2008/11/10

Tracking of State Value in the Amygdala

Moment-to-Moment Tracking of State Value in the Amygdala
Belova et al. 28 (40): 10023 -- Journal of Neuroscience

The amygdala is important for the regulation of emotion and also in learning. Neurons in the amygdala changes their response properties during learning. But what exactly are the amygdala neurons doing? what are they encoding? what is their computational role?

Previous studies have establish that amygdala neurons encode the motivational value of the associated outcome. When animals learn that a cue CS is associated with an outcome US, a subset of amygdala neurons show stronger responses to CSs that are paired with rewarding USs (positive value coding, example neuron A at 1-2 sec). Conversely, another subset of neurons code negative value and respond more vigorously to CSs paired with punishment (like airpuff, example neuron B at 2-3 sec).


The current study extends these previously findings to suggest that these amygdala neurons in fact code the value of the state in the generic sense. In essence, what the authors found was that neurons encode value not only for the CS, but also for other behavioral epochs, including the fixation point and also to the US. In the above examples, neuron A prefered rewarding CS and also showed stronger response to the fixation point, while neuron B preferred aversive CS and reduced its firing rate to the fixation point. This pattern is generally true for the population (see D).

I think this paper is really powerful because the conclusion is based not only on one behavioral epoch but on all the epochs. This comes much closer to understanding the roles of these amygdala neurons, which is to keep track of the current state value. Since neurons are active all the time and in various behavioral contexts, a thourough understanding of their physiological/behavioral roles require considering all these scenarios, not just any particular one.

This paper is also important because the coding of state value has important implications in computational theories of learning.

1 comment:

Aberie said...

Nice article. I found this also particularly interesting:
"Posttraumatic stress disorder (PTSD) is associated with enhanced noradrenergic activity. Animal and human studies demonstrate that noradrenergic stimulation augments consolidation of fear learning. Retrieval of well-established memories by presenting a learned fear cue triggers reconsolidation processes during which memories may be updated, weakened, or strengthened. We previously reported that noradrenergic blockade in the rat amygdala impairs reconsolidation of fear memories. Postretrieval β-adrenergic stimulation in the amygdala enhances reconsolidation of fear memories, making them resistant to extinction. Noradrenergic augmentation during retrieval of fear memories may thus contribute to persistence and severity of traumatic memories. Reconsolidation may be a useful tool in understanding the pathology of PTSD and may thus help in developing new and in modifying existing treatments of traumatic memories." Debiec et al.