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Chicago, IL, USA. As children know, answering the question how many are there? means you must add up individual objects in a group to get an answer. This is a cognitive ability is shared by animals as diverse as humans and birds, but the exact brain mechanisms responsible for this process remained a puzzle for years. What if the neurons that provide a means for electrochemical message handling in our brains could convey numbers (something like a computer) and learn from experience.
Scientists did eventually report evidence for the existence of neurons to compute and accumulate results. The neurons were suspected of responding to increasing numbers of items in a display with increasingly accurate computations. A team comprised of Jamie Roitman, Elizabeth Brannon, and Michael Platt (University of Illinois at Chicago — UIC) studied the process and reported their findings in PLoS Biology.
The research team focused on the parietal cortex because evidence had shown that this region of the brain is activated during functional imaging studies when people perform basic computations. Damage to the brain region disrupts basic mathematical skills.
In computing, an accumulator is a register for storing intermediate results. The processor is the brain in this case. That is, one could say 1 + 2 = 3, then add another integer to the answer, as in 3 + 1 = 4. The first simple equation is an intermediate result which is then used as long as necessary to accumulate a final result. The analog in one's brain would be accumulator neurons with similar tasks. This is numerical behavior, using one's brain to handle and arrive at an understanding of quantities.
Previous studies suggested that "accumulator neurons" may serve as the first stage in counting. To understand how the parietal cortex contributes to numerical behavior, the team had monkeys look at arrays of dots on a computer screen. By tracking fast eye movements, they could map the changes on the screen to the monkeys' responses.
A fast movement of an eye (or head or other part of an animal's body) is called a saccade. An eye saccade is a quick, simultaneous movement of both eyes in the same direction.
The research team noted that 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). This information could then be used by other neurons that respond best for a particular cardinal number, such as "4," as have been reported in prior studies.
Thus, it was shown that neurons in the lateral intraparietal area (monkeys) responded in a graded fashion to the number of items in a visual array.
It was a demonstration of accumulation during a delayed saccade task. This type of saccade is similar to the regular saccade. The difference is that the second stimulus does not start at the end of the first stimulus — it can start either before or after the first stimulus ends, an important suggestion of the brain's abaility to execute tasks in parallel.
These newer findings suggest that the neurons "sum up" individual elements to represent accumulated magnitude. It supports computer models that separate the processes of numerical identification ("that is a 1 or 2) and summing ("my answer is 3"). It may also explain the fact that parietal cortex damage causes both numerical and spatial confusion.
Abstract. 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.
Author Summary. As any child knows, to answer the question “how many,” one must start by adding up individual objects in a group. Extending beyond humans, this cognitive ability is shared by animals as diverse as birds and monkeys. Surprisingly, the exact brain mechanisms responsible for this process remain unknown. Damage to a brain area known as the parietal cortex disrupts basic mathematical skills, and functional imaging studies show that this area is activated when people perform basic computations. To understand how parietal cortex contributes to numerical behavior, we studied the activity of neurons in this area in monkeys while they looked at arrays of dots on a computer screen. We found that parietal neurons responded progressively 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 as 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 has been reported in prior studies. Our 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.
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Richard Smith, Editor-in-Chief, introduces Cases Journal. Dr. Smith urges all physicians to submit their case reports to the new open access Cases Journal, which publishes case reports from any area of healthcare.
Cases Journal will publish any case report that is understandable, ethical, authentic, and includes all essential information. A more selective companion, the Journal of Medical Case Reports, publishes original and interesting case reports that contribute significantly to medical knowledge. Article submissions are subject to potential publication by either journal. All reports will be entered in a common and open access database.
Previous studies suggested that "accumulator neurons" may serve as the first stage in counting. To understand how the parietal cortex contributes to numerical behavior, the team had monkeys look at arrays of dots on a computer screen. By tracking fast eye movements, they could map the changes on the screen to the monkeys' responses. 
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