First, she tested both autistic and typical children, ages 8 to 11 , on their ability to discriminate similar sounds from one another. Although neither group of children performed as well as adults, autistic children showed poorer performance than nonautistic children their age. This test presented a simplified version of the ability to discern the complex features of human speech.
Next, the children performed the same task while the scientists used MEG to measure brain activity from the auditory cortex, which processes the sensation of sound. MEG is a noninvasive technique that is highly sensitive to the brain's response to sounds. This work is among very few MEG studies carried out with autistic children.
The researchers measured a brain response called the M100, a neuromagnetic signal that reflects the cortex's activity after a sound is presented. The studies revealed a "flattened" brain response to sound in autistic children. "The children with autism had a much smaller dynamic range for the same sounds than did their peers," says Gage. "That smaller range may indicate a reduced ability to understand a complex, fast-moving human speech stream. For example, to accurately decode speech, we need to be able to tell the difference between sounds such as the 'b' in beach and the 'p' in peach. We do this without conscious effort, even though the distinction occurs within only 20 to 40 thousandths of a second."
For those unable to quickly and easily make this distinction, understanding everyday speech may become difficult or even impossible. "These neuroimaging findings may present the neural 'answer' to why autistic children show poor speech sound perception," says Gage.