Update TRENDS in Cognitive Sciences Vol.10 No.3 March 200694

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of stimulus-specific effects. Trends Cogn. Sci. 10, 14–232 Ulanovsky, N. et al. (2003) Processing of low-probability sounds by

cortical neurons. Nat. Neurosci. 6, 391–3983 Haenschel, C. et al. (2005) Event-related brain potential correlates of

human auditory sensory memory-trace formation. J. Neurosci. 25,10494–10501

4 Haenschel, C. et al. (2000) Gamma and beta frequency oscillations inresponse to novel auditory stimuli: a comparison of human electro-encephalogram (EEG) data with in vitro models. Proc. Natl. Acad. Sci.

U. S. A. 97, 7645–76505 Nelken, I. (2004) Processing of complex stimuli and natural scenes in

the auditory cortex. Curr. Opin. Neurobiol. 14, 474–480

Corresponding author: Goldenberg, G. (Georg.Goldenberg@extern.lrz-muenchen.de).

Available online 7 February 2006

www.sciencedirect.com

6 Friston, K. (2005) A theory of cortical responses. Philos. Trans. R. Soc.Lond. B Biol. Sci. 360, 815–836

7 Stephan, K.E. et al. Synaptic plasticity and dysconnection inschizophrenia. Biol. Psychiatry (in press)

8 Baldeweg, T. et al. (2004) Mismatch negativity potentials andcognitive impairment in schizophrenia. Schizophr. Res. 69, 203–217

9 Liegeois-Chauvel, C. et al. (1994) Evoked potentials recorded from theauditory cortex in man: evaluation and topography of the middle latencycomponents. Electroencephalogr. Clin. Neurophysiol. 92, 204–214

10 Ahissar, M. and Hochstein, S. (2004) The reverse hierarchy theory ofvisual perceptual learning. Trends Cogn. Sci. 8, 457–464

1364-6613/$ - see front matter Q 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.tics.2006.01.010

Imitation: is cognitive neuroscience neglecting apraxia?

Georg Goldenberg

Klinikum Bogenhausen, Englschalkingerstrasse 77, D 81925 Munich, Germany

Brass and Heyes [1] compare two approaches to imitation:‘specialist’ theories assume that supramodal represen-tations mediate between sensory and motor represen-tations of actions, whereas ‘generalist’ theories proposedirect links. The main difference between them might alsobe captured by designating the ‘specialist’ models ascognitive and the ‘generalist’ as sensory-motor or simply‘direct’.

Brass and Heyes favour direct models because they canexplain that observation of motor actions interferes withexecution of other motor actions and activates brainregions dedicated to motor execution. Both effects canalso be accounted for by cognitive models, with theplausible assumption that attentive observation of actionsactivates their cognitive representations. Interferencebetween observation of one action and execution ofanother can arise from their simultaneously activatedcognitive representations. The spread of activation tomotor representation can be derived from activatedcognitive representations rather than directly fromsensory representations.

A crucial point for distinguishing between sensory-motor and cognitive theories is the functional role of motorrepresentations. Properties of imitation are determined byanatomical and functional properties of motor neurons forsensory-motor theories; for cognitive theories they dependon properties of cognitive representations. According todirect theories, involvement of single body parts inimitation should be determined by the somatotopicorganization of motor control, whereas for cognitivetheories body-part specificity in imitation can varyindependently of the somatotopy of theirmotor representations.

It has been recognized since the early days of clinicalneuropsychology that apraxia resulting from unilateralhemisphere lesions affects imitation with both thecontralateral and the ipsilateral limbs (e.g. [2–4]).Deficient imitation with the ipsilateral limb is proof ofdissociation between the laterality of motor control andthe laterality of the neural substrate of imitation. More-over, the effects of left- and right-hemisphere damage arebody-part specific. Whereas imitation of whole-handpostures is impaired exclusively after left brain damage(LBD), imitation of finger configuration and of footpostures is also vulnerable to right brain damage (RBD)[5]. Similarly, imitation of mouth movements is mainlysensitive to LBD whereas imitation of movements of theupper part of the face can equally be impaired by RBD [6].Such body-part specificity of hemisphere contributionscannot result from the somatotopic organization of motorcortex within each hemisphere.

According to direct theories, imitation is achieved byconnecting visual representations of actions with motorrepresentation of the same actions. Transformation ofsensory to motor representations would therefore leadeither to reproduction of the perceived action or, if motorexecution is suppressed, to no motor action at all. Bycontrast, if a cognitive representation is built up prior to amotor representation, its content can be communicated bydifferent motor actions from those perceived.

Patients with unilateral brain lesions and apraxiacommit errors not only when imitating meaningless handpostures but also when trying to replicate them on amanikin or selecting the match for a target gesture froman array of photographs showing gestures performed bydifferent people at different angles of view [7,8]. As themotor actions of manipulating a manikin or pointing topictures are fundamentally different from the gesturesthemselves, the most likely explanation of these findings

Update TRENDS in Cognitive Sciences Vol.10 No.3 March 2006 95

is that patients build up cognitive representations of thegestures. Their inaccuracy shows up independently fromthe motor actions used for expressing their content.

Brass and Heyes note that the old literature onneurological patients with apraxia is usually neglectedbut themselves neglect both old and new studies onapraxia. These studies cast severe doubts on the postu-lated superiority of sensory-motor (‘generalist’) overcognitive (‘specialist’) theories of imitation.

AcknowledgementsI thank Otto Karnath and Wolfram Ziegler for critical discussion of a firstdraft of this letter.

References

1 Brass, M. and Heyes, C. (2005) Imitation: is cognitive neurosciencesolving the correspondence problem? Trends Cogn. Sci. 9, 489–495

Corresponding authors: Heyes, C. (c.heyes@ucl.ac.uk), Brass, M. (brass@cbs.mpg.de).

Available online 7 February 2006

www.sciencedirect.com

2 Liepmann, H. (1908) Drei Aufsatze aus dem Apraxiegebiet, Karger,Berlin

3 De Renzi, E. et al. (1980) Imitating gestures: a quantitative approach toideomotor apraxia. Arch. Neurol. 37, 6–10

4 Haaland, KY et al. (2000) Neural representations of skilled movement.Brain 123, 2306–2313

5 Goldenberg, G. and Strauss, S. (2002) Hemisphere asymmetries forimitation of novel gestures. Neurology 59, 893–897

6 Bizzozero, I. et al. (2000) Upper and lower face apraxia: role of the righthemisphere. Brain 123, 2213–2230

7 Goldenberg, G. (1995) Imitating gestures and manipulating amannikin: the representation of the human body in ideomotor apraxia.Neuropsychologia 33, 63–72

8 Goldenberg, G. (1999) Matching and imitation of hand and fingerpostures in patients with damage in the left or right hemisphere.Neuropsychologia 37, 559–566

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doi:10.1016/j.tics.2006.01.005

Letters Response

Grasping the difference: what apraxia can tellus about theories of imitationReply to Goldenberg

Cecilia Heyes1 and Marcel Brass2

1Department of Psychology, University College London, UK2Department of Cognitive Neurology, Max Planck Institute for Human Cognitive and Brain Sciences, Leipzig, Germany

We are glad that our article has stimulated interest in therelationship between work on the clinical neuropsychologyof apraxia and theories of imitation. However, we cannotagree with Goldenberg’s recasting of the distinctionbetween specialist and generalist theories [1] or, therefore,with his diagnosis of faults in the latter.

The generalist position proposes that imitation is basedon task- and species-general processes of learning andmotor control. It assumes, conventionally, that generalmechanisms of motor control are implemented, not only inthe primary sensorimotor cortex, but also in premotor andparietal association cortex. Thus, the generalist view isconcerned with a higher-order, ‘cognitive’ level of motorcontrol, and cannot therefore be contrasted, as Goldenbergsuggests, with ‘cognitive theories’.

As we outlined in our article [2], there is some evidenceof weak somatotopic organization in the premotor andparietal cortex [3]. However, nobody would claim thatthere is evidence in these areas of the simple somatotopicorganization found in the primary sensorimotor cortex [4].Therefore, the generalist position does not rest on a strongsomatotopy assumption, and is consistent with evidencethat unilateral lesions can have bilateral effects onimitative performance.

Goldenberg’s second, empirically-based objection ismore interesting. How, he asks, can a generalisttheory account for the fact that, among apraxicpatients, impairments in imitation of meaninglessgestures tend to correlate with impairments in theability to reproduce the movements on a puppet or‘manikin’ [5]. The answer lies in acknowledging that,when people are under instruction to imitate relativelycomplex and unfamiliar movements, as they are inmost clinical tests, the task enlists a range ofprocesses in addition to the visuo-motor connections,or ‘matching vertical links’, that are primarily responsiblefor solving the correspondence problem. For example,performance might depend on linguistic and non-linguis-tic processes involved in the sequencing and organizationof motor primitives [6,7]. We suggest that it is processes ofthis kind, encompassed by generalist theory with refer-ence to ‘horizontal processes’ and ‘indirect verticalassociations’ [2,8], that can contribute to both imitationand manikin-manipulation impairments inapraxic patients.

The results of the manikin study [5] draw attention tothe involvement of task-general mechanisms in imitation.Therefore, although Goldenberg disagrees with ouremphasis on visuo-motor links, we seem to be united inbelieving that generalist rather than specialist mechanismssolve the correspondence problem.