Neuroscientists have given up looking for the seat of the soul, but they are still seeking what may be special about human brains, what it is that provides the basis for a level of self-awareness and complex emotions unlike those of other animals.
Most recently they have been investigating circuitry rather than specific locations, looking at pathways and connections that are central in creating social emotions, a moral sense, even the feeling of free will. There are specialized neurons at work, as well -- large, cigar-shaped cells called spindle cells.
The only other animals known to have such cells are the great apes. These neurons are exceptionally rich in filaments. And they appear to broadcast socially relevant signals all over the brain.
The body, it turns out, is as important as the brain. Dr. Antonio Damasio, a neurologist at the University of Iowa Medical Center and the author of the book Looking for Spinoza: Joy, Sorrow and the Feeling Brain, has pioneered the argument that emotions and feelings are linked to brain structures that map the body. From human social emotions, he said, both morality and reason have grown.
Similar ideas were advanced in simpler form more than a century ago. Now, researchers can point to specific aspects of brain structure that suggest how our forebears came to develop complex social emotions, culture and other quintessential human behaviors.
The search for brain differences has not been easy. Mammalian brains are extraordinarily similar. All contain an outer rind, or cortex. The human cortex, where intelligence lies, is simply a lot bigger than that of other creatures given the human body's size.
But the size of the brain is not everything. One important feature of more complex brains is that they are rich in circuits -- linked cells from various parts of the brain that become active at the same time.
Imagine a Christmas tree with millions of lights, each representing a cell group. The thought of dogs would activate a small set of lights. The thought of a beloved dog that died last year would activate some of the same lights plus others.
The thought of a cat would activate yet another set with some overlap because animals are involved. Thinking about a sunset would activate whole new sets of lights with no overlap. Once a thought is complete, all the lights or neurons fall silent, waiting to be called into play in different combinations when new thoughts arise.
Some sets of lights are found in structures that serve as major hubs for thinking and feeling. For example, a brain region called the anterior cingulate -- a hub from which many circuits branch out -- is almost always active when human subjects are experiencing emotions or need to think about things that are difficult. Any conflict of any sort, any reward, and the anterior cingulate starts buzzing.
At least that's the judgment of the researchers who track increased blood flow with brain scans called functional magnetic resonance imaging.
One of the first circuits studied in the 1940s involved the sense of touch. Sensations from the skin, including pain and temperature, were found to be carried by nerve fibers to a part of the brain devoted to bodily sensation. Less distinct sensations from viscera and internal organs went to a small region called the insula.