Effect of language switching on arithmetic: a bilingual FMRI study.

The role of language in performing numerical computations has been a topic of special interest in cognition. The "Triple Code Model" proposes the existence of a language-dependent verbal code involved in retrieving arithmetic facts related to addition and multiplication, and a language-independent analog magnitude code subserving tasks such as number comparison and estimation. Neuroimaging studies have shown dissociation between dependence of arithmetic computations involving exact and approximate processing on language-related circuits. However, a direct manipulation of language using different arithmetic tasks is necessary to assess the role of language in forming arithmetic representations and in solving problems in different languages. In the present study, 20 English-Chinese bilinguals were trained in two unfamiliar arithmetic tasks in one language and scanned using fMRI on the same problems in both languages (English and Chinese). For the exact "base-7 addition" task, language switching effects were found in the left inferior frontal gyrus (LIFG) and left inferior parietal lobule extending to the angular gyrus. In the approximate "percentage estimation" task, language switching effects were found predominantly in the bilateral posterior intraparietal sulcus and LIFG, slightly dorsal to the LIFG activation seen for the base-7 addition task. These results considerably strengthen the notion that exact processing relies on verbal and language-related networks, whereas approximate processing engages parietal circuits typically involved in magnitude-related processing.

Parametric effects of numerical distance on the intraparietal sulcus during passive viewing of rapid numerosity changes.

A number of functional neuroimaging studies have revealed that regions in and around the intraparietal sulcus (IPS) are parametrically modulated by numerical distance, whereby there is an inverse relationship between distance and levels of activation. These areas are thus thought to contain the internal representation of numerical magnitude. Nevertheless, it has also been suggested that the IPS is involved in response selection during number comparison tasks rather than the representation of numerical magnitude per se. In order to test the independence of the effect of distance on cortical regions, we employed a passive viewing paradigm. Sixteen right-handed male participants viewed rapidly changing slides containing arrays of squares. By varying the distance between the numerosity presented in separate blocks (8 vs. 8, 8 vs. 12, and 8 vs. 16), we examined which regions exhibit a parametric effect of numerical distance. This analysis revealed such effects in the superior part of the IPS bilaterally as well as the superior parietal lobule and the supramarginal gyrus. In contrast, slides rapidly changing in area but not number (Area constant, Area x 1, and Area x 2) did not yield a parametric effect of distance in these regions. Instead, a reverse effect of area was found in a region of the calcarine sulcus. These findings suggest that areas in and around the IPS are involved in numerical magnitude discrimination in the absence of an explicit task and response requirements.

Cortical areas involved in object, background, and object-background processing revealed with functional magnetic resonance adaptation.

Previous work has suggested that object and place processing are neuroanatomically dissociated in ventral visual areas under conditions of passive viewing. It has also been shown that the hippocampus and parahippocampal gyrus mediate the integration of objects with background scenes in functional imaging studies, but only when encoding or retrieval processes have been directed toward the relevant stimuli. Using functional magnetic resonance adaptation, we demonstrated that object, background scene, and contextual integration of selectively repeated objects and background scenes could be dissociated during the passive viewing of naturalistic pictures involving object-scene pairings. Specifically, bilateral fusiform areas showed adaptation to object repetition, regardless of whether the associated scene was novel or repeated, suggesting sensitivity to object processing. Bilateral parahippocampal regions showed adaptation to background scene repetition, regardless of whether the focal object was novel or repeated, suggesting selectivity for background scene processing. Finally, bilateral parahippocampal regions distinct from those involved in scene processing and the right hippocampus showed adaptation only when the unique pairing of object with background scene was repeated, suggesting that these regions perform binding operations.

Comparison of block and event-related fMRI designs in evaluating the word-frequency effect.

Printed word frequency can modulate retrieval effort in a task requiring associative semantic judgment. Event-related fMRI, while avoiding stimulus order predictability, is in theory statistically less powerful than block designs. We compared one event-related and two block designs that evaluated the same semantic judgment task and found that similar brain regions demonstrated the word frequency effect. Although the responses were lower in amplitude, event-related fMRI was able to detect the word frequency effect to a comparable degree compared to the block designs. The detection of a frequency effect with the event-related design also suggests that stimulus-order predictability may not be as serious a concern in block designs as might be supposed.