Language localization in cases of left temporal lobe arachnoid cyst: evidence against interhemispheric reorganization.

We investigated whether left-hemisphere arachnoid cysts lead to reorganization of the language function using PET. A group analysis demonstrated that patients showed no more right-hemisphere activation than a matched control group. Several patients had clear language localizations in the left hemisphere during language comprehension; none of the patients showed right-hemisphere activation. We conclude that left-hemisphere tissue must suffer considerable compromise before reorganization of language into the right hemisphere becomes necessary. Language activations within the left hemisphere are clearly displaced. This is consistent with mere physical displacement in some patients rather than reorganization within the left hemisphere; in others intrahemispheric reorganization cannot be excluded.

Sentence comprehension and word repetition: a positron emission tomography investigation.

Using positron emission tomography, visual presentation of sentences was shown to cause increased regional cerebral blood flow relative to word lists in the left lateral anterior superior and middle temporal gyri, attributable to cognitive processes that occur during sentence comprehension in addition to those carried out during word comprehension. Additional comparisons showed that repeating words (in a different context, when subjects did not attempt to learn the initial lists) led to significant patterns of both increased blood flow (left putamen and right caudate) and decreased blood flow (left posterior temporal lobe). Increases are argued to reflect retrieval of memory traces, whereas decreases reflect diminished necessity for processing of input. A decrease in the left inferior parietal lobe was attributable to other factors.

Localizing components of a complex task: sentence processing and working memory.

Three areas of the left hemisphere play different roles in sentence comprehension. An area of posterior middle and superior temporal gyrus shows activation correlated with the structural complexity of a sentence, suggesting that this area supports processing of sentence structure. The lateral anterior temporal gyrus is more activated bilaterally by all sentence conditions than by word lists; thus the function of the area probably does not directly support processing of structure but rather processing of words specific to a sentence context. Left inferior frontal cortex also shows activation related to sentence complexity but is also more activated in word list processing than in simple sentences; this region may thus support a form of verbal working memory which maintains sentence structural information as well as lexical items.

When syntax meets semantics.

Three experiments concerning the processing of syntactic and semantic violations were conducted. Event-related potentials (ERPs) showed that semantic violations elicited an N400 response, whereas syntactic violations elicited two early negativities (150 and 350 ms) and a P600 response. No interaction between the semantic and early syntactic ERP effects was found. Sentence complexity and violation probability (25% vs. 75%) affected only the P600 and not the early negativities. The probability effect was taken as evidence that the P600 resembles the P3b. The temporal order of word processing in a sentence as suggested by the data was such that a more automatic syntactic analysis was performed (earlier syntactic-related negativities) in parallel with a semantic analysis (N400), after which a syntactic reanalysis was performed (P600). A reanalysis interpretation of the P600 could explain why the extent of the reanalysis differed with syntactic complexity and probability of ungrammaticality.