Research methods to understand the development of infants
The past five decades have seen an increasing interest in infants' world. This has produced an exponential growth in research on infants' perception, leading to advances in knowledge of infants' abilities. This essay will first succinctly review some of the theoretical perspectives influencing research in this area, namely empiricist and nativist views of infant's perception. Secondly, it will describe how new research methods employed in the study of infants' progress in perceptual abilities have widened the understanding in this complex matter, dispelling some myths. Thirdly, it will briefly describe the development of the visual system during infancy and the research methods utilised in visual perception describing some key findings in this area. The auditory system will subsequently be explained in a similar way. Lastly, the essay will define some aspects of cross-modal perception and compare single sense experiments with multisensorial research. Along the essay, an attempt will be made to point out the evidence over what parts of perception may be innate and what parts may be learned.
The human evolutionary dilemma caused by bipedalism and encephalisation is solved by an immature brain that keeps developing after birth. Immature brains have underdeveloped perceptual systems but newborns' sensory perceptivity improves rapidly with development (Smith et al., 2003). The interest in infants' perceptual world is not new although the intrinsic difficulty of accessing it has led to many misconceptions. Empiricists from Locke to Skinner theorised that the source of all knowledge comes from the perceived outside world and that perceptual abilities have to be learned and adapted to (Woodhead, 2006). William James (1890, p.489) talked about the infant's perception as 'blooming, buzzing confusion'. Somewhere in between empiricism and nativism, Piaget's constructivism suggests that although children are born with innate behaviours, an infant's early perception is chaotic. Only by interacting with the environment during the sensori-motor stage, do infants construct new knowledge into schemata (Oates et al., 2006). Contrarily, nativists, such as Spelke and Meltzoff among others, argue that infants are endowed by nature with capacities to perceive and understand events in the world, for instance, through 'core knowledge' of the physical world or some basic social knowledge (Slater & Oates, 2006).
Perception can be defined as the ability to identify, arrange and make sense of sensory data (Edgar, 2007). Sensation, perception, cognition and behaviour form a complex system that receives and responds to external stimuli. Infants have limited ability to communicate so it is hard to tell what they are experiencing. Hence, the information about infants' perception and cognition is many times collected through the window of behaviour, specifically, through the operation of attention. This way inferences are made in perceptual research, for example, that gaze duration is related to visual perception; head turning is linked to auditory attention or sucking, reaching or kicking are measures of interest (Slater & Oates, 2006). Physiological measures such as heart rate or cortical evoked potentials can be used instead of descriptive data at the behavioural level but behavioural cues are easy to measure and they have been validated by ample research (Cohen & Cashon, 2003). Better experimental techniques improve the understanding of early perceptual abilities. Experiments test hypotheses by manipulating variables with the aim of establishing causal relationships, allowing quantitative data to be collected and hence minimising the amount of interpretation required (George et al., 2006).
As implied before, the visual system in newborns is immature. Although the retina is quite developed, the foveal cones, responsible for fine detail vision, have yet to migrate, doing so around the second to third month. Accommodation is sluggish and myelinisation of the optic nerves, visual cortex and lateral geniculated bodies commences at three months but is not complete until the twelfth month (Slater & Oates, 2006). This means that acuity, colour vision, binocular vision, stereopsis and other basic processes are worse in infants than in adults and older children. For example, infants have less effective scanning abilities and their fixations in an object or face occur in fewer, usually peripheral features. This has been called the 'externality effect', and it has been attributed to the immaturity of fovea and optic muscles (Maurer & Maurer, 1988).
Fantz is, for many, the founder of modern research on infant perception (Cohen & Cashon, 2003). His viewing chamber proved to be a reliable and relatively simple technique that allowed the systematic manipulation of visual stimuli, exploring infant's natural preferences for some stimuli over others in a controlled manner. To do this, an observer records the infant's gaze direction and corneal reflection (Slater & Oates, 2006). This has been termed the 'visual preference' paradigm or 'forced choice preferential looking' and assumes that infants can discriminate between the stimuli (Cohen & Cashon, 2003). Preferential looking also is used to determine infants' visual acuity, i.e. how much fine detail can they distinguish. According to Hainline (1998), although young infants' visual acuity is similar to that of an adult cat, it is well suited to infants' needs. The 'visual preference' paradigm, testable even in very young infants, appears to support nativist arguments as it indicates the innate ability to perceive patterns and forms (Fantz, 1963).
Additionally, when there is no apparent natural preference for one stimulus over another, the answer to the question whether infants discriminate or not comes from the 'habituation' and 'novelty preference' paradigms. Infants pay decreasing attention to stimulus to which they are exposed for longer periods of time and increasingly look at a novel stimulus, consequently dishabituating. Preferring the known stimulus when presented again is called 'familiarity preference' (Slater & Oates, 2006). In this way, it has been demonstrated that infants prefer patterned rather than plain surfaces, moving over stationary things, curvilinear rather than rectilinear patterns, symmetrical over asymmetrical stimuli and three-dimensional objects rather than two dimensional representations (Slater & Oates, 2006). It is not surprising that faces, fulfilling many of these characteristics, appear to be preferred stimuli for newborns, something already noted by Fanzt (1963). Babies less than 12 hours old prefer their mother's over a stranger's face (Bushnell, 2003). Similarly, Slater et al. (1998) demonstrated that 3 day old infants preferred attractive over unattractive faces in what has been called the 'attractiveness effect'. Again these findings, together with the discovery that minutes old newborns are capable of imitating faces (Reissland, 1988) appear to support nativists views as they suggest the existence of innate abilities to recognise faces and respond to them, an essential part of bonding with caregivers (Slater & Oates, 2006).
Infants also discriminate faces on the basis of gender. Quinn et al. (2002) showed that 3 month old infants prefer to look at female faces when paired with male faces. They used familiarisation to either female or male faces and novelty preference technique to find that those infants familiarised with male faces preferred female faces but those familiarised with female faces showed no preference. To test if this preference for female faces reflected a gender bias towards the primary caregiver, the researchers identified a population of infants raised by male caregivers. These infants preferred male faces. These findings suggest that the social environment influences face processing very early in life, supporting empiricist views (Slater & Oates, 2006).
In contrast to the visual system, the auditory system develops in utero and is functional the last trimester of pregnancy. The auditory nerve is fully myelinated at birth but the auditory cortex continues its development post-natally. Consequently, newborns' auditory acuity is much better than their visual acuity. They are quite competent to make sense of their acoustic world although their auditory threshold improves with age (Slater & Oates, 2006).
Auditory preferences, expressed by head turning, have been demonstrated towards parentese or baby-talk over normal adult talk in 4 month old infants (Fernald, 1985) but only if the former is emotionally positive, as demonstrated by Singh et al. (2002).
Using sucking rates to interpret discrimination between sounds, and a habituation-dishabituation method, DeCasper and Fifer (1980) demonstrated preference for the sound of the infant's own mother's over a stranger's voice in 2 day old infants. The possibility of this finding representing a prenatal learning of their mother's voice was further investigated by DeCasper and Spence (1986) who requested mothers to read a story during pregnancy and after birth. Babies showed preference towards the known reading over an unfamiliar one, indicating very early learning. Further research into sound discriminations and environmental influences has demonstrated interesting results regarding the perception of languages. Jusczyk (2002) believes that babies become quite proficient in discriminating their own language from others by 4-5 months and start recognising individual words around 8 months.
Most perceptual developments broaden the perceptual world of infants; however, there are areas in which there is evidence of narrowing of perception. For example, young infants can discriminate monkey faces as well as human faces (Pascali et al., 2002) and non-native speech phonemes (Werker, 1989) in a way that older infants no longer do. Older infants become more adept at distinguishing human faces and native language. This may mean that infants, although predisposed at birth to attend faces and speech, lose some of the competencies they are born with whilst developing perceptual specialisation (Pascali et al., 2002), something that can be seen as another example of very early learning.
In most instances in everyday life, however, visual and auditory stimuli combine. This is cross-modal perception and it appears very early as Morrongiello et al. (1998) demonstrated by exposing newborns to sight-sound pairs that were either in the same place (co-located) or different places (dislocated). Babies learnt the object-sound pair only in the co-located condition. The synchrony of sound-sight was further investigated by Slater et al. (1999) reflecting that 2 day old infants learn to associate sounds and images if they belong together. Integration of information from different channels appears then to be innate. However, inter-sensory redundancy, i.e. the evidence that presentation of repeated information through two sensory modalities is linked, such viewing a person's mouth movements and voice, is learned around 3 months of age. This is the building block for more sophisticated cross-modal relationships, such as objects and what they are called (Bahrick & Lickliter, 2002).
Many of the experiments in infant perception studies utilise simple stimuli, for example static instead of moving images, to isolate a particular characteristic and to control possible confounding variables. However, the real world is full of dynamic perceptual experiences that question the ecological validity of these studies. Experiments that test infants' proprioception demonstrate more sophisticated abilities than simpler, single sense experiments. Butterworth (1981) proved appropriate postural control, recorded as head movements, in a moving room in sitting infants as young as 2 months. Hence the new research methods into infants' perception has produced evidence that points towards a middle ground between empiricism and nativism, with infants born with more perceptual abilities than empiricists imply and with more capacity to learn from experience than nativists suggest (Smith et al., 2003).
In conclusion, infants' perceptual system is immature, although the auditory anatomical structures and function are far more developed at birth than the visual system is, with even evidence of pre-natal hearing. Research methods utilise mostly behavioural data such as gaze direction, head turning or sucking, to make assumptions about infants' perception. In recent years, increasingly sensitive research methods into infants' perceptual abilities have found that infants not only are born equipped with some strategies to get to know the world around them but also that experience plays a crucial role in the normal development of these perceptual functions. Nature and nurture are then interdependent. Newborns seem to be hardwired to recognise and react to faces and speech, especially to those of their caregivers, but these abilities need fine tuning as visual and auditory inputs during early infancy influence the development of these competencies by specialisation or perceptual narrowing in some cases. This reveals a high degree of sophistication that is especially exposed by cross-modal perception research and more ecologically valid experiments, for example, involving proprioception. Further research is still required to advance our understanding of infants' perceptual world.
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