An interesting series of experiments has recently been reported in Science. (Regani et al 2015). These strongly support the idea that many animals and humans represent numbers by a mental number line where smaller values are located on the left and larger values on the right. See Spatial Representation of Number
In the first experiment 3-day-old chicks, once familiarized with a target number 5, spontaneously associated a smaller number (2) with their left side and a larger number (8) with their right.
In the second experiment, when the trained target number was 20, the smaller number (8) was associated with the left side, rather than right side as it was in the first experiment, and the larger number (32) was associated with the right side. Showing that the effect is relative to the trained number and importantly using numbers that are outside the normal subitising range (greater than 5).
In both experiments once trained the chicks were presented with pairs of the same number, thus 2 versus 2, 8 versus 8 and 32 versus 32.
In humans and animals there is a distance effect. Numerical comparisons become easier as the difference between the numbers increases. In both experiments the difference between the target number and the smaller and larger numbers was the same, (5 – 2) = (8 – 5) = 3 in experiment 1 and (20 – 8) = (32 – 20) = 12 in experiment 2.
There is also a size effect with comparisons between larger numbers becoming more difficult.
Mental Number Line Direction
This research strongly supports the idea that the direction of the default mental number line is from left to right. However there is evidence that this default direction is fairly easy to un-train, for instance by asking subjects to image the numbers as being on a clock face. (Bächtold et al 1998) Habitual reading direction can also effect the direction of the number line. (Shaki et al 2009) Notwithstanding this, it is clear that cultural preferences, like reading or finger counting direction, cannot in any way cause the effects being exhibited here.
With brain asymmetry, there is a right hemisphere dominance in attending to visuospatial and or numerical information (Rogers et al 2013)
So when patients with a right parietal lesion and therefore a spatial deficit for left-side stimuli, are asked to point to the mid-point of a line they point to a position right of centre. Or if asked for a number halfway between 2 and 6 might reply 5. (Zorz1 et al 2002)
Brain asymmetry, like that described above, is a common and ancient attribute of vertebrates so that:
“Interspecific similarities suggest a continuous and analogical nonverbal representation of numerical magnitude. This indicates that numerical competence did not emerge de novo in linguistic humans but was probably built on a precursor nonverbal number system “
Thanks to my biologist son Noah, for bringing this research to my attention and knowing that I would be interested in it.
Bächtold D, Baumüller M, Brugger P. (1998) Stimulus-response compatibility in representational space Neuropsychologia 36(8):731-5
Dehaene S., Bossini S., & Giraux P. (1993). The mental representation of parity and number magnitude. Journal of Experimental Psychology: General 122: 371-396.
Regani R., Vallortigara G., Priftis K., & Regolin L. (2015) Number-space mapping in the newborn chick resembles humans’ mental number line Science. DOI:10.1126/science.aaa1379
Rogers, L. J., Vallortigara G. , R. J. Andrew R. J. (2013) Divided Brains Cambridge Univ. Press, New York.
Shaki S., Fischer M., & Petrusic W. (2009). Reading habits for both words and numbers contribute to the SNARC effect Psychonomic Bulletin & Review, 16 (2), 328-331
Zorzi M., Priftis K. and Umila C. (2002) Brain Damage: Neglect disrupts the mental number line. NATURE | VOL 417 | 9 MAY 2002 | http://www.nature.com 138-139