Dijkerman & de Haan (D&dH) propose that somatosensory processing for the guidance of action can be dissociated from the processing leading to perception and memory. Leisman (Reference Leisman1989a; Reference Leisman1989b) showed that voluntary movement, like all other movements, consists of operations in time and space specified by physical parameters. When, for example, a person lifts a cup to his or her lips, the trajectory of the teacup, force vectors, acceleration, velocity at every point, total length of the path, locus of origin, and the time of onset specify the voluntary movement. Neurologically normal adults usually perform voluntary movements of this kind quickly, precisely, and without information concerning the total mass or its contents.
There are two types of explanations for the surprising precision with which we move our limbs. One is, that the motor system calculates in advance the values of movement parameters sufficiently accurately to assure successful performance. The other explanation is based on the fact that every muscular contraction changes the state of receptors in muscles and tendons. These receptors measure parameters of voluntary contraction and transmit this information to the motor system. The motor system is then thought to control voluntary contraction under the guidance of sensory feedback from these receptors. Although there is no contradiction between these two explanations, there is disagreement as to the relative importance of specific motor versus sensory feedback.
Other equally important dimension in the understanding of the organisation of voluntary motor control include the question of: (1) what are the levels of the central nervous system at which the desired values of voluntary contraction parameters are calculated? (2) what is the extent to which voluntary contractions are automatic, given that humans are able to consciously vary the parameters of voluntary contractions in an infinite number of ways?
D&dH suggest that the posterior parietal cortex (PPC) subserves both perception and action, whereas the insula subserves perceptual recognition and learning. The authors infer a close relationship between the auditory system and the dorsal and ventral visual systems. D&dH have provided an intellectual genealogical extension of the work by Milner and Goodale, who reinterpreted Ungerleider and Mishkin's (1982) distinction between the “what” and “where” visual systems (Goodale & Milner Reference Goodale and Milner1992; Milner & Goodale Reference Milner and Goodale1995). Ungerleider and Mishkin (Reference Ungerleider, Mishkin, Ingle, Goodale and Mansfield1982) suggested that the ventral visual stream (geniculostriate pathway projecting to the inferotemporal cortex) subserves object identification, whereas the dorsal stream (projections from the striate cortex and colliculi to the posterior parietal cortex) subserves object localization. This suggests that the function of the dorsal stream is better described as mediating visually guided actions. Therefore, D&dH replace Ungerleider and Mishkin's what versus where distinction with a distinction between what and how.
In the visual system, we see the dorsal and ventral stream process different types of visual information. Specifically, the dorsal system has a global bias focusing on lower spatial frequency information, whereas the ventral system focus has a local processing bias utilizing higher spatial frequency information. The dorsal system tends to focus on global form, whereas the ventral system focuses on details or parts of wholes.
Autism is an excellent example where there exists a weighting of one system and diminished processing of the other. Frith (Reference Frith1996) proposed a theory of weak central coherence in autism spectrum disorder (ASD). Additionally, her theory of enhanced perceptual discrimination (Shah & Frith Reference Shah and Frith1993) attempted to explain the uneven profile of abilities and difficulties in ASD. Central coherence refers to the ability to put information together to extract meaning, to remember the gist of a story rather than its details. Individuals with ASD show a bias for part over wholes – often excelling at noticing and recalling detailed information. Perception and processing features are believed to be superior, possibly at the expense of processing global information. Mottron et al. (Reference Mottron, Peretz and Menard2000) showed this same type of bias in the auditory system of autistic individuals, along with sensory motor deficits, explained partly by a more ventrally based sensory motor system focusing more on action and less on perception. Autistics are believed to have poor body schema and spatial localization of body parts. Many autistics cannot identify their body parts in a mirror (Mitchell Reference Mitchell1997). Even if they know the word “nose,” they may still identify the wrong body part. They have poor proprioception and are generally clumsy (Minshew et al. Reference Minshew, Sung, Jones and Furman2004; Schmitz et al. Reference Schmitz, Martineau, Barthelmy and Assaiante2003). These examples emphasize increased action and decreased perception. This parallels what we see in vision and audition with the emphasis on the ventral system and decrease in the dorsal system. The mirror neuron system is dysfunctional in autistics (Hadjikhani et al. Reference Hadjikhani, Joseph, Snyder and Tager-Flusberg2006). This system also seems to utilize similar processes to recognize movements in an implicit manner for the extraction of meaning of intent, and for emotion.
In normal individuals, motor activity suppresses mu wave activity in the sensory motor cortex, but it is also suppressed in normals when they observe someone else performing a motor act (Oberman et al. Reference Oberman, Hubbard, McCleery, Altschuler, Ramachandran and Pineda2005). In autistics, we see that the mu wave is suppressed only with their own actions but not when they observe others. This again would seem to show unevenness in sensory motor modalities with an emphasis on action and diminished perception.
Lateralized differences have also been noted in these modalities. It has been well established that, in vision, the right hemisphere processes information primarily with the more globally focused dorsal system (Melillo & Leisman Reference Melillo and Leisman2004). The left hemisphere tends to focus on detail similarly to the ventral visual system. The same right/left hemisphere differences exist in the auditory system. The right hemisphere is more spatially oriented toward the dorsal where, whereas the left hemisphere is focused on the ventral what. This is also believed to exist within the somatosensory system, where the right hemisphere is more focused on dorsal perceptual/sensory systems and proprioception, as well as implicit knowledge of egocentric relationships, and the left hemisphere is more focused on action or motor activity and conscious awareness of body parts.
The multi-modal dysfunction observed in ASD may suggest an issue of the adequacy of hemispheric function in the argument presented by D&dH.
