Human’s ability to work upon, or manipulate objects in the environment depends on the psychological function to perceive objects’ size appropriately. In daily situation, human recognizes object size by using visual and haptic cues concurrently. In experimental psychology, human visual information processing of object size has long been a major research area, and substantial amount of knowledge has been accumulated, but relatively little is known concerning the haptic information processing. In this study, characteristics of haptic size perception were examined in discrimination and magnitude estimation (ME) experiments for ‘one-dimensional (1D)‘ stimuli (bars of which lengths were varied) and ‘two-dimensional (2D)’ stimuli (circles, triangles and squares of which areas were varied). In the experiments, participants held the 1D standard and comparison stimuli with two fingers (thumb and index or middle finger), or 2D stimuli with three (or four) fingers (thumb, index, and middle (plus third) fingers), and discriminated or evaluated the sizes of the stimuli. The results of the discrimination experiment showed that Weber ratios, defined as discrimination threshold/standard stimulus size, for the two 2D stimuli (circles and squares, but not for triangles) were lower (i.e., discrimination sensitivities were higher) than those for the 1D stimuli. The difference in the discrimination sensitivities was explained by taking into account probability summation between multiple inter-finger distance cues available for the 2D stimuli. Weber ratio for the triangles may be affected by the uncertain factor that the figures were not identified only by the information of contact points. The results of the ME experiment showed that the evaluated value vs. stimulus size functions had different slopes for the 1D and 2D stimuli, suggesting that size perception for the 1D and 2D stimuli may be mediated by different mechanisms specialized for processing of the length and the area of haptic stimuli.
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