What evidence is there that infants� ability to distinguish shapes and patterns, depth and voice onset times in speech sounds are not learned?

James� dismissal of a baby�s ability to make sense of its initial experiences as �one great, booming, buzzing confusion� has been turned on its head by many studies such as Steiner�s on newborns� differential responses to taste in the form of �gustofacial expressions�, or Cernoch and Porter�s demonstration that two week old breastfed babies can recognise their mother�s scent. Although a baby�s nervous system is still immature and their optic nerves are thinner and shorter than an adult�s and only partially myelinated, their eyeballs for instance are otherwise anatomically identical to an adult�s in content.

There are considerable difficulties for experimenters in answering the question of whether infants� perceptual system is innate or learned, i.e. a trait acquired through experience. With short attention spans and unsystematic behaviour, devising a technique to test and measure the responses of babies is fiendishly difficult. Until recently, the unsatisfying solution involved depending on indirect results, such as those yielded from blind adults whose sight had been restored or animals brought up under visually-deprived conditions. Recently though, besides direct study of babies� visual apparatus, the approach for determining what perceptual equipment babies possess, i.e. their structural and physical attributes, has usually involved inferring from what the baby does, including physiological responses such as heart and breathing rate.

Progress in learning about their ability to distinguish shapes and patterns was first made with two experimental techniques: preference of looking and the habituation technique.

Robert Fantz�s innovation was in monitoring neonates� responses by measuring the length of time for which an infant looks at different objects. By offering infants a view of two shapes or patterns, and recording whether there was a preference in how long the infant gazed at one over the other, he hoped to show that they discriminated between them according to which interested them more, demonstrating an ability to distinguish between them in the first place.

However, his results were only partly conclusive: although there was a marked preference for the square over the checkerboard pattern, and the result for the two identical triangles (the control pair) was negative as expected, the other two pairs also yielded negative and thus inconclusive results. Because the pairs of shapes had been carefully chosen to test infants� ability to perceive differences in contour, contrast and pattern, it was important to find out whether the bulls-eye/stripes and circle/cross pairs were indistinguishable or simply of equal interest to the babies.

By taking into account a species-wide preference for novelty, Slater et al. split the experiment into two parts, first habituating the babies to one of the shapes, and then measuring whether, when presented with a new and different shape, the babies give it more of their attention by gazing at it longer. The advantage of this method is that if the babies do not demonstrate a preference, it indicates that they are unable to distinguish between the novel (and different) shape and the original.

Fantz�s initial limited success was bolstered by this new means of controlling the interest to the babies of the stimuli presented to them. A 65% preference in babies of between one and twenty weeks for the novel shape, either placed side-by-side with the familiar one or shown separately, was sufficient to indicate that babies believed too young to have had time to acquire the perceptual mechanisms could distinguish a variety of patterns and shapes.

Unlike shape perception where the retina only has to build up an image of a flat world, seeing in three dimensions requires a variety of visual clues.

Because depth perception is something that any mobile subject will almost certainly have to acquire in a short time, testing whether depth perception is learned has to be conducted on the very young, usually using avoidance as the monitored response. In the past, when young animals are placed near a cliff, an avoidance reaction is apparent. amd so in order to eliminate any non-visual clues, Gibson and Walk developed the �visual cliff�. This device involved a transparent table with one side being apparently �deep� and the other �shallow�. As expected, perambulatory neonates avoid the deep end, indicating early depth perception. However, babies do not crawl until ten months old and so, again, a solution which measured another sort of systematic behaviour was needed in order to test younger infants. The heart rate solution works on the principle that distress or upset, such as the fear of falling, raises the baby�s heart rate, while something novel or of interest lowers it. Compos et al. found that when placed on the shallow end, six week old babies� heart rate did not change, but not even their mother on the other side of the table to could induce them to travel over the deep end, while the heart rate of babies placed in the deep end actually went down, perhaps indicating very early perception of depth differences.

Experimenters also took advantage of human subjects� propensity to divide continua such as colour, shape and angle into categories. One such perceptual category is voice onset time (VOT), the interval between release of breath and vocal cord movement, used by adults in recognising different speech sounds such as �b� and �p�. Adult English speakers can distinguish between but not within a VOT perceptual category with a cut-off point of 30 milliseconds, less than which a �b� sound is heard, and greater than which a �p� sound is heard.

With the technique of High Amplitude Sucking (HAS), babies are given a dummy wired up so that sucking causes a sound to be heard, which they soon learn to associate and suck harder as a consequence. Eimas et al. used babies of one and four months, first habituating them to a sound of a certain VOT, and then changing to a second sound after and measuring whether the infants registered the difference by sucking harder again. The experimenters used three groups: one where the changes of sound were both up and down over the 30 millisecond cut-off point for adult English speakers� �b�-�p� perceptual category; one where the changes stayed either side of the 30 millisecond boundary, indistinguishable to an adult; and one control where the sound did not change at all.

Changes in sound across the boundary resulted in a large increase in the rate of sucking for both age-groups of baby, indicating that even infants as young as one month old have the ability to distinguish such small changes in the VOT. The results were supported by the control group, where the sucking rate decreased over time, especially for the younger babies. Interestingly however, the group of babies where the sound involved a change in the VOT within perceptual categories noticeable to adult English-speakers, differentiated between the one month and four month old babies. The older babies� sucking rate diminished by exactly the same amount as the control group�s, while the younger babies seemed able to distinguish even sound changes which did not cross the 30ms cut-off point; thus it seems that this more precise perceptual ability is lost over time as we become used to hearing only the sounds employed by speakers around us, i.e. we lose categories.

Certainly, much of the evidence mentioned above would seem to indicate that at least the mechanisms particular to perceiving shapes and patterns, depth and VOT in speech sounds are largely in place at an early age. This evidence hinges on the validity of the various assumptions behind the inferences, such as those of preference of novelty and spontaneous visual preference.

However, one of the major problems of interpreting evidence of newborns� abilities in the light of the nature-nurture debate is the fact that even a 36-hour old baby has had at least a day in the world, and nine months in the womb before that, making it very difficult to ascertain how much has already been acquired. Moreover, Slater and Morrison�s experiments with infants of differing ages on 6 familiar shapes indicated that although infants of two months old can discriminate between shapes, it is only later that this extends beyond lower-order variables towards true form perception. This evolution in the nature and complexity of babies� perceptual mechanism would seem to be acquired, rather than being genetically time-released, which adds further complications to any experiments trying to establish nativism versus empiricism.