Constraints on haptic integration of spatially shared object dimensions

A study of the haptic integration of texture, shape, and hardness of nonplanar solid objects is reported. In experiment 1 the relative discriminability of the objects along each dimension was assessed. While levels of texture and shape were equally discriminable, hardness discriminations proved considerably more difficult. The extent of dimensional integration in a speeded classification task when both dimensions could be extracted from the same local patch was investigated in experiments 2 and 3. In experiment 2 subjects were initially encouraged to attend to a nontargeted dimension covarying with a targeted one. The nontargeted dimension was subsequently held constant (withdrawn). In experiment 3 dimensional variation was introduced which was uncorrelated with the targeted property during the course of categorization and hence discouraged subjects from attending to the nontargeted property. The results of these two studies converged in showing evidence of bidirectional dimensional integration between texture and shape and unidirectional integration when hardness was the targeted dimension. The failure to integrate hardness into categorization based on texture or shape was attributed to the difficulty of hardness discriminations. Integration effects in experiment 3 were not consistently smaller than those in experiment 2, which suggests a strong involuntary component to dimensional integration. The results of an analysis of the accompanying hand movements are interpreted in terms of constraints on dimensional integration. Implications for visual, cross-modal, and two-handed codimensional processing are also discussed.