Electrical Neuroimaging of Gustatory Perception in Humans

[1] O'Doherty, J. et al. (2001). Representation of pleasant and aversive taste in the human brain. J Neurophysiol., 85, 13151321. [2] Small, D. M. et al. (2003). Dissociation of neural representation of intensity and affective valuation in human gustation. Neuron, 39, 701-711. [3] Kobayakawa, T. et al. (1999). Spatio-temporal analysis of cortical activity evoked by gustatory stimulation in humans. Chem Senses, 24, 201-209. [4] Mizoguchi, C. et al. (2002). Gustatory evoked cortical activity in humans studied by simultaneous EEG and MEG recording. Chem Senses, 27, 629-634. [5] Ohla et al. (2009). The cortical chronometry of electrogustatory event-related potentials. Brain Topogr. 22(2), 73-82. [6] Ohla et al. (under review). Electrical neuroimaging reveals intensity-dependent activation of human cortical gustatory and somatosensory areas by electric taste. Questions of how in general, and where and when in particular, gustatory percepts are represented in the human brain remain largely unanswered despite decades of research. Electrical neuroimaging of gustatory perception has been hampered by difficulties with stimulus control because the recording of event-related brain electrical (electroencephalography, EEG) and magnetic (magnetoencephalography, MEG) responses requires temporally precise stimulus presentation to obtain good summation of the signal across trials. This is difficult to achieve with flowing stimuli in solution.

Chem Senses, 27, 629-634.Questions of how in general, and where and when in particular, gustatory percepts are represented in the human brain remain largely unanswered despite decades of research.Electrical neuroimaging of gustatory perception has been hampered by difficulties with stimulus control because the recording of event-related brain electrical (electroencephalography, EEG) and magnetic (magnetoencephalography, MEG) responses requires temporally precise stimulus presentation to obtain good summation of the signal across trials.This is difficult to achieve with flowing stimuli in solution.
Our results show the dynamics of taste-evoked responses with a temporal resolution in the range of milliseconds.Overall amplitudes varied between tastants.However, amplitude differences between tastants were not correlated with perceived taste intensity.The topographical analyses indicated that EEG responses of different tastants share common components but exhibit also differences, which will be subjected to future analyses.Using source estimations as a function of time we found cortical activations within areas previously described as key for gustatory processing, i.e. the bilateral insulae [1,2] and the adjacent opercula [3,4], but also common somatosensory areas in the superior temporal lobe.Interestingly, the left insula was activated first while the right insula was activated only later as reported previously for electric taste [5,6].
Take home message: Given an appropriately precise stimulation technique, the spatio-temporal dynamics of taste-evoked cortical responses can be assessed with EEG.
Taste qualities were identified well by participants who were asked to rate taste intensity and pleasantness on a 10-point scale.Intensity was significantly elevated for sour.Pleasantness was highest for sweet.

METHODS
We used a gustometer (GU002, Burghart) with short rise times (<15 ms to 70%) to present taste solutions to 12 human participants.The tastants were embedded in a stream of compressed air and sprayed on the tip of the tongue.Stimuli were repeated 70 times for each taste quality: salty (sodium chloride), umami (monosodium glutamate), bitter (quinine), sour (citric acid), and sweet (sucrose).Brain responses were recorded from 64 EEG electrodes during tasting and analyzed with respect to their time courses, spatial distributions and neuronal generators.
A series of stable maps (microstates) summarizes the spatio-temporal dynamics of all tastants.LAURA distributed source estimations for each map (exemplified for salty) yielded areas previously associated with the processing of taste and food-related stimuli [5] Ohla et al. (2009).The cortical chronometry of electrogustatory event-related potentials.Brain Topogr.22(2), 73-82.[6] Ohla et al. (under review).Electrical neuroimaging reveals intensity-dependent activation of human cortical gustatory and somatosensory areas by electric taste.
[1-6].Event-related potentials (ERPs) exhibited an early (150-220 ms) positivity over frontal electrodes (left) and a later slow positivity (400-800 ms) over central electrodes (right) for all tastants.Circles on the topographical voltage maps indicate electrodes used to plot the ERPs.SUMMARY and DISCUSSION Contact: kathrin.ohla@rdls.nestle.comButterfly plots display the time c o u r s e of the a m p l i t u d e s from 64 electrodes for each tastant.The taste stimulations l a s t e d f r o m 0 to 900 ms.The EEG responses were most pronounced for salty and umami.