1994; Greenspan and Wieder, 1997; Stone and Hogan, 1993; Wing and Gould, 1979). In general, attempts have not yet been made to replicate these findings across studies. However, the variability in motor and sensory processing symptoms in children with autism, like other domains of development, may be related to developmental factors such as age or developmental stage. For example, few stereotypies are reported by parents of very young children with autism. Repetitive behaviors and mannerisms became more common in the same children after age 3 (Cox et al., 1999; Lord, 1995; Stone et al., 1997). Others have found no differences in sensory manifestations across ages or autism severity levels in school-aged children with autism (Kientz and Dunn, 1997). Maturational factors may affect sensory responses differently at varying developmental periods in children with autism.
Some of the unusual sensory processing and motor patterns seen in autism have been thought to result from problems in arousal modulation or habituation that result in withdrawal, rejection, or lack of response to sensory stimuli. Both physiological overarousal to novel events and underarousal and slower rates of habituation have been reported in children with autism (Hutt et al., 1964; James and Barry, 1984; Kinsbourne, 1987; Kootz and Cohen, 1981; Kootz et al., 1982; Rimland, 1964; Zentall and Zentall, 1983). A pattern of sensory rejection of external stimuli was associated with higher levels of arousal on measures of blood pressure, heart rate, and peripheral vascular resistance in children with autism, which was greatest in lower functioning children; this finding was attributed to problems in filtering and modulating responses to novelty (Cohen and Johnson, 1977; Kootz et al., 1982). It has been theorized that unpredictable and complex tasks may increase arousal modulation difficulties in both social and nonsocial situations (Dawson and Lewy, 1989).
Some studies suggest that physiological abnormalities relate to the bizarre behavioral symptoms seen in children with autism, particularly the need to preserve sameness. The children may be more sensitive to the environment and may use behavioral strategies, such as avoiding environmental change and social interaction, as methods of reducing further disorganizing experiences. In particular, tactile hypersensitivies were found to be related to behavioral rigidities (Baranek et al., 1997). Other studies have found no evidence of overarousal, and some have found evidence of underarousal (James and Barry, 1980; Corona et al., 1998). The overall circadian regulation of cortisol production, a physiologic marker of response to stress, was not found to be significantly different in autistic children; however, a tendency toward cortisol hypersecretion during school hours was found, and it appeared to be an environmental stress response (Richdale and Prior, 1992).