Object-use and the left hemisphere: the implications of apraxia and brain stimulation on perception for action

Evans, Carys (2015) Object-use and the left hemisphere: the implications of apraxia and brain stimulation on perception for action. Doctoral thesis, Northumbria University.

Preview

Text evans.carys_phd.pdf - Submitted Version Download (3MB) | Preview

Abstract

Apraxia, a high-level movement disorder, is associated with performance errors during gesture imitation, demonstrating the use of familiar objects that are present (actual object-use) or absent (pantomime), or all three scenarios. Focusing on objects, apraxia has an isolated effect on manipulation judgements regarding skilled object-use. These manipulation deficits are potentially attributed to damage to a purported ventro-dorsal stream resulting in impaired internal representations of movement (i.e. motor imagery). Instead, patients over-rely on visual affordances during object-directed motor behaviour. The cortical regions associated with the ventro-dorsal stream correspond to those damaged in apraxia, in particular the left inferior parietal lobe (IPL), adding weight to this proposal.

Using a perceptual matching task with familiar objects and an action execution grasping task with novel objects, behavioural work with left hemisphere stroke patients assessed whether apraxic deficits are specific to object manipulation and whether these patients over-rely on object affordances during skilled object-manipulation. In parallel, the effect of neuromodulation technique transcranial direct current stimulation (tDCS) on motor imagery was explored with healthy populations. Performance changes during left IPL stimulation was assessed during classic motor mental rotation and the same perceptual matching task used with patients.

Apraxic patients showed a selective impairment during object manipulation judgements of the perceptual matching task, which increased with apraxia severity. Despite tDCS over the left or right IPL equally affecting motor mental rotation performance in healthy populations, during the perceptual matching task only modulation of the left IPL slowed reaction times when making manipulation judgements but not functional semantic judgements regarding object-use. These results suggest that disruption of ventro-dorsal processing specifically disturbs motor representations of object-use. When repeatedly grasping novel objects of differing weight distribution, most apraxic patients consistently selected a structurally afforded grasp-point, indicating that apraxic patients over-rely on visual affordances after ventro-dorsal disruption.

These results confirm that the ventro-dorsal stream, in particular the left IPL, is critical in integrating perceptual internal representations of skilled movement into context-dependent action plans based on visual information. Over-reliance on visual affordances caused by disruption to this pathway not only affects perceptual manipulation judgements of familiar objects but also experience-based learning when grasping novel objects.

Actions (login required)

View Item

Downloads