Gustatory cortex
The primary gustatory cortex (GC) is a brain structure responsible for the perception of taste. It consists of two substructures: the anterior insula on the insular lobe and the frontal operculum on the inferior frontal gyrus of the frontal lobe.[1] Because of its composition the primary gustatory cortex is sometimes referred to in literature as the AI/FO(Anterior Insula/Frontal Operculum).[2] By using extracellular unit recording techniques, scientists have elucidated that neurons in the AI/FO respond to sweetness, saltiness, bitterness, and sourness, and they code the intensity of the taste stimulus.[3]
Role in the taste pathway
Like the
Functionality and stimulation
There have been many studies done to observe the functionality of the primary gustatory cortex and associated structures with various chemical and electrical stimulations as well as observations of patients with lesions and GC epileptic focus. It has been reported that electrical stimulation of the
Chemosensory neurons
Chemosensory neurons are those that discriminate between tastant as well as between the presence or absence of a tastant. In these neurons, the responses to reinforced (stimulated by tastant) licks in rats were greater than to those for the unreinforced (not stimulated by tastant) licks.[11] They found that 34.2% of the GC neurons exhibited chemosensory responses. The remaining neurons discriminate between reinforced and unreinforced licks, or process task related information.
Taste coding
How GC encodes taste qualities and representations has been a source of major debate. Cortical representation theories have been greatly influenced by peripheral taste coding models. In particular, there are two main model of peripheral taste coding: a labelled-line model, which posits that each taste receptor codes for a specific taste quality (sweet, sour, salty, bitter, umami); and an across-fiber model, which proposes that taste perception arises from the combined activity of multiple unspecific taste receptors.
Some researchers have noted that the AI/FO neurons are intrinsically multimodal, that is, they respond to other modalities in addition to taste (often to olfaction and/or somatosensation).[21] These findings could imply that GC is not strictly involved in taste perception but also in more domain general functions, such as decision making regarding consummatory behaviors[21] and valence processing.[13]
Tastant concentration-dependent neuronal activity
GC chemosensory neurons exhibit concentration-dependent responses. In a study done on GC responses in rats during licking, an increase in MSG (monosodium glutamate) concentration lingual exposure resulted in an increase in firing rate in the rat GC neurons, whereas an increase in sucrose concentration resulted in a decrease in firing rate.[11] GC neurons exhibit rapid and selective response to tastants. Sodium chloride and sucrose elicited the largest response in the rat gustatory cortex in rats, whereas citric acid causes only a moderate increase in activity in a single neuron. Chemosensory GC neurons are broadly tuned, meaning that a larger percentage of them respond to a larger number of tastants (4 and 5) as compared to the lower percentage responding to a fewer number of tastants (1 and 2). In addition, the number of neurons responding to a certain tastant stimulus varies.[11] In the rat gustatory complex study, it was shown that more neurons responded to MSG, NaCl, sucrose, and citric acid (all activating approximately the same percentage of neurons) as compared to the compounds quinine (QHCl) and water.
Responsiveness to changes in concentration
Studies using the Gustatory cortex of the rat model have shown that GC neurons exhibit complex responses to changes in concentration of tastant. For one tastant, the same neuron might increase its firing rate whereas for another tastant, it may only be responsive to an intermediate concentration. In studies of chemosensory GC neurons, it was evident that few chemosensory GC neurons monotonically increased or decreased their firing rates in response to changes in concentration of tastants (such as MSG,
GC neurons cohere and interact during tasting. GC neurons interact across milliseconds, and these interactions are taste specific and define distinct but overlapping neural assemblies that respond to the presence of each tastant by undergoing coupled changes in firing rate. These couplings are used to discriminate between tastants.[22] Coupled changes in firing rate are the underlying source of GC interactions. Subsets of neurons in GC become coupled after presentation of particular tastants and the responses of neurons in that ensemble change in concert with those of others.
Taste familiarity
GC units signal taste familiarity at a delayed temporal phase of the response. An analysis suggests that specific neuronal populations participate in the processing of familiarity for specific tastants. Furthermore, the neural signature of familiarity is correlated with familiarization with a specific tastant rather than with any tastant. This signature is evident 24 hours after initial exposure. This persistent cortical representation of taste familiarity requires slow post-acquisition processing to develop. This process may be related to the activation of
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