The amygdala
Many of the amygdala’s connections are similar to those of the orbitofrontal cortex, and indeed it has many connections to the orbitofrontal cortex itself. Bilateral damage to the temporal lobes of primates, including the amygdala, leads to the Kluver–Bucy syndrome, in which, for example, monkeys place non-food as well as food items in their mouths and fail to avoid noxious stimuli (Aggleton & Passingham, 1982; Baylis & Gaffan, 1991; Jones & Mishkin, 1972; Kluver & Bucy, 1939; Murray et al., 1996). Rats with lesions in the basolateral amygdala display similar altered food selections. Given the neural connectivity between the orbitofrontal and amygdalar regions, we might relate these phenomena to the finding that lesions of the orbitofrontal region lead to a failure to correct inappropriate feeding responses. Further evidence linking the amygdala to reinforcement mechanisms is illustrated when monkeys perform physical work in exchange for electrical stimulation of the amygdala. For example, they might be prepa red to press a lever for a long period of time to receive amygdalar stimulation (via an electrode which has been implanted in their brain), implying that this stimulation is significantly rewarding. In addition, single neurons in the monkey amygdala have been shown to respond to taste, olfactory and visual stimuli (Rolls, 2000a). But although the amygdala is similar in many ways to the orbitofrontal cortex, there is a difference in the speed of learning. When the pairing of two different visual stimuli with two different tastes (e.g. sweet and salt) is reversed, orbitofrontal cortex neurons can reverse the visual stimulus to which they respond in as little as one trial. In other words, neurons in the orbitofrontal cortex that previously ‘fired’ in response to a sweet taste can start responding to a salty taste, and neurons that previously ‘fired’ in response to a salty taste can start responding to a sweet taste, very quickly. Neurons in the amygdala, on the other hand, are much slower to reverse their responses (Rolls, 2000a). To explain this in an evolutionary context, reptiles, birds and all mammals possess an amygdala, but only primates show marked orbitofrontal cortex development (along with other parts of the frontal lobe). So the orbitofrontal cortex may be performing some of the functions of the amygdala but doing it better, or in a more ‘advanced’ way, since as a cortical region it is better adapted for learning, especially rapid learning and relearning or reversal (Rolls, 1996, 1999, 2000c).
Many of the amygdala’s connections are similar to those of the orbitofrontal cortex, and indeed it has many connections to the orbitofrontal cortex itself. Bilateral damage to the temporal lobes of primates, including the amygdala, leads to the Kluver–Bucy syndrome, in which, for example, monkeys place non-food as well as food items in their mouths and fail to avoid noxious stimuli (Aggleton & Passingham, 1982; Baylis & Gaffan, 1991; Jones & Mishkin, 1972; Kluver & Bucy, 1939; Murray et al., 1996). Rats with lesions in the basolateral amygdala display similar altered food selections. Given the neural connectivity between the orbitofrontal and amygdalar regions, we might relate these phenomena to the finding that lesions of the orbitofrontal region lead to a failure to correct inappropriate feeding responses. Further evidence linking the amygdala to reinforcement mechanisms is illustrated when monkeys perform physical work in exchange for electrical stimulation of the amygdala. For example, they might be prepa red to press a lever for a long period of time to receive amygdalar stimulation (via an electrode which has been implanted in their brain), implying that this stimulation is significantly rewarding. In addition, single neurons in the monkey amygdala have been shown to respond to taste, olfactory and visual stimuli (Rolls, 2000a). But although the amygdala is similar in many ways to the orbitofrontal cortex, there is a difference in the speed of learning. When the pairing of two different visual stimuli with two different tastes (e.g. sweet and salt) is reversed, orbitofrontal cortex neurons can reverse the visual stimulus to which they respond in as little as one trial. In other words, neurons in the orbitofrontal cortex that previously ‘fired’ in response to a sweet taste can start responding to a salty taste, and neurons that previously ‘fired’ in response to a salty taste can start responding to a sweet taste, very quickly. Neurons in the amygdala, on the other hand, are much slower to reverse their responses (Rolls, 2000a). To explain this in an evolutionary context, reptiles, birds and all mammals possess an amygdala, but only primates show marked orbitofrontal cortex development (along with other parts of the frontal lobe). So the orbitofrontal cortex may be performing some of the functions of the amygdala but doing it better, or in a more ‘advanced’ way, since as a cortical region it is better adapted for learning, especially rapid learning and relearning or reversal (Rolls, 1996, 1999, 2000c).
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