Abstract
Our experience and prejudice concerning food play an important role in modulating gustatory information processing; gustatory memory stored in the central nervous system influences gustatory information arising from the peripheral nervous system. We have elucidated the mechanism of the “top-down” modulation of taste perception in humans using functional magnetic resonance imaging (fMRI) and demonstrated that gustatory imagery is mediated by the prefrontal (PFC) and insular cortices (IC). However, the temporal order of activation of these brain regions during gustatory imagery is still an open issue. To explore the source of “top-down” signals during gustatory imagery tasks, we analyzed the temporal activation patterns of activated regions in the cerebral cortex using another non-invasive brain imaging technique, magnetoencephalography (MEG). Gustatory imagery tasks were presented by words (Letter G-V) or pictures (Picture G-V) of foods/beverages, and participants were requested to recall their taste. In the Letter G-V session, 7/9 (77.8%) participants showed activation in the IC with a latency of 401.7±34.7 ms (n = 7) from the onset of word exhibition. In 5/7 (71.4%) participants who exhibited IC activation, the PFC was activated prior to the IC at a latency of 315.2±56.5 ms (n = 5), which was significantly shorter than the latency to the IC activation. In the Picture G-V session, the IC was activated in 6/9 (66.7%) participants, and only 1/9 (11.1%) participants showed activation in the PFC. There was no significant dominance between the right and left IC or PFC during gustatory imagery. These results support those from our previous fMRI study in that the Letter G-V session rather than the Picture G-V session effectively activates the PFC and IC and strengthen the hypothesis that the PFC mediates “top-down” control of retrieving gustatory information from the storage of long-term memories and in turn activates the IC.
Highlights
Noninvasive brain imaging techniques including positron emission tomography (PET) and functional magnetic resonance imaging have demonstrated that the anterior insular cortex (IC) and frontal operculum are activated by gustatory stimulation to the tongue and mediate gustatory processing in humans [1,2,3,4,5,6,7,8,9]
We have demonstrated that gustatory imagery tasks activate the IC and frontal operculum [15]; this result was supported by several functional magnetic resonance imaging (fMRI) studies [16,17]
We identified magnetic responses in the occipital cortex, the IC, and the prefrontal cortex (PFC)
Summary
Noninvasive brain imaging techniques including positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) have demonstrated that the anterior insular cortex (IC) and frontal operculum are activated by gustatory stimulation to the tongue and mediate gustatory processing in humans [1,2,3,4,5,6,7,8,9] These findings are supported by cumulative results obtained by classical neurological observation of patients. By contrasting activation sites during gustatory and visual imagery tasks using fMRI, we proposed that the middle and superior frontal gyri are likely to be the source of ‘‘top-down’’ signals that retrieve gustatory memories [15] To test this hypothesis, it is critical to examine the order of activation of the PFC and IC. The temporal resolution of fMRI is not sufficient for examining the order of activation in the cerebral cortex, because these processes are thought to take place within a second [20]
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