Equivalent activation of the hippocampus by face-face and face-laugh paired associate learning and recognition.

The human hippocampus is known to play an important role in relational memory. Both patient lesion studies and functional-imaging studies have shown that it is involved in the encoding and retrieval from memory of arbitrary associations. Two recent patient lesion studies, however, have found dissociations between spared and impaired memory within the domain of relational memory. Recognition of associations between information of the same kind (e.g., two faces) was spared, whereas recognition of associations between information of different kinds (e.g., face-name or face-voice associations) was impaired by hippocampal lesions. Thus, recognition of associations between information of the same kind may not be mediated by the hippocampus. Few imaging studies have directly compared activation at encoding and recognition of associations between same and different types of information. Those that have have shown mixed findings and been open to alternative interpretation. We used fMRI to compare hippocampal activation while participants studied and later recognized face-face and face-laugh paired associates. We found no differences in hippocampal activation between our two types of stimulus materials during either study or recognition. Study of both types of paired associate activated the hippocampus bilaterally, but the hippocampus was not activated by either condition during recognition. Our findings suggest that the human hippocampus is normally engaged to a similar extent by study and recognition of associations between information of the same kind and associations between information of different kinds.

Left-hemisphere specialization for the processing of acoustic transients.

Previous work suggests that speech sounds incorporating short-duration spectral changes (such as the formation transitions of stop consonants) rely on left hemisphere mechanisms for their adequate processing to a greater degree than do speech sounds incorporating spectral changes of a longer duration (such as vowel sounds). Ten normal subjects were scanned using positron emission tomography while discriminating pure tone stimuli incorporating frequency glides of either short or long duration. A comparison of these two conditions yielded significant activation foci in left orbitofrontal cortex, left fusiform gyrus, and right cerebellum. Because non-linguistic stimuli were used, these foci must reflect some basic low level aspect of neural processing that may be relevant to speech but cannot be a consequence of accessing the speech system itself.

The neural substrates underlying word generation: a bilingual functional-imaging study.

We used positron emission tomography to investigate word generation in subjects whose first language was English but who were also proficient in French. These subjects performed three types of lexical search: rhyme generation based on phonological cues, synonym generation requiring a semantic search, and translation involving access to a semantic representation in the other language. Two control tasks required word repetition in each language. We investigated whether phonological and semantic word-generation activate similar regions and whether the same neural substrates subserve the second language as subserve the first. A series of cerebral blood flow increases, corresponding to Brodmann's areas 47, 46, 45, and 8, were observed in the left frontal cortex when the baseline repetition task was subtracted from each of the respective generation tasks. The results suggest that common neural substrates are involved in within- and across-language searches and that the left inferior frontal region is activated irrespective of whether the search is guided by phonological or semantic cues.