Central Organization of Taste Perception

Less is known about the organization and function of the gustatory system than either vision, audition, somatosensation, or olfaction. Over the last twenty years we have used anatomical and electrophysiological techniques to examine the central organization of the gustatory system. Behavioral experiments have provided additional insights into the detection and discrimination of taste stimuli and the animal's preferences for those stimuli. More recently we have begun a series of neuropsychological studies in patients who have sustained focal brain damage in primary taste cortex.

Neuroanatomical Research
At a time when many believed that primary taste cortex was located in the parietal lobe adjacent to the somatosensory representation of the face, we demonstrated that primary taste cortex is located in the dorsal insula and the inner operculum of the frontal lobe. These anatomical findings have been confirmed in subsequent electrophysiological studies in our laboratory and elsewhere. The significance of a smaller projection to the outer operculum of the frontal lobe is unknown but the fact that this projection arises from a separate population of neurons within the gustatory thalamus suggests that these two cortical areas participate in different aspects of taste perception. Clinical studies in this laboratory and elsewhere have demonstrated that damage to the insular cortex impairs perception of taste intensity and taste quality recognition (see below).

Neurophysiological Research
Electrophysiological recordings from awake animals allow us to watch the brain work. Our recordings from the gustatory thalamus suggest that the thalamus is not a passive relay for information bound for primary taste cortex. Interestingly, taste neurons account for fewer than half of the cells in the gustatory thalamus and are intermixed with other neurons that respond to tactile and/or thermal stimulation. Gustatory neurons constitute an even smaller minority in primary taste cortex. Another minority population of gustatory neurons is located in the orbitofrontal cortex (OFC). Our current research is examining the response properties of those neurons (see below).

Lesion-Behavioral Research
Beginning with the premise that taste perception is assembled in a hierarchical fashion as sensory information ascends from one level of the nervous system to the next, we have attempted to define the degree of gustatory proficiency of the caudal brainstem. We tested animals with bilateral lesions of the thalamic taste relay with a battery of behavioral tests and found that they show only mild impairments in taste preference and taste discrimination and show near normal responses to the motivational properties of taste stimuli. These animals also are able to learn and retain a conditioned taste aversion. Ongoing experiments are examining the ability of these animals to demonstrate anticipatory contrast, which previous studies in other laboratories have shown is impaired by thalamic lesions.

Clinical Research
Our previous neuroanatomical, neurophysiological, and lesion-behavioral research was the point of departure for our current neuropsychological studies on brain-damaged patients. Patients with stroke or tumor-related damage to the rostral, dorsal insula are impaired in both their qualitative and quantitative assessment of taste stimuli. As expected, taste intensity was impaired when sapid stimuli were applied to the side of the tongue ipsilateral to the cortical damage. The results of the qualitative identification test were more complex. Damage to the right insula, which prevents taste information from the right side of the tongue from entering the cerebral cortex, produced an ipsilateral deficit in taste quality recognition. Damage to the left insula produced a similar deficit in taste quality recognition for the left side of the tongue; unexpectedly, these patients showed a significant decrement in taste quality recognition on the right (contralateral) side of the tongue. These data suggest that taste information taste quality recognition for both sides of the tongue takes place in the left hemisphere (see Figure).

Current Research
The Role of Orbitofrontal Cortex in Satiety Initiation, maintenance, and termination of an individual meal are influenced by oral (gustatory) and post-oral (post-ingestive) factors, the latter originating in the stomach, duodenum, and venous circulation. Through separate and combined manipulations of these three compartments we are attempting to pinpoint the peripheral mechanism(s) that modulate neurophysiological changes in taste coding that accompany satiety. Collectively, these experiments will provide the first systematic neurophysiological investigation of how gustatory and visceral factors contribute to satiety and meal termination.

Lateralization of Taste Affect in the Cerebral Cortex.
Our demonstration that a language-based taste task such as identification of quality is performed in the left hemisphere suggests that other aspects of taste perception (e.g. gustatory affect) may be lateralized as well. At present, we know little about how taste information is integrated with higher-order processes such as language, emotion, and memory. Psychophysical techniques can be used to address these issues today; tomorrow, functional MRI will help us locate the precise areas in the insula and the orbitofrontal cortex that participate in higher-order processing of taste information and its integration with other systems.