1. Krummel and Allison (1996); Walunas et al. (1996)
2. Krummel and Allison (1996); Walunas et al. (1996)
3. Krummel and Allison (1996); Walunas et al. (1996)
4. Brunner et al. (1999)
5. Krummel and Allison (1996); Walunas et al. (1996); Brunner et al. (1999)