Semantic congruity affects numerical judgments similarly in monkeys and humans

Abstract

Monkeys (Macaca mulatta) were trained to order visual arrays based on their number of elements and to conditionally choose the array with the larger or smaller number of elements dependent on a color cue. When the screen background was red, monkeys were reinforced for choosing the smaller numerical value first. When the screen background was blue, monkeys were reinforced for choosing the larger numerical value first. Monkeys showed a semantic congruity effect analogous to that reported for human comparison judgments. Specifically, decision time was systematically influenced by the semantic congruity between the cue (''choose smaller'' or ''choose larger'') and the magnitude of the choice stimuli (small or large numbers of dots). This finding demonstrates a semantic congruity effect in a nonlinguistic animal and provides strong evidence for an evolutionarily primitive magnitudecomparison algorithm common to humans and monkeys.nonhuman primates ͉ numerical cognition ͉ analog magnitude ͉ distance effect H umans and nonhuman animals discriminate numbers in a way that obeys the psychophysical tenets of Weber's law (e.g., refs. 1-6; see ref. 7 for review). That is, animals and humans are faster and more accurate at comparing two numerical values as the ratio between them (min͞max) decreases. For humans, the same pattern of ratio-dependent performance emerges regardless of whether the numerical values are presented as Arabic numerals, arrays of dots, or sequences of tones (e.g., refs. 1, 8-10). This response pattern is taken to indicate that humans and animals represent approximate numerical values as imprecise mental magnitudes (e.g., refs. 4 and 7). Thus, animals and humans are thought to represent approximate numerical values in fundamentally the same way. However, the specific process by which numbers are compared in monkeys and humans has not been specified. In this study, we investigate whether monkeys show a response signature of adult human comparison judgments: the semantic congruity effect.When adult humans are asked ''Which is smaller: an ant or a rat?'', they are much quicker to respond than when asked ''Which is larger: an ant or a rat?'' (e.g., refs. 11 and 12). In contrast, when adult humans are asked to compare two large animals, such as a cow and an elephant, they are much quicker to respond when asked ''Which is larger?'' than ''Which is smaller?'' This effect is known as the semantic congruity effect and has been reported for adult humans when they compare stimuli along a variety of continua, including the distance between two cities (13), line length (14), brightness (15), the intelligence of animals (16), adjectives of ordinal quality (e.g., good, fair, poor, or excellent; ref. 17), surface area (18), and Arabic numerals (19,20). In all cases, the semantic relationship between the direction of the choice objective and the perceived magnitude of the to-be-compared entities affects the rapidity of human decision-making.For numerical comparisons, the semantic congruity effect occurs in adults when th...

The discovery and comparison of symbolic magnitudes

“…We assume that because the unmarked form of the question requires reversing the natural scale (e.g., ''smaller'' focuses attention on low magnitudes), precision diminishes more quickly with distance from the reference point in the case of the marked comparative. Our approach thus provides a mechanism by which polarity could impact magnitude judgments made by non-linguistic animals (Cantlon & Brannon, 2005). This interpretation supports the hypothesis that the linguistic differences associated with markedness in human languages can be traced to more fundamental representational differences in magnitude continua.…”

“…Like the distance effect, semantic congruity effects have also been obtained with monkeys (Cantlon & Brannon, 2005). A further phenomenon, the markedness effect, refers to the fact that for some pairs of polar adjectives, one (the ''unmarked'' form) is easier to process overall than the other (Clark, 1969).…”

“…Often the marked term is aptly applied only to the range of magnitudes extending from the negative pole to the midpoint, whereas the unmarked term can be aptly applied across the full magnitude range (Clark, 1969). However, the finding of a markedness effect in monkeys, in a design in which the two forms of the implicit query occurred on an equal number of trials during training, suggests that markedness effects cannot be fully explained by unequal frequency of linguistic use (Cantlon & Brannon, 2005).…”

“…Although the model provides a good quantitative fit to some data sets (Banks et al, 1976), it faces a number of problems as a general explanation of symbolic comparisons. Because it is based on linguistic codes, the model is severely strained by the fact that distance, congruity and markedness effects are also observed with non-linguistic primates, such as monkeys (Cantlon & Brannon, 2005;Cantlon et al, 2009). Also, the model cannot explain evidence that similar effects are observed in direct judgments of discriminability among ordered items (e.g., the form of comparative used in the question influences the relative spacing of cities along an east-west dimension as recovered by scaling methods; Holyoak & Mah, 1982).…”

The role of reference points in ordinal numerical comparisons by rhesus macaques (macaca mulatta).

“…In a separate study, we have recently obtained evidence that monkeys show similar congruence effects when making numerical comparisons. That study provides further evidence of an anchor point in the pairwise numerical judgments of monkeys (Cantlon & Brannon, 2005).…”