1. Centre for Augmentative and Alternative Communication, University of Pretoria, Pretoria, South Africa
2. Department of Speech-Language Pathology and Audiology, Sefako Makgatho Health Sciences University, Pretoria, South Africa
3. Department of Speech Language Pathology and Audiology, University of the Witwatersrand, Johannesburg, South Africa and
4. Department of Special Education and Department of Speech, Language and Hearing Sciences, San Francisco State UniversitySan Francisco, USA