Introduction: Studies on mental arithmetic show that location on the number line can be malleable such that addition causes location biases to the right and subtraction to the left (Pinhas & Fischer, 2008). Motion can cause illusory shifts in location (eg Ramachandran & Anstis, 1990) so we wondered if motion could shift location on the number line. Method: Participants viewed displays of a single-digit number at fixation with moving-dot patterns in the background. The primary, timed task was to categorize numbers as 5 (digits 2,3,4), =5, or 5 (6,7,8). Half of the trials had target 5 to reduce the response time to that target; this also caused target 5 to be treated as a spatial anchor that could be compared to the other numbers. Response times tend to be slower to numbers close to category boundaries compared with numbers farther away (Dehaene, 1997) so we used RT as a measure of distance to boundaries. In the display background, coherent moving dots were shown traveling rightward or leftward, randomly from trial to trial. A secondary, untimed task was to indicate the direction of the background motion, right or left. Results: Reaction times for target 4 were slower with rightward background motion suggesting the motion shifted 4 to the right, closer to the category border with target 5. RTs for target 6 were faster with rightward motion suggesting that 6 was also shifted to the right and farther from target 5. Leftward background motion had the opposite effect. Conclusion: Background motion caused number categorization RTs to be asymmetric, depending on the motion direction. Motion shifted apparent distances between anchor target 5 and other digits.
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