Early intervention (EI) using naturalistic behavioural methods have shown benefits for the development of communication and other skills for young children with autism spectrum disorder. The publicly funded autism EI program in Nova Scotia (NS) is based on such a method, Pivotal Response Treatment (PRT), and pre-post studies indicate benefits for children and families. However, not all children benefit equally. In this study, the investigators test the efficacy of a brief parent-mediated intervention designed to prime responsivity to PRT in children with a minimal responder profile derived from previous studies of the PRT-based EI program.

Investigators will enroll preschoolers with the minimal responder profile who are scheduled to begin the PRT-based public EI program. Consenting parents will be offered a virtual intervention in which they will be randomized to 1 of 2 arms to receive coaching in either (a) PRIMeR intervention, designed to target areas of weakness in the minimal responder profile, or (b) PRT, the treatment model used in the EI program. Coaching will take place on a virtual (video-conferencing) platform. Each child's progress on treatment targets will be assessed in a single case experimental design (SCED) using data from blind-coded video-recordings of parent-child play episodes using a standard set of toys. Overall study effects will be based on aggregated data for an anticipated n of 20 participants assigned to each arm (4 SCED series with 5 participants each contributing to each arm). The primary outcome is gains in children's social initiations (video-coded); the secondary outcome is gains in children's communication levels (multi-method assessment). Mixed methods will be used to evaluate aspects of parents' experiences,

Inclusion Criteria:

1. upcoming enrollment in NS EI program for preschoolers with autism spectrum disorder (ASD), based on confirmed or provisional ASD diagnosis by a qualified clinician
2. significant delay in cognitive abilities (assessed formally by psychologist or estimated by psychologist or developmental pediatrician at time of diagnosis)
3. current spontaneous functional use of ≤ 10 words (clinician's observations; parent report using MacArthur-Bates Communicative Development Inventory - Words & Gestures (CDI-WG)
4. limited use of toys / objects (clinician's impression or parent's report with no contrary observation by clinician)
5. low levels of expressed positive affect (smiles, laughter) or limited positive and high negative affect (clinician's impression or parent's report with no contrary observation by clinician)

Exclusion Criteria:

1. Severe sensory or motor impairment in child
2. Parent unable to complete consent process (and receive coaching) in English

Primary outcome will be coded by raters who are trained to 80% reliability on a coding scheme with operational definitions, and naive to treatment condition and study phase