Simultaneous cortical responses to multiple written words

Abstract

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 a rapid sequence 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 words in the sequence: zero words; one word; two words sequentially; or two words simultaneously. Behavioral task accuracy was worse for detecting two words presented simultaneously than sequentially. BOLD responses increased linearly with the number of words presented in several reading-related regions of the left hemisphere: text-selective occipito-temporal regions, the superior temporal sulcus, the intraparietal sulcus, and the inferior frontal sulcus. In all of those regions, responses did not differ between sequential and simultaneous presentation of two words. Nonetheless, the sensitivity of ventral temporal text-selective regions to the lexical frequencies of two words was attenuated by simultaneous presentation. To account for these patterns of activity and task performance, we suggest that the reading network can detect two strings of letters simultaneously, but there is interference during lexical access.