Before revision:  It is a non-parametric, fully featureless indicator , in that it uses only the time stamps of the firing activity , without resorting to any a priori covariate or invoking any specific  tuning curve for neural activity.
After revision:  It is a non-parametric, fully featureless indicator , in that it uses only the time stamps of the firing activity , without resorting to any a priori covariate or invoking any specific structure in the tuning curve for neural activity.