When two eyes are better than one in prehension: monocular viewing and end-point variance

Abstract

Previous research has suggested that binocular vision plays an important role in prehension. It has been shown that removing binocular vision affects (negatively) both the planning and on-line control of prehension. It has been suggested that the adverse impact of removing binocular vision is because monocular viewing results in an underestimation of target distance in visuomotor tasks. This suggestion is based on the observation that the kinematics of prehension are altered when viewing monocularly. We argue that it is not possible to draw unambiguous conclusions regarding the accuracy of distance perception from these data. In experiment 1, we found data that contradict the idea that a consistent visuomotor underestimation of target distance is an inevitable consequence of monocular viewing. Our data did show, however, that positional variance increases under monocular viewing. We provide an alternative explanation for the kinematic changes found when binocular vision is removed. Our account is based on the changes in movement kinematics that occur when end-point variance is altered following the removal of binocular vision. We suggest that the removal of binocular vision leads to greater perceptual uncertainty (e.g. less precise stimulus cues), resulting in changes in the kinematics of the movement (longer duration movements). Our alternative account reconciles some differences within the research literature. We conducted a series of experiments to explore further the issue of when binocular information is advantageous in prehension. Three subsequent experiments were employed which varied binocular/monocular viewing in selectively lit conditions. Experiment 2 explored the differences in prehension measured between monocular and binocular viewing in a full cue environment with a continuous view of the target object. Experiment 3 required participants to reach, under a monocular or binocular view, for a continuously visible self-illuminated target object in an otherwise dark room. In Experiment 3, the participant could neither see the target object nor the reaching hand following initiation of the prehension movement. Our results suggest that binocular vision contributes to prehension by providing additional information (cues) to the nervous system. These cues appear to be weighted differentially according to the particular constellation of stimulus cues available to the participants when reaching to grasp. One constant advantage of a binocular view appears to be the provision of on-line information regarding the position of the hand relative to the target. In reduced cue conditions (i.e. where a view of the target object is lost following initiation of the movement), binocular information regarding target location appears to be particularly useful in the initial programming of reach distance. Our results are a step towards establishing the specific contributions that binocular vision makes to the control of prehension.

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Notes

  1. 1.

    Terminal grip aperture refers to the grip aperture at the end of the transport component of prehension (as specified by velocity criterion, typically 2 cm/s). This is often larger than the width of the object. The final stage of prehension thus sometimes consists of a final ‘squeeze’ component at the end of which the fingers make contact with the object (the ‘final’ grip aperture is equal in magnitude to the object width).

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Acknowledgements

The research was funded in part by a grant from the Natural Sciences and Engineering Research Council of Canada to M.A. Goodale and P. Servos.

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Correspondence to Mark Mon-Williams.

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Loftus, A., Servos, P., Goodale, M.A. et al. When two eyes are better than one in prehension: monocular viewing and end-point variance. Exp Brain Res 158, 317–327 (2004). https://doi.org/10.1007/s00221-004-1905-2

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Keywords

  • Binocular vision
  • Human
  • Motor control
  • Perception
  • Prehension
  • Stereopsis