Abstract
Our research aimed to evaluate whether size perception and saccadic amplitude were affected before and after grasping a variable-size target under the presence (TF) or absence of tactile feedback (NoTF). Twenty-one volunteers (mean-age: 25.05±4.08years) participated. Volunteers were tested in four blocks of 30 trials. Each trial consisted of 3 consecutive phases: pre-grasping estimation, grasping and post-grasping estimation. During pre- and post-, participants were asked to perform a saccadic eye movement to a horizontal bar and manually indicate its horizontal size by extending index and thumb fingers. In the grasping phase, participants were instructed to make a grasping action towards the bar. While grasping, 30% of the trials had a random horizontal size perturbation, which meant a lengthening or shortening of 33% of the original size. In sixty trials, grasping action was assisted by TF implemented through mobile phone vibration motors. A motion caption system recorded and assessed participant’s hand position. Participant’s gaze was recorded by a mobile eye-tracker. During the post-estimation phase, the non-perturbation condition revealed an inverse relationship between saccadic amplitude and bar size (r=-0.69 and P=0.03, for both NoTF and TF). However, the same indirect relationship during shortening (NoTF: r=-0.17 and P=0.66; TF: r=-0.56 and P=0.15) and lengthening (NoTF: r=-0.46 and P=0.25; TF: r=-0.24 and P=0.50) conditions was not found. Grip aperture showed a significant correlation in all post-phase conditions for both NoTF and TF (r>0.93 and P<0.001). Comparing pre- and post-phase values, both NoTF and TF evidenced that lengthening condition caused a significant reduction in grip aperture (p=0.02 and p=0.04, respectively), whereas shortening did not reveal significant changes (P=0.74 and P=0.53). A significant increase in saccadic amplitude after shortening condition in TF was observed (P<0.001), but not in NoTF (P=0.22). Horizontal target size perturbation and tactile feedback during grasping execution modifies size perception and saccadic amplitude.