Perceptual decisions regarding object manipulation are selectively impaired in apraxia or when tDCS is applied over the left IPL.
This study evaluated whether apraxia can be understood as due to impaired motor representations or motor imagery necessary for appropriate object-use, imitation, and pantomime. The causal role of the left inferior parietal lobe (IPL), which is heavily implicated in apraxia, is also evaluated. These processes are appraised in light of the proposed ventro-dorsal sub-stream of the classic two visual pathway model, where perceptual information from the ventral stream and the dorsal action stream are integrated and essential for object manipulation. Using a task assessing object-use perception, stroke patients with apraxia demonstrated a selective deficit during perceptual decisions reliant on the integration of visible and known object properties to select the appropriate grasp for object-use. This deficit increased with apraxia severity. A dissociation was evident in these patients showing intact non-motoric perceptual decisions regarding the functional semantic relationship between two objects in the absence of the actor (e.g. how a hammer hits a nail). Converging evidence was found using a modified version of the same task in a neuromodulation study that directly targeted the left IPL in healthy participants using transcranial direct current stimulation (tDCS). Application of inhibitory stimulation over the left IPL reduced performance during perceptual decisions regarding object manipulation whilst performance was unaffected during functional semantic decisions. Excitatory stimulation of the left IPL did not affect performance in either task. Combined, these results suggest that the left inferior parietal lobe is critical for motor imagery, and that apraxia may be caused by an inability to use internal motor representations of object manipulation. These results are discussed in terms of motoric and non-motoric perceptual processes and the proposal of an additional ventro-dorsal sub-stream within the dorsal and ventral visual pathways model.
Department of Psychology, Goldsmiths, University of London, New Cross, London SE14 6NW, UK. Electronic address: carysemilyevans@gmail.com.
Carys
C
Evans
Institute of Research in the Psychological Sciences, Université catholique de Louvain, Belgium.
Martin G
MG
Edwards
Department of Psychology, Faculty of Health and Life Sciences, Northumbria University, Newcastle upon Tyne, UK.
Lawrence J
LJ
Taylor
Psychology, University of Stirling, Scotland, UK.
Magdalena
M
Ietswaart
D000368:Aged; D000369:Aged, 80 and over; D000704:Analysis of Variance; D001072:Apraxias / Q000150:complications / Q000000981:diagnostic imaging / Q000473:pathology*; D001931:Brain Mapping; D016022:Case-Control Studies; D003657:Decision Making / Q000502:physiology*; D005260:Female; D007839:Functional Laterality / Q000502:physiology*; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D008875:Middle Aged; D010296:Parietal Lobe / Q000000981:diagnostic imaging / Q000502:physiology*; D010465:Perception / Q000502:physiology*; D011930:Reaction Time / Q000502:physiology; D012660:Semantics; D012720:Severity of Illness Index; D020521:Stroke / Q000150:complications / Q000000981:diagnostic imaging; D015898:Tomography Scanners, X-Ray Computed; D065908:Transcranial Direct Current Stimulation / Q000009:adverse effects*
Apraxia; Dorsal and ventral streams; Left inferior parietal lobe; Motor imagery; Object manipulation; Perception and action