1. A method for assessing the onset or progression of impairment in the nervous system functioning of a subject associated with a possible nervous system disorder, injury, derangement, or toxicity, the method comprising the steps of: performing a facial emotion sensitivity test on a computer screen using a user interface for determining the lowest signal-to-noise ratio and stimulus sequence durations at which a subject can accurately and consistently identify a target stimulus from at least one non-target stimulus, wherein said user interface comprises a specific pattern of visual form or motion, wherein said target stimulus and non-target stimulus is comprised of a plurality of faces, said plurality of faces comprising a plurality of facial expressions that forms a gradient of facial expressions, wherein said target stimulus is comprised of at least one facial expression that is more extreme on said gradient of facial expressions than the facial expressions on said at least one non-target stimulus, and further wherein said facial emotion sensitivity test manipulates a task difficulty associated with visual form or motion characteristics of said specific stimulus pattern, said facial emotion sensitivity test further comprising the following steps executed on a computer processor: presenting at least two stimuli simultaneously such that one of said at least two stimuli comprises a target stimulus satisfying a given target criterion and the other one(s) of said at least two stimuli comprises at least one non-target stimulus not satisfying said given target criterion; moving a position of said target stimulus and said non-target stimulus on said computer screen according to said specific pattern of visual form or motion while changing a signal-to-noise ratio of all of said at least two stimuli; monitoring the subject's indication of said position of said target stimulus as said target stimulus moves along said specific pattern of visual form or motion and said signal-to-noise ratio and stimulus sequence duration changes, wherein the signal-to-noise ratio is increased until the subject has correctly identified the target stimuli analyzing the subject's positional error with respect to said position of said target stimulus and observed subject response motion dynamics errors associated with the subject's ability to respond to said movement of said target stimulus; creating a subject score deriving from the subject's accuracy, speed, and precision in responding to said changes in said signal-to-noise ratios and stimulus sequence durations of said at least two stimuli; adjusting said subject score relative to a normal range derived from at least one comparison subject groups studied under at least one stimulus condition; deriving critical performance parameters from said subject score, said critical performance parameters associated with brain functional disorders or injury from diverse causes in the subject; controllably recording and displaying on said computer screen said critical performance parameters and confidence interval parameters associated with said critical performance parameters; and using said critical performance parameters in assessing, evaluating, or determining a possible presence, onset, progression, or therapeutic response of performance impairments related to the central nervous system functioning of the subject.