Researchers in the Stanford Human Intracranial Cognitive Electrophysiology program recently identified the specific cluster of neurons in the brain that recognizes numerals.
The team’s findings were published on April 17 in the Journal of Neuroscience. The study was co-authored by Associate Professor of Neurology and Neurological Sciences Josef Parvizi; postdoctoral research fellows Dora Hermes-Miller, Brett Foster and Mohammad Dastjerdi; research associate Jonathan Winawer; research assistant Vinitha Rangarajan; neurosurgery resident Kai Miller and medical student Jennifer Shum M.D. ’13.
“We have been working on this for a while,” Hermes-Miller said. “This is a very exciting finding. It is great that we have been able to test for and find the existence of a number region in the brain.”
Parvizi said that the discovery originated in his study of patients with drug-resistant epilepsy. For those patients, Parvizi uses intracranial electrodes to track when and how a seizure is triggered.
“The electrodes record data from a group of neurons and will detect epileptic activity from any group of pathological neurons,” he said. “The coverage is different for each patient depending on clinical signs, and we look for the site of seizure origination — say, in the occipital lobe, or the frontal lobe, or so on, depending on the patient.”
Researchers have to wait for up to seven days to record a seizure signal, according to Parvizi. During this time, the researchers asked patients to volunteer for other studies about brain functions.
While these experiments varied in purpose, Parvizi said that they shared the common theme of helping the researchers determine the location of brain functions.
“We try to discover how certain functions of the brain are localized in certain areas of the brain and how different areas with related functions work together,” Parvizi said.
According to Parvizi, the most interesting results came from an experiment to test whether or not a certain group of neurons in the brain was preferentially responsive to visual numerals compared to other symbols.
Each patient first looked at numerals and other similar symbols like letters while researchers tracked which portion of the patient’s brain was most responsive to each symbol.
The patients then viewed numerals, number-words like “three” and words that sounded similar to the number-words, with researchers again monitoring brain activity.
Through the experiments, the researchers were able to confirm the existence of a specific group of neurons that preferentially responded to numerals. This clump of neurons, which is less than one-fifth of an inch wide, is located within the inferior temporal gyrus, a superficial region of the brain’s outer cortex.
“The location of these neurons, or population of neurons, was strikingly consistent from one subject to another, at least in the right-hand hemisphere where we had good evidence and good sensor coverage,” Parvizi said.
According to Parvizi, these neurons are able to recognize numerals because of the pattern of their wiring with other parts of the brain, as they are “connected to the areas of the brain which decode magnitude and numerosity.”
According to Parvizi, the study is unique because it independently examines each patient’s brain rather than looking at a normalized brain composed of aggregated data. The research also confirmed that the strong correlation found in the brains of different patients existed only for numbers, and not for colors and shapes.
Although the team’s discovery sheds some light on how the brain functions, Parvizi said that the finding leaves more questions unanswered about the fundamental processes of the brain, such as how human brains develop these functions.
“When do you acquire the ability to recognize numerals, and how is it acquired?” Parvizi said. “That is a fascinating question, and one that I cannot answer.”