Ferrets, looking loudly, hear the light.

Ferrets, looking loudly, hear the light

In a series of unusual experiments, scientists have rewired the brains of newborn ferrets so the animals, in a sense, hear things they would normally see. The research provides the strongest confirmation yet for a theory of brain function that deems the visual, auditory and other "higher" parts of the brain as fundamentally alike in computational function -- resembling, at least in early stages of development, interchangeable parts.

Moreover, the research supports the notion that these higher, or cortical, parts of the brain "learn" how to perform many of their sensory or motor functions from early cues in the environment. While that theory is not new, the experiments appear to underline the importance of sensory experiences before birth and during infancy in determining an individual's ability to process information later in life.

Mriganka Sur and his co-workers at the Massachusetts Institute of Technology in Cambridge rerouted retinal neurons -- which normally send sensory data from the eyes to the visual cortex in the brain -- in 16 ferrets so that the data went instead to the animals' auditory cortex. Cortical areas process raw bits of data into more useful "patterns" of information. The researchers studied the response patterns of cells in the auditory cortex while showing the ferrets various visual cues.

"The basic issue is: Does all cortex perform basically the same operation, and do the different outcomes only depend on putting different inputs in?" says Jon Kaas, an experimental psychologist at Vanderbilt University in Nashville, Tenn. "Functionally, each area of the cortex is doing something quite different. But is each area somehow doing the same sort of calculations with whatever input it gets?"

The answer appears to be yes, the MIT researchers report in the Dec. 9 SCIENCE. They found that some cells in the auditory cortex "transform" raw data into "oriented rectangular receptor fields" -- a type of patterned response to stimuli that has until now been clearly identified only in the visual cortex.

The finding is somewhat surprising, Sur and others say, since auditory information processing -- which includes calculations of frequency changes and phase shifts to locate sound in space -- seems in some respects quite different from the operations required to sense visual patterns. So while the finding supports the theory that all cortical tissue organizes information similarly, Sur says it also suggests that whatever detailed differences may exist among auditory, visual and other cortical operations are "learned" differences -- the result of specific neural wiring patterns somehow programmed by early sensory inputs.

"This means there is nothing intrinsic about the auditory cortex that makes it auditory," Sur says. "It depends on what kind of input it gets" early in life. The finding, he adds, could help explain the enormous capacity of the young brain for recovery of function (SN: 4/30/88, p.280). "So if early in life there are...lessons in some part of the brain, other parts of the brain have the capacity to sort of chip in or help in the recovery of function."

Moreover, Kaas says, the research has potential significance for learning theory. "As we understand the role of the environment in the developing nervous system, we'll understand how to modify [prenatal and early childhood experiences] in ways that are desirable, or perhaps more importantly to prevent stimuli that are undesirable."