Gaze–grasp coordination in obstacle avoidance: differences between binocular and monocular viewing

Abstract

This is the accepted version of the paper.This version of the publication may differ from the final published version. Permanent repository link AbstractMost adults can skillfully avoid potential obstacles when acting in everyday cluttered scenes. We examined how gaze and hand movements are normally coordinated for obstacle avoidance and whether these are altered when binocular depth information is unavailable. Visual fixations and hand movement kinematics were simultaneously recorded while 13 right-handed subjects reached-toprecision grasp a cylindrical household object presented alone or with a potential obstacle (wine glass) located to its left (thumb's grasp-side), right or just behind it (both closer to the finger's grasp-side) using binocular or monocular vision. Gaze and hand movement strategies differed significantly by view and obstacle location. With binocular vision, initial fixations were near the target's centre of mass (COM) around the time of hand movement onset, but usually shifted to end just above the thumb's grasp-site at initial object contact, this mainly be made by the thumb, consistent with selecting this digit for guiding the grasp. This strategy was associated with faster binocular hand movements and improved end-point grip precision across all trials than with monocular viewing, during which subjects usually continued to fixate the target closer to its COM despite a similar prevalence of thumb-first contacts. While subjects looked directly at the obstacle at each location on a minority of trials and their overall fixations on the target were somewhat biased towards the grasp-side nearest to it, these gaze behaviours were particularly marked on monocular vision-obstacle behind trials which also commonly ended in finger-first contact. Subjects avoided colliding with the wine glass under both views when on the right (finger-side) of the workspace by producing slower and straighter reaches, with this and the behind obstacle location also resulting in 'safer' (i.e., narrower) peak grip apertures and longer deceleration times than when the goal-object was alone or the obstacle was on its thumb-side. But monocular reach paths were more variable and deceleration times were selectively prolonged on finger-side and behind obstacle trials, with this latter condition further resulting in selectively increased grip closure times and corrections. Binocular vision thus provided added advantages for collision avoidance, known to require intact dorsal cortical stream processing mechanisms, particularly when the target of the grasp and potential obstacle to it were fairly closely separated in depth. Different accounts of the altered monocular gaze behaviour converged on the conclusion that additional perceptual and/or attentional resources are likely engaged compared to when continuous binocular depth information is available. Implications for people lacking binocular stereopsis are briefly considered.

Gaze when reaching to grasp a glass

“…Thus, if the extent of the occlusion is an important factor, people might look near the index finger's contact point around the time of the grasp when the glass is full of water. When it is full of milk, we expect people to look near the thumb's contact point in accordance with earlier findings (Grant, 2015). When the glass is at hip height, the index finger's contact point is close to the occluding edge because the glass is largely seen from above, so we expect participants to fixate near their index finger's contact point irrespective of whether the glass is full of water or milk.…”

“…Either of these explanations is consistent with occlusion of the finger's contact point making one look closer to that contact point unless the index finger is completely occluded by the object, in which case constraints are invisible and extrapolation is impossible. This might explain why people seem to fixate near their thumb's contact point at the time of their grasp when the target object is at eye height and is grasped with the index finger on the far side of the object so that it completely occludes the last part of the index finger's path (Grant, 2015;Johansson et al, 2001). Changes in how the hand occludes parts of the object might also be one of the reasons why gaze differs between normal grasping at eye height and awkward, upside-down grasping of the same objects in order to turn them around (Belardinelli et al, 2016).…”

Fixation Biases towards the Index Finger in Almost-Natural Grasping

“…That they were looking at the contact points, rather than at the top of the object, is supported by the shifts in gaze when different grip orientations were adopted ( Fig 4 ). In the subsequent experiments we will examine several possible reasons for looking closer to the index finger’s contact point than to the thumb’s contact point in this experimental configuration (with the object at eye height gaze appears to be directed towards the thumb’s contact point [ 26 ]). Not finding a significant correlation between contact asynchrony and critical gaze bias is inconsistent with the idea that gaze is related to which digit is the first to make contact with the object [ 19 , 26 ].…”

“…It was proposed that the bias of looking towards the index finger’s contact point might be related to the index finger generally contacting the object earlier than the thumb [ 19 ]. In accordance with this proposal, people tend to look closer to their thumb’s contact point when the thumb is the first to make contact with the object, which is the case if the object that is to be grasped is at eye height so that the index finger’s contact point is behind the object, well out of sight [ 26 ]. However, there did not appear to be any correlation between which digit first touched the object and where people directed their gaze in an earlier study in which both contact points were visible [ 18 ].…”

“…For instance, the way the object is removed from such a frontal surface might result in a need to fixate near the index finger’s contact point. In some more natural grasping situations people look closer to their thumb’s contact point [ 10 , 26 ], perhaps because this is the only visible contact point since the index finger’s contact point is behind the object, and therefore occluded by the object itself. We therefore decided to re-examine where people look when grasping under natural grasping conditions.…”