The CHecklist for Autism in Toddlers (CHAT) is a screening instrument that identi®es children aged 18 months who are at risk for autism. This article explains how the CHAT was developed and how it should be used. First we offer a brief introduction to autism. WHAT IS AUTISM? Autism, ®rst described by Kanner in 1943 1 , is one of a family of`pervasive developmental disorders 2. The most severe of the childhood psychiatric conditions, it is characterized by a triad of impairmentsÐin socialization, communication and¯exible behaviour. The exact cause is unclear but family and twin studies suggest a genetic basis 3±5 ; molecular genetic studies are underway 6. Altered central nervous system function is evident in several different brain regions including the medial prefrontal cortex 7 and the amygdala 8±10. Autism occurs in about 1 per 1000 children 11. The general view is that autistic conditions exist in a spectrum, with classic autism at the extreme. In DSM±IV this is referred to as autistic disorder, and in ICD±10 as childhood autism. To qualify for this diagnosis, the dif®culties in social interaction, communication, and exible behaviour must have begun before the age of three years. Atypical autism and`pervasive developmental disorder not otherwise speci®ed' (PDD-NOS) also lie on the autistic spectrum, but children with these conditions do not meet criteria for autism because of late age of onset, atypical symptoms, symptoms which are not very severe, or all of these. Asperger's syndrome is thought to be another condition on the autistic spectrum: individuals with this syndrome have the social interaction dif®culties and restricted patterns of behaviour and interests but their IQ is normal and there is no general delay in language. A ®nal subtype are individuals with`high functioning autism' (HFA), who are diagnosed when all the signs of Asperger's

The perceptual integration of a mixture of two complex tones was studied in experiments on adult subjects. Each tone was formed by amplitude modulation (AM)of a carrier sinusoid of frequency (CF) by a raised sinusoid with modulation frequency (MF). One tone always had CF = 1500 Hz and MF = 100 Hz. The other had different CFs around 500 Hz and different MFs around 100 Hz. Both harmonic and inharmonic partials, produced by AM, were employed. The method involved studying the competition between two perceptual organizations: (1) the fusion of the two complex tones, and (2) the tendency of the higher tone to be stripped out of the mixture by a competing sequential organization. Fusion was best when the higher and lower complex tones had the same MF, even when the resulting partials did not form part of the same harmonic series. When MF was the same for the higher and lower tones and all partials were harmonically related, the tones fused best when the AM applied to the two tones was in phase. Results are discussed in relation to the problem of perceptual separation oftwo simultaneous voices and are seen as favoring a theory in which basilar membrane outputs that are amplitude-modulated by the same glottal pulse will be allocated to the same voice.