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Most developmental scientists now agree that both nature (genetics) and nurture (environment) are essential for the normal development of perception. However, there is still much dispute about the extent to which either nature or nurture is the more important factor. Points of view on this issue are more than just philosophical musings, they affect the kinds of experiments that are undertaken. I argue here that classifying particular aspects of perceptual development as either innate or learned presents us with an overly passive view in which either genes or environment imposes structure on the developing brain. In contrast, I suggest that perceptual development is better characterized as an activity-dependent process involving complex and subtle interactions at many levels.

To begin to illustrate my point, let's consider some recent neurobiological work on the prenatal (before-birth) development of the brain in rodents. Neurons are specialized cells that transmit impulses or messages to other neurons, glands, and muscles. [■]The neurons studied in these experiments are those involved in binocular vision. [■]Experiments show that the prenatal tuning (training) of these neurons arises through their response to internally generated waves of electrical activity. [■]In other words, the response properties of these visual neurons are shaped by a "virtual environment" generated by cells elsewhere in the brain and eye. [■]Although the term "innate" can be stretched to cover this example of development, we could equally well describe this process as the cells "learning" from the input provided. Further, after birth the same neurons continue to be tuned in the same way, except that now their input also reflects the structure of the world outside. When we examine development in detail, it becomes harder to argue, as some theorists do, that "innate knowledge" is fundamentally different from learning.

Another example of the role of activity-dependent processes in perceptual development comes from the ability to detect and recognize faces. Because regions of the human brain are specialized for processing faces, some researchers have argued that this ability is innate. However, experiments with infants reveal a more complex story. The tendency for newborns to look more toward faces turns out to be based on a very primitive, reflexlike system that is triggered by a stimulus as simple as three high-contrast blobs in the approximate locations of the eyes and mouth. This simple bias is sufficient to ensure that newborns look much more at faces than at other objects and patterns over the first weeks of life. One consequence of this is that developing circuits on the visual recognition pathway of the brain get more input related to faces and thus are shaped by experience with this special type of visual stimulus. We can now study this process by using new brain-imaging methods. Such studies have shown that the brains of young infants show less-localized and less-specialized processing of faces than do the brains of adults. It is not until they are one year old that infants show the same patterns of brain specialization for processing faces as do adults, by which time they have had as much as a thousand hours of exposure to human faces.

Another example comes from the study of infants' eye movements to visual targets. Although newborns are capable of some primitive reflexive eye movements, only much later in the first year can they make most of the kinds of complex and accurate eye movements seen in adults. One view is that the very limited ability present in newborns is just sufficient to allow them to practice and develop new brain circuits for the more complex integration of visual and motor information necessary for adult eye movements. Once again, it appears that infants actively contribute to their own subsequent development.

These considerations should make us skeptical about the many claims that are made for innate perceptual abilities based on experiments with babies of four months and older. In fact, when the same experiments were done with younger infants, quite different results have often been obtained, suggesting dramatic changes in perceptual abilities over the first few weeks and months after birth.

Infants are not passively shaped by either their genes or their environment. Rather, perceptual development is an activity-dependent process in which, during postnatal life, the infant plays an active role in generating the experience it needs for subsequent development.

OFFICIAL62 Look at the four squares[■]that indicate where the following sentence could be added to the passage. Where would the sentence best fit? Click on a square[■]to add the sentence to the passage.

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