Imagine trying to read a word – even this very sentence – and the letters all looking like a jumbled mess. You can see letters but they no longer make sense. This recently happened to patients who were in a unique study to investigate the origins of reading in the brain.
These patients, who had electrodes implanted in their brains for locating and treating epileptic seizures, have provided some of the most compelling evidence to date isolating where in the brain we recognize letters and read words. In the new study published in PNAS, researchers reported that electrically stimulating a region of the fusiform cortex (left mid-fusiform gyrus) resulted in a disruption in the patients’ ability to read.
One patient reported feeling like she was trying to mix two words together when she only saw a single word and thinking there was an “n” in the word “message.” In a second patient, the disruption was even more dramatic, with him misperceiving letters, first responding “A” when viewing the letter “X” and then confidently reporting seeing the letters “F” and “H” when viewing the letter “C”. Many of these observations were directly captured in this video:
The researchers were also able to use using machine-learning methods to analyze the electrophysiological data recorded from the patients’ electrodes. They found that they could decode which word a person was reading at a particular moment.
Senior author on the paper Avniel Ghuman of the University of Pittsburgh spoke with CNS about this study, its implications for our understanding of literacy and reading disorders in particular, and current and future applications of invasive electrophysiology techniques such as the one used in this new study.
CNS: How did you become personally interested in this area of study?
Ghuman: Reading is fascinating because it is a uniquely human skill and one that has only relatively recently become prevalent in historical terms, unlike something like face perception, which can be seen in our evolutionary ancestors. Therefore, studying reading provides a unique insight into how this visual expertise acquired during childhood shapes the brain.
Indeed, the fact that literacy is a relatively recent development and the fact that we translate what we read into the sound of the words in our heads has led to a fierce debate, which can be tracked back to the 19th century, as to whether or not reading-specific neural circuits even exist. More broadly, it is the capacity to use modern recording and analysis tools to answer questions that people have been asking about cognition for centuries that drew me to cognitive neuroscience.
CNS: Why use invasive, neurosurgical techniques to study reading in the brain?
Ghuman: Invasive measurements provide perhaps the highest spatial and temporal resolution one can record from the human brain coupled with extremely strong signals. This provides an unprecedented sensitivity and specificity for studying human cognitive neuroscience.
Additionally, invasive techniques provide the ability to precisely stimulate an area and profoundly disrupt its activity, allowing us to make inferences about causal relationships between the function of that area and particular behaviors.
CNS: What was it like working with epilepsy patients for the experiment?
Ghuman: Working with epilepsy patients is humbling. These are individuals who just had one major brain surgery and are going to have another one and yet they are willing to volunteer to participate in research because they understand the unique contribution they can make to cognitive neuroscience. The patients often have very understandable fatigue from the surgery, so we design our experiments to learn as much as possible in a limited amount of time. I know that everyone who is part of this research, including myself, sincerely appreciates the time and energy the patients are willing to give us to help us better understand the brain.
CNS: What were you most excited by in the study results?
Ghuman: It was stunning to be in the room to see the effects of stimulation to the left mid-fusiform during reading. I was blown away when one of the subjects confidently reported seeing the letters “F” and then “H” when a “C” was on the screen. It was as if he simply saw different letters on the screen than what was actually there due to the stimulation (see video above).
CNS: Can you briefly describe how you were able to identify in the brain recordings the specific words patients were reading?
Ghuman: We applied machine-learning algorithms designed to detect which aspects of the neural signals are different between conditions, specifically in our case, between different words. We give these algorithms multiple examples of brain recordings for each word and the algorithm learns to detect the patterns in those recordings associated with particular words. We then gave the algorithm an unfamiliar recording and the algorithm attempted to decode what word the brain activity came from. Using these algorithms, based on the brain recordings, we were able to identify the words the patient was reading significantly more accurately than as if we were randomly guessing.
Interestingly, we found that there were two critical windows of time for this decoding. First, based on the brain activity from 100-250 milliseconds after the subjects read a word where we could tell the difference between words that were completely different, such as “hint” and “dome.” Then from about 300-500 milliseconds, we could now even tell the difference between words that were only one letter different, such as “hint” and “lint.” This demonstrates that the left mid-fusiform gyrus plays an important role in refining the neural representation of words in at least two, temporally distinct stages of information processing.
CNS: What do we now know about language and the brain that we did not previously and how will those results affect people?
Ghuman: The reading-specific effects seen with disrupting the left mid-fusiform gyrus and the ability to decode individual words based on the brain activity from this area, provide very strong evidence that this area is dedicated to reading. This helps to address the debate regarding the neural basis of reading that has persisted for almost 150 years.
Our finding also show that there are multiple critical windows of information processing, with an early gist-level representation and a later, more precise representation that differentiates even very similar words from one another. These findings are important to incorporate in models of reading and suggest a mechanism in which the left mid-fusiform gyrus interacts with many regions of the brain involved in language and visual processing to refine the representation of words over time.
The results of our study naturally lead to the question of what role abnormalities in the left mid-fusiform gyrus, and abnormal interactions between this area other brain areas, play in reading disorders. It also suggests that normalizing the function of this area may be important for remediating disordered reading.
CNS: Can you describe other work you have done using invasive human electrophysiology?
Ghuman: Much of the work my lab does at this point involves invasive human electrophysiology. Besides reading, we study the neural basis of face perception and social and affective perception.
As an example, we published a study recently using machine learning methods to examine neural information processing for face identity using electrodes that were in an area of the brain critical for face recognition. Interestingly, in that study we also found that there were two very similar critical time windows for face perception as the ones we found for word recognition. The fact that similar time windows are seen for both faces and words suggests that these are critical periods for neural information processing.
CNS: What do you see as the future of invasive human electrophysiology, including do you see a future of it beyond research studies?
Ghuman: Right now invasive human electrophysiology is rapidly expanding and, while still rare, it has a bright future in cognitive neuroscience research. Beyond research, brain stimulation is increasingly being used to treat neurological disorders and shows promise for the treatment of some mental health disorders. It is already an important treatment for Parkinson’s disease and essential tremor and shows promise for treating epilepsy, chronic pain, and obsessive compulsive disorder. It is being evaluated for the treatment of other cognitive disorders. Indeed, an important aspect of my lab’s research is to do the basic cognitive neuroscience research that will lay the groundwork for potential future applications of brain stimulation in mental health.
CNS: What are next steps for this research?
Ghuman: We would like to understand word recognition in context, for example, how neural information processing differs for money “bank” compared to river “bank.” We would ultimately like to understand how information flows among broadly distributed brain regions involved in visual, phonological, and language processing to support reading.
-Lisa M.P. Munoz