Because haptic perception of objects generally involves manipulation, the question arises as to whether the perception of particular object properties is influenced by other object properties or by the way in which the object is handled. For example, does the perceived weight of an object depend on the angle of its contact surfaces or the friction between the object and the digits, both of which influence the grip force required to lift the object? Here, one question is whether the grip forces in lifting influence weight perception even though the grip forces are not directly involved in overcoming the force ofgravity. For example, does the greater effort required to lift a slippery object give rise to the perception of it being heavier than a less slippery object of the same weight?
More than 150 years ago, Ernst Heinrich Weber observed that the ability to discriminate weight is better when the weights are actively lifted by the hand than when they are supported by a passive hand. This observation suggests that a sense of effort, associated with voluntary muscular exertion, contributes to the perception of weight. Although afferent signals contribute to weight perception, at least under some conditions there is ample evidence that effort, defined as the level of central or efferent drive, contributes to weight perception. The idea is that when we generate motor commands to lift an object, a copy of the commands (efference copy) generates an internal sensation (corollary discharge) that influences perceived weight. The centrally generated sensation is referred to as the sense of effort.
Figure 8A shows the results of an experiment in which people were asked to compare the weights of a reference object and a series of randomly presented test objects of varying weight both heavier and lighter than the weight of the reference. The test objects had the same size and shape as the reference object, and the objects were lifted using a precision grip with the tips of the index fingers on either side. In one condition, the reference object was covered in less slippery sandpaper and the test objects were covered in more slippery satin (Fig. 8, solid circles and solid curve), whereas in a second condition the reference object was covered in satin and the test objects were cover in sandpaper
80.1 94.3 108.5+122.7 136.9 151.1 80.1 94.3 108.5+122.7 136.9 151.1
Test object weight (grams) Test object weight (grams)
Figure 8 Probability (n=14) of responding that the test canister is lighter than the previously lifted reference canister as a function of the test canister weight. In different experiments, the canisters were lifted with either a vertical (A) or horizontal (B) precision grip. Open circles and dashed lines code the condition in which the test canister was covered in less slippery satin, and the closed circles and solid lines code the condition in which the test canister was covered in less slippery sandpaper. The triangles indicate the reference weight (modified with permission from Flanagan, J. R., Wing, A. M., Allison, S., and Spencely, A., Perception Psychophys. 57, 282-290, 1995).
(Fig. 8, open circles and dashed curve). Figure 8A shows the probability of judging the test object to be heavier than the reference as a function of the weight of the test object. In both conditions, when the test object is much heavier (151.1 g) than the reference (115.6 g) the test object is always judged to be heavier. Conversely, when the test object is much lighter (80.1 g), it is never judged to be heavier. However, in between these extremes, the probability of judging the test object to be heavier is greater when the test object is covered in slippery satin. (Note that there is a general tendency to judge the second of two successively lifted weights, in this case the test object, to be heavier.) This indicates that when lifting with the fingertips on the sides of the object, a more slippery object is judged heavier than an equally weighted object that is less slippery. One interpretation of the results shown in Fig. 8A is that humans judge the more slippery object to be heavier because the grip force used in lifting is greater. When people hold the reference and test objects with a horizontal grip (Fig. 8B), in which surface slipperiness has little influence on the required grip force, there is no effect of surface slipperiness on weight perception.
The results shown in Fig. 8A suggest that people fail to fully distinguish between the effort related to grip force and that related to load force when judging weights lifted with a precision grip. However, this overflow effect may only pertain to muscle actions that are functionally related. Support for this view comes from the observation that the perceived heaviness of a given weight, lifted by one digit, increases if a concurrent weight is lifted by any other digit of the same hand. When the foot or other hand lifts the concurrent weight, the perceived heaviness is not affected.
Although differences in grip force influence weight perception when these differences are determined by frictional conditions, grip force does not appear to influence perceived heaviness when it is manipulated by changing surface shape. When people compare the weights of triangular blocks lifted either on the angled or flat side, there is no effect of angle of perceived weight. It may be that when the grip force requirements strongly match those prescribed by visual cues, people suppress the effort related to grip force differences in evaluating weight. Recall that visual cues related to surface angle can be used effectively for feedforward force control but that there is no evidence that visual information related to frictional condition can be exploited for anticipatory force control.
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