RESEARCH ARTICLE Dissociation between vergence and binocular disparity cues in the control of prehension Dean R. Melmoth Æ Mithu Storoni Æ Georgina Todd Æ Alison L. Finlay Æ Simon Grant Received: 16 June 2006 / Accepted: 20 June 2007 / Published online: 31 July 2007 Ó Springer-Verlag 2007 Abstract Binocular vision provides important advanta- ges for controlling reach-to-grasp movements. We exam- ined the possible source(s) of these advantages by comparing prehension proficiency under four different binocular viewing conditions, created by randomly placing a neutral lens (control), an eight dioptre prism (Base In or Base Out) or a low-power (2.00–3.75 dioptre) Plus lens over the eye opposite the moving limb. The Base In versus Base Out prisms were intended to selectively alter ver- gence-specified distance (VSD) information, such that the targets appeared beyond or closer than their actual physical position, respectively. The Plus lens was individually tailored to reduce each subject’s disparity sensitivity (to 400–800 arc s), while minimizing effects on distance estimation. In pre-testing, subjects pointed (without visual feedback) to mid-line targets at different distances, and produced the systematic directional errors expected of uncorrected movements programmed under each of the perturbed conditions. For the prehension tasks, subjects reached and precision grasped (with visual feedback available) cylindrical objects (two sizes and three loca- tions), either following a 3 s preview in which to plan their actions or immediately after the object became visible. Viewing condition markedly affected performance, but the planning time allowed did not. Participants made the most errors suggesting premature collision with the object (shortest ‘braking’ times after peak deceleration; fastest velocity and widest grip at initial contact) under Base In prism viewing, consistent with over-reaching movements programmed to transport the hand beyond the actual target due to its ‘further’ VSD. Conversely, they produced the longest terminal reaches and grip closure times, with multiple corrections just before and after object contact, under the Plus lens (reduced disparity) condition. Base Out prism performance was intermediate between these two, with significant increases in additional forward movements during the transport end-phase, indicative of initial under- reaching in response to the target’s ‘nearer’ VSD. Our findings suggest dissociations between the role of vergence and binocular disparity in natural prehension movements, with vergence contributing mainly to reach planning and high-grade disparity cues providing particular advantages for grasp-point selection during grip programming and application. Keywords Reaching  Grasping  Pointing  Stereopsis  Visuomotor behaviour  Human Introduction The proficient control of prehension requires visual com- putation of the precise three-dimensional (3D) properties of the goal object within the scene, combined with vision of the moving limb (see Jeannerod 1986). A major interest in prehension research is to determine whether specific types of visual information are required to mediate skilled per- formance from among the range of potential 3D-cues D. R. Melmoth  M. Storoni  G. Todd  A. L. Finlay  S. Grant (&) Department of Optometry and Visual Science, The Henry Wellcome Laboratories for Visual Sciences, City University, Northampton Square, London, EC1V 0HB, UK e-mail: s.grant@city.ac.uk Present Address: M. Storoni Department of Ophthalmology, Norfolk and Norwich University Hospital, Colney Lane, Norwich, NR4 7UY, UK 123 Exp Brain Res (2007) 183:283–298 DOI 10.1007/s00221-007-1041-x