Grasping follows Weber's law: How to use response variability as a proxy for JND.

Weber's law is a fundamental psychophysical principle. It states that the just noticeable difference (JND) between stimuli increases with stimulus magnitude; consequently, larger stimuli should be estimated with larger variability. However, visually guided grasping seems to violate this expectation: When repeatedly grasping large objects, the variability is similar to that when grasping small objects. Based on this result, it was often concluded that grasping violated Weber's law. This astonishing finding generated a flurry of research, with contradictory results and potentially far-reaching implications for theorizing about the functional architecture of the brain. We show that previous studies ignored nonlinearities in the scaling of the grasping response. These nonlinearities result from, for example, the finger span being limited such that the opening of the fingers reaches a ceiling for large objects. We describe how to mathematically take these nonlinearities into account and apply this approach to our own data, as well as to the data of three influential studies on this topic. In all four datasets, we found that-when appropriately estimated-JNDs increase with object size, as expected by Weber's law. We conclude that grasping obeys Weber's law, as do essentially all sensory dimensions.

The SNARC Effect in Chinese Numerals: Do Visual Properties of Characters and Hand Signs Influence Number Processing?

The SNARC effect refers to an association of numbers and spatial properties of responses that is commonly thought to be amodal and independent of stimulus notation. We tested for a horizontal SNARC effect using Arabic digits, simple-form Chinese characters and Chinese hand signs in participants from Mainland China. We found a horizontal SNARC effect in all notations. This is the first time that a horizontal SNARC effect has been demonstrated in Chinese characters and Chinese hand signs. We tested for the SNARC effect in two experiments (parity judgement and magnitude judgement). The parity judgement task yielded clear, consistent SNARC effects in all notations, whereas results were more mixed in magnitude judgement. Both Chinese characters and Chinese hand signs are represented non-symbolically for low numbers and symbolically for higher numbers, allowing us to contrast within the same notation the effects of heavily learned non-symbolic vs. symbolic representation on the processing of numbers. In addition to finding a horizontal SNARC effect, we also found a robust numerical distance effect in all notations. This is particularly interesting as it persisted when participants reported using purely visual features to solve the task, thereby suggesting that numbers were processed semantically even when the task could be solved without the semantic information.

Semantic grasping escapes Weber's law.

According to Weber's law, the just noticeable difference between stimuli increases proportionally with stimulus magnitude, suggesting that perception becomes more variable when a stimulus becomes larger. Surprisingly, this basic psychophysical principle appears to be violated in grasping because the variability of grasping movements does not increase with object size. This dissociation between perception and grasping has been interpreted either as evidence for different neuronal processing of real-time visual size information [Ganel, T., Chajut, E., Algom, D. (2008a). Current Biology, 18(14), R599-R601], or for the idea that grasping ignores stimulus size and is based on position information only [Smeets, J. B. J., and Brenner, E. (2008). Current Biology, 18(23), R1089-R1090]. Both accounts assume that it is the processing of visual information that leads to the absence of Weber's law in grasping. We show that even if neither visual nor any real-time sensory information about the stimulus is presented (but only abstract, semantic information about its size), grasping does not follow Weber's law. This indicates that other mechanisms must be responsible for the unexpected behavior of grasping.