LA JOLLA, CA -- "Green" means "go," but what does "red" mean? Just about everybody says "stop" since we all have learned to imbue certain colors with meaning (or we would be road kill by now). Long thought to be limited to higher levels of information processing, researchers at the Salk Institute for Biological Studies successfully traced this type of associative learning to early stages of the visual processing pathway.
"Sensory neurons in the visual cortex that handle incoming information are very plastic and what they 'see' is determined by our experience in the world," says lead investigator Thomas D. Albright, director of the Vision Center Laboratory. Their findings, reported in the March 14 issue of the journal Neuron, will help scientists to better understand how such learning takes place in the brain based on our daily experiences.
Human memory relies mostly on association and objects frequently seen together to become linked in our mind; when we try to retrieve information, one thing reminds us of another, which reminds us of yet another, and so on. Not surprisingly, neurobiologists have been trying to uncover the underlying mechanisms for decades.
The acquisition of associated memories is believed to result from the establishment or strengthening of connections between neurons that represent the associated objects. Once trained and intricately linked, a neuron that responds to the sight of a keyboard might respond to the sight of a computer monitor, a coffee cup or reading glasses - depending on the previously forged links.
In the past, studies on associative learning primarily focused on a special area of the brain called the "inferior temporal cortex" (ITC), a high level stage of visual processing. It is known to be critical for object recognition and for storage of this type of learning.
"We wanted to know whether associative plasticity is unique to such higher levels of processing or whether it is a more general property of the brain that can happen even at lower, 'sensory' areas," explains first author Anja Schlack, a post-doctoral researcher in the Albright lab.
Our eyes take in the visual environment and break the incoming images down into simple features such as color, brightness, motion and form. These pieces of information are channeled from the eye to the brain along specialized pathways. The ventral pathway, for examples, carries information about form while the dorsal pathway is sensitive to space and motion.
Schlack trained monkeys to associate a stationary arrow pointing upward or downward -a meaningless object for the monkey - with dots moving up or down. While the monkeys watched arrows or moving dots, Schlack observed signals from neurons located in the middle temporal or MT area, an early way station along the dorsal pathway. It's also nicknamed the "motion area" since over 90 percent of all neurons in this area respond to movement in a particular direction but are relatively impervious to color or form.
Before the start of the training session and just as the researchers had predicted, stationary arrows meant nothing to neurons in the MT area while moving dots elicited clear signals. After the learning process had taken place, the cells responded to both because experience had changed their tuning. "After the training, the arrows elicit a recall of the motion and this is what the MT neurons then respond to," concludes Schlack.
These results might explain the observations made recently in a different lab with the help of functional magnetic resonance imaging (fMRI). When shown photographs of athletes in motion, the human equivalent of the macaque area MT lit up in human observers. The Salk studies suggest that these brain activations probably result from learned associations, strengthened by daily experience.
"We are constantly faced with a complex and ever changing environment," says Albright. "The ability to use information based on learned relations between objects helps us to make sense out of what we see faster and more efficiently. This ability allows us to make the right decisions in a timely manner: Even when presented with a complex visual scene during rush hour we stop at the red light and avoid getting hit by the oncoming traffic."
The Salk Institute for Biological Studies in La Jolla, California, is an independent nonprofit organization dedicated to fundamental discoveries in the life sciences, the improvement of human health and the training of future generations of researchers. Jonas Salk, M.D., whose polio vaccine all but eradicated the crippling disease poliomyelitis in 1955, opened the Institute in 1965 with a gift of land from the City of San Diego and the financial support of the March of Dimes.