Biology Reference
In-Depth Information
I.
I
NTRODUCTION
In recent years, functional neuroimaging and neuropsychological studies have begun to
refine our understanding of the functions of the human amygdala. Studies in animals have
implicated the amygdala in emotional and social behaviors (LeDoux 2000), especially those
related to fear and aggression, whereby emotionally valenced stimuli need not reach
conscious awareness to engage amygdala processing. Lesion and functional imaging studies
in humans have demonstrated the participation of the amygdala in recognizing emotional
facial expressions. After anterior temporal lobectomy or selective amygdalo-
hippocampectomy in drug-resistant temporal lobe epilepsy patients, it has been also shown
that subjects with considerable amygdala damage were significantly impaired in learning
emotional facial expressions when compared with control subjects (Orman and Stewart 2007)
In addition, the amygdala appears to be an important component of the neural systems
that help retrieve socially relevant knowledge on the basis of facial appearance (Adolphs et al.
1998). Recently, it has been shown that complete amygdala lesions result in a severe
reduction in direct eye contact during conversations with real people, together with an
abnormal increase in gaze to the mouth (Spezio et al. 2007). These novel findings from real
social interactions are consistent with the hypothesized role for the amygdala in autism
(Baron-Cohen et al. 2000; Sweeten et al. 2002). Imaging studies have also revealed that the
amygdala response during facial recognition is altered in patients with schizophrenia (Kosaka
et al. 2002). In humans, as well as animals, activation of the amygdala has been shown to be
closely correlated with memory for both aversive and pleasant stimuli (Hamann et al. 1999;
LeDoux and Muller 1997). Data suggest that the amygdala is not a critical long-term
information storage site but that its role is to regulate memory consolidation in other brain
regions (McGaugh 2002).
Emotion is central to the quality and range of everyday human experience. It is known
that emotional items when presented in a neutral context interfere with episodic encoding of
temporally contiguous non-emotional items, resulting in dissociable valence-dependent
retrograde and arousal-dependent anterograde modulatory effects. By studying two rare
patients with congenital lipoid proteinosis (Urbach-Wiethe) and a focal disease emphasis on
the basolateral nucleus of the amygdala (BLA), it has been demonstrated that this
bidirectional modification of episodic encoding by emotion depends on the integrity of the
amygdala, as both retrograde and anterograde modulatory effects are absent (Hurlemann et al.
2007). These findings implicate the amygdala in a neural circuitry that orchestrates rapid
retrograde and anterograde regulation of episodic memory access upon criteria of behavioral
significance.
Long-term potentiation (LTP) is a mnemonic model in which particular patterns of
activation of incoming excitatory fibers (representing the learning experience) may induce
long-lasting enhancement of the communication between the involved pre- and post-synapses
(representing the memory) (Richter-Levin and Yaniv 2001). Hippocampal LTP, the most
prominent cellular model of memory formation, can be modulated by stimulation of the BLA
in its induction and early maintenance. It has been shown that specific areas of the rat
amygdala project to the entorhinal cortex, hippocampus, subiculum, and parasubiculum
(Pikkarainen et al. 1999) and that the amygdala differently controls hippocampal subregions
as well as memory processes involving the hippocampal CA1 region of the hippocampus and
