Brains have emulators with brains: Emulation economized

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. Emulators with brains. The emulation account provides a formal way to apply the idea that the brain’s default mode is not passive waiting but active prediction, not only in motor control and imagery, but also in perception and perceptual imagery – an extension which fits perfectly with a long series of fMRI studies we performed on voluntary anticipatory processes. These studies made use of the serial prediction task, which requires participants to predict perceptual events on the basis of stimulus sequences. The lateral premotor cortex (PM), pre-supplementary motor area (pre-SMA), and corresponding parietal/temporal areas are engaged in active anticipation of sensory events. Note that this network is activated in absence of motor behavior, and that perceptual input is controlled by contrast computation. Several functional characteristics of the considered areas render them candidate components of an emulator network. First, in the aforementioned studies each PM field’s response is restricted to specific stimulus features: PM fields for vocal movements are engaged in rhythm and pitch prediction, those for manual movements, in object prediction, and those for reaching and pursuit, in spatial prediction. A simplified synopsis of the results indicates that the anticipation of sensory events activates the PM fields of those effectors that habitually cause these sensory events (Schubotz & von Cramon 2001; 2002; Schubotz et al. 2003). This “habitual pragmatic body map” (Schubotz & von Cramon 2003) in PM may precisely reflect Grush’s description of an “articulated” body/environment emulator. Second, our findings would also be in line with an emulation network that entails both amodal and modal representations. Grush proposes motor regions to reflect the controller, and ventral and dorsal processing streams to be the core environmental emulator. We would rather suggest that multiple PM-parietal loops (including the ventral/dorsal stream) function as emulators, with each loop linking both heteromodal and unimodal representations (following the terminology in Benson 1994). One may even hold articulated emulation to be the default mode of PM-parietal loops which are exploited for perception, action, and imageries (see Fig. 1). Visual, auditory, or somatosensory imagery might be generated by efferent signals to and feedbacks from the corresponding unimodal association cortices. We argue that such a modal emulation cannot be considered to be independent from amodal emulation. Rather, the same signal is concurrently sent to both unimodal and heteromodal association areas, even though current internal and external requirements may then determine which feedback becomes causally effective. Visual, auditory, hand, and foot imagery may introspectively feel different possibly because the controller exploits different premotor-parietal-subcortical loops. But all these networks, first, are made of both unimodal and heteromodal cortices which, second, communicate with ease. Possibly this in turn renders an extra measurement process redundant, as we also argue. On the other hand, “controller” functions (or perhaps better, competitive filter functions) may be realized more restrictedly within preSMA, in turn under the influence of anterior median frontal cortices, lateral prefrontal cortex, and extensive feedback projections. Don’t introduce independent modal emulators – even if imagery sometimes feels purely visual . . . An introspectively compelling reason for suggesting independent modal emulation is that some kinds of modal imagery (e.g., a vase) feel purely visual and not at all motor. However, our fMRI findings reveal introspective reports to be unreliable (because introspection does not tell us that motor areas are engaged in non-motor anticipation). Likewise, we are introspectively blind to the empirical fact that perceiving an object includes perceiving what is potentially done with that object (see Gibson [1979/1986] for the notion of an object’s affordance, and, e.g., Fadiga et al. [2000] for premotor responses to mere object perception in the monkey). Conversely, it is conceptually inconsistent to assume amodal emulators to be independent of modal emulators, because in the emulation account, perception is sensation, given an interpretation in terms of amodal environment emulators, whereas sensation in turn is the on-line running of modal emulators. It therefore appears that amodal and modal emulation have to be conceptualized as reciprocally dependent1. . . . And don’t measure the emulators – even if imagery sometimes feels proprioceptive. An introspectively compelling reason for suggesting extra measurement is that motor imagery feels proprioceptive and not at all dynamic/kinematic. This also builds the core premise for splitting emulation from simulation: A motor plan is a dynamic/kinematic plan, whereas full-blown motor imagery is (mock) proprioceptive by nature and therefore must be previously transformed from the former by intermediate emulation and measurement.2 However, exactly this premise would be rejected by accounts based on the ideomotor principle (e.g., theory of event coding; Hommel et al. 2001). These take motor acts to be planned in terms of desired action effects, that is, expected sensory events, and therefore plans and effects most likely share a common neural code. Comfortingly, emulation theory is not committed to the view that efferent signals are motor by nature. To be an efferent signal is nothing more than to be a delivered signal, no matter whether motor, sensory, sensorimotor, or amodal. Let us assume that the controller speaks “Brainish,” the lingua franca spoken by every subsystem in the brain, and that “measurement” is nothing but (and therefore should be termed) feedback from Commentary/Grush: The emulation theory of representation: Motor control, imagery, and perception 414 BEHAVIORAL AND BRAIN SCIENCES (2004) 27:3 the unimodal components of the general-purpose emulator. Grush correctly reminds us that “the emulator is a neural system: any and all of its relevant states can be directly tapped” (target article, sect. 5.1, para. 5).