Abstract
The orientation of the body in space can influence perception of verticality leading sometimes to biases consistent with priors peaked at the most common head and body orientation, that is upright. In this study, we investigate haptic perception of verticality in sighted individuals and early and late blind adults when tilted counterclockwise in the roll plane. Participants were asked to perform a stimulus orientation discrimination task with their body tilted to their left ear side 90° relative to gravity. Stimuli were presented by using a motorized haptic bar. In order to test whether different reference frames relative to the head influenced perception of verticality, we varied the position of the stimulus on the body longitudinal axis. Depending on the stimulus position sighted participants tended to have biases away or toward their body tilt. Visually impaired individuals instead show a different pattern of verticality estimations. A bias toward head and body tilt (i.e., Aubert effect) was observed in late blind individuals. Interestingly, no strong biases were observed in early blind individuals. Overall, these results posit visual sensory information to be fundamental in influencing the haptic readout of proprioceptive and vestibular information about body orientation relative to gravity. The acquisition of an idiotropic vector signaling the upright might take place through vision during development. Regarding early blind individuals, independent spatial navigation experience likely enhanced by echolocation behavior might have a role in such acquisition. In the case of participants with late onset blindness, early experience of vision might lead them to anchor their visually acquired priors to the haptic modality with no disambiguation between head and body references as observed in sighted individuals (Fraser et al., 2015). With our study, we aim to investigate haptic perception of gravity direction in unusual body tilts when vision is absent due to visual impairment. Insofar, our findings throw light on the influence of proprioceptive/vestibular sensory information on haptic perceived verticality in blind individuals showing how this phenomenon is affected by visual experience.
Highlights
Perceiving the direction of gravity, i.e., verticality, is of great importance in building references that are used to maintain balance and move through space
The shift of the psychometric function is a measure of the bias in perceiving verticality and it is indicated by any deviations from 0 of the point of subjective equality (PSE)
We investigated haptic perception of verticality in sighted and visually impaired individuals when tilted counterclockwise
Summary
Perceiving the direction of gravity, i.e., verticality, is of great importance in building references that are used to maintain balance and move through space. Already in 1861, Aubert discovered a systematic bias in estimating verticality when tilted on a side (Aubert, 1861): the so called A-effect shows that in these conditions, estimation of verticality is tilted toward body tilt This phenomenon has often been interpreted as an expression of body tilt underestimation due to the influence of an idiotropic vector signaling the upright position (Mittelstaedt, 1983; De Vrijer et al, 2008, 2009) that in Bayesian terms could be modeled as a prior set at the most common body orientation relative to gravity, that is 0◦ on the roll plane (MacNeilage et al, 2007). Functional perception of verticality is important in order to maintain postural stability as it signals the direction of gravity when standing upright. Further studies observed that perceived verticality switches from A- to E-effects when the tilt overcomes 135–150◦ (Kaptein and Van Gisbergen, 2004; Kaptein et al, 2005) and this effect is related to a change in precision that likely depends on the effectiveness of the otolith organs in transducing head orientation relative to gravity (Tarnutzer et al, 2009)
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