the subject's allocation of spatial attention. In a study that investigated eye movement while normal subjects were asked to look at a stationary horizontal line with MB, the authors showed that fixations occurred in the opposite direction to the background movement—i.e., in the same direction as the IM (Jeong et al., 2004). Normally, people look at (foveate) the area of the environment to which they are attending. In the presence of IM, their attention is therefore directed toward the portion of the line that appears to be on the leading side.

Patients with hemispatial neglect perceive the line segment located in the contralesional side of their body (unattended segment) as shorter than its actual size whereas they perceive the line segment located in the ipsilesional side (attended segment) longer (Harvey et al., 1995; Milner et al., 1993). The effect of orientation on size estimation (orientation/estimation hypothesis) was also found in neglect patients with “cross over” phenomenon (Mennemeier et al., 2002). Another study reporting the underestimation of size in neglected space attributed the size misjudgement to a distortion of “representational medium” of patients rather than disruptions of attentional or perceptual processing (Bisiach et al., 2004). This attention-magnitude relationship was also found to affect line bisection performances in normal subjects (Chatterjee et al., 1994; Milner et al., 1992).

Since the part of a stimulus to which people attend or orient appears to have a greater magnitude than the part that is relatively unattended, line bisection should be biased in the direction of the IM—that is, opposite to the direction of the MB. This prediction, however, is in conflict with prior studies that demonstrated an attentional bias in the direction of MB (Bisiach et al., 1996; Mattingley et al., 1994; Pizzamiglio et al., 1990). These latter studies, however, did not assess their subjects for the presence of IM.

To learn how IM induced by MB influences line bisection, Na et al. (2002) used a large screen and a beam projector to maximize the effect of MB. In conditions where subjects reported the presence of IM, LBEs deviated in the direction of the IM and opposite to that of the MB. In conditions where subjects reported that there was no IM, LBEs were in the same direction of MB. Unfortunately, this study did not employ conditions that systematically varied the degree of IM, and thus failed to determine whether there is a direct relationship between LBEs and IM.

Based on these prior studies, we predicted that MB would induce IM in the opposite direction and LBE would deviate in direction of the IM. To be correctly perceived, objects that move faster might require higher levels of attention. Thus, we would expect that as the degree of IM increases, the allocation of attention will be more focused in the direction of movement and thus the LBEs will also increase. Conversely, in those conditions where IM is absent, the only movement perceived will be the MB and thus the line bisection bias will occur in the same direction as MB. An investigation on whether MB can induce IM and whether IM can alter the spatial allocation of attention might have both pragmatic and theoretical implications. So we carried out a series of experiments. In Experiment I, we established the MB conditions that are capable of inducing varying degrees of IM. In Experiment II, we attempted to learn by having subjects perform line bisections whether the MB conditions that induce IM in Experiment I also induce an attentional bias. Finally, we conducted Experiment III to examine whether LBEs would occur in the direction of MB in those conditions where IM is absent.

EXPERIMENT I. DETERMINATION OF THE BACKGROUND MOVEMENT CONDITIONS THAT INDUCE VARYING DEGREES OF IM

Materials and Methods

Subjects

Twenty-five right-handed subjects (11 men, mean age 25.1±3.0, mean education 16.6±2.4) with no history of psychiatric or neurological disease participated in Experiment I. All subjects demonstrated binocular visual acuity above 0.7, either uncorrected or corrected. An informed consent approved by the Institutional Review Board of Samsung Medical Center was obtained in writing from all subjects.

Experimental apparatus

The stimuli programmed by Microsoft Visual C++ (Version 6.0) and DirectX (Version 6.0) were projected by a Sony VPL-X600M LCD projector on a large screen (500 × 360 cm, visual angle 45.24 × 33.40°). As illustrated in Figure 1A, the MB was composed of alternating yellow and black vertical stripes that were 23.38 cm (2.23°) in width. The spatial frequency of the MB was approximately 0.23 cycle/deg. In the center of this background, a red horizontal line, 156.0 cm (14.81°) long and 2.8 cm (0.27°) thick, was superimposed. The horizontal line was stationary; the background moved leftward or rightward at six different velocities that were selected through a pilot study: Speed condition 1 (S1), 15.98 cm/sec (1.53 °/sec); Condition 2 (S2), 77.93 cm/sec (7.43 °/sec); Condition 3 (S3), 139.89 cm/sec (13.30 °/sec); Condition 4 (S4), 387.72 cm/sec (35.81 °/sec); Condition 5 (S5), 511.63 cm/sec (46.18 °/sec); and Condition 6 (S6), 759.46 cm/sec (64.66 °/sec). Thus, overall there were 12 conditions (six speeds in two directions). As depicted in Figure 1B, the distance between the subject's eye and the center of the screen was 600 cm. The horizontal line in the middle of the screen was located 137.6 cm (13.26°) above the subject's eyes level. The center of the screen was adjusted so that it intersected with the midsagittal plane of the subjects' body and head.

(A) Stimuli used in Experiment I and II. (B) The stimuli were projected on a large screen by a beam projector.

Experimental procedure

Subjects were seated on a chair in a dark and silent auditorium. They were asked to rate the magnitude of the IM for the 12 conditions on a five-point scale, indicating whether the line appeared to be ‘not moving’ (0), ‘moving a little’ (1), ‘moving moderately’ (2), ‘moving much’ (3), and ‘moving very much’ (4). Subjects were also asked to report the direction of IM if they perceived the presence of IM. Post and Lott (1990) had normal subjects look at stimuli for 30 seconds and found that IM increased as a function of time. Therefore, in this study, we instructed subjects to look at each stimulus for 15 seconds before they rated the IM.

The experiment was divided into two blocks: one for the six leftward MB conditions and the other for the six rightward MB conditions. We asked subjects to rate the same conditions twice: once for each of the two blocks. The six conditions (one trial per condition) were randomly presented for a first rating, and then repeated in a different order for a second rating. The first series of ratings served as practice trials and only the results of the second ratings were used in the analysis. Half of the subjects rated the leftward background movements followed by rightward background movements. The other half rated rightward background movements first.

In a pilot study, when the subjects were asked to simply rate the IM from 0 to 4, their responses were extremely variable. This may be related to the fact that they did not have a standard to which they could compare each trial and that each subject's definition of being ‘fast’ is subjective. Thus, to decrease variability in each block, subjects were instructed to compare the IM in the six conditions, rating the condition with maximum IM as 4, and the other conditions with reference to this maximum. Comparison of the six conditions after watching each condition serially would require perceptive recall. Thus, in each block, subjects were allowed to go back and correct the IM rating for conditions that they had previously rated. For the same reason, as described above, we had the subjects repeat the whole rating process twice and analyzed the data from the second rating only, hoping that this procedure would enhance reliability. When analyzing the results, IM perceived in the opposite direction to the MB was coded as a positive number, while IM perceived in the same direction as the MB was coded as a negative number. Thus, possible IM ratings ranged over nine values from −4 to +4. It took about 20 minutes for each subject to complete Experiment I.

Measuring the test–retest reliability of IM ratings

To measure the reliability of the IM ratings, we had 13 of 25 subjects (six men and seven women) perform the entire experiment twice. A retest was performed within 48 hours (mean 24.65, SD 1.14) after the first test. The test and the retest were identical except that sequence of trials in each block was randomized.

Results

The ratings of IM magnitude

The results of the IM ratings are presented in Figure 2. The mean IM rating for the left MB conditions showed that IM magnitude was greatest in S3, followed by S2, S1, S4, S5, and S6 in that order. In the rightward MB Conditions, S3 also showed greatest IM, followed by S2, S4, S1, S5, and S6.

Results of Experiment I. Rating of illusory motion (IM) at various speed conditions of moving background (MB) (S1: 1.53 °/sec; S2: 7.43 °/sec; S3: 13.30 °/sec; S4: 35.81 °/sec; S5: 46.18 °/sec; S6: 64.66 °/sec). The figures within graph represent means of IM magnitudes rated by subjects on each MB direction and speed condition. Vertical bars represent standard errors.

To investigate the influence of the direction and speed of the MB on the IM rating, data were analyzed using a general linear model (GLM). The main effect for MB direction [F(1,24) = .00, p > .05] and the interaction between direction and speed [F(5,120) = .66, p > .05] were not significant. But the main effect for MB speed was significant [F(5,120) = 8.75, p < .001]. Speed conditions were compared after the data for the rightward and leftward MB conditions were pooled. After Bonferroni correction, significant differences were found between S1 and S2 [F(1,24) = 28.17, p < .005]; S2 and S3 [F(1,24) = 8.78, p < .05]; and S3 and S4 [F(1,24) = 10.08, p < .05]. However, no significant differences were found between S 4 and S5 [F(1,24) = 7.33, p > .05]; and S5 and S6 [F(1,24) = 1.54, p > .05].

Judgment of IM direction

Subjects' judgment about the direction in which the horizontal line appeared to move was as follows. In S1, S2, and S3 for both directions, 100% of subjects who perceived an IM reported that the horizontal line appeared to move in the opposite direction to that of the MB. In S4, S5, and S6, however, the percentage of subjects who perceived an IM ranged from 40% to 88%: S4 (leftward 76%, rightward 88%); S5 (leftward 68%, rightward 60%); and S6 (leftward 40%, rightward 60%). Almost all the subjects who perceived an IM in S4, S5 and S6 reported that the horizontal line appeared to move to a direction opposite to that of the MB. The exceptions where subjects reported that the horizontal line appeared to move in the same direction as that of the MB included one subject in S6 with respect to a leftward MB, one subject in S4 (rightward MB), two subjects in S5 (rightward MB), and one subject in S6 (rightward MB).

Reliability of IM ratings

Test–retest reliability for the IM rating showed good correlations: r = .619 (p < .001) for leftward MB conditions and r = .669 (p < .001) for rightward MB conditions.

EXPERIMENT II. LINE BISECTION PERFORMANCE

Experiment I demonstrated that the direction and speed of MB influences the direction and magnitude of IM. In Experiment II we expected that the LBEs would occur in the direction of IM and increase in proportion to the increase of IM. To test these predictions, we carried out line bisection performance in various IM conditions.

Materials and Methods

Subjects

Twenty-seven right-handed subjects with good visual acuity (13 men, aged 25.9 ± 3.2, mean education 15.6 ± 2.1) participated in Experiment II.

Experimental apparatus

The apparatus for Experiment II was the same as in Experiment I, but the speeds of MBs were selected on the basis of results of Experiment I, as follows: (1) S6 with the lowest IM; (2) S3 with the highest IM; (3) S1 and S2, both with medium-range IMs. Conditions S4 and S5 were not tested in this experiment because in these conditions some subjects did not perceive IM motion or reported the direction of IM in the same direction as the MB. In addition to these eight experimental conditions (four different speeds in two directions), there was a control condition that was identical to the moving conditions except that the vertical stripes were absent and replaced by a gray background. Therefore, subjects performed line bisections in a total of nine conditions.

Subjects were requested to bisect the line by moving the cursor with the computer mouse held in their right hand, and clicking on the line. There was no time limit for the performance on this line bisection task. The initial position of the cursor was on the upper margin of the screen, but was randomly presented 14 cm to the left or right of the midpoint. This laterality of initial position was evenly distributed with 50% of left and 50% of right. On completion of each trial, represented by the clicking of the mouse, the cursor automatically returned to the original position, the new line appeared on the screen, and the speed of the MB changed to that of the next trial. LBEs were automatically computed by the computer in terms of pixels (later converted to cm). Negative values represented leftward errors, and positive values represented rightward errors, from the true midpoint. Before the experiment, we calibrated the settings to ensure that the computer indicated zero when the cursor was clicked on the actual midpoint of the line on the screen. The computer also measured the response time in ms from the appearance of the line to the bisection.

Experimental procedure

Subjects performed eight trials of bisection per condition. This total of 72 trials was divided into three blocks. The first block consisted of eight control trials. The second and third blocks consisted of 32 trials of the four leftward and four rightward movement conditions (S6, S3, S1, and S2). The first block always preceded the second and third blocks, but the second and the third blocks alternated in sequence so that half of the subjects performed the leftward MB block first, and the other half performed the rightward MB block first. In the second and the third blocks, the sequences of the speed conditions were randomized. There was a 2-minute break between the second and third blocks. There were four practice trials before the experiment. The total duration of the experiment for each subject was about 20 minutes.

Results

Line bisection errors

Mean response time for leftward and rightward MB condition was 4862 (SD 2211) ms and 5141 (SD 2385) ms, respectively. The results of LBEs are presented in Figure 3. The LBEs were displayed as a function of the order of IM magnitude as assessed in Experiment I. In the control condition without MB, subjects bisected slightly leftward (−0.27 cm, SD 1.6 cm) but this deviation from the veridical midpoint was not significant (t = .869, p > .05). In leftward and rightward conditions, LBEs deviated in a direction opposite to that of the MB with respect to the control condition. That is, in leftward MB conditions, LBEs in all four conditions deviated rightward when compared to those of the control condition, whereas in rightward MB conditions, LBEs in all four conditions deviated leftward when compared to those of the control condition. Students' t-tests with Bonferroni correction were performed to test whether the LBEs in leftward and rightward MB conditions differed significantly from those of the control condition. In five of the eight MB conditions, deviations were significant (leftward S2, t = −3.19, p < 0.05; leftward S3, t = −3.35, p < .05; rightward S1, t = 4.19, p < .001; rightward S2, t = 5.95, p < .001; rightward S3, t = 4.53, p < .001). But in the remaining three conditions, deviations were not significant.

Results of Experiment II. Line bisection errors (LBE) at various speed conditions of moving background (MB) plotted as an order of magnitude of illusory motion (IM). (S6: 64.66 °/sec; S1: 1.53 °/sec; S2: 7.43 °/sec; S3: 13.30 °/sec). Vertical bars represent standard errors.

An analysis using GLM showed that the main effect of MB direction was significant [F(1,26) = 103.20, p < .001]. The interaction of MB direction and MB speed was also significant [F(3,78) = 10.24, p < .001]. The main effect of MB speed was not significant [F(3,78) = .52, p > .05]. Simple main effects for MB speed conditions at leftward MB direction [F(3,78) = 7.16, p < .001] and at rightward MB direction [F(3,78) = 3.64, p < .05] were significant. A linear relation was significant [F(1,26) = 18.32, p < .001] when the LBEs of leftward MB conditions in the order of IM magnitude were submitted to a trend analysis. On the other hand, it was apparent that LBEs of rightward MB conditions increased until the third IM magnitude but decreased slightly in the fourth IM magnitude. The trend analysis also showed a linear relation [F(1,26) = 4.97, p < .05].

These results indicate that LBEs in leftward and rightward MB conditions occurred in opposite directions, and LBEs increased as the IM magnitude increased in both the leftward and rightward MB conditions.

EXPERIMENT III. LINE BISECTION PERFORMANCE IN CONDITIONS WITHOUT IM

Unlike our findings, results of studies from other laboratories that assessed line bisection with MB found that errors deviated toward the direction of the MB. To account for these differences, we posited that experimental conditions in those prior studies did not induce sufficient IM and that it was IM, like real movement, that induces a line bisection bias in the direction of the IM. Thus, the purpose of Experiment III was to test the postulate that MB without IM would induce a bias in the direction of MB. It has been suggested that IM is present when the target stimulus (line to be bisected) is of smaller dimensions than the MB and absent when the target stimulus is larger than the MB (cited from Palmer, 1999). In this Experiment III we, therefore, had subjects attempt to bisect the stationary lines that were larger than the MB and thus might not produce IM and induce LBEs in the same direction as MB movement.

Materials and Methods

Subjects

Twenty-seven right-handed subjects (17 men, aged 23.1 ± 4.3, mean education 14.0 ± 2.2 years) participated in Experiment III.

Experimental apparatus

The apparatus for Experiment III was the same as in Experiment II except for some modifications. As presented in Figure 4A, the width of the MB was smaller than the horizontal line. The size of MB was reduced to 400 × 288 cm, 80% of the size used in Experiment II, but the width of the vertical stripes was unchanged. The horizontal line was increased in length to 470 cm, approximately three times as long as the line used in Experiment II, but its thickness was unchanged at 2.8 cm.

(A) Stimuli for Experiment III. (B) Results of Experiment III. Line bisection errors (LBE) at various speed conditions of moving background (MB) (S6: 64.66 °/sec; S1: 1.53 °/sec; S2: 7.43 °/sec; S3: 13.30 °/sec). Vertical bars represent standard errors.

Experimental procedure

In this study, the background moved at the same four different speeds as in Experiment II, with the conditions labeled the same as in Experiment II. There was a control condition that was identical to the moving conditions except that the vertical stripes were absent and replaced by a gray background. Therefore, subjects performed line bisection in nine conditions that were divided into three blocks. There were eight trials per condition. The first block consisted of eight trials in the control condition. The second and the third blocks consisted of four leftward MB conditions and four rightward MB conditions. The sequence of the second and the third block was counterbalanced as in Experiment II. The 32 trials in each of the second and third blocks were presented in a randomized order. There was a two-minute break between the second and third blocks. Before the experiment, there were four practice trials. The total duration of the experiment for each subject was about 20 minutes.

Results

Line bisection errors

Mean response time for leftward and rightward MB condition was 4435 (SD 1808) ms and 4037 (SD 1396) ms, respectively. The results of the LBEs are shown in Figure 4B. The LBEs were displayed in the order of S6, S1, S2, and S3. In the control condition without MB, subjects bisected slightly rightward (0.27 cm, SD 4.70 cm), but the deviation from the veridical midpoint was not significant (t = .296, p > .05). In leftward and rightward conditions, LBEs deviated in the same direction as the MB. That is, in leftward MB conditions, LBEs in all four conditions deviated leftward when compared to those of the control condition, whereas in rightward MB conditions, LBEs in all four conditions deviated rightward when compared to those of the control condition. The results of t-tests with Bonferroni correction showed that S2 and S3 for both leftward and rightward directions differed significantly from LBEs of the control condition (leftward S2, t = 4.37, p < .005; leftward S3, t = 3.76, p < .01; rightward S2, t = −4.46, p < .005; rightward S3, t = −4.88, p < .001). In the remaining four conditions, deviations did not differ from those of the control condition.

An analysis using GLM was performed to find the effect of MB direction and speed online bisection performance. This analysis revealed that the main effect of MB direction was significant [F(1,26) = 26.86, p < .001]. The interaction of MB direction and speed was also significant [F(3,78) = 30.15, p < .001]. The main effect of MB speed was not significant [F(3,78) = 1.31, p > .05]. Simple main effects for MB speed at both leftward MB [F(3,78) = 23.61, p < .001] and rightward MB direction [F(3,78) = 17.21, p < .001] were significant. These results indicated that LBEs in leftward and rightward MB conditions deviated in the same direction of MB and the magnitude of LBEs differed according to the MB speed.

Lastly, we performed another experiment (Experiment IV) about motion aftereffects.a

Prolonged viewing of a stimulus moving in one direction and then immediately looking at stationary stimuli make the stationary stimuli appear to move in the opposite direction to the moving stimuli. This illusion, called motion aftereffects (MAE), lasts for about 8–16 sec (Hoffmann et al., 1999; Tootell et al., 1995). In Experiment II and III, the absence of intertrial interval might have caused MAE and confounded our results. Thus, we conducted Experiment IV in which normal subjects were asked to bisect lines in the same setting used in Experiment II and III except that there was an intertrial interval of 20 seconds. The results replicated those of Experiment II and III. However, absolute values of LBEs of Experiment IV were smaller than those of Experiment II or III, indicating that the MAE might have partly affected the LBEs in Experiment II and III.

DISCUSSION

The aim of this study was to examine in normal subjects the effects of an MB on inducing alteration in the allocation of spatial attention and how the presence of IM influence attention as determined by the line bisection task. It was first necessary to establish the relationship between the MB conditions and the degree of IM they induce. For this purpose, we asked the subjects in Experiment I to assess the magnitude of the IM of stationary horizontal lines against MB. Being a subjective phenomenon, IM is difficult to measure objectively. However, the test–retest reliability of IM measurements using a semiquantitative five-point scale was acceptable.

The results of IM ratings for the six MB speed conditions in Experiment I showed that IM was reported as increasing as the MB speed increased up to a certain level (S3: 13.30 °/sec), and that after a certain point, further increases of MB speed caused reported IM to decrease. These results replicate those of Post and Lott (1990), although, to the best of our knowledge, the physiologic mechanisms for why IM peaks at a certain speed of MB are unknown yet.

Our first hypothesis was that LBEs would deviate in a direction opposite to that of the MB with the presence of IM because the presence of IM would induce subjects to allocate their attention to the forward part of the line. Accordingly, Experiment II was conducted with four different speed conditions selected on the basis of the data from Experiment I. As expected, the results showed that LBEs deviated in the opposite direction to that of the MB, that is, in the same direction of the IM. This is consistent with the results of Na et al. (2002), which showed that when the background moved at a slow speed (9.4 °/sec), bisection marks deviated in the opposite direction to that of the background motion. The results of the present study and those of Na et al. (2002) are, however, inconsistent with those of previous studies (Bisiach et al., 1996; Mattingley et al., 1994; Pizzamiglio et al., 1990) which reported that LBEs of normal subjects or patients without hemispatial neglect show bisection errors in the direction of the MB.

The discrepancy between our results and those of other studies may be related to differences in the MB speed used in each study. Our study suggests that slower background movements (less than about 15 °/sec) induce IM of a line, whereas more rapid background movements (more than about 60 °/sec) do not. Thus, the discrepancy can be explained if prior studies had used rapid MB. Although attributes of background stimulus were not the same across studies, in Bisiach et al. (1996) the background consisted of vertical stripes moving at 13.13 °/sec. In Pizzamiglio et al.'s (1990) study, the background consisted of luminous dots moving at 71.1 °/sec, and in Mattingley et al.'s (1994) study the backgrounds consisted of dots dispersed around the middle horizontal line moving at either 5.1 °/sec or 10.2 °/sec. Therefore, the speed of background movement cannot be the only variable that influenced our results.

The second reason for the discrepancy may be that the experimental apparatus used in those previous studies by other investigators might have been inadequate to induce IM. Imaginary movements might be induced when the frame of the MB is sufficiently large such that subjects are not able to perceive that portions of the environmental that are not moving. For example, if your car is fully stopped and there is a large truck that is next to your car that starts to move forward, you might have the illusion that your car is rolling backward. If, however, there is a car next to you and you can see other items in the environment beside this car when this car starts to move forward you might not perceive that your car is rolling backwards. Therefore, to enhance the IM effect, the size of moving background should be much larger than the stationary central target, and the stimulus should be presented in a dark room (Palmer, 1999). Previous studies used computer monitors (Bisiach et al. 1996; Mattingley et al., 1994) or plexiglass screen (Pizzamiglio et al., 1990), and thus the viewing angle might have been smaller than that of our screen (45.24° × 33.40°).

When a person must interact with an object, as the object moves faster, the observer might have to allocate more attention to this object. Thus, the second hypothesis of our study was that the line bisection errors would increase as a function of the magnitude of IM. The results of Experiment II showed that LBEs do tend to increase along with IM. This result therefore adds further support to the notion that IM does influence the allocation of attention as determined by line bisection performance of normal subjects. As described earlier, we presumed that the opposing results of our study compared to those of previous studies are most likely to be associated with the motion-illusion factor. To test this and to further confirm the relationship between the IM and line bisection, we conducted Experiment III. Here, the stationary target (horizontal line) was made larger than the moving background presumably to avoid the IM effect. The results proved similar to those of previous studies (Bisiach et al., 1996; Mattingley et al., 1994; Pizzamiglio et al., 1990) such that line bisection mark deviated in the direction of MB.

It is not known why MB, in the absence of IM, would induce a bias on the line bisection task in the direction of the background movements, but it is possible that the background movement influences the spatial locus of attention toward the direction of movement. When IM is present against MB, however, the background movement and the IM are in different directions, but because the subjects' task is to bisect the line, they may attend primarily to the IM of the line and be relatively inattentive to the background. An alternative explanation for the influence of background movement on line bisection performances may be the direction of visual scanning. Prior studies involving line bisection of normal subjects suggested that left-to-right visual or attentional scanning induces leftward errors occur, and vice versa (Jewell & McCourt, 2000; McCourt et al., 2000). A background movement will induce pursuit (i.e., scanning) eye movements in the same direction as the MB, thus the LBEs in Experiment II will occur in the opposite direction of MB. However, this visual scanning theory is less likely an account because it is not consistent with the results of Experiment III in which LBEs occurred in the same direction of MB (i.e., scanning eye movements).

Overall, our experiments demonstrate that the perception of the motion of environmental stimuli induces subjects to attend toward the perceived direction of that motion. That is, in conditions with IM (Experiment II), subjects perceived the stationary line as moving, resulting in line bisection biases that are in the direction of the IM of the line. In those conditions where IM may not be implicated (Experiment III), subjects perceived the background movement rather than line movement, resulting in bisection biases in the direction of the background motion. These results might also help explain the results of previous studies that demonstrated attentional biases in normal subjects are influenced by rotatory vestibular stimulation (Shuren et al., 1998). The reason that vestibular stimulation improves spatial neglect has not been fully explained, but after a person is rotated, the vestibular system perceives that either the person or the environment is rotating. This account is also consistent with the results obtained by Rubens (1985). With cold-water stimulation in control subjects' left ear, the environment and objects in the environment such as horizontal lines appeared to be moving leftward. Thus, the bias induced in normal subjects by vestibular stimulation might be related to the effect of illusory movement on the lateralized attentional systems. However, this illusory movement account might not fully explain the improvement of the rightward error on line bisection in patients with left spatial neglect treated with caloric stimulation (Rubens, 1985), because most of these patients after stroke failed to report illusory movements. Thus, the improvement observed in these neglect patients with left ear cold water stimulation might be related to other mechanisms.

ACKNOWLEDGMENTS

This study was supported by a grant of the Korea Health 21 R&D Project, Ministry of Health & Welfare, Republic of Korea (A050079). The authors thank Sue Yeon Lee for her assistance in the preparation of the manuscript.

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Figure 0

(A) Stimuli used in Experiment I and II. (B) The stimuli were projected on a large screen by a beam projector.

Figure 1

Results of Experiment I. Rating of illusory motion (IM) at various speed conditions of moving background (MB) (S1: 1.53 °/sec; S2: 7.43 °/sec; S3: 13.30 °/sec; S4: 35.81 °/sec; S5: 46.18 °/sec; S6: 64.66 °/sec). The figures within graph represent means of IM magnitudes rated by subjects on each MB direction and speed condition. Vertical bars represent standard errors.

Figure 2

Results of Experiment II. Line bisection errors (LBE) at various speed conditions of moving background (MB) plotted as an order of magnitude of illusory motion (IM). (S6: 64.66 °/sec; S1: 1.53 °/sec; S2: 7.43 °/sec; S3: 13.30 °/sec). Vertical bars represent standard errors.

Figure 3

(A) Stimuli for Experiment III. (B) Results of Experiment III. Line bisection errors (LBE) at various speed conditions of moving background (MB) (S6: 64.66 °/sec; S1: 1.53 °/sec; S2: 7.43 °/sec; S3: 13.30 °/sec). Vertical bars represent standard errors.