Effects of numeric magnitude on the cortical valuation network

Effects of Numeric Magnitude on the Cortical Valuation Network Frank Kanayet (kanayet.1@osu.edu) Department of Psychology, 255 Psychology Building Columbus, OH 43206 USA John Opfer (opfer.7@osu.edu) Department of Psychology, 245 Psychology Building Columbus, OH 43206 USA William Cunningham (cunningham@psych.utoronto.ca) Department of Psychology, University of Toronto, 100 St George St Toronto, ON M5S 3G3 Canada Abstract rewarded just one. A special case of using magnitude for valuation is the use of monetary rewards. By using money, dissimilar goods can be compared on the same scale (e.g., dollars or cents) and can be described with just one value, its numeric magnitude (e.g., +300 or -300). Although translation of value into a numeric scale has many benefits, it may also come with a price. Numeric magnitude, like luminance and loudness, has a compressive psychophysical scale (Fechner, 1860/1966; Weber, 1846/1948; Dehaene, 1997). Thus, the difference between 10 and 15 appears larger than the difference between 120 and 125. The compressive nature of numerical judgments is important because it may play a large role in how the brain tracks monetary value and makes economic decisions (Furlong & Opfer, 2009; Peters et al., 2008). Indeed, “unit effects” on decision-making have been known for many decades. For example, Kahneman and Tversky (1981) observed that participants were willing to trade 20 minutes of their time to save $5 on a $15 calculator, but not on a $125 jacket, even though in both cases they are trading 20 minutes of their time for the same amount of money (i.e. $5). Although, these effects can be explained by assuming that subjects pay more attention to the proportional gains, the compressive function of numeric representations might provide another explanation. Because larger numerals have smaller psychological distances between them, the difference between a $125 and $120 jacket is subjectively less than the difference between a $15 and $10 calculator. In this paper, we were interested in why neural activation also appears to devalue marginal monetary gains. Specifically, we addressed whether the neural response to increasing quantities of money are caused by increases in objective monetary value (the value hypothesis), by increases in the numeric magnitude used to represent the magnitude of the monetary reward (the number hypothesis), both, or neither. This issue is important because neuroeconomists typically assume that the brain areas responsible for processing monetary rewards are not affected by the magnitudes of the numerals that represent the rewards, but this assumption has never been tested. Previous work has identified a distributed, network of neural systems involved in appraising the value of rewards, such as when winning $100. We hypothesized that involvement of intraparietal sulcus (IPS) in this network is specialized for processing numeric rather than monetary value. To test our hypothesis, we manipulated numeric magnitude and units to construct a range of economic rewards (e.g., +$1, +100¢) in response to simple decisions. Consistent with our hypothesis, BOLD activity in IPS was related to changes in numeric magnitude, independent of monetary value, whereas activity in OFC was associated with monetary value, independent of numeric magnitude. Finally, by using representation similarity analysis, we found that the information represented in IPS and OFC was more consistent with the patterns expected if representations of numeric magnitudes or monetary values, respectively, were in a compressive scale. Together, these findings show the importance of numerical cognition for understanding how the brain processes monetary rewards. Keywords: monetary rewards; IPS; OFC; fMRI, representation similarity analysis, numerical cognition Introduction Humans are continuously faced with choices, often involving incommensurable options. When choosing among options for a romantic date, for example, it is not clear how to compare the esthetic pleasure of a movie with the gustatory pleasure of a nice dinner. Making decisions apparently requires computing the value of each option in some way that would register the difference in values (Montague, King-Casas & Cohen, 2006). Economic models have long assumed that humans behave ‘as if’ they compute value for each option in a common mental currency (i.e., subjective value), and experiments on the distributed neural correlates of valuation (i.e., valuation network) suggest that this assumption may be correct (Kable & Glimcher, 2009; Padoa-Schioppa & Asad, 2006, 2008; but see Vlaev, et al., 2011; Tremblay & Schultz, 1999; Seymour et al., 2007; Nieuwenhuis et al., 2005). One of the sources of information that comprise this common mental currency is the magnitude of the reward. Being rewarded two cookies feels better than being