The importance of response type to the relationship between temporal order and numerical magnitude.

Time, number, and space may be represented in the brain by a common set of cognitive/neural mechanisms. In support of this conjecture, Schwarz and Eiselt (Journal of Experimental Psychology: Human Perception and Performance, 35, 989-1004, 2009) found that numerically smaller digits were perceived to occur earlier than larger digits, and they concluded that this difference reflected faster processing of smaller numbers. This difference, however, could have been related to a response bias, whereby participants map responses of "which first" onto the "first" number along the mental number line. In Experiment 1, participants made temporal order judgements between digits presented to the left or to the right. The point of subjective simultaneity was shifted so that the 9 had to be presented before the 2 in order for simultaneity to be perceived. This difference could reflect either faster processing of the 2 or a response bias. Experiments 2a and 2b eliminated response biases by using simultaneity judgements, which have no logical stimulus mapping. Both of the latter experiments established that the 2 was not processed faster than the 9. Although the present results relate specifically to numerical magnitude and temporal order associations, they also have broader implications. Other studies have reported associations between dimensions such as size, duration, and number and have attributed these to parietal mechanisms. Such associations, however, may also be an artefact of response biases.