Making mice see red25 Mar 2007
I just wrote about the alteration of a behavior pattern (decision-making) resulting from injury to the prefrontal cortex. That is the kind of functional specificity that might be expected as a result of "modularization" of mental functions. In that case, injuries to different people in the same place have the same kind of effect on behavior. Arguably, some part of the ventromedial prefrontal cortex functions as a module involved in moral decision-making (and as I noted, decision-making applied to gambling and other kinds of risks).
In a different study this week, a research team created knock-in mice, expressing the human photopigment allowing trichromatic vision in humans and other primates. They found that, even though mice belong to a lineage that hasn't had trichromatic vision for more than 100 million years, the mice immediately started perceiving the new color.
Here's an account from New Scientist, by Roxanne Khamsi:
The study demonstrates that the mouse brain is "primed" for expanded colour vision, the researchers say. It suggests that a simple mutation giving rise to the L receptor protein in our primate ancestors immediately expanded their visual perception.
"It has been unclear whether the simple addition of a photopigment is sufficient to yield a new dimension of colour vision, or whether you might need, in addition, some changes in the nervous system," says Gerald Jacobs, a vision researcher at the University of California, Santa Barbara, US, who took part in the study.
This is very interesting because it means that simple mutational changes in sensory systems are readily incorporated into sensory and cognitive processes by the brain. The paper suggests that the occurrence of novel sensory receptors might lead to selection for more and more efficient mechanisms for discriminating information, based on an initial, imperfect effect. That is basically the explanation for the evolution of all sensory systems, and is a powerful one.
The addition of new sensory receptors may be even more relevant for smell than for sight, since smell depends on hundreds of different olfactory receptor proteins, which originated by duplications and are eliminated by deactivations in different lineages of mammals. In retrospect, the idea that a novel sensory receptor might have immediate effects is sort of obvious, considering that humans are polymorphic for many olfactory receptors and some taste receptors. People with red-green colorblindness don't have catastrophic failure of their visual perception; instead, the perception system develops normally in the context of the lack of information from the missing receptor. Likewise, the senses of smell and taste bootstrap themselves based on the information present throughout development.
It's not remarkable that the brain should be plastic to new inputs; we would have noticed much sooner if it weren't!
Jacobs GH, Williams GA, Cahill H, Nathans J. 2007. Emergence of novel color vision in mice engineered to express a human cone photopigment. Science 315:1723-1725. doi:10.1126/science.1138838