Summary: According to researchers, the adult subcortex processes numbers at the same level of skill as infants.
Source: Carnegie Mellon University.
Despite major brain differences, many species from spiders to humans can recognize and differentiate relative quantities. Adult primates, however, are the only ones with a sophisticated cortical brain system, meaning that the others rely on a subcortex or its evolutionary equivalent.
Carnegie Mellon University scientists wanted to find out whether the adult human subcortex contributes to number processing at all. Published in the Proceedings of the National Academy of Sciences, their study found that the adult subcortex processes numbers at the same level as infants and perhaps other lower-order species, such as guppies and spiders.
“This study tells us a great deal about the human subcortex, most importantly that it does not appear to improve from its number abilities in infancy, while the cortex, which is more developed in humans than in any other species, does continuously develop,” said Elliot Collins, a Ph.D. student in psychology within CMU’s Dietrich College of Humanities and Social Sciences and a M.D. student in the School of Medicine at the University of Pittsburgh.
Because the subcortex’s location and small size make it hard to observe in humans using imaging techniques, the researchers conducted a series of experiments using a stereoscope. The stereoscope allowed them to present two consecutive visual stimuli either sequentially to one eye at a time or sequentially to both eyes. This was crucial since signals that enter one eye remain separated in the subcortical part of the visual system.
One hundred adults made decisions about two groups of dots to the same eye or different eyes. The results showed that numerical judgments in the one eye trials were better under one key condition: when the first and second stimuli’s quantity differed greatly, such as having a ratio of 4:1 or 3:1.
“The subcortex is not good at making fine grain number discriminations, and these findings support that,” Collins said. “Our results suggest, however, that adults with a fully operational cortex still have a subcortex with the ability to distinguish number, yet it operates on a similar level to what is found in babies, other primates and lower level species who can make coarse computations of large ratios such as, for example, which shoal of fish is bigger and should be joined. This provides evidence of a potential evolutionary bridge between the human adult subcortex and the brain of lower order species.”
CMU’s Marlene Behrmann, the Cowan University Professor of Cognitive Neuroscience, and the University of Massachusetts’ Joonkoo Park, who received his master’s in human-computer interaction from CMU, also participated in the study.
Funding: Grants from the National Science Foundation, Temporal Dynamics of Learning Center, NIH Medical Scientist Training Program and NIH Predoctoral Training supported this research.
Source: Shilo Rea – Carnegie Mellon University
Image Source: NeuroscienceNews.com image is credited to Melissa Neely for Carnegie Mellon University.
Original Research: Abstract for “Numerosity representation is encoded in human subcortex” by Aristeidis Sotiras, Elliot Collins, Joonkoo Park, and Marlene Behrmann in PNAS. Published online March 20 2017 doi:10.1073/pnas.1613982114
Numerosity representation is encoded in human subcortex
Certain numerical abilities appear to be relatively ubiquitous in the animal kingdom, including the ability to recognize and differentiate relative quantities. This skill is present in human adults and children, as well as in nonhuman primates and, perhaps surprisingly, is also demonstrated by lower species such as mosquitofish and spiders, despite the absence of cortical computation available to primates. This ubiquity of numerical competence suggests that representations that connect to numerical tasks are likely subserved by evolutionarily conserved regions of the nervous system. Here, we test the hypothesis that the evaluation of relative numerical quantities is subserved by lower-order brain structures in humans. Using a monocular/dichoptic paradigm, across four experiments, we show that the discrimination of displays, consisting of both large (5–80) and small (1–4) numbers of dots, is facilitated in the monocular, subcortical portions of the visual system. This is only the case, however, when observers evaluate larger ratios of 3:1 or 4:1, but not smaller ratios, closer to 1:1. This profile of competence matches closely the skill with which newborn infants and other species can discriminate numerical quantity. These findings suggest conservation of ontogenetically and phylogenetically lower-order systems in adults’ numerical abilities. The involvement of subcortical structures in representing numerical quantities provokes a reconsideration of current theories of the neural basis of numerical cognition, inasmuch as it bolsters the cross-species continuity of the biological system for numerical abilities.
“Numerosity representation is encoded in human subcortex” by Aristeidis Sotiras, Elliot Collins, Joonkoo Park, and Marlene Behrmann in PNAS. Published online March 20 2017 doi:10.1073/pnas.1613982114