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<p> It has been known for over a century that the temporal lobe,<sup></sup>including the amygdala, is involved in emotion. In 1888, Brown<sup></sup>and Schafer (<a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF21">21</a>) described taming in monkeys affect associated<sup></sup>with temporal lobe retraction. Klüver and Bucy (<a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF102">102</a>) elaborated<sup></sup>on this finding by characterizing a collection of emotional<sup></sup>disturbances caused by temporal lobe damage, which became known<sup></sup>as Klüver-Bucy syndrome. Monkeys with temporal lobe lesions<sup></sup>exhibited an absence of anger and fear, increased exploration,<sup></sup>visual agnosia, hyperorality, hypersexualtity, and loss of social<sup></sup>interactions. Subsequent work has shown that lesions restricted<sup></sup>to the amygdala produced many of these effects including a<sup></sup>loss of fear and anger, increased exploration, and hyperorality<sup></sup>(<a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF288">288</a>, <a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF300">300</a>). The reduced fear and anger, "taming" effect,<sup></sup>of amygdalar lesions is seen in many animal species (<a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF71">71</a>). While<sup></sup>amygdala damage in humans rarely results in full-blown Klüver-Bucy<sup></sup>syndrome, it is associated with some emotional deficits (<a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF2">2</a>)<sup></sup>including loss of the recognition of fear in others (<a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF1">1</a>).<sup></sup> </p> <p> Our understanding of the amygdala and its role in emotion is<sup></sup>hampered by the abstract nature of emotion itself. In humans,<sup></sup>bilateral damage restricted to the amygdala is extremely rare.<sup></sup>Animal studies are limited by their inability to tell us how<sup></sup>they "feel." Thus much of our understanding of the role of<sup></sup>the amygdala in emotion comes from the animal studies on fear<sup></sup>(<a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF114">115</a>). Fear, conditioned and unconditioned, elicits a constellation<sup></sup>of autonomic and hormonal responses that include cardiac effects<sup></sup>(increased blood pressure, changes in heart rate), hormonal<sup></sup>effects (release of stress hormones, adrenaline), defecation,<sup></sup>vocalization, freezing, and a potentiated startle response<sup></sup>(<a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF20">20</a>, <a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF114">115</a>, <a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF115">116</a>, <a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF141">141</a>). These fear response patterns are similar<sup></sup>in animals and humans (<a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF110">110</a>). Electrical stimulation of the<sup></sup>amygdala elicits fear and anxiety responses in both humans (<a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF34">34</a>, <a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF70">70</a>) and animals (<a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF94">94</a>), and lesions of the amygdala block<sup></sup>the expression of certain, but not all, types of unconditioned<sup></sup>fear. For example, rats with amygdala lesions show reduced<sup></sup>freezing in response to cats (<a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF17">17</a>) or cat hair (<a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF276">276</a>), attenuated<sup></sup>analgesia and heart rate responses to a loud noise (<a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF14">14</a>, <a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF298">298</a>),<sup></sup>and have reduced taste neophobia (<a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF182">182</a>). However, amygdala<sup></sup>lesions do not affect other measures of fear such as open arm<sup></sup>avoidance in an elevated plus maze (<a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF271">271</a>, <a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF272">272</a>) or analgesia<sup></sup>to shock (<a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF287">287</a>).<sup></sup> </p> <p> The amygdala is also necessary for many types of fear-motivated<sup></sup>learning. Amygdala lesions disrupt the acquisition, but not<sup></sup>the retention, of both active avoidance (escape from fear)<sup></sup>(<a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF212">212</a>) and passive avoidance (<a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF124">124</a>, <a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF235">235</a>, <a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF271">271</a>) conditioned<sup></sup>responses. Moreover, emotional processing in the amygdala is<sup></sup>not limited to fear and aversive stimuli. The amygdala is also<sup></sup>involved in conditioning using appetitive stimuli such as food,<sup></sup>sex, and drugs. Amygdalar lesions disrupt appetitive Pavlovian<sup></sup>conditioning (<a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF66">66</a>, <a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF266">266</a>), conditioned place preference (<a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF54">54</a>,<sup></sup><a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF55">55</a>, <a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF166">166</a>), and conditioned taste aversion (<a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF180">180</a>, <a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF182">182</a>, <a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF230">230</a>,<sup></sup><a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF292">292</a>) and reduce gustatory neophobia (<a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF51">51</a>, <a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF266">266</a>). Finally, in<sup></sup>addition to the direct role of the amygdala in learning and<sup></sup>memory, activation of the amygdala also has a modulatory effect<sup></sup>on the acquisition and consolidation of memories that evoke<sup></sup>an emotional response (<a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF168">168</a>, <a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF169">169</a>, <a href="http://physrev.physiology.org/cgi/content/full/83/3/803#REF195">195</a>). Although it is well<sup></sup>recognized that the amygdala is involved in a myriad of memory-<sup></sup>and learning-related tasks, the best studied is its role in<sup></sup>Pavlovian fear conditioning and fear-motivated operant conditioning.<sup></sup>The remainder of this review will therefore focus on the neural<sup></sup>substrates of classical fear conditioning.<sup></sup> </p>
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