Perceived versus actual head-on-trunk orientation during arm movement control

Abstract

Static roll head tilt induces bias in the trajectory of upper limb voluntary movements. The aim of the experiment was to investigate whether this bias is dependant on the perception of body configuration rather than on its actual configuration. We used the 'return' phenomenon as a method to produce dissociation between perceived and actual head tilt. Static roll head tilt in supine subjects was sustained for 15 min during which subjects were periodically required to estimate verbally the tilt of their head respective to their trunk and draw, with their right index finger, straight lines aligned with their trunk. After 15 min, subjects' head were realigned with the trunk, and subjects continued to give verbal estimate of head position and perform the motor task. Results showed that the initial angular deviation of the lines in the direction opposite to head tilt gradually diminished. The adaptation was noticeable within the first 3-5 min of tilt and subsequently diminished. Verbal estimates confirmed the return phenomenon, i.e. subjects perceived their head as slowly returning towards its neutral position after a few minutes of sustained tilt. When realigned with the trunk, subjects experienced the illusion that their head was tilted in the opposite direction to the initial head tilt and a line deviation in the opposite direction to those made on initial exposure was observed (after-effect). These results indicate that the angular deviation in motor production observed in condition of static head tilt were largely related to the perceived body configuration and therefore favour the hypothesis that the conscious perception of body configuration plays a key role in organising sensorimotor tasks.

Visual-Shift Adaptation Is Composed of Separable Sensory and Task-Dependent Effects

“…This change is likely due to shifts in felt eye and head position (Lackner, 1973;Crawshaw and Craske, 1974). In traditional prism-based studies of adaptation, these shifts resulted in part from prolonged deviation of the eyes or head from midline during the exposure period (Ebenholtz, 1974;Paap and Ebenholtz, 1976;Mars et al, 1998;Guerraz et al, 2006). While the virtual feedback setup eliminates this particular source of adaptation, it is likely that other peripheral or early sensory factors play some role in the sensory recalibration we have observed.…”

The internal representation of head orientation differs for conscious perception and balance control

“…These procedures were used to maximize the elicited perceptual illusion of head return (Ivanenko & Grasso, ; Guerraz et al . ). The experiment consisted of participants indicating their perceived head‐turned posture and multiple trials of EVS as described above.…”

Postural reorientation does not cause the locomotor after-effect following rotary locomotion

“…Following prolonged rotation between body segments, after-effects concerning their perceived orientation have previously been reported (Gurfinkel et al 1989;Guerraz et al 2006;Mars et al 1998). Gurfinkel et al (1989) described a so-called return phenomenon in which subjects perceive a slow displacement of their head towards the neutral position after holding it in a rotated position for several minutes.…”

“…For example, a prolonged static head turn with eyes closed leads to a perceived gradual return of the head to the forward facing position (Gurfinkel et al 1989). If the head is then realigned with the body, it is perceived as oriented in the opposite direction to the original turn or tilt (Guerraz et al 2006;Mars et al 1998). It is therefore evident that a prolonged change in body segment orientation can modify their subsequent perceived relative orientation.…”