Introduction
Amygdala is a subcortical collection of nuclei, that are primarily responsible for the modulation of flight and fight response. It is considered to be a part of the limbic system and adds emotional value to our memories. The amygdala is a bilateral structure which means that there are 2 nuclei for each brain hemisphere. Residing close to the brain stem, it is evolutionary speaking an older structure, which means that it is involved in more autonomic responses and therefore harder to control by our volition
Amygdala and the modulation of hypothalamic signals
In the research from Schoenfeld and Hamilton [1] the function of the amygdala was observed in the relationship with food behavior. Rats, with lesions in the amygdala, performed different activities in a span of 24 hours. In this research, it was established, that the function of the amygdala was dissociated from the functions of the hypothalamus. While the amygdala on itself can not initiate eating behavior, it is a profound modulator of the signals coming from the hypothalamus. Interestingly, different areas of the amygdala differently modulated the response. Medial regions fortified positive signals, while the lateral posterior areas reduced the strength of aversive signals expressing sickness. In the research from Schulkin et al. [2], it was established that the amygdala also specifically modulates the desire for salt intake. In the case of the lesioned amygdala, the desire for salt did not change, even if deprived through a diet or by surgical removal of the adrenal gland.
All of these modulation effects of the signal coming from the hypothalamus are a result of different connections between particular parts of the amygdala and other brain cells residing in the brain stem or higher-order associative cortical structures.
Amygdala and its involvement in memory
As was mentioned before, the amygdala has a tight connection with the lower order brain units, such as the hypothalamus and the brain stem. However, as a part of the limbic system, it is also connected to the higher-order cortical structures, which could potentially initiate a desire to eat, even though homeostatically, we should still be in a good state. Moriss and Dolan [3] have found out, that the activity of the amygdala was related to increased focus for the food object, when the rats were in a deprived state i.e. hungry. The higher the activity of the amygdala, the more desired the food object was and the better memorization and predictability of it was expressed. In comparison, the researchers also analyzed the activity of the orbitofrontal cortex, which on the other hand expressed the hunger levels of the rat. The hungrier the rat was, the higher the activity of the orbitofrontal cortex, when a food object was observed.
As was already noted, the amygdala was involved in the memorization of food objects and experiences with them. In that respect, if the amygdala evokes experiences with food objects, only perceiving a food object should therefore elicit a desire or move towards it (if the experience was pleasant). This was established by Petrovich et al. [4], where the recalled memories were able to override current homeostatic signals of satiety. Interestingly, the connection between the basolateral area – BLA and the lateral area of the hypothalamus – LHA was specifically responsible for food-related conditioning (positive or negative).
Following that, the amygdala has established itself as a strong modulator of food behavior, based on our experience, and is one of the structures that can elicit or suppress homeostatic signals of hunger or satiety to a certain extent, through increasing motivation or eliciting fear/ Additional, the amygdala has been found to be involved in difference recognition and fear of novel food i.e. gustatory neophobia, which is especially prevalent in rodents such as rats [5].
Conclusions
Amygdala is our brain center for eliciting flight or fight responses. And as food is such an important aspect of our survival, the amygdala involvement in this aspect is quite profound. Amygdala represents the first brain structure, that demonstrates modulation of food behavior, based on our experience, which is a very important aspect to allow us to seek foods, that we had good, and avoid those that we had bad experiences with. It however also creates a crack in our armor of over or underrating, since these memories and learned incentives can modulate the more essential signals of satiety and hunger.