People can discriminate real words from nonwords even when the latter are orthographically and phonologically word-like, presumably because words activate specific lexical and/or semantic information. We investigated the neural correlates of this identification process using event-related functional magnetic resonance imaging (fMRI). Participants performed a visual lexical decision task under conditions that encouraged specific word identification: Nonwords were matched to words on orthographic and phonologic characteristics, and accuracy was emphasized over speed. To identify neural responses associated with activation of nonsemantic lexical information, processing of words and nonwords with many lexical neighbors was contrasted with processing of items with no neighbors. The fMRI data showed robust differences in activation by words and word-like nonwords, with stronger word activation occurring in a distributed, left hemisphere network previously associated with semantic processing, and stronger nonword activation occurring in a posterior inferior frontal area previously associated with grapheme-to-phoneme mapping. Contrary to lexicon-based models of word recognition, there were no brain areas in which activation increased with neighborhood size. For words, activation in the left prefrontal, angular gyrus, and ventrolateral temporal areas was stronger for items without neighbors, probably because accurate responses to these items were more dependent on activation of semantic information. The results show neural correlates of access to specific word information. The absence of facilitatory lexical neighborhood effects on activation in these brain regions argues for an interpretation in terms of semantic access. Because subjects performed the same task throughout, the results are unlikely to be due to task-specific attentional, strategic, or expectancy effects.

Neuropsychological and neurophysiological evidence point to a role for the left fusiform gyrus in visual word recognition, but the specific nature of this role remains a topic of debate. The aim of this study was to measure the sensitivity of this region to sublexical orthographic structure. We measured blood oxygenation (BOLD) changes in the brain with functional magnetic resonance imaging while fluent readers of English viewed meaningless letter strings. The stimuli varied systematically in their approximation to English orthography, as measured by the probability of occurrence of letters and sequential letter pairs (bigrams) comprising the string. A whole-brain analysis showed a single region in the lateral left fusiform gyrus where BOLD signal increased with letter sequence probability; no other brain region showed this response pattern. The results suggest tuning of this cortical area to letter probabilities as a result of perceptual experience and provide a possible neural correlate for the 'word superiority effect' observed in letter perception research.Much evidence supports the idea that perceptual systems become selectively efficient at processing inputs that are encountered frequently. In learning a written language, for example, human brains appear to become tuned to the recurring visual patterns of the language represented in its orthographic structure. This is illustrated by the fact that letters embedded in words (such as S in the English word FLASH) or in word-like letter strings (S in FRISH) are more efficiently recognized than letters embedded in unusual letter strings (S in RFHSL) (McClelland and Rumelhart, 1981;Reicher, 1969). Such evidence suggests that normal readers use information about frequently recurring letter combinations, encoded as a result of experience with a specific written language, to more efficiently perceive and identify letters and letter strings.Neuropsychological evidence for an orthographic processor in the brain comes from patients who exhibit 'letter-by-letter reading' after left occipitotemporal brain injury (Binder and Mohr, 1992;Cohen et al., 2003;Leff et al., 2001;Sakurai et al., 2000). Such patients have normal language functions, including good recognition of single letters, but show profoundly impaired processing of letter strings, suggesting focal damage to systems responsible for storing or using orthographic information (Behrmann et al., 1998;Patterson and Kay, 1982;Warrington and Shallice, 1980). This localization is supported by neuroimaging experiments in normal readers, which have identified a region in the lateral left fusiform (occipitotemporal) gyrus that responds more strongly to words and word-like nonwords than to consonant letter strings or nonsense characters Dehaene et al., 2001;Polk and Farah, 2002 1999). Several elegant studies showed that this orthographic system employs an abstract code that is unaffected by changes in letter case (Dehaene et al., 2001Polk and Farah, 2002).Although activation in this brain region appears to be relat...

Object-Based SemanticsMany models of semantics have attempted to address the preceding questions. These models can be categorized A specification of the structural characteristicsof the mental lexicon is a central goal in word recognition research. Of various word-level characteristics,semantics remains the most resistant to this endeavor. Although there are several theoretically distinct models of lexical semantics with fairly clear operational definitions (e.g., in terms of feature sharing, category membership, associations, or cooccurrences), attempts to empirically ad judicate between these different models have been scarce. In this paper, we present several experiments in which we examined the effects of semantic neighborhood size as defined by two models of lexical semantics-one that defines semantics in terms of associations, and another that defines it in terms of global co-occurrences. We present data that address the question of whether these measures can be fruitfully applied to examinations of lexical activation during visual word recognition. The findings demonstrate that semantic neighborhood can predict performance on both lexical decision and word naming.

No abstract

The effect of semantic distance (Lund & Burgess, 1996) was examined in three semantic categorization experiments. Experiment 1, a yes/no task that required participants to make animal/nonanimal judgments by responding to both sets of stimuli (Forster & Shen, 1996), revealed no effect of semantic distance. Experiment 2, a go/no-go task that required participants to respond to only the experimental (i.e., nonanimal) items, revealed a large effect of semantic distance. In addition, response latencies were longer and error rates were lower to the experimental items in Experiment 2 than to those in Experiment 1. These findings were replicated in Experiment 3, in which semantic distance and task condition were manipulated within subjects. We conclude that these results are consistent with (1) the view that the go/no-go tasks elicited more extensive processing of the experimental items and (2) a connectionist account of semantic activation, whereby processing is facilitated by the presence of semantic neighbors.