The Cortical Localization of the Lexicons

Abstract

Positron emission tomography was used to investigate changes in regional cerebral blood flow (rCBF) in neurologically normal subjects during word reading and word repetition. The blood flow in these conditions was compared with control conditions where subjects were presented with stimuli of comparable auditory and visual complexity to real words and said the same word on presentation of each stimulus. The control condition for word repetition (hearing spoken words presented backwards) resulted in bilateral activation of the superior temporal gyrus. Word repetition caused a significant increase in rCBF over this control condition in the left superior and middle temporal gyri. The control condition for word reading (seeing stimuli written in 'false fonts', i.e. non-existent letter-like forms) resulted in significant changes in rCBF bilaterally in the striate and extrastriate cortex. Word reading caused a significant increase in blood flow relative to this control in the posterior part of the left middle temporal gyrus. The implications of these results are discussed, and it is argued that they are consistent with localization of a lexicon for spoken word recognition in the middle part of the left superior and middle temporal gyri, and a lexicon for written word recognition in the posterior part of the left middle temporal gyrus.

Equivalent Responses to Lexical and Nonlexical Visual Stimuli in Occipital Cortex: A Functional Magnetic Resonance Imaging Study

“…1C, and included the level of the activation focus reported for pseudowords (z 5 14) for five of eight subjects. Our finding that this activation disappears with false font strings as control is in line with previous PET studies (Howard et al, 1992;Price et al, 1994Price et al, , 1996. Price et al (1996), however, do report more posterior medial ''cuneus (extrastriate cortex)'' activation in a feature detection task contrasting lexical stimuli with length-matched false font strings as controls.…”

“…These activations may correspond to the ''lateral extrastriate'' activations reported without giving the exact location by Petersen et al (1990) not only for word-like stimuli but also for consonant and (heterogeneous) false font strings when contrasted with fixation cross-control. Similar activations have been reported by Howard et al (1992) and Price et al (1994Price et al ( , 1996 for words and pseudowords with length-matched control stimuli. Since these activations did not disappear when control stimuli were matched for length, they cannot be related to the string length of visual stimuli, but may rather be associated with some higher order aspect of visual stimulus complexity.…”

“…The data leading to this conclusion were obtained in a PET experiment in which words, wordlike stimuli, i.e., pseudowords, and nonlexical stimuli, i.e., consonant strings and false font strings, were visually presented with a frequency of 1/s and a duration of 150 ms. Only the lexical stimuli evoked significant activation in the left medial extrastriate cortex when contrasted with a fixation cross-stimulus serving as the baseline condition for all stimuli. These findings, however, could not be reproduced in a subsequent PET study with longer (1000 ms) stimulus duration and length-matched strings of false fonts as the baseline condition (Howard et al, 1992). Stimulus duration was ruled out as a variable accounting for these differences by comparing short and long stimulus durations (Price et al, 1994).…”

Tuning of the human left fusiform gyrus to sublexical orthographic structure

“…1) or modulation of activation (Fig. 2) in the posterior superior temporal lobe, a region often activated by words and pronounceable nonwords in comparison to unpronounceable strings (Booth et al, 2002;Herbster et al, 1997;Howard et al, 1992;Indefrey et al, 1997;Price et al, 1996;Tagamets et al, 2000). We propose that this posterior superior temporal region (centered on the posterior superior temporal sulcus) is sensitive to the pronounceability of letter strings, consistent with its hypothesized role in phonological access and visual-auditory integration (Callan et al, 2005;Hickok et al, 2000;Indefrey and Levelt, 2000;Van Atteveldt et al, 2004;Wise et al, 2001).…”

Some neurophysiological constraints on models of word naming

“…To the extent that lexical and semantic systems are spatially distinct in the cortex, the two models might also predict different specific areas of activation in a contrast between words and nonwords. For example, semantic memory systems have often been localized to ventral areas in the temporal lobe and to the angular gyrus (Binder and Price, 2001;Damasio et al, 2004;Gainotti, 2000;Martin, 2001), whereas some studies suggest representation of lexical codes in more dorsolateral temporal regions (Howard et al, 1992;Perani et al, 1996;Price et al, 1996;Small et al, 1996). To date, however, few studies have shown any activation for words over nonwords during simple pronunciation tasks, making adjudication of this matter difficult.…”