Simultaneous cortical responses to multiple written words

We have learned much about the brain regions that support reading by measuring neuronal responses to single words, but we know little about how the brain processes multiple words simultaneously. This fMRI study fills that gap by varying the number of English words presented while holding the amount of visual stimulation constant. We adapted the "simultaneous suppression" paradigm, which has demonstrated that the response to multiple stimuli presented simultaneously is typically smaller than the sum of responses to the same stimuli presented sequentially. On each trial, participants viewed rapid sequences of three frames. Each frame contained two character strings, most of which were pseudo-letters with visual features matched to familiar letters. The experimental conditions differed in the number of English words in the sequence: zero words; one word; two words sequentially; or two words simultaneously. Behaviorally, participants performed worse at detecting two words presented simultaneously than sequentially. In the brain, the BOLD response increased linearly with the number of words presented in several reading-related regions of the left hemisphere: the inferior frontal sulcus, the intraparietal sulcus, the superior temporal sulcus, and text-selective occipito-temporal regions. But the mean response magnitudes did not differ between sequential and simultaneous presentation of two words. Nonetheless, the sensitivity of ventral temporal text-selective regions to two words lexical frequencies was attenuated by simultaneous presentation. This suggests that words were processed less deeply when they competed simultaneously. Our new paradigm, therefore, reveals processing capacity limits in the reading circuitry.