**2.3 Bottom-up and top-down influences on heaviness perception**

As peripheral or bottom-up processing issues related to perceptual system of heaviness, individual learning and sensitivity in weight discrimination are important factors. The Weber fraction, i.e., weight sensitivity, widely differs among individuals (0.02~0.16) (Holway et al., 1937; Raj et al., 1985). Age, especially, is a crucial factor leading to a decrease of sensitivity to weight or heaviness (Gandevia, 1996; Dijker, 2008). Serious deterioration is also reported for neural disorders including leprous neuropathy (Raj, et al., 1985), lesions to inferior-frontal cortex including PMv (Halstead, 1945), and left parietal and temporal lesions (Li et al., 2007).

As for higher level, cognitive-based or top-down processing, individual learning or experience is also an important factor to influence perception of heaviness (Fig. 1). An expectation that a larger object should be heavier than a smaller object, for example, is thought to affect perceived heaviness. Accordingly, when lifted, the larger of two objects of equal weight tends to be perceived as lighter than the smaller (H.E. Ross, 1969; Davis & Roberts, 1976; Gordon et al., 1991; Rabe et al., 2009; Buckingham & Goodale, 2010). This expectation factor is also experienced in relation to object colour (Payne, 1958), object material (Ellis & Lederman, 1999; Buckingham et al., 2009), and even the human conditions of gender and age (Dijker, 2008). For example, when viewed, darker or metallic objects are judged to be heavier than brighter or wooden ones, but are then perceived as lighter when actually lifted under same-weight conditions. When confronting conflicting issues, such as with the SWI, when two objects have different size but are of identical weight, subjects tend to rationalize that if the weight is the same, the larger object should be lighter, rather than depending on the current sensations of heaviness (Mon-Williams & Murray, 2000). This tendency seems to be more pronounced when subjects are subject to the forced-choice condition in which they must choose either Heavier or Lighter (Mon-Williams & Murray, 2000; Buckingham & Goodale, 2010). This raises the question, what is the best way to obtain accurate and natural responses from subjects: using the two category method "Heavier and Lighter", the three category method "Heavier, Lighter, and Similar", the bimanual or unilateral matching methods, or the magnitude estimation method? Other questions posed by experimenters such as, "Which is heavier?" or "Which is the heaviest" might cognitively bias perception of heaviness.
