Neurons For Numerosity: Parietal Neurons ‘Sum Up’ Individual Items In A Group
As any child knows, to answer the question “how many,” one must start by adding up individual objects in a group. This cognitive ability is shared by animals as diverse as humans and birds.
Neurons in the lateral intraparietal area of the monkey brain have been shown to integrate information about space and time. A new study asks whether they also “add up” numerical quantity. (Credit: Article authors and PLoS Biology)
Surprisingly, the exact brain mechanisms responsible for this process remained unknown until now. In from the University of Illinois at Chicago now report novel evidence for the existence of “accumulator neurons,” which respond to increasing numbers of items in a display with progressively increasing activity, in the parietal cortex of monkeys.
The authors focused on the parietal cortex based on evidence that damage to this brain region disrupts basic mathematical skills, and is activated during functional imaging studies when people perform basic computations. To understand how parietal cortex contributes to numerical behavior, the authors studied the activity of neurons in the lateral intraparietal area in monkeys while they looked at arrays of dots on a computer screen.
Parietal neurons responded with progressively increasing activity as the total number of elements in the display was varied across a wide range of values (2-32). These neurons resemble “accumulator neurons” that have been suggested to serve the first stage in counting.
This information could be used by other neurons that respond best for a particular cardinal number, such as “4,” as have been reported in prior studies. These findings support computer models that separate the processes of summing and numerical identification, and may also explain the fact that parietal cortex damage causes both numerical and spatial confusion.
Neurons in the lateral intraparietal area in monkeys respond in a graded fashion to the number of items in a visual array during a delayed saccade task, suggesting that the neurons “sum up” individual elements to represent accumulated magnitude.
Citation: Roitman JD, Brannon EM, Platt ML (2007) Monotonic Coding of Numerosity in Macaque Lateral Intraparietal Area. PLoS Biol 5(8): e208 doi:10.1371/journal.pbio.0050208
Monotonic Coding of Numerosity in Macaque Lateral Intraparietal Area
1 Department of Neurobiology, Duke University, Durham, North Carolina, United States of America, 2 Department of Psychology and Neuroscience, Duke University, Durham, North Carolina, United States of America, 3 Center for Cognitive Neuroscience, Duke University, Durham, North Carolina, United States of America
As any child knows, the first step in counting is summing up individual elements, yet the brain mechanisms responsible for this process remain obscure. Here we show, for the first time, that a population of neurons in the lateral intraparietal area of monkeys encodes the total number of elements within their classical receptive fields in a graded fashion, across a wide range of numerical values (2–32). Moreover, modulation of neuronal activity by visual quantity developed rapidly, within 100 ms of stimulus onset, and was independent of attention, reward expectations, or stimulus attributes such as size, density, or color. The responses of these neurons resemble the outputs of “accumulator neurons” postulated in computational models of number processing. Numerical accumulator neurons may provide inputs to neurons encoding specific cardinal values, such as “4,” that have been described in previous work. Our findings may explain the frequent association of visuospatial and numerical deficits following damage to parietal cortex in humans.