title: Numerical acuity of fish is improved in the presence of moving targets, but only in the subitizing range.
abstract: There is controversy in comparative psychology about whether on the one hand non-symbolic number estimation of small (≤4) and large numbers involves a single mechanism (an approximate number system), or whether on the other hand enumeration of the numbers 1-4 is accomplished by a separate mechanism, an object tracking system. To date, support for the latter hypothesis has come only from the different ratio-dependency of performance seen in the two numerical ranges, a reading that has been criticized on several grounds. In humans, the two-system hypothesis is supported by evidence showing that manipulation of the physical properties of the stimuli (e.g., the motion of the items) has dissimilar effects on small- and large-number discrimination. In this research, we studied this effect on guppies. Initially, fish were trained to simultaneously discriminate two numerical contrasts having the same easy ratio (0.50): one in the small-number (2 vs. 4) range and one in the large-number (6 vs. 12) range. Half of the fish were presented with moving items; the other half were shown the same stimuli without motion. Fish were then subjected to non-reinforced probe trials in the presence of a more difficult ratio (0.75: 3 vs. 4 and 9 vs. 12). Under both static and moving conditions, the fish significantly discriminated 6 versus 12, but not 9 versus 12 items. As regards small numbers, both groups learned to discriminate a 0.50 ratio, but only fish tested with moving stimuli also discriminated 3 and 4 items. This differential effect suggests that fish may possess two separate systems for small- and large-number discrimination.
authors.affiliation: Department of General Psychology, University of Padova, Via Venezia 8, 35131, Padua, Italy, christian.agrillo@unipd.it.
authors.forename: Christian
authors.initials: C
authors.lastname: Agrillo
authors.forename: Maria Elena
authors.initials: ME
authors.lastname: Miletto Petrazzini
authors.forename: Angelo
authors.initials: A
authors.lastname: Bisazza
mesh_terms: D000818:Animals; D004193:Discrimination Learning / Q000502:physiology; D004192:Discrimination, Psychological; D005260:Female; D008297:Male; D009038:Motion; D011036:Poecilia / Q000502:physiology; D014796:Visual Perception