A Brain pathway rapidly converts written words into speech, challenging old models of reading and opening doors for dyslexia interventions.
How does the brain transform written words into spoken ones within milliseconds? Findings published in The Journal of Neuroscience highlight an unexpected role of a key brain region traditionally linked to speech production. Evidence suggests the left posterior inferior frontal cortex (pIFC) is activated during reading far earlier than previously believed. Through targeted brain stimulation, it was demonstrated that this region is crucial for translating written words into spoken language just 100 milliseconds after encountering a word—well before conventional theories indicate.
For decades, research has sought to uncover the mechanisms behind reading, particularly the sequence of processes involved in converting written text into spoken words. Traditional models describe a “serial cascade,” where visual recognition occurs in the fusiform gyrus, phonological conversion in the supramarginal gyrus, and speech production in the pIFC. These models suggest that each stage operates sequentially, waiting for input from the previous one.
Recent neuroimaging studies, however, reveal simultaneous activation of these regions during reading, challenging the assumption of independent operations. To better understand the role of the pIFC, transcranial magnetic stimulation (TMS)—a non-invasive technique for temporarily disrupting brain activity—was employed. This method provided new insights into how the brain integrates written and spoken language processes.
“Traditional cognitive models of reading assume that speech production occurs after initial visual and phonological processing of written words,” noted Kimihiro Nakamura, principal investigator at the Systems Neuroscience Section of the National Rehabilitation Center Research Institute.
This seems a plausible and reasonable a priori assumption, but a series of more recent magnetoencephalography (MEG) studies show that the pIFC, classically associated with spoken production, responds to print at 100-150 ms after word onset, almost simultaneously with posterior brain regions for visual and phonological processing. Moreover, the functional significance of this fast neural response is also unclear, because the left pIFC is now known to mediate different aspects of linguistic/non-linguistic processing. We therefore wanted to fill this gap between cognitive models and empirical data from functional brain imaging.
— Kimihiro Nakamura
The Study
A study involving 50 adults explored these findings through two experiments. In the first, three tasks were performed: reading words aloud, making semantic judgments (distinguishing whether a word referred to an animal or a plant), and identifying the color of text as a perceptual control task. The second experiment added an object-naming task to compare reading-related processes with those used in general spoken language production.
During these tasks, transcranial magnetic stimulation (TMS) pulses were applied to three brain regions at intervals of 50, 100, 150, and 200 milliseconds after written words were presented. This method provided precise insight into when specific brain regions became active and how disrupting their function influenced task performance. Measurements of reaction times and accuracy helped reveal the effects of TMS on each task’s outcomes.
The stimuli for the reading tasks included words written in a phonologically regular script, where each character consistently corresponded to a specific sound. This approach reduced variability in how text was converted into speech sounds, allowing for a clearer identification of the contributions made by each brain region.
Findings revealed an unexpected role of the pIFC, which was shown to be critical early in the reading process. When transcranial magnetic stimulation (TMS) was applied to the pIFC 100 milliseconds after a written word appeared, the ability to read aloud was significantly disrupted. This effect was specific to reading and did not interfere with semantic or color-judgment tasks. These results indicate that the pIFC plays a direct role in the rapid conversion of written words into speech sounds.
The fusiform gyrus also demonstrated early involvement in reading. Disruption of its function at 100 milliseconds impaired both reading and semantic tasks, emphasizing its role in visual word recognition. However, unlike the pIFC, the fusiform gyrus exhibited no task-specific effects; its disruption impacted tasks requiring both phonological and semantic processing.
“Most of the current knowledge of spatiotemporal dynamics in reading is derived from functional neuroimaging data with high-temporal resolution, such as ERP and MEG, according to which posterior brain systems responsible for visual and phonological processing respond to print at 250-500 ms after stimulus-onset,” stated Kimihiro Nakamura, principal investigator at the Systems Neuroscience Section of the National Rehabilitation Center Research Institute.
“While the main goal of the study was to dissect the causal role of early pIFC activation in reading, our TMS results revealed that those other systems for reading also act much faster than assumed by most neurocognitive models of reading derived from ERP/MEG data. Because TMS is a brain stimulation method for transiently disrupting local neural activity, we argue that the observed gap could be attributed to possible differences in timing between actual neuronal firing and peak response latencies estimated from ERP/MEG waveforms.” — Kimihiro Nakamura
The supramarginal gyrus displayed delayed activation, with performance disruptions occurring only at 150 milliseconds or later when transcranial magnetic stimulation (TMS) was applied. This observation aligns with its recognized role in phonological processing, which takes place following the initial visual recognition of words.
A second experiment provided further clarity regarding the specific role of the left posterior inferior frontal cortex (pIFC) in reading. Tasks included oral reading and object naming, with TMS applied at similar intervals. Disruption of the pIFC affected reading but had no impact on object naming, even though both tasks involved spoken responses. This distinction highlights the pIFC’s specialized involvement in converting text into speech sounds, rather than general speech production.
These results challenge the long-standing “serial cascade” model of reading, which assumes that visual and phonological processing must be completed before speech production begins. Instead, findings suggest parallel processing by the pIFC and fusiform gyrus, with the pIFC playing a pivotal role in a “sublexical” pathway that rapidly connects visual input to speech motor systems.
“Our TMS data provide the first causal evidence showing that the early activation of the left pIFC specifically mediates rapid generation of speech motor codes during reading, which probably relies on the enhanced long-distance connectivity between occipitotemporal and frontal cortices that developed with the extensive experience in reading,” explained Kimihiro Nakamura, principal investigator at the Systems Neuroscience Section of the National Rehabilitation Center Research Institute.
“Our results also show that this region starts to respond to print approximately 30 milliseconds faster than thought previously, but not necessarily in an ordered cascade as assumed by cognitive models of visual word processing.” — Kimihiro Nakamura
Additionally, the study suggests that the brain may utilize a previously underestimated sublexical pathway for print-to-sound conversion. While this pathway is known to support early reading development and text decoding in children or individuals with brain damage, its role in proficient adult readers remains less understood. The results indicate that this fast, sublexical mechanism is fully functional in literate adults, even when whole-word recognition systems are predominantly used.
These findings deepen insights into how the brain processes reading and carry significant implications for addressing reading-related challenges, including dyslexia. By identifying the pIFC’s early and critical role, opportunities are created for exploring how these pathways develop and may be enhanced through targeted interventions.
“The precise temporal dynamics during reading is of critical importance for understanding the neurophysiology of dyslexia and related disorders,” Nakamura stated.
“By combining such temporal dynamics information with high-temporal-resolution methods (e.g., EEG and electrical cortical stimulation), interest lies in developing novel neuromodulation methodologies for effective remediation and training for these disorders.” — Kimihiro Nakamura
Early pIFC activation in reading was first documented in 2004 and subsequently supported by multiple magnetoencephalography (MEG) studies. However, the theoretical implications of this activation remained unclear, largely because MEG could only infer correlations between brain structures and functions. By using TMS to suppress activity in specific cortical areas during reading tasks, this study resolved lingering questions, providing compelling evidence of a causal link between early pIFC activation and reading behavior.
https://doi.org/10.1523/JNEUROSCI.0194-24.2024
Abstract
Cognitive models of reading assume that speech production occurs after visual and phonological processing of written words. This traditional view is at odds with more recent magnetoencephalography studies showing that the left posterior inferior frontal cortex (pIFC) classically associated with spoken production responds to print at 100–150 ms after word-onset, almost simultaneously with posterior brain regions for visual and phonological processing. Yet the theoretical significance of this fast neural response remains open to date. We used transcranial magnetic stimulation (TMS) to investigate how the left pIFC contributes to the early stage of reading. In Experiment 1, 23 adult participants (14 females) performed three different tasks about written words (oral reading, semantic judgment, and perceptual judgment) while single-pulse TMS was delivered to the left pIFC, fusiform gyrus or supramarginal gyrus at different time points (50–200 ms after word-onset). A robust double dissociation was found between tasks and stimulation sites—oral reading, but not other control tasks, was disrupted only when TMS was delivered to pIFC at 100 ms. This task-specific impact of pIFC stimulation was further corroborated in Experiment 2, which revealed another double dissociation between oral reading and picture naming. These results demonstrate that the left pIFC specifically and causally mediates rapid computation of speech motor codes at the earliest stage of reading and suggest that this fast sublexical neural pathway for pronunciation, although seemingly dormant, is fully functioning in literate adults. Our results further suggest that these left-hemisphere systems for reading overall act faster than known previously.