The subtlety of these effects indicates that the motor system’s influence on perception is modulatory rather than comprising a necessary component of speech sound recognition. In sum, there is unequivocal neuropsychological evidence that a strong version of the motor theory of speech perception, one in which the motor system is necessary component, is untenable. However, there is suggestive evidence

that the motor system is capable of modulating the perceptual system to some degree. Models of speech perception will need to account for both sets of observations. During the last decade a great deal of progress has been made in mapping the neural organization of sensorimotor integration for speech. Early functional imaging Luminespib studies identified an Selleck OSI744 auditory-related area in the left planum temporale region that was also involved in speech production ( Hickok et al., 2000 and Wise et al., 2001). Subsequent studies showed that this left dominant region, dubbed Spt for its location in the Sylvian fissure at the parietal-temporal

boundary ( Figure 2A) ( Hickok et al., 2003), exhibited a number of properties characteristic of sensorimotor integration areas such as those found in macaque parietal cortex ( Andersen, 1997 and Colby and Goldberg, 1999). Most fundamentally, Spt exhibits sensorimotor response properties, activating both during the passive perception of speech and during covert (subvocal) speech articulation (covert speech was used to ensure that overt auditory feedback was not driving the activation) ( Buchsbaum et al., 2001, Buchsbaum et al., 2005 and Hickok et al., 2003). Further, different subregional patterns

of activity are apparent during the sensory and motor phases of the task ( Hickok et al., 2009), likely reflecting the activation of different neuronal subpopulations ( Dahl et al., 2009) some sensory- and others motor-weighted. Figures 2B–2D show examples of the sensory-motor response properties of Spt and the patchy new organization of this region for sensory- versus motor-weighted voxels ( Figure 2C, inset). Spt is not speech specific; its sensorimotor responses are equally robust when the sensory stimulus is tonal melodies and (covert) humming is the motor task (see the two curves in Figure 2B) ( Hickok et al., 2003). Activity in Spt is highly correlated with activity in the pars opercularis ( Buchsbaum et al., 2001 and Buchsbaum et al., 2005), which is the posterior sector of Broca’s region. White matter tracts identified via diffusion tensor imaging suggest that Spt and the pars opercularis are densely connected anatomically (for review see Friederici, 2009 and Rogalsky and Hickok, 2010).