|
Click here for PDF version
DIRECTION AND
RELIABILITY OF HEAD TILT IN HUMANS
Susan Putnam,
Michael Noonan, Claire Bellia
Canisius College, Buffalo, New York, USA
and Fred
H. Previc
Armstrong Laboratory, Brooks Air Force Base, Texas, USA
ABSTRACT
It has been
proposed that asymmetry in the inner ear underlies various
manifestations of brain-behavior asymmetry in the human.
Specifically, Previc (1991) argued that an otolith imbalance
manifests itself in an asymmetrical head posture, and later (1994)
suggested that head tilt may be consonant with other measures of
human laterality. The present study tested the reliability of head
tilt across days and assessed its relationship with handedness,
footedness, and eyedness. As in Previc’s earlier studies, a
majority of our subjects tilted rightward. Head tilt proved to be
highly stable across days but was not correlated with the other
laterality measures. These findings suggest that head tilt may
reflect an underlying asymmetric substrate which appears not to be
directly related to other measures of cerebral hemispheric
dominance.
INTRODUCTION
The direction in
which posture deviates from vertical when people are instructed to
stand erect has recently been offered as an important index of human
laterality. Previc (1991) proposed that postural imbalances arise
from an asymmetry of the otolithic pathways of the vestibular
system, with people characteristically leaning towards the side of
their weaker otolith organ (cf. Gresty et al, 1992).
In a subsequent
study, Previc (1994) examined human head tilt using measurements
performed on photographs of individual subjects. In that study, the
percentage of right-head-tilted individuals was found to be slightly
over twice that of the left-head-tilted group, indicating that this
index of laterality falls into the general pattern of
rightward-biased behaviors.
An important
next step in investigating this phenomenon is the determination of
the degree to which the direction of tilt is reliable within
subjects across days, i.e. to ask whether the direction of head tilt
is a stable characteristic of normal human subjects. Such
reliability tests are critical in assessing the stability of
directional behaviors when questioning whether such behaviors are
directionally consonant with others (Noonan & Axelrod, 1989). An
investigation of head-tilt reliability constituted the primary goal
of the present study.
Previc's (1991,
1994) arguments for a potential relationship between otolith
asymmetry and cerebral lateralization suggest that direction of head
tilt might be expected to be consonant with other indices of
laterality such as handedness and eyedness. Alternatively,
Geschwind (1975, 1985) has suggested that there may be at least two
independent clusters of lateral behavioral biases, one for the
distal bilateral appendages (i.e., handedness and footedness), the
other for the axial musculature (which would underlie asymmetries in
trunk flexion and turning biases). Within this view, one might
expect head tilt to fall into the latter category and be independent
of the handedness-eyedness cluster. Previc’s (1994) study has
already shed partial light on this issue, and seems to support the
latter point of view in that he found head tilt to be unrelated to
handedness. However, there was evidence of an indirect link between
head tilt and sighting dominance. A secondary goal of the present
study, therefore, was to provide further data relevant to this
question by obtaining handedness, footedness, and eyedness measures
along with head tilt in order to further assess the degree of
consonance among these measures.
METHOD
Subjects
The subjects who
completed this study were recruited from the students and staff of
Canisius College. Eighty-six individuals were originally
recruited. Nine subjects failed to return for their second
session. Upon debriefing, three revealed medical conditions which
may have influenced their posture or laterality and were
subsequently dropped from consideration. Five subjects were judged
by a “blind” observer to have noticeably turned (as well as tilted)
their heads despite instructions to face ahead, and those were
discarded. The remaining 69 subjects constituted the subject sample
for this investigation. They included 32 males and 37 females,
ranging in age from 18 years to 55 years (mean age = 21.8 years).
Volunteers were
not recruited solely on the basis of handedness, although
left-handers, when they were surreptitiously observed while engaged
in normal campus activities, were specifically targeted for
recruitment. Forty-seven subjects were right-handed; 17 were
left-handed, and five showed mixed-hand dominance (as later
defined). Neither general recruits nor left-handers were ever
informed that the study would focus on head tilt, nor was there any
mention of laterality. In an effort to keep the subjects blind to
the actual purpose of the study, they were told that they were
posing for pictures to be used later in a “computerized mug-shot
face recognition study”
Procedures
The initial
session lasted approximately five minutes. In it, two images (e.g.
see Fig. 1) of the head and shoulders of each subject were obtained
about one minute apart using a tripod-mounted video camera. Upon
initially entering the testing room, the subject was instructed to
stand in front of a grid, with shoes removed, and to look straight
ahead at the camera with both arms down at the side. The
experimenter stood out of the subject's view, observing events on a
video monitor on the opposite side of a screen that served to divide
the testing room into two halves. The experimenter counted to three
and clicked a dummy camera shutter as an audible signal to the
subject that an image had been captured. (In actuality, the entire
session was videotaped in real time.) After the first image had
been "captured", the subject was asked to step aside and write
his/her name on a list. The subject then returned to the original
position in front of the grid so that a second image could be
obtained in the same way.
Between two and
nine days later, in a second session, two additional video images
were obtained, using the same procedure. Because it has been
demonstrated that what may appear to be minor environmental
asymmetries can strongly influence the expression of some lateral
preferences (e.g., Noonan & Axelrod, 1981), we attempted to
counterbalance any possible influence of external asymmetries in the
testing room by having the camera and subjects positioned on the
opposite sides of the room during this second session.
After the two
video images were captured in the second session, each subject was
asked to perform a series of motor tasks which were designed to
assess sighting dominance, handedness, and footedness. (In an
attempt to continue to observe behavior that remained unaffected by
any conscious focus by the subjects on laterality, we told the
subjects that this next task sequence was being developed for an
upcoming study on children's motor development.)
Eyedness
Sighting
dominance was assessed in two ways. First, the subject looked
through the wide end of a small megaphone with both eyes open,
aligning the small end so that the experimenter's nose could be
seen. The eye of the subject visible to the experimenter through
the megaphone was recorded as dominant. Each subject was then asked
to read letters visible inside a monocular slide viewer, and the eye
used by the subject was recorded. Use of the left eye was coded as
-1, and use of the right eye was coded as +1. The average eyedness
of each subject was then calculated as the average of the two tests
(i.e., left = -1, mixed = 0, right = +1).
Handedness
First, the hand
used by the subject when asked to sign in was noted. Each subject
was also asked to throw three bean bags into a box and the throwing
hand was noted. Again, left was coded as -1 and right as +1, and
average handedness was calculated as the mean of the two scores.
Footedness
Footedness was
assessed by two tests. In the first, the subject kicked three bean
bags, and the foot used was noted. Second, each subject was
asked to stand on a tape line and to get into position as if about
to run a race, and the foot placed in back was observed. Once
again, the left foot was coded as -1 and the right as +1, and the
average of the two tasks was used for analysis.
Assessment of
Head Tilt
A laboratory
assistant, blind to the other laterality assessments, was
responsible for capturing four images (digitized video frames) for
each subject (two from each session) from the video tapes. The
moment of frame capture was synchronized with the audible click of
the dummy shutter on the sound track of the video tape. Each of the
four images was then saved on the computer in its normal orientation
and also in its mirror-reversed orientation (yielding eight images
per subject). The mirror-reversals were carried out to guard
against, and counterbalance, any asymmetries inherent in our
processing of the images -- due to handedness of experimenter, etc.
Using the Image
Pro computer program, one of us (CB), while remaining blind to the
subjects' laterality measures, determined the direction and extent
of head tilt for each image by obtaining the deviation of a line
joining the outer canthus of each eye (the lateral junctions of the
upper and lower eyelids, see Fig 1) from a horizontal grid line
behind the subject. This measurement was expressed in angular
degrees from the horizontal, with leftward tilt in negative numbers
and rightward tilt in positive numbers. Each subject's head tilt
for any image was computed as the average of the angular value
obtained on the two measures of the same image (normal orientation
and mirror reversal -- with the mathematical sign for the
mirror-reversed image inverted, of course). Each subject's average
head tilt was computed as the mean of all eight angular values
(again, with the sign inverted for the reversed images).
In order to
assess inter-rater reliability, eight other blind assistants also
assessed head tilt in the same way for one set of the eight images
per subject. To assess comparability across laboratories, one of us
(FP) independently assessed the head tilt on one printed image of
each of our subjects, using the facial landmarks and procedures
described in his earlier report (Previc, 1994).
Figure One:
Example of subject, in this case tilting 5 degrees leftward.
RESULTS
The head tilt
measurements obtained on the same images in normal and
mirror-reversed orientation were very close to one another (the mean
deviation was only 0.33 deg; range .00 to 2.97 deg), and highly
correlated (mean Pearson-r value .987; range .978 to .994).
There were also very high correlations for all ratings of the same
images by different individuals (that is, between those values
obtained by CB and those obtained by the eight independent blind
research assistants). The Pearson-r statistics comparing the
eight sets of independently obtained angular values with the values
obtained by CB for the same eight image sets ranged between .979 and
.991. Additionally, there was a very high (r=.974)
correlation between those angles determined independently by FP in
Texas when compared to either assessment of the same images at
Canisius in New York.
The sample-wide
mean head tilt was 0.36 deg, i.e. slightly rightward. The largest
left-leaning angle observed in any one image was -10.0 deg and the
largest right-leaning angle was +9.5 deg. Forty-one (59.4%) of the
subjects were right-head-tilted and 28 (40.6%) were left-head-tilted
(X2 (1) = 2.4; p = .12). Thirty subjects exhibited
averaged head tilt scores leaning rightward by more than one
degree. Nineteen leaned leftward on average by more than one
degree. The average head tilt for the remaining 20 subjects was
within one degree of the horizontal. There was no significant
difference in the mean head tilts for females as compared to males,
F(1)=0.490.
Substantial
reliability in measured head tilt was found both within and between
days. For the two images obtained moments apart on the same day,
the Pearson’s r was .832 for the first session and .735 for
the second session (both p < .001). For the average of the
two angles from the first session compared to the average of the two
angles derived from the second session, the Pearson’s r was
.820 (p<.001). The direction (left or right) of the mean
tilt in the first session was the same as that obtained in the
second session for 57 (82.6%) of our subjects. The mean difference
between the average head tilt from the first session and the average
head tilt from the second session was only 0.44 deg, and this
difference was not influenced by the number of days intervening
between the sessions (r = -.06).
Not
surprisingly, there was a high point bi-serial correlation between
the results of our two handedness measures (writing hand and
throwing hand): r = .836, p < .001, and between our
two eyedness measures (megaphone eye and viewer eye): r =
.731, p <.001. Our two footedness measures were not
significantly related however: r = .082, ns. There was a
significant correlation between our subjects’ average handedness
scores and their average eyedness scores: r = .458, p
<.001, and between average handedness and the foot used for
kicking: r = .733, p < .001. The foot placed
rearward in the run-a-race position test was not significantly
related to handedness or eyedness.
Direction of
head tilt was not found to be significantly correlated with average
handedness either before (r = -.099) or after (r =
-.085) the five mixed-handed subjects were excluded. Neither was
there a significant relationship between average footedness and head
tilt (r=.052), nor between average eyedness and head tilt (r=.045).
Correlations between average head tilt and the measures on each
laterality subtest (i.e., writing hand, throwing hand, megaphone
eye, etc.) were, in all cases, negligible and nonsignificant. An
analysis of variance (ANOVA) on average head tilt using average
handedness, average footedness, average eyedness, and sex as factors
revealed no significant relationship among the variables, whether
the mixed-dominance subjects were included or excluded. There were
likewise no significant Chi Square relationships found when average
head tilt was taken as a dichotomous variable (left or right) and
compared separately, or collectively, (using the BMDP4F computer
program) with similarly dichotomized versions of the handedness,
footedness, and eyedness variables.
DISCUSSION
Clearly, head
tilt is a reliable laterality index, as well as one that is easy to
measure with high inter-rater reliability. Subjects
characteristically exhibited consistent head tilt between images
taken within the same videotaping session, and also between images
taken across days, and this was not affected by the interval between
sessions. As Previc (1991) proposed, head tilt appears to be a
consistent internally mediated postural asymmetry, with a rightward
bias in the adult human population. We found no sex differences in
degree of head tilt which contrasts to an earlier report by Ragan
(1982).
In our study,
the direction of handedness, footedness and eyedness were all
partially correlated with each other, a finding which corresponds to
evidence from numerous other studies (e.g., Coren & Porac, 1978).
Such consonance presumably reflects a common hemispheric-dominance
underlying these behavioral biases. The fact that, in the present
study, head tilt was itself stable yet not significantly correlated
with the other indices of laterality suggests the presence of an
independent underlying neural mechanism. This can be viewed as
compatible with the view advanced by Geschwind (1975, 1985) that
motor biases involving the axial musculature are regulated
independently from handedness and eyedness. It will be interesting
in future studies to see if head tilt is consonant with other biases
of the trunk, such as turning biases. In work on rodent laterality,
there is evidence for separate underlying neural substrates for
various directional preferences (cf. Glick & Shapiro, 1985; Robinson
et al., 1985; Noonan & Axelrod 1989). It should not be surprising
if a similar arrangement is ultimately recognized in humans as well.
Alternative
strategies for measuring head tilt under different circumstances are
possible, and some have already been employed in other
investigations (Greenberg, 1960; Peters, 1983; Previc, 1994). It is
possible (indeed likely) that head posture varies with the subject’s
physical environment (e.g., with or without visual input), task
(e.g., standing vs writing), and social/emotional circumstances (cf.
Ragan, 1982). It will be important in future studies to further
compare the angles of tilt assessed by different measures, and to
similarly assess their association with handedness and eyedness.
Our data cannot
address whether or not the direction of head tilt arises from
asymmetrical functioning of the vestibular organ as hypothesized
(Previc, 1991; Gresty et al., 1992; Curthoys & Halmagyi, 1995). It
remains to conduct future investigations in which the proposed
relationship between head tilt and otolith imbalance is more
directly tested, as well as the hypothesized connection between
otolith imbalance and other manifestations of cerebral
lateralization.
REFERENCES
COREN, S., &
PORAC, C. (1978). The validity and reliability of self-report items
for measurement of lateral preference. British Journal of
Psychology, 69, 207-211.
CURTHOYS, I. &
HALMAGYI, M. (1995) Vestibular compensation: A review of the
oculomotor, neural, and clinical consequences of unilateral
vestibular loss. Journal of Vestibular Research, 5, 67-107.
GESCHWIND, N.
(1975) The apraxias: Neural mechanisms of disorders of learned
movement. American Scientist, 63, 188-195.
GESCHWIND, N.
(1985) Implications for evolution, genetics and clinical
syndromes. In S. D. Glick (Ed.), Cerebral Lateralization in
Nonhuman Species (pp.247-278). Orlando, FL: Academic Press.
GREENBERG, G.
(1960). Eye-dominance and head-tilt. American Journal of
Psychology, 73, 149-151.
GRESTY, M.A.,
BRONSTEIN, A.M., BRANDT, T. & DIETERICH, M. (1992). Neurology of
otolith function. Peripheral and central disorders. Brain, 115,
647-673.
GLICK, S.D., &
SHAPIRO, R. M. (1985). Functional and neurochemical mechanisms of
cerebral lateralization in rats. In S. D. Glick (Ed.), Cerebral
Lateralization in Nonhuman Species (pp.157-183). Orlando, FL:
Academic Press.
NOONAN, M. &
AXELROD, S. (1981). Earedness (Ear choice in monaural
tasks): Its measurement and relationship to other lateral
preferences. The Journal of Auditory Research, 21,
263-277.
NOONAN, M. &
AXELROD, S. (1989). The stability and intertest consonance of
lateral postural-motor biases in rats: results and implications.
Behavioral and Neural Biology, 52, 386-405.
PETERS, M.
(1983) Inverted and noninverted lefthanders compared on the basis
of motor performance and measures related to the act of writing.
Australian Journal of Psychology, 35, 405-416.
PREVIC, F.H.
(1991). A general theory concerning the prenatal origins of
cerebral lateralization in humans. Psychological Review, 98,
299-334.
PREVIC, F.H.
(1994). The relationship between eye dominance and head tilt in
humans. Neuropsychologia, 32, 1297-1303.
RAGAN, J. M.
(1982) Gender displays in portrait photographs. Sex Roles,
8, 33- 43.
ROBINSON, T. E.,
BECKER, J. B., CAMP, D. M., & MANSOUR, A. (1985). Variation in the
pattern of behavioral and brain asymmetries due to sex differences.
In S. D. Glick (Ed.), Cerebral lateralization in nonhuman
species (pp. 185-231). Orlando, FL: Academic Press.
|
