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If who we are is shaped by what we remember, and if memory is a function of the brain, then synapses--the interfaces through which neurons communicate with each other and the physical structures in which memories are encoded--are the fundamental units of the self. That was the message of a talk given at APA's 2002 Annual Convention by New York University psychologist and neuroscientist Joseph LeDoux, PhD.

"Synapses are pretty low on the totem pole of how the brain is organized, but I think they're pretty important," said LeDoux.

The self, he suggested, is the sum of the brain's individual subsystems, each with its own form of "memory," together with the complex interactions among the subsystems. Without synaptic plasticity--the ability of synapses to alter the ease with which they transmit signals from one neuron to another--the changes in those systems that are required for learning would be impossible.

LeDoux's research has focused on the network of brain regions responsible for detecting and responding to threatening stimuli. At the center of that network is the amygdala, the almond-shaped cluster of neurons near the base of the brain that stores memories of fearful stimuli and triggers fear responses.

Studies have shown that there are two pathways through which the amygdala's fear responses can be triggered: a fast "low road" from the thalamus to the amygdala, and a slower "high road" that passes from the thalamus to the neocortex and only then to the amygdala, said LeDoux. The two paths do not always reach the same conclusions, he explained. The relatively crude "low road" may respond to a long, thin object as a dangerous snake--and trigger an immediate fear response--while the slower "high road" is determining that the object is a harmless stick.

Evolutionarily speaking, it may make sense for the faster pathway to err on the side of caution, said LeDoux; after all, "it's probably better to treat a stick as a snake than a snake as a stick." But the disconnection between "low" and "high" roads, which was first discovered in rats but has since been corroborated in humans, could also be responsible for some psychopathologies. "We know that lots of people have fears that they can't come to conscious terms with," said LeDoux. "People who have pathological fears may be treating sticks as snakes all the time, metaphorically."

One of the biggest challenges in neuroscience, said LeDoux, is discovering how individual systems like the fear network are related to other systems, and how all of the brain's systems together create the collection of behaviors we call the "self." Although we know of a number of factors that could bind the brain's subsystems together--including shared inputs, neural "convergence zones," and the diffuse effects of neurotransmitters like serotonin--the details remain a mystery, said LeDoux.

The insights we gain from exploring the brain can give us new ways of thinking about psychology, he added. For instance, research on the neural underpinnings of emotion and cognition has shown that the amygdala sends projections to almost every part of the brain, including regions responsible for high-level cognition, but the number of projections back to the amygdala from cognitive regions is small. That insight may help explain why emotion can sometimes overpower cognition.

"Emotional systems tend to monopolize brain resources," said LeDoux. "It's much easier for an emotion to control a thought than for a thought to control an emotion."