Gemma A. Calvert

Dehydration affects brain structure and function in healthy adolescents

It was recently observed that dehydration causes shrinkage of brain tissue and an associated increase in ventricular volume. Negative effects of dehydration on cognitive performance have been shown in some but not all studies, and it has also been reported that an increased perceived effort may be required following dehydration. However, the effects of dehydration on brain function are unknown. We investigated this question using functional magnetic resonance imaging (fMRI) in 10 healthy adolescents (mean age = 16.8, five females). Each subject completed a thermal exercise protocol and nonthermal exercise control condition in a cross‐over repeated measures design. Subjects lost more weight via perspiration in the thermal exercise versus the control condition (P < 0.0001), and lateral ventricle enlargement correlated with the reduction in body mass (r = 0.77, P = 0.01). Dehydration following the thermal exercise protocol led to a significantly stronger increase in fronto‐parietal blood‐oxygen‐level‐dependent (BOLD) response during an executive function task (Tower of London) than the control condition, whereas cerebral perfusion during rest was not affected. The increase in BOLD response after dehydration was not paralleled by a change in cognitive performance, suggesting an inefficient use of brain metabolic activity following dehydration. This pattern indicates that participants exerted a higher level of neuronal activity in order to achieve the same performance level. Given the limited availability of brain metabolic resources, these findings suggest that prolonged states of reduced water intake may adversely impact executive functions such as planning and visuo‐spatial processing. Hum Brain Mapp, 2010.

Tool use changes multisensory interactions in seconds: evidence from the crossmodal congruency task

Active tool use in human and non-human primates has been claimed to alter the neural representations of multisensory peripersonal space. To date, most studies suggest that a short period of tool use leads to an expansion or elongation of these spatial representations, which lasts several minutes after the last tool use action. However, the possibility that multisensory interactions also change on a much shorter time scale following or preceding individual tool use movements has not yet been investigated. We measured crossmodal (visual-tactile) congruency effects as an index of multisensory integration during two tool use tasks. In the regular tool use task, the participants used one of two tools in a spatiotemporally predictable sequence after every fourth crossmodal congruency trial. In the random tool use task, the required timing and spatial location of the tool use task varied unpredictably. Multisensory integration effects increased as a function of the number of trials since tool use in the regular tool use group, but remained relatively constant in the random tool use group. The spatial distribution of these multisensory effects, however, was unaffected by tool use predictability, with significant spatial interactions found only near the hands and at the tips of the tools. These data suggest that endogenously preparing to use a tool enhances visual-tactile interactions near the tools. Such enhancements are likely due to the increased behavioural relevance of visual stimuli as each tool use action is prepared before execution.

Lexical and sentential processing in British Sign Language

Studies of spoken and written language suggest that the perception of sentences engages the left anterior and posterior temporal cortex and the left inferior frontal gyrus to a greater extent than nonsententially structured material, such as word lists. This study sought to determine whether the same is true when the language is gestural and perceived visually. Regional neural activity was measured using functional MRI while Deaf and hearing native signers of British Sign Language (BSL) detected semantic anomalies in well‐formed BSL sentences and when they detected nonsense signs in lists of unconnected BSL signs. Processing BSL sentences, when contrasted with signed lists, was reliably associated with greater activation in the posterior portions of the left middle and superior temporal gyri and in the left inferior frontal cortex, but not in the anterior temporal cortex, which was activated to a similar extent whether lists or sentences were processed. Further support for the specificity of these areas for processing the linguistic—rather than visuospatial—features of signed sentences came from a contrast of hearing native signers and hearing sign‐naïve participants. Hearing signers recruited the left posterior temporal and inferior frontal regions during BSL sentence processing to a greater extent than hearing nonsigners. These data suggest that these left perisylvian regions are differentially associated with sentence processing, whatever the modality of the linguistic input. Hum Brain Mapp, 2005.