Selective vulnerability and task difficulty. , i = 1, 2, tmin ≤ t ≤ t
max,.For the orange curve the parameters of the formula in Figure 1 are: a
1 = b
1 = 1, c
1 = -1/6. For the green curve the parameters are: a
2 = b
2 = 1, c
2 = -1/20. The orange curve represents a symptom with a ceiling effect at the beginning of clinical disease (e.g. praxia in Alzheimer's disease), the green curve represents a symptom with a floor effect early during progression of disease (e.g. episodic memory in Alzheimer's disease).