Two seminal studies, conducted independently on essentially the same topic, both published in 1958, radically reduced the potential complexity of the experimental method and led to a dramatic change in the nature of research on infant perception and cognition. Berlyne (1958) measured the visual fixations of 3- to 9-month-old infants. On each trial, two black and white checkerboard patterns that differed in brightness or complexity were placed on a display board in front of each infant. An observer who could not see the patterns called out the direction of gaze of the infant—a technique that allowed Berlyne to determine to which pattern the infant first fixated. One of Berlyne's findings was that infants first looked at a complex pattern, such as a checkerboard with many squares, more than at a simple pattern, such as a checkerboard with few squares.
At the same time Fantz, known to many as the founder of modern research on infant perception, began a series of studies (Fantz, 1958,1961,1963; Fantz, Ordy, & Udelf, 1962) on infants' visual preferences. Patterns, such as checkerboards with differing numbers of squares, vertical stripes of different thicknesses, and drawings of regular versus scrambled faces were shown to infants two at a time. Fantz's procedure was a methodological advance over Berlyne's in that because of the placement of the infant in a testing chamber, the experimenter could actually see a reflection of the stimulus on the infants' cornea. Also, Fantz measured total looking time rather than just the direction of first look. Among Fantz's findings were that infants tend to prefer patterned surfaces to uniform surfaces and complex patterns to simple patterns.
Both Berlyne's and Fantz's studies represented real advances over previous research in the field. Their innovations included demonstration of a simple, reliable, inexpensive technique for measuring infant visual attention, systematic manipulation of the stimuli presented to infants, and examination of differences in the pattern of visual attention over age. Their technique capitalized on what may be considered infants' natural preferences for some stimuli over others; it has come to be called the visual preference paradigm.
The visual preference paradigm is still a very popular technique. Numerous studies by many investigators over the past 40 years have used some version of this visual preference paradigm to examine topics ranging from infant visual acuity to pattern perception, preferences for complexity, and even face perception. Several of these topics are discussed in greater detail later in this chapter.
The visual preference technique works well when infants have a natural preference for certain stimuli—that is, when from the outset, infants have the tendency to look at some stimuli longer than at others. When this occurs, we can infer not only that infants have a preference for one stimulus over another, but also that infants can discriminate between those stimuli. However, many cases exist in which infants do not display an initial preference, yet investigators need to know whether the infants can discriminate between the stimuli. In such cases, investigators often rely on a paradigm that combines the visual preference technique with habituation. Once again, the field is indebted to Fantz for leading the way. In 1964, Fantz reported a study in which infants were shown two magazine pictures simultaneously, side by side, and the infants' looking times to the pictures were recorded. As trials progressed, the picture on one side remained the same, but the one on the other side changed from trial to trial. Over the course of the experiment, infants gradually came to look more and more at the side with novel pictures.
This preference for novelty has become the underlying basis of the most widely used research tool for investigating infant perception and cognition—the infant visual habituation paradigm. Although many variations of this paradigm exist, a prototypical example would be to repeatedly present one visual stimulus until an infant's looking time habituates to some criterion level, such as 50% of the infant's initial looking time. Novel and familiar test stimuli would then be presented to see if the infant looks longer at (i.e., recovers to) the novel ones. Doing so indicates that the infant can differentiate between the novel and the familiar stimuli, even though initially the infant may not have had a natural preference for one over the other.
As we shall see later in this chapter, the infant visual habituation paradigm has been used for over three decades to investigate basic and esoteric questions related to infant perception, attention, memory, language acquisition, object knowledge, categorization, and concept formation. Differences in habituation and recovery have been reported between normal and aberrant infants, and both habituation rates and preferences for novelty appear to be moderately correlated with later IQ.
As simple and straightforward as infant preferences for novelty and habituation appear to be, the situation is actually more complicated. Many people assume that infants always have a preference for novelty. In reality, several studies have shown that a preference for familiarity often precedes a preference for novelty. Furthermore, this early preference for familiarity seems to be stronger in younger infants. It is also stronger when the information-processing task is more complex or difficult for the infant.
Hunter and Ames (1988) have summarized these conditions. According to them, the time it takes for an infant to be familiarized to a stimulus—that is, show a novelty preference—depends upon both the age of the infant and the complexity of the stimulus. For example, the familiarity preferences for older infants (e.g., those over 6 months of age) should be very brief compared to those for younger infants
(e.g., those under 6 months of age), and within an age group the familiarity preferences should vary according to stimulus complexity. The bottom line is that if younger infants are repeatedly shown very simple stimuli—or if older infants are shown moderately complex stimuli—both groups are likely to produce the classic monotonically decreasing habituation curve. On the other hand, if infants at either age are shown dynamic moving scenes involving multiple objects, they are likely to prefer familiar scenes prior to preferring novel ones. Therefore, it becomes important in such studies to habituate all infants to a relatively stringent criterion and to include both familiar and novel stimuli at the end to test that the infant indeed prefers novelty. Unfortunately, many infant habit-uation studies today do not adhere to these procedures.
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