help answer if the state of affairs were visible. So a reasonable interpretation of ‘‘significant respects’’ is whether visual images are capable of yielding the same answers vision itself would provide. Obviously, this varies with the question. But for some substantial range of questions, images appear to have such a capacity.
A stock example from the literature is visualizing one’s previous living room in order to answer the question ‘‘How many windows did the living room of your previous domicile have?’’ One could answer accurately if the living room were currently visible, but one may also answer correctly by visualizing the living room. No doubt there would be differences of vividness and detail between an image generated from memory and an actual percept of the living room. But there would be sufficient resemblance that both image and percept could generate correct answers. Similarly, consider Shepard’s famous ‘‘mental rotation’’ task (Shepard and Metzler, 1971; Shepard and Cooper, 1982). The task was to determine whether pairs of three-dimensional objects, shown in line drawings from different perspectives, were congruent or noncongruent with one another. Subjects’ performances indicated that they visualized one object being rotated into correspondence with the second, so that they could then visualize congruence or noncongruence. No doubt these acts of visualization differed in numerous respects from perceptual states that would have occurred, had subjects actually observed a rotation and resultant juxtaposition. Nonetheless, the imagery states apparently resembled the hy-pothetical visual states in some significant respects, because the imagery states enabled subjects to answer the series of congruence questions with an accuracy rate of nearly 97 percent, an accuracy rate that must be very close to what would have been achieved if actual observations had been made.
We can now give tentative answers to our earlier questions about visual imagination’s capacity to produce states that resemble their perceptual counterparts, sufficient resemblance to yield accurate mindreading attri-butions. Psychology and neuroscience have revealed extensive and often surprising correspondences between visual imagery and perception. This suggests that the power of E-imagination is very considerable, at least in the visual domain. It also suggests that if visualization were used for mind-reading, it could indeed yield accurate attributions. Much depends, of course, on the specific attributions, but thus far it is quite credible that mindreading applications of visual E-imagination could attain high levels of accuracy.
representation or imagination of executing bodily movement, a representation from the inside. Unlike visualization, motor imagination is not a conspicuous part of consciousness. Nonetheless, it is very common, as we shall see.
What kinds of tasks involve motor imagination? Here is an example from the research literature. If someone is shown a hand and asked whether it is a left or a right hand, he will imagine his own hand moving from its cur-rent orientation into the stimulus orientation for comparison (Parsons, 1987, 1994). The trajectory imagined for the left hand is strongly influenced by the biomechanical constraints on actual left-hand movements, and likewise for imagined right-hand trajectories. Parsons, Gabrieli, Phelps, and Gazzaniga (1998) showed that imaginative processes for the two hands are controlled by their opposite (contralateral) hemispheres, just like ordinary manipulation.
When patients with disconnected cerebral hemispheres judged the handed-ness of drawings of left and right hands in various positions, their accu-racy was high when a hemisphere judged the handedness of a contralateral hand but not above chance when the judged hand was ipsilateral to the perceiving hemisphere. Thus, imagining a hand movement is apparently executed by the same cerebral mechanism that actually executes movements of that hand.
As this example illustrates, motor imagination occurs in tasks quite dif-ferent from mindreading. However, at the moment our discussion is not directed at mindreading, though it will shortly turn to that. It is meant to show that E-imagination is a robust phenomenon, capable of producing outputs that correspond closely to counterpart states. What exactly are the counterpart states of the outputs of motor imagination? They are events of motor pro-duction, events occurring in the motor cortex that direct behavior. Such events have minimal levels of consciousness and little, if any, phenomenol-ogy, so it would be difficult to advance a thesis of phenomenological re-semblance between motor imagination and genuine motor guidance. But lots of experimental research support a fairly strong neural resemblance thesis.
To avert possible confusion, motor imagination should be distinguished from mirror-neuron activity. Mirroring activity is an involuntary response to perceptual stimuli, whereas motor imagination is subject to voluntary control and not normally driven by any distinctive class of perceptual stimuli. Re-searchers assume voluntary control of motor imagination when they instruct subjects to imagine doing certain movements, and subjects routinely report complying with such instructions. Compliance seems to be genuine, because in many studies highly significant changes occur. A salient example is en-hancement in athletic performance. In one dramatic demonstration, Yue and Cole (1992) compared the increase in muscular strength among subjects who actually trained with the increase in subjects who generated mere motor imagery. Actual training produced a 30 percent increase in maximal force;
motor imagery produced a 22 percent increase. The effect is the product of
cortical activity, not covert muscular activity, because subjects did not make covert muscular contractions during motor imagery.
Many studies of motor imagery have used chronometric methods. Mental simulation time mimicked real movement time, and many other kinematic properties were also preserved in the simulations. Decety, Jeannerod, and Preblanc (1989) measured the time it took subjects to walk to a target. When they were blindfolded and encouraged to imagine walking to the target, imagined walking times were very similar to actual walking times. Similarly, subjects either actually walked or imagined walking on beams with different widths. It was assumed that the narrower the width, the more difficult the task. A clear effect of task difficulty was found in both actual movement times and mental movement times. This suggests that mental simulations of action are supported by sensorimotor structures used for real action.
If common use of sensorimotor structures obtains, motor pathologies should affect motor imagery in the same ways they affect motor performance.
This hunch is indeed borne out. Dominey, Decety, Brouselle, Chazot, and Jeannerod (1995) examined patients with Parkinson’s disease who were significantly slower on a finger-sequencing task with one hand than with the other. In an imagined performance of the same task, this timing asymmetry between affected and unaffected hands was closely matched. Sirigu et al.
(1995) tested a subject with motor cortex damage on finger, arm, and leg movements. A correlation was found between actual and imagined movement times. One test involved a finger movement task to keep up with a metro-nome. Doing the task in imagination, the patient kept up with the metronome to 95 beats per minute in the impaired hand and 160 beats with the intact hand. Her later actual performance was very close to this: maximum speeds were 90 and 170 beats.
Brain imaging studies provide additional evidence of shared motor repre-sentations as between motor execution and imagery of action. Experiments with patients who had impairments in motor imagery following lesions in the parietal area provide clear evidence of this sharing. Sirigu et al. (1996) studied mentally simulated hand movements in four patients with unilateral left or right parietal lobe lesions. All patients experienced movement difficulties restricted to the hand and fingers. Nine normal individuals showed excellent congruence be-tween maximum imagined and executed movement speeds. In contrast, patients with parietal cortex lesions produced estimates that were systematically inac-curate or that were inconsistent from one trial to the next. Thus, the ability to estimate manual motor performance through mental imagery is disturbed after parietal lobe damage. For two patients with right parietal lesions, imagined movements of the intact hand accurately predicted actual motor performance, indicating that their deficit is a selective incapacity to generate a mental rep-resentation of the relevant hand’s movements. Sirigu and colleagues concluded that mental rehearsal of a motor act involves internal simulation.
Marc Jeannerod (2001) reviews a large number of regions in which neural activity for executed action and neural activity for imagined action is very similar. With regard to primary motor cortex, for example, fMRI studies demonstrate that pixels activated during contraction of a group of muscles are also activated during imagery of a movement involving the same muscles (Roth et al., 1996). The level of activation during imagery is about 30 percent of the level observed during execution, which is still substantial. Lotze et al.
(1999) did additional fMRI tests to study other motor-related subsystems, such as the premotor cortex and supplementary motor area (SMA). Both the premotor cortex and SMA were equally activated during both actual and imagined movement. They characterized their results as ‘‘support[ing] the hypothesis of functional equivalence of motor imagery and motor prepara-tion’’ postulated by Jeannerod (Lotze et al., 1999: 494).
If motor imagery (in normal subjects) so closely resembles the ‘‘real thing,’’ how does the system distinguish an imagined event from its genuine counterpart? Why doesn’t motor imagery result in muscular responses? The same question arises equally for visual imagery. Why doesn’t visualization produce hallucination (radically false belief about the environment)? In the case of motor imagery, the accepted wisdom is that motor imagery is like motor production except for an added inhibitory signal that prevents overt movement (Lotze et al., 1999). One study provides striking confirmation for this accepted wisdom. Schwoebel and colleagues (2002) report the case of C.W., a patient with bilateral parietal damage from two separate strokes.
When C.W. imagines movements, he actually produces them, but without being aware of doing so. He gives every indication of understanding in-structions to merely imagine certain hand movements, and he reports no awareness of overtly moving his hands when following the instructions. So it appears that the inhibitory signal has been selectively removed by his parietal damage. In his case, motor imagination has the unintended consequence of following through to execution (M. Wilson, 2003). This study lends further support to the case for close similarity between motor imagery and actual motor production.8 It also alerts us to a phenomenon important for under-standing simulational mindreading, namely, that simulations can ‘‘leak out’’
or ‘‘spill over’’ into genuine, nonsimulational activity.9