Objectives: The present study examined the development of left and right superior temporal gyrus (STG) 50ms (M50) and 100ms (M100) auditory responses in typically developing children (TD) and in children with ASD. It was hypothesized that (1) left and right STG M50 responses would be observed equally often in younger than older children, (2) left and right M100 responses would be observed more often in older than younger children indicating later development of secondary auditory areas, and (3) M100 but not M50 would be observed less often in ASD than TD in both age groups, reflecting slower development of later developing auditory areas in ASD.
Methods: 35 controls, 63 ASD without LI (ASD-LI), and 38 ASD with LI (ASD+LI) were recruited. Binaural tones were presented. Using single-dipole source localization, the presence or absence of a STG M50, M100, and M200 was scored. A median split separated subjects into younger (6 to 10-years-old) and older groups (11 to 15-years-old).
Results: Although M50 responses were observed equally often in older and younger subjects and equally often in TD and ASD, M50 responses were delayed in ASD-LI and ASD+LI bilaterally. For M100, an effect of age was observed only in ASD-LI (young =76%, older = 95%; p=0.01). In contrast, left and right STG M100 responses were observed in almost all young and old TD subjects, and left and right STG M100 responses observed less frequently in young and old ASD+LI. Group comparisons showed that in younger subjects, M100 responses were observed more often in TD than ASD+LI (90% vs 66%, p=0.04), with no differences between TD and ASD-LI (90% vs 76% p=0.14) or between ASD-LI and ASD+LI (76% vs 66%, p=0.53). In older subjects, whereas no differences were observed between TD and ASD+LI (91% vs 75%, p=0.24), responses were observed more often in ASD-LI than ASD+LI (95% vs 75%, p=0.03). Given similar differences between TD versus ASD+LI and ASD-LI versus ASD+LI, the non-significant TD versus ASD+LI finding is likely due to a smaller N in the TD group.
Conclusions: TD subjects have an identifiable left and right M50 and M100 by 6 years of age. Although M50 responses were present in all groups, M50 responses were temporally delayed in ASD, suggested delayed development of primary auditory areas. Whereas there was a significant increase in the presence of M100 responses in older than younger ASD-LI, many older ASD+LI subjects continued to have a missing M100. Examining the TD data, present findings indicated that by 11 years a right M100 should be observed in 100% of subjects and a left M100 in 80% of subjects. Thus, by 11years, lack of a left and especially right M100 offers neurobiological insight into abnormal sensory processing that may underlie language or cognitive impairment.