Several behavioral studies have been published on infants at risk who are 6 months or younger, using primarily standardized measures of cognitive and language development, and specially developed structured assessments of atypical behavior in infants that might be early indicators of autism symptoms (Bryson et al., 2006
). The consensus from these investigations is that during the earliest months of life there are no clear differences between infants who later meet criteria for ASD, other high risk infants, or low risk controls. These babies all show clear social interest and engagement, they smile at other people, especially their primary caregivers, they follow eye gaze, vocalize and have good eye contact during face-to-face interactions (see Rogers, 2009
). In one recent study, which followed a relatively large cohort of infants at risk, the infants with an ASD outcome at age 3 showed more
frequent social and communication behaviors at 6 months than the low risk infants who had a normal outcome (Ozonoff et al., 2010
). Thus, prospective studies provide no evidence for very early deficits in social engagement during the infancy period.
By 12 months, however, there are significant differences in the infants who go on to develop ASD, hinting at a fundamental change in social communication as well as other behaviors taking place between 6 and 12 months. What are some of the behaviors that have been identified at 12 months? A small number of children begin to show slight delays in fine or gross motor development at this age, though for most children this is a pattern that emerges later during the second year of life (Landa & Garrett-Mayer, 2006
). Within the motor domain, some children show unusual repetitive behaviors that involve atypical arm waving movements (Iverson & Wozniak, 2007
; Loh et al., 2007
). Differences in visual attention are also risk signs for ASD. One example is atypical looking at objects, such as looking at objects from the corner of one’s eyes or staring for an unusually long time at simple objects (Ozonoff et al., 2008
). Other attentional problems include difficulty disengaging attention from one visual stimulus and shifting attention toward a novel stimulus (Zwaigenbaum et al., 2005
). In addition to these motor and visual behaviors, more direct signs of social-communication impairment are also found at 12 months. These infants show reduced eye contact, social smiling, social interest, imitation, reduced responding to bids for joint attention, as well as reduced responding to their own name (Nadig et al., 2007
; Ozonoff et al., 2010
; Presmanes et al., 2007
; Zwaigenbaum et al., 2005
). It appears that these differences in joint attention and response to name reflect social interest and motivation (and not more basic visual or attention deficits) in that they correlate most closely with other behaviors related to initiating social engagement with other people (Rogers, 2009
). Finally, even at 12 months delays are already evident in both nonverbal gestural communication and in language. Infants who later develop ASD are less likely to intentionally communicate with others using either pointing or other social gestures (Mitchell et al., 2006
Importantly, no single atypical behavior has been found that is shared by all 12 month olds who later go on to meet criteria for ASD. Instead, as with the diagnosis of ASD itself, there is a constellation of social, attentional and motor behaviors that may be considered risk markers (Zwaigenbaum et al., 2005
). If a baby exhibits several of these behaviors at 12 months, specifically, 7 or more risk markers, they are more likely to meet diagnostic criteria for ASD at a later age (Zwaigenbaum et al., 2005
). In this way studies of infants at risk reflect the complex
nature of the disorder. ASD is complex (heterogeneous) in terms of its genetic basis, range of behaviors and severity of phenotypic expression, and now has been shown to be complex with respect to the range of phenotypic risk markers. Current genetic studies suggest that the majority of ASD cases may be explained by a large number of variable genetic risk factors, each variant conferring small risk. On this model, ASD likely requires multiple risk genes that interact with each other and the environment (Abrahams & Geschwind, 2008
). There is also growing evidence that between 10% and 20% of ASD cases may be explained by unusual chromosomal rearrangements, mutations in specific ASD genes, or copy number variants (deletions or duplications), often arising de novo
and presenting as more severe cases accompanied by intellectual disability. Among infants at risk, occasionally a young infant (below the age of 12 months) already shows atypical behaviors, usually sensory or motor patterns, although as noted, this is not found in the majority of infants (cf. Bryson et al., 2007
). These infants invariably go on to have more severe ASD symptoms as well as intellectual disability. One possibility is that these infants who show very early signs may also have unusual copy number variants or other submicroscopic chromosomal anomalies, which would be consistent with their phenotype.
To summarize: the vast majority of infants at risk who later go on to have ASD show no signs at 6 months; at 12 months although there is no single atypical behavior that signals risk for ASD, there is a set of social communicative, motor and attention behaviors that together can predict later diagnosis. A few rare cases of infants do show unusual behaviors before the first birthday. One possibility is that these cases are marked by atypical genetic risk factors too, in the form of chromosomal anomalies. The findings suggest, then, that the pathway to atypical social-communicative impairment defining of ASD begins to emerge during the latter half of the first year of life, and varies considerably among infants.