On Earth, humans need to have a sense of verticality. In sensorimotor terms, our upright, bipedal, postural stance is mediated by vestibular, somesthetic and visual inputs that serve as indicators of any deviation from the vertical. On the cognitive level, our vertical perception defines a gravitational reference frame, which subserves the coding of the location and orientation of objects in the environment independently of the observer's own orientation. Consequently, the subjective vertical (SV, i.e. the subjective estimation of gravitational direction) is commonly considered as an indicator of the sense of orientation. The SV is measured by asking the observer to align a light bar with the direction of gravity (i.e. the subjective visual vertical, SVV). Other modalities (such as the haptic modality) have been also used (e.g. [1
The SVV arises from the complex integration of inputs from vestibular, visual, proprioceptive and tactile receptors. It has been clearly established that subcortical structures are involved in the vestibular contribution to oculomotor control (vestibulo-oculomotor reflexes) and postural control (vestibulo-collic and vestibulo-spinal reflexes) (for a review, see [10
]). However, it is not known precisely how and where in the cortex the vestibular information on spatial cognition (and on the sense of verticality, in particular) is processed. Otoliths and semicircular canals give rise to vestibular inputs, which run from the eighth nerve to the vestibular nuclei at the pontine level. Studies in monkeys [11
] have shown that after thalamic projection, the signals reach (directly or indirectly) several areas of the vestibular cortex (areas 2v, 3a and 7, in particular), the parieto-insular vestibular cortex (PIVC) and the ventral intraparietal area. Functional MRI studies [12
] have revealed that similar areas are involved in humans (particularly the PIVC and temporal areas), with right-hemisphere dominance. According to Barra et al. [14
], the representation of verticality may depend on neural circuits that include thalamoparietal projections (for somesthetic graviception) and thalamo-insular projections (for vestibular graviception).
Lesions or impairment at any of the steps in vestibular information processing can induce pathological deviations of the SVV or the subjective visual horizontal. Indeed, this paradigm has frequently been used to detect acute, unilateral vestibular defects in disease settings [15
]. However, thalamic infarction [16
] and cortical damage (especially in parietal areas and, more specifically, damage to the PIVC) can also induce deviation of the SVV (for a review, see [17
]). For example, in the study by Brandt et al. [18
], contraversive tilts of the SVV were found in 33 out of 52 patients with brain damage in the PIVC area; ocular torsion was ruled out as a possible cause of the deviation. Most studies on stroke patients have reported an altered sense of verticality and a subjective vertical tilted towards the contralesional side - especially in patients suffering from hemineglect syndrome [19
]. Nowadays, a growing number of researchers use the SVV paradigm to investigate other diseases, such as paraplegia [14
] and psychiatric conditions [3
However, even healthy observers will suffer from non-negligible, biased accuracy if visual cues are not available and the head axis is no longer relevant (when roll-tilted, for example). Many literature reports show that (i) head tilts of up to 60° can give rise to contraversive displacement of the SVV (the Müller effect, also known as the E-effect) and (ii) greater tilt angles induce systematic deviations in the head tilt direction (the Aubert effect, also known as the A-effect). Nevertheless, a review of the literature on head-tilt effects as a function of the amplitude of head tilt reveals strong disagreements with respect to the specific values and conditions that yield A- or E-effects. For example, several researchers have found systematic A-effects with moderate head tilts of about 30° [2
], whereas others always observed an E-effect under similar conditions [26
]. Furthermore, some researchers report high, between-subject variability rather than an average, systematic effect of head tilt [28
]. Table provides a summary of these various experiments and their main results.
Results obtained in different studies on the Subjective Visual Vertical (SVV) or Subjective Visual Horizontal (SVH).
In fact, if we consider moderate tilts and normal subjects, two main factors appear to vary strongly from one study to another (see Table ): the angle size and the participants' gender (when this information is mentioned). In experiments showing a systematic A-effect [2
], very large stimuli were used (from 19.85° to 32.44°). Although the effect of the stimulus angle size was tested some time ago by Wade [26
], it merits renewed investigation. In fact, Wade found that a large angle size could diminish the E-effect but did not find an A-effect - even with a large angle size (28.52°). Unfortunately, data concerning other aspects of study design (such as whether participants of both genders or just one gender were included) were not specified.
Several spatial tasks show gender effects, although the latter are not completely understood. Anatomical explanations have been proposed [31
]. The otolithic organs (i.e. the utricle and saccule) and the superior semicircular canals appear to be larger in men than in women [33
]; this may explain (at least in part) gender-related differences in vestibular information processing (see [31
]). Since the pioneering work by Asch and Witkin [34
], it has been well documented that women are usually more affected by visual, contextual cues (such as those used as in the rod-and-frame test and the water-level task, for example [35
]). Gender effects are also frequently mentioned in spatial attention tasks, with poorer performance levels by women (although a correlation with the functional differences revealed by fMRI has not been found [37
]). In navigation tasks, significant differences between men and women have also been evidenced [38
]. However, gender differences in spatial tasks do not appear to be limited to paradigms involving visual contextual cues, since Tremblay et al. [31
] found gender differences in judgment of the morphological horizon in different body orientations. A few studies have sought to identify a gender difference for the SVV during head-tilt (and in the absence of visual context) [36
] but failed to do so.
In order to explain some of discrepancies concerning head-tilt effects for the SVV, we decided to investigate the possible impact of methodological factors (the line's angle size and the participant's gender) with a moderate (30°) head tilt. This question is important in view of the growing body of research using the SVV paradigm in various disease settings. Forty healthy participants (20 men and 20 women) were asked to make visual vertical adjustments of a light bar with their head positioned vertically or roll-tilted by 30° to the left or the right. Line angle sizes of 0.95° and 18.92° were presented.