Zebrafish excel in number discrimination under an operant conditioning paradigm

Abstract

Numerical discrimination is widespread in vertebrates, but this capacity varies enormously between the different species examined. The guppy (Poecilia reticulata), the only teleost examined following procedures that allow a comparison with the other vertebrates, outperforms amphibians, reptiles and many warm-blooded vertebrates, but it is unclear whether this is a feature shared with the other teleosts or represents a peculiarity of this species. We trained zebrafish (Danio rerio) to discriminate between numbers differing by one unit, varying task difficulty from 2 versus 3 to 5 versus 6 items. Non-numerical variables that covary with number, such as density or area, did not affect performance. Most fish reached learning criterion on all tasks up to 4 versus 5 discrimination with no sex difference in accuracy. Although no individual reached learning criterion in the 5 versus 6 task, performance was significant at the group level, suggesting that this may represent the discrimination threshold for zebrafish. Numerosity discrimination abilities of zebrafish compare to those of guppy, being higher than in some warm-blooded vertebrates, such as dogs, horses and domestic fowl, though lower than in parrots, corvids and primates. Learning rate was similar in a control group trained to discriminate between different-sized shapes, but zebrafish were slightly more accurate when discriminating areas than numbers and males were more accurate than females. At the end of the experiment, fish trained on numbers and controls trained on areas generalized to the reciprocal set of stimuli, indicating they had used a relational strategy to solve these tasks.

Symbolic quantitative cognition in wild zebrafish (Danio rerio)

“…Initial trials did not distinguish between individual fish, although we observed considerable interindividual differences, consistent with the literature which finds that half 97 or even substantially more 109 individuals among zebrafish do not reach learning criteria for these tasks. The population trend was also compared with batch-specific results and no departure from population-level results were found (data not shown).…”

“…Therefore, in line with the literature 97,109 , significant associative learning does occur in this task among a subset (n=5) of fish tested (n=12), while the remaining fish tested showed freezing or frantic movement in response to the stimulus or considerable side bias in their responses within the first few days of training. Since all fish tested in this experiment were individually tracked, we could compare their side bias and WSLS score alongside their choices between the two visual stimuli (data for all five fish shown in Figure 5).…”

“…While Bisazza and Santaca 97 do not attempt to explain the difference much, methodological differences may explain differing outcomes in numerosity experiments on a given model system 152,153 . The salience of the visual stimuli used may also impact outcomes 149 - Bisazza and Santaca 97 used simpler and more ecologically relevant circular visual stimuli than Agrillo et al 109 , who used a variety of abstract shapes. In fact, it was this consideration that prompted us to use simple lines of the kind seen in Roman numerals rather than the more abstract symbolic stimuli used by Agrillo et al 109 , or Arabic numerals.…”

“…This is therefore a first attempt to investigate associative quantitative learning of a visual predictor of reward quantity in wild zebrafish - where both visual stimulus and food reward bear numerosity along with magnitude, and the stimulus functions as a symbol with predictive value for the associated reward quantity. Given what is known about nonsymbolic numerosity in zebrafish 97,109 , the investigation of symbolic quantitative cognition is a next step necessary to examine the ability of zebrafish to perform numerical operations, in order to understand the cognitive abilities of a model system in which calcium imaging can cast a wide net across the brain in examining the neural basis of quantitative abilities as well as the genetic underpinnings of clinical variations such as dyscalculia 97,142 .…”

“…If so, this would impact performance in any quantitative learning task involving sets of visual stimuli, including our own. However, a recent set of work contests both these findings 109 , suggesting that zebrafish perform well 97 , and equivalently to other fish (guppies 150 ) in numerical tasks as well as in tasks involving visual shape discrimination 151 . While Bisazza and Santaca 97 do not attempt to explain the difference much, methodological differences may explain differing outcomes in numerosity experiments on a given model system 152,153 .…”