Exp Brain Res DOI 10.1007/s00221-014-3851-y
Research Article
Flanker interference effects in a line bisection task Sergio Chieffi · Tina Iachini · Alessandro Iavarone · Giovanni Messina · Andrea Viggiano · Marcellino Monda
Received: 9 October 2013 / Accepted: 18 January 2014 © Springer-Verlag Berlin Heidelberg 2014
Abstract Previous studies have shown that flanking distractors influence line bisection. In the present study, we examined if reaching the flanker after bisecting the line resulted in a variation of flanker interference on line bisection. Right- and left-handed participants were asked to bisect a horizontal line flanked by a dot (bisection task, B-task) or to bisect the line and then to reach the dot (bisection plus reaching task, BR-task). The dot was placed laterally to, and above or below, the line edge. The results showed that in both tasks the subjective midpoint was shifted away from the position of the dot. However, this effect was greater in the BR-task than in the B-task. We suggest that the requirement to perform an action to the flanker in the BR-task induced participants to pay more attention to the dot, enhancing its salience and distorting effects on line bisection. Keywords Line bisection · Distractor · Flanker interference · Flanker salience · Attention · Perception
S. Chieffi (*) · G. Messina · M. Monda Department of Experimental Medicine, Second University of Naples, Naples, Italy e-mail: [email protected] T. Iachini Department of Psychology, Second University of Naples, Naples, Italy A. Iavarone Neurological and Stroke Unit, CTO Hospital, AORN “Ospedali dei Colli”, Naples, Italy A. Viggiano Department of Medicine and Surgery, University of Salerno, Salerno, Italy
Introduction The line bisection task is commonly used to obtain evidence of hemispatial neglect. Unilateral hemispheric lesions, especially of the right hemisphere, produce neglect of the contralesional (left) hemispace in the horizontal plane. Patients with neglect misbisect horizontal lines to the right of the true center (Bartolomeo 2007; Bisiach and Vallar 1988; Heilman et al. 2003; Rizzolatti and Berti 1990). Line bisection has been also studied in healthy participants. Some authors (Bowers and Heilman 1980; Bradshaw et al. 1987; Scarisbrick et al. 1987) indicated that normal righthanders tended to systematically bisect lines to the left of center. This phenomenon was called pseudoneglect because normal subjects’ errors were in the opposite direction to those made by patients with neglect. It was suggested that leftward deviation of the subjective center might be linked to the spatial nature of the task which activated selectively the right hemisphere with resulting enhancement of the left perceptual field (Bradshaw et al. 1987; Scarisbrick et al. 1987). However, other authors found consistent rightward errors (Halligan and Marshall 1989; Nichelli et al. 1989) or failed to find any constant error (Halligan et al. 1990, 1991). Recently, by employing the landmark task (in which participants had to judge whether a transaction mark appeared closer to the right or left end of a line), some researchers showed that stimulus and context factors may influence the localization of the subjective midpoint. Generally, participants demonstrate a leftward bias in their landmark judgement. However, the magnitude of the leftward bias in the landmark judgement appears to decrease as a function of decreasing line length (McCourt and Jewell 1999; Rueckert et al. 2002), fatigue or lowering alertness (Manly et al. 2005), and prolonging performance on the task (Benwell et al. 2013b; Dufour et al. 2007; Manly et al.
13
2005). Furthermore, Thiebaut de Schotten et al. (2011) suggested that differences in bisection biases may be driven by differences in brain lateralization. Greater agreement exists in bisection of radial and vertical lines. Normal participants usually misbisect radial lines farther than the true midpoint and vertical line above the true midpoint (Chieffi et al. 2008; Post et al. 2006; Shelton et al. 1990). Shelton et al. (1990) attributed this tendency to attentional/perceptual factors. During visual exploration, attention is preferentially distributed away from the body, because the visual system should be specialized for detecting distant stimuli (Shelton et al. 1990). Experimental evidence suggests that contextual stimuli may influence bisection performance. A host of studies investigated the influence on line bisection of a cue presented at one end of the line (Harvey et al. 1995; McCourt et al. 2005; Milner et al. 1992; Nichelli et al. 1989). Generally, the cue biases bisection performance to the side where it is localized, e.g., to the right with a cue on the right side and to the left when a cue is presented on the left side of the line. Milner et al. (1992) explained cueing effects in terms of attentional mechanisms. They proposed that attention is drawn toward the cued portion of the line that makes the cued side more salient and leads to a relative overestimation of that side. However, other authors pointed out that since the cue is positioned beyond the true endpoint of the line, it may extend the cued side of the line (Fischer 1994; Mattingley et al. 1993). Other studies addressed the role of perceptual distortion of the line to-be-bisected in visual illusions (Chieffi 1996; Mattingley et al. 1995; Vallar et al. 2000) and found significant bisection errors in the predicted direction. Mattingley et al. (1995) showed that participants were susceptible to the illusory effects elicited by variants of the Muller-Lyer illusion, in which inducing fins were located at one or both ends of a horizontal line, one projecting inward and the other projecting outward. Participants produced significant bisection errors toward outward-projecting fins and away from inward-projecting. Similarly, Vallar et al. (2000) used the Brentano form of the Muller-Lyer illusion and observed that participants displaced midpoint toward the side expanded by the illusion. Chieffi (1996) used a different illusory configuration, the Baldwin illusion, in which the line to-be-bisected was terminated at each end by two different-sized bars. Participants bisected the line toward the smaller bar. Finally, other studies investigated the influence on line bisection produced by contextual stimuli placed laterally to, and above and/or below, the line edge (Chieffi 1999; Chieffi and Ricci 2002; Chieffi et al. 2012). In this case, normal participants tended to bisect the line away from the location of the flanker. Chieffi and Ricci (2002) hypothesized that instead of perceiving the line as a whole,
13
Exp Brain Res
participants exposed to a distractor might have segmented the line in two parts: one part flanked by the distractor and the other part not. In this case, the representation of the line as a whole was made more difficult requiring the sum of the two parts. This greater difficulty might result in ‘neglecting’ the flanked part of the line. Chieffi et al. (2012) also proposed that the flanker decreased the salience of the segment of the line that it flanked by increasing the attentional load to extract that segment from the background (white paper plus flanker). This, in turn, led to an underestimation of the flanked part of the line and a relative overestimation of the non-flanked part. Toba et al. (2011), using a landmark task, have reported a similar phenomenon. In their study, normal observers saw prebisected lines preceded by a cue. The cue was presented adjacent to, and above, the left or right endpoint of the pre-bisected line (see Toba et al. 2011, p. 240). On each trial, observers indicated whether they saw the bisection mark at the left or at the right of the midpoint. Results showed that left-sided cues shifted the apparent bisection point to the right (and vice versa). Authors (Toba et al. 2011) proposed that their findings were consistent with the attentional repulsion effect (ARE) previously demonstrated with vernier offsets (Suzuki and Cavanagh 1997). Accordingly, cues-induced exogenous attention repelled the perceived location of the bisection marker away from the attentional focus. According to the studies cited above, it is worthwhile to emphasize that the effect of the contextual stimulus seems to vary in function of its position with respect to the line. When the contextual stimulus is placed beyond the line end, i.e., along the virtual extension of the line, subjects tend to localize the subjective midpoint toward the contextual stimulus location (Fischer 1994; Harvey et al. 1995; Mattingley et al. 1993; McCourt et al. 2005; Milner et al. 1992; Nichelli et al. 1989). Conversely, when the contextual stimulus is placed laterally to, and above and/or below, the line edge, subjects tend to localize the subjective midpoint away from the contextual stimulus position (Chieffi 1999; Chieffi and Ricci 2002; Chieffi et al. 2012; Toba et al. 2011). The aim of the present study was to examine if reaching the flanker after bisecting the line resulted in a variation of flanker interference on line bisection. Participants were asked to either (1) bisect lines flanked by a dot (bisection task, B-task) or (2) bisect the flanked lines and then reach the dot (bisection plus reaching task, BRtask). It should be emphasized that in the B-task the dot acted only as a distractor; in the BR-task, it had a double function: It served as a distractor for the initial bisection movement and as a target for the subsequent reaching movement. It is plausible that the requirement to perform an action to the dot in the BR-task induced participants to
Exp Brain Res
pay more attention to it (e.g., to precisely localize the dot) than during simple viewing in B-task. This, in turn, might enhance the salience of the dot and produce a variation of its distorting effects on line bisection. If this was true, two expectations can be put forward. If the bisection bias in the direction opposite to the dot position was greater in the BR-task than in the B-task, then the requirement to reach the dot might have further decreased the salience of the flanked segment (Chieffi et al. 2012) or increased the attentional repulsion away from the focus of attention, i.e., from the dot itself (Toba et al. 2011). Conversely, if the bisection bias away from the dot was greater in the B-task than in the BR-task, then the requirement to reach the dot might have shifted the point of gravity of the whole configuration toward the side in which the dot was presented, enhancing in this way the salience of the flanked segment.
Materials and methods Participants Thirty-eight right-handed and forty-two left-handed subjects participated in this study. Hand dominance was assessed by administering the Edinburgh Handedness Inventory (Oldfield 1971). Each group was assigned to one of the two experimental conditions. The right-handed group assigned to the B-task comprised nine women and ten men: mean age = 23.8 (SD = 3.2); the one assigned to the BR-task comprised nine women and ten men: mean age = 24.7 (SD = 4.6). The left-handed group assigned to the B-task included nine women and twelve men: mean age = 24.8 (SD = 4.0); the one assigned to the BR-task comprised eleven women and ten men: mean age = 25.2 (SD = 4.2). The subjects were naïve to the tasks. All the subjects reported having normal or corrected to normal vision. The experiment was approved by the ethics committee and was performed in accordance with the 1964 Declaration of Helsinki. Subjects gave written informed consent to take part in the study. Stimuli The stimuli were black lines drawn and centered on A3 (420 × 297 mm) white paper. The lines were 2 mm thick and 28 cm long. The line could be flanked by a black dot 6 mm in diameter (experimental conditions). The dot was placed either on the left or on the right of the line center, 3 cm above or below the line. The center of the dot was either aligned with one line end (0 cm), or placed more internally at 3.0 and 6.0 cm from line end (dot distance) (Fig. 1). As control, there was the line without flanking dot.
Fig. 1 Examples of experimental stimuli used in both bisection and bisection plus reaching tasks. The dot is placed on the left and above the horizontal line
Procedure The participants sat in a comfortable chair in front of a table in a uniformly lit room. The experimenter sat at the opposite side of the table and administered the stimuli one at a time. Lines were oriented horizontally, at the intersection of the transverse and frontal planes. Line midpoint laid along the participant’s midsagittal axis, 30 cm from the trunk. In the B-task, the participants were asked to bisect the horizontal line, and in the BR-task to bisect the line and then to reach and cross out the dot with a small line. Participants used a pencil held with their preferred hand. In both tasks, a total of 104 trials per subject were given (eight control trials plus 96 experimental trials (two horizontal dot positions (left, right) × two vertical dot positions (above, below) × three dot distances from the line end (0 cm, 3 cm, 6 cm) × eight presentations). Stimuli were presented in a pseudo-randomized order. Line bisection error (LBE) was measured in both control and experimental conditions with an accuracy of 0.5 mm from the true midpoint. Deviations to the right of the true midpoint were assigned a positive value, whereas deviations to the left were given negative values. Then, we computed the experimental LBE (mm). For left dot conditions, experimental LBE was calculated by subtracting LBE measured in the control condition from LBE measured in each left dot condition. For right dot conditions, experimental LBE was calculated by subtracting LBE measured in each right dot condition from LBE measured in the control condition. In this way, we defined the sign of the experimental LBE by taking as a reference point the position of the dot (the left dot for the left experimental conditions and the right dot for the right experimental conditions). A positive value of experimental LBE indicated that the subjective midpoint in the experimental condition was localized farther than that in the control condition, relatively to the dot position.
13
Exp Brain Res
Table 1 Mean values of the line bisection error (LBE) and experimental LBE (Exp. LBE) (SD in brackets) of right- and left-handed participants in the bisection and bisection-reaching tasks Dot position
Left
Dot distance
0 cm
Right-handed (bisection task) LBE (mm) −0.49 Exp. LBE (mm) 0.55 (1.48) Right-handed (bisection-reaching task) LBE (mm) 0.69 Exp. LBE (mm) 2.76** (2.06) Left-handed (bisection task) LBE (mm) −3.42 Exp. LBE (mm) −0.57 (1.55) Left-handed (bisection-reaching task) LBE (mm) −0.25 Exp. LBE (mm)
2.08** (2.17)
Right 3 cm
6 cm
0 cm
3 cm
6 cm
−0.19 0.85* (1.44)
0.65 1.69** (1.68)
−1.39 0.35 (1.54)
−1.82 0.78 (1.63)
−2.36 1.32** (1.64)
1.61 3.68** (2.07)
1.95 4.02** (2.35)
−2.83 0.76 (2.07)
−3.50 1.43* (2.35)
−4.34 2.27** (2.40)
−1.83 1.02* (2.19)
−1.13 1.72** (2.24)
−3.15 0.30 (1.38)
−4.33 1.48** (1.65)
−4.15 1.30* (2.48)
0.85
0.66
3.18** (2.20)
2.99** (2.61)
−4.03
1.70** (1.72)
−4.65
2.32** (2.43)
−4.39
2.06** (2.74)
For LBE, deviations to the right of the true midpoint were assigned a (+) value, whereas deviations to the left were given (−) values. The way of computing the Exp. LBE is reported in the Procedure section. For Exp. LBE, positive values indicate that the subjective midpoint in the experimental condition was located farther than that in the control condition, relatively to the dot position * Significance levels of departure from zero are shown for experimental LBEs (* p
Research Article
Flanker interference effects in a line bisection task Sergio Chieffi · Tina Iachini · Alessandro Iavarone · Giovanni Messina · Andrea Viggiano · Marcellino Monda
Received: 9 October 2013 / Accepted: 18 January 2014 © Springer-Verlag Berlin Heidelberg 2014
Abstract Previous studies have shown that flanking distractors influence line bisection. In the present study, we examined if reaching the flanker after bisecting the line resulted in a variation of flanker interference on line bisection. Right- and left-handed participants were asked to bisect a horizontal line flanked by a dot (bisection task, B-task) or to bisect the line and then to reach the dot (bisection plus reaching task, BR-task). The dot was placed laterally to, and above or below, the line edge. The results showed that in both tasks the subjective midpoint was shifted away from the position of the dot. However, this effect was greater in the BR-task than in the B-task. We suggest that the requirement to perform an action to the flanker in the BR-task induced participants to pay more attention to the dot, enhancing its salience and distorting effects on line bisection. Keywords Line bisection · Distractor · Flanker interference · Flanker salience · Attention · Perception
S. Chieffi (*) · G. Messina · M. Monda Department of Experimental Medicine, Second University of Naples, Naples, Italy e-mail: [email protected] T. Iachini Department of Psychology, Second University of Naples, Naples, Italy A. Iavarone Neurological and Stroke Unit, CTO Hospital, AORN “Ospedali dei Colli”, Naples, Italy A. Viggiano Department of Medicine and Surgery, University of Salerno, Salerno, Italy
Introduction The line bisection task is commonly used to obtain evidence of hemispatial neglect. Unilateral hemispheric lesions, especially of the right hemisphere, produce neglect of the contralesional (left) hemispace in the horizontal plane. Patients with neglect misbisect horizontal lines to the right of the true center (Bartolomeo 2007; Bisiach and Vallar 1988; Heilman et al. 2003; Rizzolatti and Berti 1990). Line bisection has been also studied in healthy participants. Some authors (Bowers and Heilman 1980; Bradshaw et al. 1987; Scarisbrick et al. 1987) indicated that normal righthanders tended to systematically bisect lines to the left of center. This phenomenon was called pseudoneglect because normal subjects’ errors were in the opposite direction to those made by patients with neglect. It was suggested that leftward deviation of the subjective center might be linked to the spatial nature of the task which activated selectively the right hemisphere with resulting enhancement of the left perceptual field (Bradshaw et al. 1987; Scarisbrick et al. 1987). However, other authors found consistent rightward errors (Halligan and Marshall 1989; Nichelli et al. 1989) or failed to find any constant error (Halligan et al. 1990, 1991). Recently, by employing the landmark task (in which participants had to judge whether a transaction mark appeared closer to the right or left end of a line), some researchers showed that stimulus and context factors may influence the localization of the subjective midpoint. Generally, participants demonstrate a leftward bias in their landmark judgement. However, the magnitude of the leftward bias in the landmark judgement appears to decrease as a function of decreasing line length (McCourt and Jewell 1999; Rueckert et al. 2002), fatigue or lowering alertness (Manly et al. 2005), and prolonging performance on the task (Benwell et al. 2013b; Dufour et al. 2007; Manly et al.
13
2005). Furthermore, Thiebaut de Schotten et al. (2011) suggested that differences in bisection biases may be driven by differences in brain lateralization. Greater agreement exists in bisection of radial and vertical lines. Normal participants usually misbisect radial lines farther than the true midpoint and vertical line above the true midpoint (Chieffi et al. 2008; Post et al. 2006; Shelton et al. 1990). Shelton et al. (1990) attributed this tendency to attentional/perceptual factors. During visual exploration, attention is preferentially distributed away from the body, because the visual system should be specialized for detecting distant stimuli (Shelton et al. 1990). Experimental evidence suggests that contextual stimuli may influence bisection performance. A host of studies investigated the influence on line bisection of a cue presented at one end of the line (Harvey et al. 1995; McCourt et al. 2005; Milner et al. 1992; Nichelli et al. 1989). Generally, the cue biases bisection performance to the side where it is localized, e.g., to the right with a cue on the right side and to the left when a cue is presented on the left side of the line. Milner et al. (1992) explained cueing effects in terms of attentional mechanisms. They proposed that attention is drawn toward the cued portion of the line that makes the cued side more salient and leads to a relative overestimation of that side. However, other authors pointed out that since the cue is positioned beyond the true endpoint of the line, it may extend the cued side of the line (Fischer 1994; Mattingley et al. 1993). Other studies addressed the role of perceptual distortion of the line to-be-bisected in visual illusions (Chieffi 1996; Mattingley et al. 1995; Vallar et al. 2000) and found significant bisection errors in the predicted direction. Mattingley et al. (1995) showed that participants were susceptible to the illusory effects elicited by variants of the Muller-Lyer illusion, in which inducing fins were located at one or both ends of a horizontal line, one projecting inward and the other projecting outward. Participants produced significant bisection errors toward outward-projecting fins and away from inward-projecting. Similarly, Vallar et al. (2000) used the Brentano form of the Muller-Lyer illusion and observed that participants displaced midpoint toward the side expanded by the illusion. Chieffi (1996) used a different illusory configuration, the Baldwin illusion, in which the line to-be-bisected was terminated at each end by two different-sized bars. Participants bisected the line toward the smaller bar. Finally, other studies investigated the influence on line bisection produced by contextual stimuli placed laterally to, and above and/or below, the line edge (Chieffi 1999; Chieffi and Ricci 2002; Chieffi et al. 2012). In this case, normal participants tended to bisect the line away from the location of the flanker. Chieffi and Ricci (2002) hypothesized that instead of perceiving the line as a whole,
13
Exp Brain Res
participants exposed to a distractor might have segmented the line in two parts: one part flanked by the distractor and the other part not. In this case, the representation of the line as a whole was made more difficult requiring the sum of the two parts. This greater difficulty might result in ‘neglecting’ the flanked part of the line. Chieffi et al. (2012) also proposed that the flanker decreased the salience of the segment of the line that it flanked by increasing the attentional load to extract that segment from the background (white paper plus flanker). This, in turn, led to an underestimation of the flanked part of the line and a relative overestimation of the non-flanked part. Toba et al. (2011), using a landmark task, have reported a similar phenomenon. In their study, normal observers saw prebisected lines preceded by a cue. The cue was presented adjacent to, and above, the left or right endpoint of the pre-bisected line (see Toba et al. 2011, p. 240). On each trial, observers indicated whether they saw the bisection mark at the left or at the right of the midpoint. Results showed that left-sided cues shifted the apparent bisection point to the right (and vice versa). Authors (Toba et al. 2011) proposed that their findings were consistent with the attentional repulsion effect (ARE) previously demonstrated with vernier offsets (Suzuki and Cavanagh 1997). Accordingly, cues-induced exogenous attention repelled the perceived location of the bisection marker away from the attentional focus. According to the studies cited above, it is worthwhile to emphasize that the effect of the contextual stimulus seems to vary in function of its position with respect to the line. When the contextual stimulus is placed beyond the line end, i.e., along the virtual extension of the line, subjects tend to localize the subjective midpoint toward the contextual stimulus location (Fischer 1994; Harvey et al. 1995; Mattingley et al. 1993; McCourt et al. 2005; Milner et al. 1992; Nichelli et al. 1989). Conversely, when the contextual stimulus is placed laterally to, and above and/or below, the line edge, subjects tend to localize the subjective midpoint away from the contextual stimulus position (Chieffi 1999; Chieffi and Ricci 2002; Chieffi et al. 2012; Toba et al. 2011). The aim of the present study was to examine if reaching the flanker after bisecting the line resulted in a variation of flanker interference on line bisection. Participants were asked to either (1) bisect lines flanked by a dot (bisection task, B-task) or (2) bisect the flanked lines and then reach the dot (bisection plus reaching task, BRtask). It should be emphasized that in the B-task the dot acted only as a distractor; in the BR-task, it had a double function: It served as a distractor for the initial bisection movement and as a target for the subsequent reaching movement. It is plausible that the requirement to perform an action to the dot in the BR-task induced participants to
Exp Brain Res
pay more attention to it (e.g., to precisely localize the dot) than during simple viewing in B-task. This, in turn, might enhance the salience of the dot and produce a variation of its distorting effects on line bisection. If this was true, two expectations can be put forward. If the bisection bias in the direction opposite to the dot position was greater in the BR-task than in the B-task, then the requirement to reach the dot might have further decreased the salience of the flanked segment (Chieffi et al. 2012) or increased the attentional repulsion away from the focus of attention, i.e., from the dot itself (Toba et al. 2011). Conversely, if the bisection bias away from the dot was greater in the B-task than in the BR-task, then the requirement to reach the dot might have shifted the point of gravity of the whole configuration toward the side in which the dot was presented, enhancing in this way the salience of the flanked segment.
Materials and methods Participants Thirty-eight right-handed and forty-two left-handed subjects participated in this study. Hand dominance was assessed by administering the Edinburgh Handedness Inventory (Oldfield 1971). Each group was assigned to one of the two experimental conditions. The right-handed group assigned to the B-task comprised nine women and ten men: mean age = 23.8 (SD = 3.2); the one assigned to the BR-task comprised nine women and ten men: mean age = 24.7 (SD = 4.6). The left-handed group assigned to the B-task included nine women and twelve men: mean age = 24.8 (SD = 4.0); the one assigned to the BR-task comprised eleven women and ten men: mean age = 25.2 (SD = 4.2). The subjects were naïve to the tasks. All the subjects reported having normal or corrected to normal vision. The experiment was approved by the ethics committee and was performed in accordance with the 1964 Declaration of Helsinki. Subjects gave written informed consent to take part in the study. Stimuli The stimuli were black lines drawn and centered on A3 (420 × 297 mm) white paper. The lines were 2 mm thick and 28 cm long. The line could be flanked by a black dot 6 mm in diameter (experimental conditions). The dot was placed either on the left or on the right of the line center, 3 cm above or below the line. The center of the dot was either aligned with one line end (0 cm), or placed more internally at 3.0 and 6.0 cm from line end (dot distance) (Fig. 1). As control, there was the line without flanking dot.
Fig. 1 Examples of experimental stimuli used in both bisection and bisection plus reaching tasks. The dot is placed on the left and above the horizontal line
Procedure The participants sat in a comfortable chair in front of a table in a uniformly lit room. The experimenter sat at the opposite side of the table and administered the stimuli one at a time. Lines were oriented horizontally, at the intersection of the transverse and frontal planes. Line midpoint laid along the participant’s midsagittal axis, 30 cm from the trunk. In the B-task, the participants were asked to bisect the horizontal line, and in the BR-task to bisect the line and then to reach and cross out the dot with a small line. Participants used a pencil held with their preferred hand. In both tasks, a total of 104 trials per subject were given (eight control trials plus 96 experimental trials (two horizontal dot positions (left, right) × two vertical dot positions (above, below) × three dot distances from the line end (0 cm, 3 cm, 6 cm) × eight presentations). Stimuli were presented in a pseudo-randomized order. Line bisection error (LBE) was measured in both control and experimental conditions with an accuracy of 0.5 mm from the true midpoint. Deviations to the right of the true midpoint were assigned a positive value, whereas deviations to the left were given negative values. Then, we computed the experimental LBE (mm). For left dot conditions, experimental LBE was calculated by subtracting LBE measured in the control condition from LBE measured in each left dot condition. For right dot conditions, experimental LBE was calculated by subtracting LBE measured in each right dot condition from LBE measured in the control condition. In this way, we defined the sign of the experimental LBE by taking as a reference point the position of the dot (the left dot for the left experimental conditions and the right dot for the right experimental conditions). A positive value of experimental LBE indicated that the subjective midpoint in the experimental condition was localized farther than that in the control condition, relatively to the dot position.
13
Exp Brain Res
Table 1 Mean values of the line bisection error (LBE) and experimental LBE (Exp. LBE) (SD in brackets) of right- and left-handed participants in the bisection and bisection-reaching tasks Dot position
Left
Dot distance
0 cm
Right-handed (bisection task) LBE (mm) −0.49 Exp. LBE (mm) 0.55 (1.48) Right-handed (bisection-reaching task) LBE (mm) 0.69 Exp. LBE (mm) 2.76** (2.06) Left-handed (bisection task) LBE (mm) −3.42 Exp. LBE (mm) −0.57 (1.55) Left-handed (bisection-reaching task) LBE (mm) −0.25 Exp. LBE (mm)
2.08** (2.17)
Right 3 cm
6 cm
0 cm
3 cm
6 cm
−0.19 0.85* (1.44)
0.65 1.69** (1.68)
−1.39 0.35 (1.54)
−1.82 0.78 (1.63)
−2.36 1.32** (1.64)
1.61 3.68** (2.07)
1.95 4.02** (2.35)
−2.83 0.76 (2.07)
−3.50 1.43* (2.35)
−4.34 2.27** (2.40)
−1.83 1.02* (2.19)
−1.13 1.72** (2.24)
−3.15 0.30 (1.38)
−4.33 1.48** (1.65)
−4.15 1.30* (2.48)
0.85
0.66
3.18** (2.20)
2.99** (2.61)
−4.03
1.70** (1.72)
−4.65
2.32** (2.43)
−4.39
2.06** (2.74)
For LBE, deviations to the right of the true midpoint were assigned a (+) value, whereas deviations to the left were given (−) values. The way of computing the Exp. LBE is reported in the Procedure section. For Exp. LBE, positive values indicate that the subjective midpoint in the experimental condition was located farther than that in the control condition, relatively to the dot position * Significance levels of departure from zero are shown for experimental LBEs (* p
