Reply: apraxia: a gestural or a cognitive disorder?

Sir, It is intriguing that our article ‘Critical brain regions for tool-related and imitative actions: a componential analysis’ was the sole subject of a recent positive Scientific Commentary in Brain by Professor Georg Goldenberg ‘Challenging traditions in apraxia’ (Goldenberg, 2014) noting that the paper ‘excels by virtue of its methodical astuteness, large sample size and the clarity of its results’, as well as a more critical Letter to the Editor by Francois Osiurak and Didier Le Gall (2014). We are very pleased that our paper has stimulated discussion. Drs Osiurak and Le Gall raise three points of concern, all focused on our interpretation of results. The first concern is with our suggestion that the inferior parietal lobe provides the basis for the kinematic component of praxis actions, contributing to the planning of movement trajectories in terms of extent, direction, and timing. The second is with our claim that the posterior temporal lobe supports the stored, representational components of praxis. Finally, they note a concern that our account has been modified since it was originally proposed in 2001. We will address each of these criticisms in turn. Osiurak and Le Gall suggest that the kinematic hypothesis is unable to account for the difficulty of many patients with left brain damage to select the appropriate tool to solve problems. Rather, they prefer to conceptualize the difficulty as a failure of technical reasoning. But what are the core underlying mechanisms that enable one to ‘reason’ about physical object properties? We suggest that selection of the appropriate tool (and pantomime of tool use) require a simulation (which some have likened to a predictive ‘forward model’) (Tian and Poeppel, 2012; Pickering and Clark, 2014) of how the arm, hand, and tool will move through space once the tool is picked up and used. In support of the claim that apraxic patients are deficient in predicting how movements will unfold (in advance of actually moving), we have shown that apraxics are unable to predict how they would position the hand to comfortably grasp a 3D shape were they to actually move, in the context of normal performance once permitted to actually reach out and grasp. Moreover, the magnitude of the prediction deficit is strongly correlated with the magnitude of deficit on tool pantomime and imitation, and with parietal lesions (Buxbaum et al., 2005). These and other findings (Ochipa et al., 1997; Jax et al., 2006, 2014; Mutha et al., 2010; Sunderland et al., 2011, 2013; Eidenmuller et al., 2014) support our proposal that a kinematic planning deficit is a core component of the apraxia syndrome. There are many fascinating questions to be explored here, including the format of the kinematic planning deficit, that is, whether it is restricted to the planning of body movements per se, or rather, a deficit in a more abstract process that is relevant to representing any complex trajectory, body-related or not (see Wong et al., 2014 for the latter view). Osiurak and Le Gall are also critical of our interpretation of the posterior temporal involvement in the semantic aspects of tool actions seen both in the present study and our previous paper in Brain assessing gesture recognition (Kalenine et al., 2010). They note that there are reports of patients with isolated semantic deficits who are able to use both familiar and novel tools. Although the organization of the semantic system is a matter of continued debate, a view held by many is that conceptual knowledge is feature-based and distributed across visual, auditory, tactile, olfactory, and action properties (Allport, 1985). Without further specification it is unclear that a given ‘semantic deficit’ will disrupt action semantics, specifically. In fact, apraxics are frequently intact in knowledge of the function (purpose) of tools, while deficient in manipulation knowledge (i.e. action semantics) (Buxbaum and Saffran, 1998, 2002). There are a number of lines of evidence supporting the claim that knowing how to use a tool does not emerge solely from mechanical reasoning (although certainly, kinematic planning doi:10.1093/brain/awu240 Brain 2015: 138; 1–3 | e334