A non-invasive system, and method for simple, quantitative screening for mTBI and other forms of mid cognitive impairment using a visuo-motor performance test (for example, a submaximal grip test) in response to visual feedback to quantitatively measure the intra-individual variability of performance metric for initial screening of patients with mild traumatic brain injury (mTBI), and other neurological disorders. The system and method can be administered in minutes, by any level of caregiver, in any environment including military in-the-field or sports on-field deployments, and is useful in screening those truly injured from those disguising or mimicking injury. In addition to screening, the system and method can be used to monitor and/or detect changes to intra-individual variability over time by comparison to a baseline, which in turn is helpful in determining estimated recovery trajectory or other related information.