Autism spectrum disorder (ASD) is characterized by persistent deficits in social communication and social interaction. In addition to that, there is a presence of restricted, repetitive patterns of behaviour, interests or activities. The symptoms cause clinically significant impairment in social, occupational or other important areas of functioning.

The Early Start Denver Model (ESDM) is a treatment that addresses the needs of children with ASD under three years of age. It is designed for children ages 12 to 60 months who are diagnosed with ASD or are at risk for ASD. The goal is to accelerate children’s developmental rates in cognitive, social-emotional, language, and adaptive domains and to reduce the disabling effect of ASD symptoms.

The Denver Model states that it is important to build close relationships between children with ASD and other children and adults to facilitate development. Following this model, EDSM includes sensory social routines. This includes seeking out social partners, initiate routines and continue them through non-verbal and verbal communication. Other principles of ESDM include a focus on assessing and teaching to all developmental areas (1), the use of an interdisciplinary team (2), a focus on teaching play and imitation skills (3), utilizing imitation to teach skills in other domains (4), an emphasis on both verbal and non-verbal communication (5) and partnership with parents (6).

Children with ASD may have impairments in imitation skills, which are necessary to learn a lot of social skills. ESDM focuses on strengthening imitation skills in young children to improve this. The social motivation hypothesis states that children with ASD demonstrate impaired sensitivity to the reward value of social stimuli. This leads to reduced attention to and interaction with such stimuli and fewer opportunities for social learning. The lack of social interaction leads to increased impairments in communication an social-emotional skills. A primary goal of ESDM is to enhance the reward value of social interaction.

ABA holds that antecedent (A), behaviour (B) and consequence should be used to teach people at risk for ASD or diagnosed with ASD. This means that the antecedent should be clear. There should be appropriate consequences for children showing certain behaviour and there should be shaping and chaining behaviours to help children learn new skills.

When a child begins treatment, skills are assessed using the ESDM curriculum checklist. This is used to determine developmentally appropriate treatment objectives. These objectives guide the intervention. The skills that are chosen for the treatment objectives represent the skills that are the next in line, developmentally. It is then turned into measurable treatment objectives containing clear descriptions of the antecedents that should cue the demonstration of the skill (1), the skill (2) and requirements for mastery and generalization of the skill (3). The treatment objectives originate in a manualized curriculum but are individualized to a particular child. Each treatment objective is broken down into several teaching steps based on a task analysis of the skill (i.e. child’s current baseline level and ending with a fully mastered and generalized skill).

Teaching within ESDM takes place in naturalistic interactions (e.g. daily caretaking; play activities). The intervention can be delivered in different settings. It is an intensive intervention with multiple hours of therapy per week, primarily in the children’s homes. Parents are taught to use ESDM techniques in everyday interactions. The idea is that the child is surrounded by individuals providing high-quality, frequent teaching opportunities throughout the day.

There are effects for social communication (1), language (2), cognition (3), adaptive behaviours (4) and ASD symptoms (5). For parents, there are effects in their interaction skills and the levels of stress.

Children seem to improve as a result of ESDM and maintain this result over time. There are positive effects of parent coaching on child learning, especially in areas of language, play and imitation. ESDM does not necessarily improve parent coaching compared to other treatments but it can be achieved in less time and reduces parental stress. ESDM used in preschool or day care had positive outcomes and better outcomes compared to another treatment. ESDM seems to reduce symptoms at a later age for infants.

Early and intensive behavioural intervention based on ESDM is associated with improvements in brain activity. People who received ESDM showed greater cortical activation while viewing faces and this is associated with improved social behaviour. Early behavioural intervention can thus alter brain developmental outcome in children with ASD. ESDM leads to increased attention toward and engagement with people and enhances brain activity in regions related to social processing. There are several neural mechanisms which may contribute to this result:

• The experience-expectant plasticity of the immature brain may allow for positive results as ESDM capitalizes on this.
• Children with ASD have deficits in social motivation and attention. This may be due to a dysfunction in dopaminergic projections to the striatum and frontal cortex (i.e. mediator of the effects of reward on approach behaviour). This is important for the anticipated reward value of a stimulus as this motivates people and directs attention. They may also have a deficient functional connectivity which makes complex social stimuli difficult to comprehend and less rewarding. ESDM draws a child’s attention to social information and make social interactions (i.e. use of sensory social routines).
• ESDM employs strategies that simultaneously target multiple objectives and multisensory experiences. This may help the coordination of multiple brain regions and promotes enhanced long-range neural functional connectivity.

Children with ASD typically show atypical patterns of hemispheric specialization during speech processing.

It is difficult to screen for ASD. ASD is defined by behaviour and not all infants show problematic behaviour as most children develop ASD into their 2^nd and 3^rd year of life. Next, there are no infant screeners that are good at predicting later diagnosis in infants younger than 12 months. Lastly, symptoms associated with ASD in infants under 12 months of age are not specific to ASD, making it difficult to predict ASD diagnosis in later life.