1. Royal Visio, National Foundation for the Visually Impaired and Blind, Huizen, The Netherlands,
2. Behavioural Science Institute, Department of Special Education, Radboud University Nijmegen, The Netherlands
3. Royal Visio, National Foundation for the Visually Impaired and Blind, Huizen, The Netherlands
4. Behavioural Science Institute, Department of Special Education, Radboud University Nijmegen and Royal Effatha Guyot Viataal Group, Sint Michielsgestel, The Netherlands