As fundamental researchers in the neuroethology of efference copy, we were stimulated by Grush's bold and original synthesis. In the following critique, we draw attention to ways in which it might be tested in the future, we point out an avoidable conceptual error concerning emulation that Grush seems to share with other workers in the field, and we raise questions about the neural correlates of Grush's schemata that might be probed by neurophysiologists.

Dualistic approaches to the mind-body relationship are commonplace; however, the adoption of dualistic thinking can often obscure aspects of the way the organism functions as a whole biological entity. Future versions of the emulation theory will, it is hoped, address some of these issues, including the nature of process noise, how distinct iterations can occur, and how to deal with non-emulated aspects of motor control.

Grush's framework has epistemological implications and explains how it is possible to acquire offline empirical knowledge. It also complements the extended-mind thesis, which says that mind leaks into the world. Grush's framework suggests that the world leaks into the mind through the offline deployment of emulators that we usually deploy in our experience of the world.

The general applicability of forward models in brain function has previously been recognized. Grush's contribution centers largely on broadening the extent and scope of forward models. However, in his effort to expand and generalize, important distinctions may have been overlooked. A better grounding in the underlying physiology would have helped to illuminate such valuable differences and similarities.

I focus on the distinction between sensation and perception. Perceptions contain additional information that is useful for interpreting sensations. Following Grush, I propose that emulators can be seen as containing (or creating) hidden variables that generate perceptions from sensations. Such hidden variables could be used to explain further cognitive phenomena, for example, causal reasoning.

We briefly address three issues that might be important to evaluate the validity of the “emulation theory”: (1) Does it really say something new? (2) Are similar processes engaged in action, imagery, and perception? (3) Does a brain amodal emulator exist?

The Kalman filtering technique is considered as a part of concurrent data-processing techniques also related to detection, parameter evaluation, and identification. The adaptive properties of the filter are discussed as being related to symmetrical brain structures.

Inspired by Rick Grush's emulation theory, we reinterpreted a series of our neuroimaging experiments which were intended to examine the representations of complex movement, modality-specific imagery, and supramodal imagery. The emulation theory can explain motor and cognitive activities observed in cortical motor areas, through the speculation that caudal areas relate to motor-specific imagery and rostral areas embrace an emulator for amodal imagery.

In this commentary we discuss a predictive sensorimotor illusion, the size-weight illusion, in which the smaller of two objects of equal weight is perceived as heavier. We suggest that Grush's emulation theory can explain this illusion as a mismatch between predicted and actual sensorimotor feedback, and present preliminary data suggesting that the cerebellum may be critical for implementing the emulator.

Grush makes extensive use of von Holst and Mittelstaedt's (1950) efference copy hypothesis. Although his embellishment of the model is admirably more sophisticated than that of its progenitors, I argue that it still suffers from the same conceptual limitations as entailed in its original formulation.

Exclusively focused on data that are consistent with the proposed ideas, the target article misses an opportunity to review data that are inconsistent with them. Weaknesses of the emulation theory become especially evident when one tries to incorporate physiologically realistic muscle and reflex mechanisms into it. In particular, it fails to resolve the basic posture-movement controversy.

Aspects of “emulation theory” have been seminal to our understanding of spatial orientation for more than 50 years. Sometimes called internal models, existing implementations include both traditional observers and optimal observers (Kalman filters). This theoretical approach has been quite successful in helping understand and explain spatial orientation – successful enough that experiments have been guided by model predictions.

Grush makes an important contribution to a promising way of viewing mental representation: as a component activity in sensorimotor processes. Grush shows that there need be no entities in our heads that would count as representations, but that, nevertheless, the process of representation can be defined so as to include both natural and artificial (e.g., linguistic or pictorial) representing.

Much can be gained by specifying the operation of the emulation process. A brief review of studies from diverse domains, including complex motor-skill representation, emotion perception, and face memory, highlights that emulation theory offers precise explanations of results and novel predictions. However, the neural instantiation of the emulation process requires development to move the theory from armchair to laboratory.

Illusory kinesthetic sensation was influenced by motor imagery of the wrist following tendon vibration. The imagery and the illusion conditions commonly activated the contralateral cingulate motor area, supplementary motor area, dorsal premotor cortex, and ipsilateral cerebellum. This supports the notion that motor imagery is a mental rehearsal of movement, during which expected kinesthetic sensation is emulated by recruiting multiple motor areas, commonly activated by pure kinesthesia.

Grush's emulation theory comprises both modality-specific and amodal emulators. I suggest that the amodal variety be replaced by multisensory emulators. The key distinction is that multisensory processing retains the characteristics of individual sensory modalities, in contrast to amodal processing. The latter term is better reserved for conceptual and linguistic systems, rather than perception or emulation.

This commentary addresses the neural implementation of emulation, mostly using findings from functional Magnetic Resonance Imaging (fMRI). Furthermore, both empirical and theoretical suggestions are discussed that render two aspects of emulation theory redundant: independent modal emulators and extra measurement of amodal emulation. This modified emulation theory can conceptually integrate simulation theory and also get rid of some problematic philosophical implications.

Grush's emulator model appears to be consistent with the idea of a body schema, that is, a detailed mental representation of the body, its structure, and movement in relation to the environment. If the emulator is equivalent to a body schema, then the next step will be to specify how the emulator accounts for neuropsychological and developmental phenomena that have long been hypothesized to involve the body schema.

One property of the emulator framework presented by Grush is that imagery operates off-line. Contrary to this viewpoint, we present evidence showing that mental rotation of a simple figure modulates low-level features of drawing articulation. This effect is dependent upon the type of rotation, suggesting a more integrative online role for imagery than proposed by the target article.

Grush describes a variety of different systems that illustrate his vision of representation through emulation. These individual data points are not necessarily sufficient to determine what level of detail is required for a representation to count as emulation. By examining one of his examples closely, this commentary suggest that salience of the information supplied is a critical dimension.