In four experiments, a multidimensional signal detection analysis was used to determine the influence of length, diameter, and mass on haptically perceived heaviness with and without vision. This analysis allowed us to test for sensory and perceptual interactions between mass and size. As in previous research, sensory interactions were apparent in all four experiments. A novel result was the appearance of perceptual interactions that became more prominent when diameter varied and when vision was allowed. Discussion focuses on how vision and the modalities of touch (i.e., haptic and dynamic) might influence which interactions appear in the data.
Perceiving the weight of an object in the hand is common to many real-world and laboratory activities. Despite its ubiquity, though, it is not a simple process. When we grasp an object to judge its weight, we sense its physical properties (e.g., its mass), process this information to form a percept of weight, and then make a decision about how to transform this internal percept into an outward report of heaviness.1 These three subprocesses-sensory, perceptual, and decisional-combine to guide our reports of perceived heaviness. These subprocesses also provide three opportunities for features other than mass (e.g., size) to influence perceived heaviness. Disentangling such effects (i.e., determining how each feature influences each subprocess) can be difficult because each can produce equivalent effects on perceptual reports of heaviness. However, contemporary psychophysical techniques, based on signal detection theory, offer a way to distinguish the effects of sensory, perceptual, and decisional subprocesses for multidimensional stimuli. In the present experiments, such multidimensional signal detection techniques were applied in order to determine the influence of length, diameter, and mass on haptically perceived heaviness with and without vision.
Sensory, Perceptual, and Decisional Subprocesses in Weight Perception
The process of generating a perceptual report of heaviness can be divided into three subprocesses (Amazeen, 1999; Oberle & Amazeen, 2003). The first is the sensory subprocess, in which the observer detects the physical property or properties relevant to perceiving weight. The second is the perceptual subprocess, in which there is a percept or other psychological or neural code associated with the object's weight. The third is the decisional subprocess, in which the observer uses a criterion or some other rule to generate a perceived heaviness response based on the experienced percept of weight. The fact that multiple processes intervene between stimulus and response presents a major challenge to psychophysical research; namely, a perceptual report is not just a measure of one's percept but represents a combination of, or interaction among, stimulus, percept, and decision. Complicating matters even further is the fact that multiple processes may produce mathematically equivalent distributions of responses (e.g., Cohen, 2003). Psychophysical techniques based on signal detection theory (Green & Swets, 1966) can be useful in making such distinctions among subprocesses. Figure 1 illustrates these effects in the context of unidimensional signal detection theory. The participant's report of heaviness (in this case, heavier or lighter) is a function of the physical variable mass, the location of the percept along the dimension of perceived weight (measured by d'), and the decision criterion (C) used.
Weight perception presents additional challenges, though, which necessitate the use of methodological and analytical techniques that go beyond unidimensional signal detection theory. The main challenge is that the physical properties relevant to perceived heaviness include more than just the mass of the object; properties such as size (Charpentier, 1891 ), shape (Dresslar, 1894), and surface material (Ellis & Lederman, 1999; Flanagan & Wing, 1997; Flanagan, Wing, Allison, & Spencely, 1995) have all been shown to have robust influences on perceived heaviness. …