The impact of mediodorsal thalamic lesions on olfactory attention and flavor perception.

Olfactory attention may be important in generating odor-induced tastes - an arguably universal form of synesthesia - by ensuring that the taste concurrent is captured by the nose and olfaction, not by the mouth and gustation (oral-capture). To examine the role of olfactory attention in generating odor-induced tastes and oral capture we tested a small sample (n=4) of participants with likely impairments in olfactory attention - individuals with mediodorsal thalamic nucleus (MDNT) lesions. These participants were compared to two sets of controls on tests of olfactory attention, oral capture, odor and flavor perception, and control tasks. MDNT participants demonstrated impaired olfactory attention and enhanced oral capture. Greater oral capture was associated with greater olfactory attentional impairment. These findings imply that olfactory attention may be important in attributing odor-induced tastes to the olfactory modality. However, unlike for visual binding and for the neurodevelopmental synesthesias, where attention may be necessary to demonstrate both phenomena, olfactory attention deficits did not impair flavor binding or the experience of odor-induced tastes.

The role of the mediodorsal thalamic nucleus in human olfaction.

Olfaction has a unique dual-route pathway to the neocortex - one being via the mediodorsal nucleus of the thalamus (MDNT). In this study we explored the role of the MDNT pathway, by comparing six patients with MDNT lesions, with 14 controls, on tests of general olfactory ability (i.e., odor acuity, discrimination, naming, recognition memory and hedonic judgement), visual control and neuropsychological tests, and tests of olfactory attention. The MDNT patients performed normally on most general olfactory tests but showed varying impairments on tests of olfactory attention. These findings suggest that the MDNT pathway is involved, either specifically or generically, in mediating human olfactory attention.

The functional role of the medio dorsal thalamic nucleus in olfaction.

Olfaction is unique relative to other sensory modalities in terms of how its neuroanatomy is organized within the brain and its perceptual properties. Olfactory information processing occurs via connections made directly from primary processing areas (piriform cortex) to neocortical structures (orbitofrontal cortex) as well as indirectly via the medio-dorsal nucleus of the thalamus (MDNT). To date, little is known about the functional significance of the MDNT in olfactory information processing. The aim of this article is to review and discuss thalamic function in olfaction. We draw upon research in human neuroimaging, neuropsychology, as well as animal and neurophysiological studies on the thalamus and MDNT in general, before focusing our discussion on the effects of MDNT lesions specific to olfactory function. Finally, although these data are currently limited and sometimes conflicting, especially those based upon human pathology, the putative roles of the MDNT in olfactory information processing and notably its role in attention, are discussed.

Phonological processing in Chinese-English bilingual biscriptals: an fMRI study.

Different activation loci have been reported for language processing in unilingual Chinese and unilingual English participants, as well as in bilingual readers of English and French, two alphabetic languages. Nevertheless, the extant imaging work on Mandarin-English bilinguals favors common neural substrates for English and Chinese, languages with contrasting oral and written forms. We investigated the phonological processes in reading for English-Chinese biscriptals using a homophone matching task with parallel behavioral (n = 28) and fMRI (n = 6) experiments. Unlike previous reports, we observed distinct regions of activation for Mandarin in the left and right frontal lobes, the left temporal lobe, and the right occipital lobe, plus distinct regions of activation for English bilaterally in both the frontal and parietal lobes. The implications of these novel findings are discussed with reference to language representation in bilinguals.