It has been proposed that the visual mechanisms that control well-calibrated actions, such as picking up a small object with a precision grip, are neurally distinct from those that mediate our perception of the object. Thus, grip aperture in such situations has been shown to be remarkably insensitive to many size-contrast illusions. But most of us have practiced such movements hundreds, if not thousands of times. What about less familiar and unpracticed movements? Perhaps they would be less likely to be controlled by specialized visuomotor mechanisms and would therefore be more sensitive to size-contrast illusions. To test this idea, we asked right-handed subjects to pick up small objects using either a normal precision grasp (thumb and index finger) or an awkward grasp (thumb and ring finger), in the context of the Ponzo illusion. Even though this size-contrast illusion had no effect on the scaling of the precision grasp, it did have a significant effect on the scaling of the awkward grasp. Nevertheless, after three consecutive days of practice, even the awkward grasp became resistant to the illusion. In a follow-up experiment, we found that awkward grasps with the left hand (in right handers) did not benefit from practice and remained sensitive to the illusion. We conclude that the skilled target-directed movements are controlled by visual mechanisms that are quite distinct from those controlling unskilled movements, and that these specialized visuomotor mechanisms may be lateralized to the left hemisphere.
"This indicator has quite practical advantages, such as the possibility of studying practice-related variations of interference throughout an experiment , which is barely possible with the block-wise manipulations inherent to the study of Garner interference. This will allow, for example, a new look at the currently debated issue of whether, for instance, unskilled actions are controlled by the dorsal visuomotor (Janczyk et al., 2010) or by the ventral perceptual system (Gonzalez et al., 2008). "
[Show abstract][Hide abstract] ABSTRACT: It has been suggested that the human brain processes visual information in different manners, depending on whether the information is used for perception or for action control. This distinction has been criticized for the lack of behavioral dissociations that unambiguously support the proposed two-visual-pathways model. Here we present a new and simple dissociation between vision for perception and vision for action: Perceptual judgments are affected by the similarity of relevant and irrelevant stimulus features, while object-oriented actions are not. This dissociation overcomes the methodological problems of previously proposed differences in terms of vulnerability to visual illusions or to variability in irrelevant object features, and it can also serve as an easily applicable behavioral indicator of underlying processing modes.
"However, we think that they indicate that the diVerence between central and peripheral Welds in itself is not crucial here. Instead, it is to be expected that grasping (or pointing at) non-foveated objects is less automatized and more awkward than grasping foveated objects, 4 and hence, may be more reliant on allocentric information (see Gonzalez et al. 2008; Van der Kamp et al. 2003). In conclusion, we examined the hypothesis that the control of movement of the left hand would be more likely to entail the use of allocentric information than movements performed with the right hand. "
[Show abstract][Hide abstract] ABSTRACT: We investigated whether the control of movement of the left hand is more likely to involve the use of allocentric information than movements performed with the right hand. Previous studies (Gonzalez et al. in J Neurophys 95:3496-3501, 2006; De Grave et al. in Exp Br Res 193:421-427, 2009) have reported contradictory findings in this respect. In the present study, right-handed participants (N = 12) and left-handed participants (N = 12) made right- and left-handed grasps to foveated objects and peripheral, non-foveated objects that were located in the right or left visual hemifield and embedded within a Müller-Lyer illusion. They were also asked to judge the size of the object by matching their hand aperture to its length. Hand apertures did not show significant differences in illusory bias as a function of hand used, handedness or visual hemifield. However, the illusory effect was significantly larger for perception than for action, and for the non-foveated compared to foveated objects. No significant illusory biases were found for reach movement times. These findings are consistent with the two-visual system model that holds that the use of allocentric information is more prominent in perception than in movement control. We propose that the increased involvement of allocentric information in movements toward peripheral, non-foveated objects may be a consequence of more awkward, less automatized grasps of nonfoveated than foveated objects. The current study does not support the conjecture that the control of left-handed and right-handed grasps is predicated on different sources of information.
Experimental Brain Research 01/2012; 218(1):91-8. DOI:10.1007/s00221-012-3007-x · 2.04 Impact Factor
"The actions were performed either by using a normal precision grip (forefinger and thumb) or an awkward precision grip (little finger and thumb). The awkward grip was introduced to control for grip familiarity . Although magicians are familiar with pantomimed actions using normal grips, magicians and controls were equally unfamiliar with awkward grips. "
[Show abstract][Hide abstract] ABSTRACT: Most of us are poor at faking actions. Kinematic studies have shown that when pretending to pick up imagined objects (pantomimed actions), we move and shape our hands quite differently from when grasping real ones. These differences between real and pantomimed actions have been linked to separate brain pathways specialized for different kinds of visuomotor guidance. Yet professional magicians regularly use pantomimed actions to deceive audiences.
In this study, we tested whether, despite their skill, magicians might still show kinematic differences between grasping actions made toward real versus imagined objects. We found that their pantomimed actions in fact closely resembled real grasps when the object was visible (but displaced) (Experiment 1), but failed to do so when the object was absent (Experiment 2).
We suggest that although the occipito-parietal visuomotor system in the dorsal stream is designed to guide goal-directed actions, prolonged practice may enable it to calibrate actions based on visual inputs displaced from the action.
PLoS ONE 02/2011; 6(2):e16568. DOI:10.1371/journal.pone.0016568 · 3.23 Impact Factor
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