Jean-Claude Lepecq

Optic flow sensitivity in neonates

The present research investigates neonatal sensitivity to optic flow. Twenty five 3-day-old infants were placed inside a dark room and observed while presented with a 10 s bilateral and backward peripheral optic motion. Seven constant flow velocity conditions were used (2.5, 5.0, 10.0, 15.0, 20.0, 25.0 and 30.0 degrees per second) and were compared to a baseline motionless condition. Sagittal deviation of the head was computer-sampled at 60 Hz frequency with pressure transducers. As indicated by the mean head pressure during optic flow exposure, infants reacted with backward leaning of the head whose magnitude was linearly related to the optic flow velocity. Additionally, isolated head postural responses were identified. The magnitude of these responses was clearly related to the optic flow velocity.

COGNITIVE EFFECTS ON VISUALLY INDUCED BODY MOTION IN CHILDREN

Cognitive effects on linear sagittal vection in children were investigated. Forty children (7 and 11 years old) were exposed to a bilateral backward optical flow in a single physical condition (seated in a stationary armchair) but in two contrasted cognitive conditions. In one cognitive condition, the children were precisely informed that the armchair could move. In the other, they were informed that the armchair could not move. In each age group, half the children were assigned to one cognitive condition, the other half to the other condition. The results indicate that knowledge about the plausibility of a physical displacement does not affect the probability of obtaining vection. However, at both ages, the latencies for reporting vection were shorter when the physical displacement was known to be possible than when it was known to be impossible. The present results indicate that exclusively cognitive factors do not affect vection occurrence but can modulate latencies for reporting vection.

The effect of linear vection on manual aiming at memorized directions of stationary targets.

Stationary observers were required to aim manually at either a straight-ahead, or a lateral, immobile target (0° and 20° of eccentricity, respectively). First, they aimed at a perceptually present target. Second, they aimed at the memorized direction of the target. Third, they aimed at the memorized direction of the target after a 4 s period of forward sagittal vection. The comparisons of the post-vection versus ante-vection aimings at the memorized direction of the targets reflect an illusory increase in the eccentricity of the lateral target, and no change in the direction of the straight-ahead target. These results objectivate the idea that forward sagittal vection (a particular case of linear vection) corresponds to an illusion of self-displacement, ie an illusory change of body place through space.

Afforded actions as a behavioral assessment of physical presence in virtual environments

A particular affordance was used as a potential candidate for behavioral assessment of physical presence in virtual environments. The subjects’ task was to walk through a virtual aperture of variable widths. In the case of presence, the subjects’ body orientation, while walking, was hypothesized to be adapted to the width of the aperture and to their own shoulder width. Results show that most subjects adapted their behavior to both their body architecture and the virtual width constraints. These subjects exhibited a behavioral transition from frontal walking to body rotation while walking through broad to narrow apertures. The same behavioral transition has already been documented in real environments (Warren and Whang in J Exp Psychol Human Percept Perform 13(3):371–383, 1987). This behavioral adjustment is thus assumed to be an objective indication of presence. Beyond these results, the present study suggests that every afforded action could be a potential tool for sensorimotor assessment of physical presence.

Interaction between Reference Frames during Subjective Vertical Estimates in a Tilted Immersive Virtual Environment

Numerous studies highlighted the influence of a tilted visual frame on the perception of the visual vertical (‘rod-and-frame effect’ or RFE). Here, we investigated whether this influence can be modified in a virtual immersive environment (CAVE-like) by the structure of the visual scene and by the adjustment mode allowing visual or visuo-kinaesthetic control (V and VK mode, respectively). The way this influence might dynamically evolve throughout the adjustment was also investigated in two groups of subjects with the head unrestrained or restrained upright. RFE observed in the immersive environment was qualitatively comparable to that obtained in a real display (portable rod-and-frame test; Oltman 1968, Perceptual and Motor Skills 26 503–506). Moreover, RFE in the immersive environment appeared significantly influenced by the structure of the visual scene and by the adjustment mode: the more geometrical and meaningful 3-D features the visual scene contained, the greater the RFE. The RFE was also greater when the subjective vertical was assessed under visual control only, as compared to visuo-kinaesthetic control. Furthermore, the results showed a significant RFE increase throughout the adjustment, indicating that the influence of the visual scene upon subjective vertical might dynamically evolve over time. The latter effect was more pronounced for structured visual scenes and under visuo-kinaesthetic control. On the other hand, no difference was observed between the two groups of subjects having the head restrained or unrestrained. These results are discussed in terms of dynamic combination between coexisting reference frames for spatial orientation.

Early perceptuo-motor development: posture and locomotion

After a brief review of the neuronal organization involved in postural control, this chapter looks at the neonatal postural repertoire and its development during the first two years of life. Then classical maturationist approaches are challenged by recent research on the effects of biomechanical constraints and intersensory integration on posturo-locomotor development. Finally, a wider framework than a purely maturationist one is proposed.

Does visually induced self-motion affect grip force when holding an object?

Accurate control of grip force during object manipulation is necessary to prevent the object from slipping, especially to compensate for the action of gravitational and inertial forces resulting from hand/object motion. The goal of the current study was to assess whether the control of grip force was influenced by visually induced self-motion (i.e., vection), which would normally be accompanied by changes in object load. The main task involved holding a 400-g object between the thumb and the index finger while being seated within a virtual immersive environment that simulated the vertical motion of an elevator across floors. Different visual motions were tested, including oscillatory (0.21 Hz) and constant-speed displacements of the virtual scene. Different arm-loading conditions were also tested: with or without the hand-held object and with or without oscillatory arm motion (0.9 Hz). At the perceptual level, ratings from participants showed that both oscillatory and constant-speed motion of the elevator rapidly induced a long-lasting sensation of self-motion. At the sensorimotor level, vection compellingness altered arm movement control. Spectral analyses revealed that arm motion was entrained by the oscillatory motion of the elevator. However, we found no evidence that grip force used to hold the object was visually affected. Specifically, spectral analyses revealed no component in grip force that would mirror the virtual change in object load associated with the oscillatory motion of the elevator, thereby allowing the grip-to-load force coupling to remain unaffected. Altogether, our findings show that the neural mechanisms underlying vection interfere with arm movement control but do not interfere with the delicate modulation of grip force. More generally, those results provide evidence that the strength of the coupling between the sensorimotor system and the perceptual level can be modulated depending on the effector.

Evidence of imagined passive self-motion through imagery–perception interaction

The existence of whole-body passive self-motion mental imagery was investigated by examining whether the perception of passive body accelerations can be affected by passive self-motion imagery. Twenty healthy subjects recognised target passive body acceleration. This recognition task was performed under three conditions: (1) a baseline condition without imagery; (2) a compatible imagery condition during which subjects imagined themselves passively moving in the same direction as the target acceleration; (3) a non-compatible imagery condition during which subjects imagined themselves passively moving in the direction opposite to that of the target acceleration. The recognition of the target acceleration was improved under compatible and degraded under non-compatible imagery. This interaction implies that perception and imaginary share common representations, and supports the existence of passive self-motion imagery.

Optical and electronic systems for spatial and temporal analysis of video images

In this paper are described two systems for spatial (two-dimensional coordinates) and temporal analysis of video images. In the optical version, the Apple IIe generates a pointer on an Apple monitor; the pointer is then superimposed on the video image of a second video monitor, by reflecting the pointer with a half-silvered mirror. In the electronic version, a special card was designed for the Apple IIe to generate a pointer directly on the other video monitor. Both systems use the same software. The procedures of spatial and temporal data acquisition and storage are described as well as the reading, writing, printing, removing, listing, chaining, and dating operations of the digital and graphics files.

Chapter 9 Frames of Reference Underlying Early Movement Coordination

Abstract The aim of this chapter is to reconsider the early development of sensorimotor coordination from a “goal-oriented” point of view. Firstly, the limitations of some classical approaches about the development of coordination will be briefly recapitulated. Secondly, the theoretical tools provided by “goal-oriented” approaches will be presented. Finally, these tools will be used to analyse examples of infant behavior in terms of coordination. From this analysis, it appears that “goal-oriented” approaches provide powerful theoretical tools on the basis of which new integrated conceptions of early sensorimotor coordination can be grounded.