Independent Sensorimotor and Perceptual Predictions of Weight

As discussed previously, people use visual information about object size and shape to estimate parametrically the impending force requirements in manipulation. Thus, people will increase grip and load force more rapidly when lifting a large object than a similar looking small object. This feedforward strategy takes advantage of the link between size and weight that normally pertains to a class or family of similar objects; for example, big cups should weigh more than small ones. However, it fails when this link is altered. In such a case, people must rely on reactive control mechanisms to correct for their erroneous prediction and on feedback mechanisms to tune the internal models used for predictive control. Such a situation arises in the classic size-weight illusion in which people are asked to compare the weights oftwo equally weighted objects of similar form but unequal size. This illusion, first documented more than 100 years ago, refers to the fact that people reliably judge the smaller of the two objects to be heavier when lifted, even after many lifting trials.

A leading theory of the size-weight illusion is that the illusion arises from a mismatch between predicted and actual sensory feedback. The idea is that when we lift the smaller object, the actual sensory feedback about liftoff will not occur when predicted and the object will thus be judged heavier. Conversely, the larger object, which is lighter than expected, will be judged heavier.

The sensory mismatch seems entirely plausible when one considers lifting the two equally weighting objects the very first time. Here, visual size cues will be misleading and we would expect people to use too much force for the larger object and too little force for the smaller object. However, we also know that people acquire sensorimotor memory related to object weight over repeated lifts. The question arises whether people will continue to misjudge the force required when repeatedly lifting large and small objects of equal weight. Figure 9 reveals the answer. People were asked to repeatedly lift a small and a large cube (Fig. 9A) in alternation. Predictably, when the two objects are lifted for the first time, the forces required for the large object are overestimated and the forces required for the small object are underestimated (Fig. 9B, left). Compensatory, reflex-mediated adjustments in force are triggered in either case. When lifting the small object, the initial increase in grip force and load force is too small and liftoff does not occur when expected. As

Figure 9 Independent sensorimotor and perceptual predictions of weight. (A) Drawing showing the relative sizes of two equally weighted cubes. Subjects lifted the cubes using a precision grip with the tips of the index finger and thumb on either side of a handle. The handle was attached by clips located on top and in the center of each object. The handle was instrumented with two sensors that measure the forces and torques applied by each digit. Plastic contact disks (3 cm in diameter) were mounted on each sensor and covered in medium-grain sandpaper. A light-sensitive diode embedded into the center of the lifting platform recorded liftoff. (B) Grip force (GF), load force (LF), grip and load force rates, and light-sensitive diode recorded in the first trial (lifts 1 and 2) and the eighth trial (lifts 15 and 16). The subjects lifted the large object (thick traces) and then the small object (thin traces) in each trial. In all trials, subjects grasped the object and increased grip and load force together until liftoff, signaled by the light diode, occurred. In the first trial, peak grip and load force rates were scaled to object size, whereas by the eighth trial the peak force rates were similar for the two objects and appropriately scaled to object weight. Although the subjects adapted their motor output to the true object weights, they still reported verbally that the small object was heavier (adapted with permission from Flanagan, J. R., and Beltzner, M. A., Nature Neurosci. 3, 737741,2000).

a result, the forces increase again until liftoff is achieved. When lifting the large object, overshoots occur in the grip and load forces and liftoff occurs earlier than expected. The unexpected early liftoff triggers a decrease in force approximately 100 msec later. However, a very different pattern of force output is observed by the time the cubes are lifted for the eighth time (Fig. 9B, right). Now the force and force rate functions for the small and large cubes are very similar and liftoff occurs at about the same time for both cubes. In contrast to the initial lift trials, grip and load force neither overshoot nor undershoot their final levels, and no corrective adjustments in force are observed. These results illustrate that people adapted their force output, and thus their sensory predictions used for force control, to the actual object weights. Thus, sensorimotor memory about object weight, obtained from previous lifts and based on somatosen-sory information, comes to dominate visual size cues in terms of feedforward force control.

Although the motor system gradually adapts force output to the true, equal weights of the size-weight stimuli, the perceptual system that mediates awareness of object weight does not adapt. After lifting the two cubes 20 times each, people still reported that the small object was heavier. Moreover, the strength of the size-weight illusion—measured using magnitude estimation techniques—is equally strong. That people experience the size-weight illusion while accurately predicting the fingertip forces required for lifting clearly debunks the theory that the perceptual illusion is accounted for by a sensory mismatch. Instead, the results indicate that the illusion can be caused by highlevel cognitive factors. Although the size-weight illusion occurs while there is no evidence of mismatch at the sensorimotor level, the mismatch theory may still operate at a purely perceptual level. For example, people may continue to make erroneous perceptual predictions about weight based specifically on visual size cues. A mismatch between these perceptual predictions and actual sensory feedback may give rise to the size-weight illusion. This implies separate comparison processes for perceptual and sensorimotor predictions.

The finding that people continue to experience the size-weight illusion even though they learn to make accurate sensorimotor predictions about object weight indicates that sensorimotor systems can operate independently of perceptual systems. This idea is supported by a growing body of research on visuomotor control showing that partly distinct neural pathways are used depending on whether the sensory information is used to control actions or make perceptual judgments.

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