A study by researchers at the University of Pittsburgh and Carnegie Mellon University has shed new light on the critical issue of how the human brain recognizes and reads words.
Their findings provide strong evidence for the existence of a specific area of the brain responsible for the visual representation of words, a still hotly-debated issue in the field. More remarkably, the researchers were also able to electronically “read out” individual words being read by neurosurgery patients with electrodes implanted in their brains for treatment of epilepsy.
Reading is a uniquely human skill and visual word recognition a particular conundrum for neuroscientists. Because it is such a relatively recent development in historical terms, the neural systems responsible for reading simply could not have evolved for this purpose. Instead, any specialization for reading must have occurred through learning and the adaptation of existing centers in the brain.
The Visual Word Form Area Debate
Whether or not a reading-specific brain region even exists has been fiercely debated by scientists for more than a century, with most of the debate revolving around a small area near the back of the brain in the left mid-fusiform gyrus, which some researchers call the “visual word form area”.
French neurologist Joseph Jules Dejerine. Wellcome Library
This debate goes back at least as far back as the 19th century work of French neurologists Jean-Martin Charcot and Joseph-Jules Dejerine, and German psychiatrist and neuropathologist Carl Wernicke.
Charcot and Dejerine believed in the existence of a center for the visual memory of words, while Wernicke firmly rejected the notion, proposing instead that reading only necessitates visual representations of individual letters that feed forward into the language system.
More recently, researchers have disagreed about whether there is a visual word form system in the brain that represents the visual forms of letter combinations, pieces of words (like phonemes and morphemes) and whole words.
Those on one of side have argued that the brain possesses a visual word form area that is “a major, reproducible site of orthographic knowledge”, while those on the other disavow any need for such reading-specific visual specialization in the brain, arguing instead for neurons that are “general purpose analyzers of visual forms”.
Electrical Brain Stimulation
In this study, the role of this area in reading was studied using electrodes implanted in the brains of patients undergoing surgical treatment for drug-resistant epilepsy. The electrodes were surgically implanted in the left fusiform gyrus and other brain regions, and were used to continuously monitor the patients’ brain activity for one to two weeks. Physicians were hoping to precisely localize the dysfunctional tissue causing their epileptic seizures so it could be excised at the same time as the electrodes were removed.
Credit: Michael J. Ward
Because this process provided an unprecedented opportunity to understand how the brain recognizes printed words, several patients agreed to participate in a study led by senior author Dr. Avniel S. Ghuman, Director of MEG research and the Laboratory of Cognitive Neurodynamics at the University of Pittsburgh, and co-first authors Dr. Elizabeth A. Hirshorn of the University of Pittsburgh’s Learning Research and Development Center and Yuanning Li, a Ph.D. candidate in the Program in Neural Computation, a joint program of the University of Pittsburgh and Carnegie Mellon University.
In the first step, mild electrical brain stimulation was applied to different parts of the brain through the electrodes while patients read words and letters. This caused the brain tissue near the electrodes to temporarily act abnormally, and disrupted the normal functioning of the stimulated area.
When stimulation was delivered to the target area in the fusiform gyrus, patients’ ability to read words was profoundly disturbed, even though both patients were otherwise proficient readers. One patient reported thinking about two different words at the same time, and trying to combine them, even though only one word — “illegal” — was on the screen.
In another case, the same patient reported thinking that there was an “n” in the word “message”. The effects were even more dramatic in a second patient who completely misperceived letters. When the letter “X” was displayed on-screen, he reported seeing “A”, and when the letter “C” was on-screen, he confidently reported seeing the letters “F” and then “H”. A video of this can be seen below.
However, when stimulation was delivered to other parts of the brain, both patients could name words and letters without difficulty. Moreover, stimulation to the fusiform region did not affect their ability to identify and name pictures and faces.
Estimating Word Waveforms
In the second part of the study, patients were asked to read words while activity from the brain area was recorded through the electrodes.
Using sophisticated analysis techniques for pattern recognition, the researchers were actually able to identify which specific word a patient was reading at a particular moment in time just by looking at the shape of the waveform. This suggests that neural activity in the area encodes learned visual words in a way that can be used to discriminate even visually similar words from one another.
”Yes, we can essentially tell the difference between different words by looking at the waveform,” said Dr. Avniel Ghuman, “But I would like to emphasize that while our accuracy is substantially greater than a random guess, it’s not near 100% at the moment. This allows us to say with confidence that the information about individual words is present in this brain area, but I don’t want to give the impression we could or would spy on peoples’ thoughts with any great accuracy.”
Dr. Ghuman and his colleagues also discovered that there were two critical windows of time for word processing during which affected how and when they could read individual words.
”Specifically, during brain activity that arose from 100 to 250 milliseconds after the subjects saw a word , we could distinguish between words that were completely different, like ‘hint’ and ‘dome’, but not words that were very similar, like ‘hint’ and ‘lint,’” he explained.
“However, in the time window right after that, about 300 to 500 milliseconds, we were able to distinguish between words just one letter different, like ‘hint’ and ‘lint.’ This means that the fusiform gyrus plays a critical role in refining the neural representation of what we are reading over hundreds of milliseconds.”
Taken together, the researchers say, the results of this study provide strong evidence that an area of the left fusiform gyrus is shaped by reading experience and plays a critical role in the accurate recognition of printed words. These results have important implications for our understanding of the neurobiological basis of many reading disorders and suggests a potential neural target for reading disorder therapies.
Moreover, according to Dr. Ghuman, these results help resolve the great debate about whether or not the brain has reading-specific areas that has raged for over a century.
“The fact that the activity in the left mid-fusiform gyrus codes for individual words, and that disrupting this area causes profoundly disturbed word recognition, while leaving the ability to recognize non-word objects relatively intact, provides some of the strongest evidence to date that this area is shaped by reading experience and becomes dedicated to the accurate recognition of printed words,” he said.