Psychophysical Properties of the Trunk Midline
Giuseppe Spidalieri and
Roberto Sgolastra
Institute of Human Physiology, University of Ferrara, I-44100 Ferrara, Italy
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ABSTRACT |
Spidalieri, Giuseppe and Roberto Sgolastra. Psychophysical properties of the trunk midline. J. Neurophysiol. 78: 545-549, 1997. This study was carry out to obtain direct evidence that the body midline actually is perceived and to assess some psychophysical properties of this line. Twelve normal, right-handed male subjects were asked to make accurate pointing movements toward the midline of the anterior trunk on the basis of their mental representation of this line. Each hand was used to point while the head was either aligned with the trunk or tilted 30° to the right or left. Analysis of end-positions of pointing on trunk images acquired by an image analysis system indicated that the trunk midline indeed is perceived as a straight line. Three putative trunk midlines were taken into consideration on the basis of anatomic landmarks, and it was found that the mental representation of the trunk midline came nearest to the line orthogonal to the intermammary line crossing its midpoint. The performing hand and the position of the head relative to the trunk both had an effect on the mental representation of the trunk midline. These findings suggest that somatosensory signals from the trunk, as well as proprioceptive input from the neck, contribute to the elaboration of the subject's mental representation of the trunk midline.
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INTRODUCTION |
The evidence that the body midline is perceived is indirect and relies mainly on the ability of normal subjects accurately to point their hand straight ahead (i.e., in the direction where they feel their midsagittal plane would project in front of them) without the aid of any visual cues (for a review, see Jeannerod 1988
). It is known that the perception of the straight ahead direction is affected by factors such as head position in relation to the trunk and gaze orientation in the horizontal plane (Jeannerod 1988
; Jeannerod and Biguer 1989
; Porac and Coren 1986
; Werner et al. 1953
). However, the concept of body midline is hampered by the fact that the trunk is not perfectly symmetrical. Hence, which anatomic landmarks anchor body midline perception is still an open question.
The present experiments focused on getting direct evidence that the trunk midline indeed is perceived and at analyzing some of its psychophysical properties. To this aim, normal right-handed male subjects were asked to point accurately toward the anterior trunk midline with either hand on the basis of the mental representation of this line while their head was being held aligned with the trunk or laterally tilted 30° to the right or left. Three questions were asked: Is your mental representation of the trunk midline a straight line? Which anatomic landmarks does your mental representation of the trunk midline come nearest to? Is your mental representation of the trunk midline affected by which hand you use or by the position of your head in relation to your trunk?
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METHODS |
Twelve neurologically intact male subjects aged between 20 and 40 yr took part in the experiment after giving their informed consent. All of them proved to be strongly right-handed on the questionnaire of Briggs and Nebes (1975)
. None had had any fractures of the sternum, clavicles, ribs, or vertebrae nor had any of them undergone to thorax or vertebral column surgery. None had pathologic deviation of the vertebral column.
Each subject, bare-trunked and blindfolded, lay supine on an examination bed with his arms extended along both sides of his trunk and wearing a plastic helmet. His nipples and two points along the sagittal groove the two pectorales majores make on the anterior surface of the sternum were marked by white and green thick tempera paints, respectively (Fig. 1A).

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| FIG. 1.
A: regression line run for end-positions of 20 pointing movements made by a representative subject in reference condition (i.e., right-hand performance while head was aligned with trunk). Note white marks on nipples, green marks (which are black in the figure) on sternum, and 21-mm calibration bars. B: same image as in A in which intermammary line and 3 putative trunk midlines, taken into consideration to compute absolute and constant errors of pointing, were traced by means of image analysis software. Same software also was used to mark centers of nipples, umbilicus, and 2 green marks ( ) as well as midpoint of intermammary line ( ).
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The subject was given a plastic stylus and instructed to point, as accurately as possible, to four points on the anterior trunk midline solely on the basis of his mental representation of their position on the trunk. There was no time limit. The target points were as follows: target 2, crossing the line running between the nipples (intermammary line); target 4, midway between the intermammary line and the center of the umbilicus; target 3, midway between target 2 and target 4; target 1, cranially to the intermammary line, at a distance from target 2 equal to the distance between target 2 and target 3. The stylus was shaped like a blunt thumb tack, the head of which was 10 mm in diam, the round shaft 4 mm in diam. Attached orthogonally to the center of its head was a 13-mm-diam grip to be held between the thumb and the forefinger. The hemispherical tip of the shaft was 6 mm from the distal end of the two fingers. To mark end-positions of pointing on the trunk, each time the subject pointed the tip of the stylus was first dipped in a pad containing thick white tempera paint (Fig. 1).
The experiment was carried out in one session of six blocks. The hand used for pointing and head position were changed for each block. The head was set so that the head midline was either parallel to the trunk midline or tilted 30° to the right or left. Each block consisted of 20 pointing movements, 5 toward each target, so that each subject performed a total of 120 pointing movements. The intertrial interval was 15-25 s. The sequence of blocks was randomized among subjects following a Latin square design. During each block the subject was asked to point to the different targets in random order. To prevent the subject from using tactile signals arising from contact of the stylus, he was never asked to point to the same target twice in a row. Indeed, the variable end-positions for pointing movements made toward each target (Fig. 1) indicates that tactile signals did not play a crucial role. The experimenter indicated the target to be pointed at by saying its corresponding number. To verify that the subject fully understood the task, eight practice trials were run but without dipping the tip of the stylus in the paint. At the end of each block, a cross-shaped calibration mark (21 × 21 mm) was laid on the trunk and an image of the trunk was acquired (Fig. 1).
Images were captured by a black and white camera (Panasonic, CCTV Wv-BL204) connected to an image analysis system (Kontron Elektronix, Vidas 2.1). This system consisted of a 486DX33 personal computer, an image acquisition board (Kontron Elektronix, KAT386), two color monitors, a digitizing tablet (Digicad Plus) and an image analysis software (Vidas 2-E005A). The camera was oriented orthogonally to the bed surface and the lens (Cosmicar, CCTV 16 mm F1.5) was ~110 cm away from the anterior surface of the trunk. Images were acquired in the 1,024 × 1,024 pixel format. The above experimental conditions made it possible to analyze images with a spatial resolution of ~0.6 mm/pixel. On each image, the image analysis software was used to calculate the coordinates of the center of gravity of the traces left by the stylus tip (end-positions of pointing), the white marks on the nipples, the umbilicus, and the two green marks (Fig. 1) with respect to the screen coordinates. The image analysis system also was used on-line to position the subject's head before running each experimental block: the image of the head and trunk was visualized on the computer screen overlaid on a line grid; the head was positioned so that the line passing through the center of the glabella (previously marked by white paint) and the center of the fissure between the two upper central incisors (head midline), and the line passing through the two green marks either coincided or made a 30° angle to the right or left. Afterward, the helmet was mechanically fixed so that the head could not move.
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RESULTS |
Three approaches were used to answer the question: is one's mental representation of the trunk midline a straight line? Analysis was carried out on the coordinates of end-positions of pointing movements made by the right hand while the head was aligned with the trunk (hereafter referred to as reference condition). As first approach, linear regression analysis was run for individual data (Fig. 1A). Because, under the present experimental conditions, the slope of each regression line was nearly zero, regression analysis was run a second time for the coordinates of the end-positions computed in relation to x-y axes rotated so that the new abscissa made a 30° angle with the previous regression line. Rotating the axes in this manner was considered appropriate because the aim was to verify whether the data were fitted reliably by a linear equation rather than to establish a correlation between independent and dependent variables. After axis rotation, the individual correlation coefficients ranged from 0.970 to 0.996 (mean = 0.989) and P < 0.0001 was found in all cases.
As a second approach, linear and quadratic regression analyses were run for individual end-positions and the deviations from linear and quadratic regressions were submitted to analysis of variance (ANOVA). A significant difference between linear and quadratic regression deviations (P < 0.01) was found for one subject only. Finally, as a third approach, ANOVA was carried out on the deviations from the linear regression line run for pooled data from all subjects. No significant differences were found among the deviations computed from end-positions of pointing aimed at the four targets (F3,36 = 1.05; P = 0.38). Therefore, it was concluded that end-positions of the pointing movements directed to the four targets uniformly distribute around a straight line. In conclusion, the results obtained by all three approaches concur to indicate that the mental representation of the trunk midline does correspond to a straight line.
To determine which anatomic landmarks subjects refer to in building up the mental representation of the trunk midline, three putative midlines were taken into consideration (Fig. 1B): the line orthogonal to the intermammary line passing through its midpoint (P-IM line), the line passing through the midpoint of the intermammary line and the center of the umbilicus (IM-U line), and the line passing through the center of the sagittal groove the two pectorales majores make on the anterior surface of the sternum (MS line). Afterward, the individual mean absolute and constant errors of the pointing movements made under the reference condition relative to the above putative midlines were computed. In 10 subjects, the smallest mean absolute error (Table 1) was found with respect to the P-IM line and in the other 2 (TFH and MVN), it was not significantly different (paired t-test; significance level P = 0.01) from the mean absolute error toward the P-IM line. As to the mean constant error (Table 2), in 11 subjects, it was the smallest with respect to theP-IM line. In the one remaining subject (TFH), the smallest mean constant error was not significantly different from the mean constant error toward the P-IM line. It was concluded, therefore, that the mental representation of the trunk midline comes nearest to the P-IM line. Hence, hereafter, it will be referred to as the effective trunk midline.
To further strengthen this conclusion, the slopes of the individual regression lines run for data from the pointing movements made under the reference condition were compared with the slopes of the equations of the three individual putative midlines. It was found that the mean value of the differences between the slopes of the regression lines and the slopes of the effective trunk midline [0.006 ± 0.038; (mean ± SD)] was lower than the mean value of the differences between the slopes of the regression lines and the slopes of the IM-N (
0.015 ± 0.024) and MS (0.022 ± 0.027) lines. Statistical analysis (paired t-test) revealed that the differences between the first mean and the other two means approached significance (t11 = 2.15; P < 0.03 and t11 = 1.74; P < 0.05, respectively).
To ascertain whether the performing hand and/or head position play a role in building up the mental representation of the trunk midline, the constant errors of all subjects toward the effective trunk midline were analyzed with a 2 × 3 repeated measures ANOVA. The hand factor proved significant (F1,55 = 12.2; P < 0.01). As shown in Fig. 2A, both hands erred to the left of the effective trunk midline, although the mean constant error of the right hand (0.48 mm) was smaller than that of the left hand (2.66 mm). The head position factor was also significant (F2,55 = 8.5; P < 0.01). The mean constant error was smallest when the head was tilted to the right (0.31 mm) and gradually increased as it was moved leftward (head aligned with the trunk, 1.6 mm; head tilted to the left, 2.8 mm). However, only the difference between the two tilted-head conditions (t55 = 3.45; P < 0.01; unpaired t-test using the Bonferroni correction for repeated contrasts) proved significant. The hand/head position interaction did not reach significance (F2,55 = 0.9; P = 0.4). A comparison of the means (Fig. 2B) showed that end-positions of right hand movements (head tilted to the left,
0.68 mm; head aligned with the trunk,
0.02 mm; head tilted to the right, 2.14 mm) were closer to the effective trunk midline than were those of the left hand movements (head tilted to the left, 1.3 mm; head aligned with the trunk, 3.22 mm; head tilted to the right, 3.46 mm).

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| FIG. 2.
Mean constant pointing errors with respect to effective trunk midline pooled across subjects as a function of performing hand (A) and performing hand plus head position (B). Positive and negative errors indicate end-positions to left and to right of effective trunk midline, respectively. Error bars are SEs. RH, right hand; LH, left hand; R, head tilted 30° to right; C, head aligned with trunk; L, head tilted 30° to left.
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Analysis of variable errors of all subjects toward the effective trunk midline by 2 × 3 repeated measures ANOVA revealed that neither the hand factor (F1,55 = 3.32; P = 0.08) nor the head position factor (F2,55 = 0.23; P = 0.79) was significant.
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DISCUSSION |
Evidence has been given in RESULTS that the trunk midline actually is perceived and that its mental representation corresponds to a straight line approximating the line orthogonal to the intermammary line and crossing its midpoint. These findings raise the questions of what information is used and what neural substrates and mechanisms are involved in the processing that leads to perception of the trunk midline. If one considers the concept of trunk midline in Euclidean terms, this line coincides with the superficial contour of the trunk intersecting the midsagittal plane, i.e., it is the sole and median locus of points dividing the right side from the left side of the trunk. It follows that somatosensory information from the trunk must have a critical, although not exclusive, role. However, the above definition runs into two conceptual difficulties. The first concerns the very idea of the representation of an immaterial line within the brain; the second involves the mechanisms by which the one median line is represented within a bihemisphered brain. Combined electrophysiological and neuroanatomic investigations carried out in cats (Manzoni et al. 1980
) and monkeys (Conti et al. 1986
) showed that two zones can be distinguished in the trunk representation of the primary somatosensory cortex: an acallosal zone, overlapping with the representation of the lateral trunk, which is endowed with neurons having contralateral receptive fields, and a callosal zone, corresponding to the representation of the medial trunk, which is connected reciprocally with the contralateral homotopic zone and endowed with neurons having bilateral receptive fields straddling the midline. Experimental data in cats (Barbaresi et al. 1984
; Cheema et al. 1983
) and neurological findings in patients with hemianesthesia (DeJong 1979
; Jeannerod et al. 1984
) suggest that the ipsilateral representation is mediated by uncrossed projections. It has been hypothesized (Manzoni et al. 1989
) that callosally projecting neurons in the homotopic cortical zones devoted to the medial trunk may, through their two-way interhemispheric circuit, act as a coordinated unit representing the only median region of the periphery centered on the trunk midline.
Nevertheless attaining the perception of the trunk midline requires further processing. In fact, first the mechanisms responsible for building up the mental representation of the trunk midline have to take into account all sources of trunk asymmetry, such as the oblique disposition of the heart, the difference in muscle size between the two sides of the trunk, and incidental lateral deviations of the vertebral column. This property suggests that the mental representation of the trunk midline may provide an egocentric frame of reference with respect to which to structure the body schema (Frederiks 1985
) and may organize actions within both peripersonal and extrapersonal space (Karnath et al. 1991
; Yardley 1990
).
Second, the angular relations between the trunk and head midline affect the mental representation of the trunk midline. In normal subjects, it has been found that unilateral posterior neck muscle vibration induces an illusory contralateral displacement of a stationary light spot viewed in the dark as well as hand pointing errors in the same direction as the apparent spot displacement. Moreover, when the light spot has to be moved until it is perceived as lying in the midsagittal plane, it is displaced on the same side, relative to the objective midsagittal plane, as the vibrated muscles (Biguer et al. 1988
; see also Karnath et al. 1994
). Taylor and McCloskey (1991)
showed that unilateral posterior neck muscle vibration also can alter the perceived position of the head. This illusion is in the same direction as that of the visual illusion. In left-sided neglect patients, left posterior neck muscle vibration reduces neglect symptoms, and it has been hypothesized that this effect is based on a shift in the subjective spatial localization of the midsagittal plane in the direction ipsilateral to the vibrated muscles (Karnath 1994
). Like stretching, muscle vibration increases muscle spindle firing (Goodwin et al. 1972
). It may be speculated that, in the present experiments, increased firing of muscle spindles on one side of the neck was obtained by lateral head tilting and may be responsible for the displacement of the mental representation of the trunk midline. On the contrary, the role of the vestibular receptors should be negligible or, at most, modest because no head rotation was imposed to our subjects. It must be pointed out, however, that under the present experimental conditions, subjects were blindfolded and, therefore, they had no spatial reference as provided by the light spot in the above-mentioned muscle vibration experiments. This fact might explain why the subjective displacement of the trunk midline was toward the side opposite that of the stretched neck muscles in our experiments, whereas, the subjective displacement of the midsagittal plane was toward the side of the vibrated muscles in the muscle vibration experiments.
The fact that differences between the variable errors of the two hands were nonsignificant indicates that the left hand is not less accurate than the right hand. The leftward bias of end-positions of pointing by the left hand may be motor or perceptual/attentional in nature. If it were motor, it might be regarded as left-side underestimation. Indeed, leftward deviations in spatial tasks such as line bisection has been documented in left-handed subjects (Bradshaw et al. 1987
; Scarisbrick et al. 1987
). In the present experiments, however, subjects were asked to point toward the trunk midline and, therefore, if the motor hypothesis was correct, the leftward bias in left hand performance also might represent a case of physiological hypokinesia of the nondominant hand (see Heilman et al. 1987
). However, the available evidence does not enable us to rule out the perceptual/attentional hypothesis, i.e., that the leftward deviation of left hand pointing might be related to the superiority of the right hemisphere over the left in coordinate spatial functions (for reviews, see Hellige 1993
; Kosslyn et al. 1989
). If the right hemisphere was also superior in pointing toward the mental representation of the trunk midline, performing by the left hand would result in an overestimation of the left side of the trunk. In this case, one might refer to this effect found in normal subjects as pseudoneglect of the left side of the trunk, analogous to the way the results of tactile line-bisection tasks were interpreted (Bowers and Heilman 1980
).
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ACKNOWLEDGEMENTS |
This work was supported by grants from the Ministero dell'Università e della Ricerca Scientifica e Tecnologica.
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FOOTNOTES |
Address reprint requests to G. Spidalieri.
Received 24 December 1996; accepted in final form 24 March 1997 .
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