Humanity? Maybe It's in the Wiring
By SANDRA BLAKESLEE
Published: December 9, 2003
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 is 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 1940's 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.
Or so the thinking of the time went. But Dr. Arthur Craig, a functional neuroanatomist at the Barrow Neurological Institute in Phoenix, says this classic view is incorrect for most sensations.
In a series of recent articles published in leading neuroscience journals, Dr. Craig has laid out a new wiring diagram for how the body talks to the brain. Tissues from all over the body, from skin surface to muscles, contain nerve endings or sensors that relay information, via long nerves, to the upper spinal cord. From this information come sensations including sharp pain, burning pain, cool or warm temperature, itching, muscle contraction, muscle burn because of lactic acid, joint movements, soft touch, mechanical stress, tickling, flushing, hunger and thirst.
The target cells, called Lamina 1 neurons, together make up a map of the state of the body. They are the first of several steps in the sorting and transmitting of sensory information, through structures in the brain stem and midbrain to the cortex.
The line ends at two thumb-size parts of the cortex called the insula, one on the left and one on the right side of the brain. But the crucial stop along the way may be a nucleus of cells in the back of the thalamus with the intimidating name of the posterior ventromedial nucleus, VMpo for short.
This structure hardly exists in most mammals. It is the size of a grain of sand in the macaque monkey, but relatively enormous in humans -- the size of a pistachio nut. It collects information on bodily states like temperature or the need for water, that need to be monitored to keep the body stable, in equilibrium. A nearby structure, another similar nucleus, also collects sensory news from internal organs. Each sends the information on to the pair of insulae.