1. A method of classifying a stimulus based on its associated stimulus response path in biological feature space, the method comprising: (a) aligning two or more stimulus response paths on a common radius, r, from a point in biological feature space where a negative control is applied; (b) calculating a separation distance, D, between points on two of the stimulus response paths having the common radius, r, from the negative control point; and (c) classifying the stimulus of at least one of the stimulus response paths on the basis of the calculated value of the separation distance, D.