Undergraduate Honors Theses

Date of Award

Winter 4-18-2018




Dr. Patrick Brown


Normal grasping actions towards real objects are target-directed, mediated via real-time visuomotor control, and provide haptic feedback. Studies of visual form agnosic patient DF suggest that pantomime (pretend) grasps are different; they recruit the visual Ventral stream (inferior temporal cortex), while normal grasps recruit the visual Dorsal stream (posterior parietal cortex). This functional duality underlies the eponymous Two Visual Systems Hypothesis (TVSH). Critics of the TVSH emphasize the multimodal nature of sensory processing and propose a model, termed the Common Source Hypothesis (CSH), of a single more localized system. Existing studies of natural prehension during interleaved trials of normal and pantomime grasps are presented as supporting the CSH, as are reports that pantomime grasps are unsusceptible to knowledge of haptic feedback availability. However, these studies have methodological shortcomings that compromise their results. The current study replicated these experiments while eliminating those methodological shortcomings. Healthy participants performed grasping tasks involving cylinders presented to the participant using a mirror setup, while data on grasp kinematics were recorded. Normal and pantomime grasps were used to recruit the dorsal and ventral streams respectively. We found that when interleaving normal and pantomime grasps and controlling for knowledge of upcoming haptic feedback, pantomime grasps displayed the expected decrements in precision, supporting the TVSH. Additionally, pantomime grasps were susceptible to manipulation of knowledge whereas normal grasps were not, indicating a bifurcation of the visual system by degree of cognitive accessibility. These findings highlight the important role of cognition in mediating grasping actions when a participant knows there will not be haptic feedback on the upcoming grasp.