... the subject's response time increasing proportionally to the number of block stimuli present, as evidence to his theory (Cohen & Blair, 1998). This was a conclusion shared by Shepard and Metzler (1971) (as cited in Cooper and Shepard, 1973) based on their initial mental rotation experiments. They reported that Reaction Time (RT), when comparing two, same shape, three-dimensional line drawings, increased linearly with the angular difference between the two (Koriat & Norman, 1985). Shepard and Metzler took this to mean that the increasing delay in responding was due to the time taken to rotate the stimulus in the subjects mind increasing in relation to the "complexity" of the stimulus (Cooper & Shepard, 1973). Cooper and Shepard's (1973) data conflicted with that of Shepard and Metzler's. They found that their results, when graphed, showed a quadratic (non-linear) function for rotated, normally presented alphanumeric stimuli, rather than the linear plot found ...

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