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, said impairment associated with brain functioning in a desired stimulus domain and relating to various different physical areas of a subject's brain, the method comprising the steps of: performing a facial emotion nulling test on a computer screen using a user interface for variably and selectably determining a 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 set of specific patterns of visual form or motion of at least one face with at least one facial expression, wherein said target stimulus comprises a particular degree of said facial expression and said non-target stimulus comprises varying degrees of said facial expression not the same as said particular degree of said facial expression of said target stimulus, and further wherein said target stimulus aligns with a stimulus domain and a predetermined physical portion of the subject's brain for which a test is desired and further wherein said target stimulus associates with a task design involving the subject's ability to process said target stimulus corresponding to the operational performance of a predetermined physical area of the subject's brain; further wherein said variable and selectable facial emotion nulling test manipulates a task difficulty associated with visual form or motion characteristics of said specific stimulus pattern, said variable and selectable facial emotion nulling 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 the target stimulus satisfying a given target criterion for associating with the desired stimulus domain and physical portion of the subject's brain and the other one(s) of said at least two stimuli comprises the 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 set of specific patterns of visual form or motion while changing a signal-to-noise ratio of all of said at least two stimuli in response to responses from the subject; monitoring the speed and accuracy of 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; 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 and from said analyzing relating to the stimulus domain and physical portion of the subject's brain; adjusting the signal-to-noise ratio relating to said target stimulus and non-target stimulus wherein the signal-to-noise ratio is increased until the subject has correctly identified the target stimuli in response to said analyzing step for assessing the subject's responses to said target stimulus as a relationship to the subject's mental processing abilities in the desired stimulus domain and physical portion of the subject's brain; 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 relating to the function of the subject's stimulus domain and operational performance of a physical portion of the subject's brain; adjusting said subject score relative to a normal range derived from at least one comparison subject group studied under at least one stimulus condition; deriving critical performance parameters from said subject score, said critical performance parameters relating to the stimulus domain and operational performance of a physical portion of the subject's brain for deriving information 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 and relating to the stimulus domain and operational performance of a physical portion of the subject's brain.