Motor imagery: never in your wildest dream

Abstract

The results from brain-lesioned patients might also contribute to our understanding of the function of the posterior parietal lobe. These new data suggest that the parietal cortex is a structure where the evaluation of motor performance, real and imaged, can occur based upon a comparison of corollary discharge and multimodal reafferent sensory signals with a stored internal representation of the motor plan as it normally unfolds. In line with this idea, it has been reported[21xDuhamel, J-R. et al. Brain. 1992; 115: 1387–1402Crossref | PubMed | Scopus (140)See all References, 22xHeide, W. et al. Ann. Neurol. 1995; 38: 739–748Crossref | PubMed | Scopus (158)See all References]that parietal-lesioned patients cannot complete the second of two, sequenced, saccadic eye movements without visual feedback of eye and target locations, which could be interpreted as a failure to use corollary discharge to evaluate movement. By contrast, the parietal-lesioned patients studied by Sirigu et al.[9xSirigu, A. et al. Science. 1996; 273: 1564–1568Crossref | PubMedSee all References[9]are able to execute properly but not image movement sequences, a finding that might indicate a dependence upon evaluating reafferent signals. It is possible that these different forms of motor-evaluation deficits result from lesions that disrupt separate subregions of the posterior parietal lobe and, consequently, separate components of the posterior parietal network for evaluating ongoing motor behavior. That the superior parietal lobe (SPL) contributes to decision-making processes by evaluating visuospatial information has been demonstrated recently in a magnetic resonance imaging (MRI) study of mental rotation[23xTagaris, G.A. et al. NeuroReport. 1996; 7: 773–776Crossref | PubMedSee all References[23]. The level of SPL activation was proportional to the number of errors made by subjects, who had to mentally rotate one of two visual images and decide whether the images were the same or mirror reflections of each other.Parallel data from physiological studies in monkeys also indicate that posterior parietal neurons, long known to have visuomotor activity unrelated to simple sensory or motor signals, might be organized into functional groups according to how each contributes to the process of movement evaluation. Within area 5, one population of cells, which discharges only following the onset of voluntary limb movement, becomes silent after peripheral deafferentation of the performing limb, while a second population, which discharged well in advance of the same limb movement, was unaffected by limb deafferentation[24xSeal, J., Gross, C., and Bioulac, B. Brain Res. 1982; 250: 229–243Crossref | PubMed | Scopus (63)See all References[24]. In addition, neuronal activity recorded from a visuomotor intraparietal cortical area prior to eye-movement onset, reflects the sensory signals that would be expected were the eyes already at the final target position[25xDuhamel, J-R., Colby, C.L., and Goldberg, M.E. Science. 1992; 255: 90–92Crossref | PubMedSee all References[25]; activity that could result only from a corollary discharge. Therefore, posterior parietal activity appears to reflect both reafferent signals and corollary discharge. Taken in conjunction with the finding of neuronal activity representing intended movements[16xKalaska, J.F. and Crammond, D.J. Cereb. Cortex. 1995; 5: 410–428Crossref | PubMed | Scopus (197)See all References[16](Fig. 3Fig. 3), the posterior parietal network has all the necessary components to transform visuospatial information[26xAndersen, R.A. Cereb. Cortex. 1995; 5: 457–469Crossref | PubMed | Scopus (230)See all References, 27xLacquaniti, F. et al. Cereb. Cortex. 1995; 5: 391–409Crossref | PubMed | Scopus (239)See all References]into the signals that are required to evaluate movements[28xSakata, H. et al. Cereb. Cortex. 1995; 5: 429–438Crossref | PubMed | Scopus (431)See all References[28]as they occur.Patients with parietal lesions can, despite their problems, mentally image movements and perform them. Thus, they do not have a failure of imagination, rather, unlike controls, the internal mental `movements' of patients with parietal lesions do not appear to obey physical laws or to be tied to physiological constraints. Perhaps this is because rather than imaging movements, they are free to imagine moving however they want to, as in a dream.