Better Working Memory for Non-Social Targets in Infant Siblings of Children with Autism Spectrum Disorder.

Bridging clinical and genetic understanding of Autism Spectrum Disorders (ASD) will require identifying the prodromal disruptions and subsequent developmental pathways through which these disruptions lead to core symptoms. One strategy for exploring the earliest differences associated with ASD is to study the broader ASD phenotype in infant siblings of children with ASD. The broader ASD phenotype refers to atypical core functions characteristic of children with ASD and some of their school-age siblings and parents. Social-communicative development and responsiveness are disrupted in some siblings of children with ASD (Zwaigenbaum et al., 2007; Constantino et al., 2006). Disrupted social-communication development, especially coordinated social attention, is among the first symptoms of ASD (Mundy, 1995; Stone, Coonrod, Turner & Pozdol, 2004). There is evidence that disrupted coordinated social attention is a component of the broader ASD phenotype affecting infant siblings of children with ASD. In the second year of life, some younger siblings of children with ASD (sibs-ASD) initiate and respond to fewer bids for coordinated social attention (Stone, McMahon, Yoder & Walden, 2007; Presmanes, Walden, Stone & Yoder, 2007). Although 5-10% of sibs-ASD are expected to develop autism, this effect does not seem to be driven by a small subset of poorly-performing infants. Studying sibs-ASD who are younger than one year of age may help us identify the precursors of this disrupted coordinated social attention. Decreased social memory ability could lead to delays in coordinated social attention. The process of coordinating social attention involves the ability to create, maintain and update representations of a social partner. During infancy, working memory (WM) is an information handling capacity with which the infant actively forms and updates transient representations (Reznick, 2007). Although we have much to learn about infant WM, there is a growing body of evidence supports expectations for how WM develops in the first year of life. Typically, WM emerges in developing infants by the middle of the first year (Reznick, Morrow, Goldman & Snyder, 2004; Schwartz & Reznick, 1999) and improves markedly in subsequent months (Pelphery et al., 2004). Based on current understanding of the emergence of infant representations, we are able to assert that infant working memory performance reflects a capacity that has been shaped by prior experience representing objects (Munakata, 2004). Some studies have reported poor WM in children with ASD (Bennetto, Pennington & Rogers, 1996; Happe, Hughes, Booth & Charlton, 2006) but others have not (Ozonoff & Strayer, 2001; Dawson et al., 2002). Although some parents of children with autism have been found to have spatial WM deficits (Kaoczat, Rogers, Pennington & Ross, 2002), we are aware of no research assessing WM in children younger than 3 years of age who have received a diagnosis of ASD or are at-risk for ASD. Thus, there is some limited precedence for a global WM decrease seen in infant siblings of children with ASD. However, there is an insufficient basis to consider WM in infants and WM in older children (and adults) a unitary construct. As a result, the suggestion that familial differences in WM might extend down to infants has to be considered exploratory. An alternative to a global WM difference is a group difference that is specific to the nature of the target being represented. Social stimuli are less likely to draw the attention of young children with ASD than non-social stimuli (e.g., children with ASD pay less attention to a person humming than to a phone ringing; Dawson et al., 2004). In infant sibs-ASD, atypical interaction patterns have been reported as early as 4 months of age (Yirmiya et al., 2006). It should be noted that while like Yirmiya and colleagues, some researchers have found evidence of early differences in sibs-ASD (McCleery, Allman, Carver & Dobkins, 2007; Merin, Young, Ozonoff & Rogers, 2007; Ibanez, Messinger, Newell, Sheskin & Lambert, in press) others studies have found no sibs-ASD differences (Nadig et al., 2007; Zwaigenbaum et al., 2005; Landa & Garrett-Mayer, 2006). Nonetheless, early differences in social engagement in sibs-ASD could contribute to both reduced social WM and disrupted social-communication abilities. Investigating infant WM for different types of targets not only allows us to make direct comparisons of which target an infant represents more readily, but also provides a window into the representational histories that have lead up to their current information handling capacity with those types of targets. We tested WM in infants at 6.5 and 9 months of age in a task that challenged them to remember the location of social and non-social targets, and to update their WM across a sequence of trials involving three different locations sampled with replacement. We used a delayed-response task (Pelphrey et al., 2004; Reznick et al., 2004; Schwartz & Reznick, 1999) that is a modification of the peek-a-boo game that infants typically find intrinsically rewarding. We tested two groups of infants: sibs-ASD and infants whose older sibling(s) were typically developing (sibs-TD). Each infant participated in two conditions: a social target trial block and a non-social target trial block. If WM is globally compromised in at-risk infants, then WM performance would be lower for the sibs-ASD in both social and non-social conditions. If elevated risk is associated with a relative decrease in social WM, then this liability would emerge in an interaction of target type condition and risk-status. An interaction could also be driven by sibs-ASD having better non-social WM than sibs-TD. An early sibs-ASD advantage in non-social WM would be consistent with the finding that high-functioning individuals with ASD and their parents have relative strengths in reasoning about non-social objects (Baron-Cohen et al., 1997). In summary, we tested simultaneously for two possibilities: 1) a risk group (sibs-ASD vs. sibs-TD) difference in WM independent of target type, and 2) an interaction of risk group and target type. Further, to investigate the degree to which any group difference was specific to WM, we also compared the groups on their responsiveness to the social and non-social stimuli when WM was not required. There were two measures of non-WM stimuli interest: latency to orient and preferential looking.