associated with brain contusion. until the 17th century, a concussion was usually described by its clinical features, but after the invention of the microscope, more physicians began exploring underlying physical and structural mechanisms. however, the prevailing view in the 17th century was that the injury did not result from physical damage, and this view continued to be widely held throughout the 18th century. the word "concussion" was used at the time to describe the state of unconsciousness and other functional problems that resulted from the impact, rather than a physiological condition. in 1839, guillaume dupuytren described brain contusions, which involve many small hemorrhages, as contusio cerebri and showed the difference between unconsciousness associated with damage to the brain parenchyma and that due to concussion, without such injury. in 1941, animal experiments showed that no macroscopic damage occurs in concussion. society and culture costs due to the lack of a consistent definition, the economic costs of mtbi are not known, but they are estimated to be very high. these high costs are due in part to the large percentage of hospital admissions for head injury that is due to mild head trauma, but indirect costs such as lost work time and early retirement account for the bulk of the costs. these direct and indirect costs cause the expense of mild brain trauma to rival that of moderate and severe head injuries. terminology the terms mild brain injury, mild traumatic brain injury (mtbi), mild head injury (mhi), and concussion may be used interchangeably; although the term "concussion" is still used in sports literature as interchangeable with "mhi" or "mtbi", the general clinical medical literature uses "mtbi" instead, since a 2003 cdc report outlined it as an important strategy. in this article, "concussion" and "mtbi" are used interchangeably. the term "concussion" is from latin concutere, "to shake violently" or concussus, "action of striking together". research minocycline, lithium, and n-acetylcysteine show tentative success in animal models. measurement of predictive visual tracking is being studied as a screening technique to identify mild traumatic brain injury. a head-mounted display unit with eye-tracking capability shows a moving object in a predictive pattern for the person to follow with their eyes. people without brain injury will be able to track the moving object with smooth pursuit eye movements and correct trajectory while it is hypothesized that those with mild traumatic brain injury cannot. grading systems main article: concussion grading systems national and international clinical practice guidelines do not recommend a concussion grading system for use by medical professionals. historical information on grading systems in the past, the decision to allow athletes to return to participation was frequently based on the grade of concussion. however, current research and recommendations by professional organizations including the national athletic trainers' association recommend against such use of these grading systems. currently, injured athletes are prohibited from returning to play before they are symptom-free during both rest and exertion and until results of the neuropsychological tests have returned to pre-injury levels. three grading systems have been most widely followed: by robert cantu, the colorado medical society, and the american academy of neurology. each employs three grades, as summarized in the following table: comparison of historic concussion grading scales – not currently recommended for use by medical professionals guidelines grade igrade iigrade iii cantu post-traumatic amnesia <30 minutes, no loss of consciousnessloss of consciousness <5 minutes or amnesia lasting 30 minutes–24 hoursloss of consciousness >5 minutes or amnesia >24 hours colorado medical society confusion, no loss of consciousnessconfusion, post-traumatic amnesia, no loss of consciousnessany loss of consciousness american academy of neurology confusion, symptoms last <15 minutes, no loss of consciousnesssymptoms last >15 minutes, no loss of consciousnessloss of consciousness (iiia, coma lasts seconds, iiib for minutes) at least 41 systems measure the severity, or grade, of a mild head injury, and there is little agreement about which is best. in an effort to simplify, the 2nd international conference on concussion in sport, meeting in prague in 2004, decided that these systems should be abandoned in favor of a 'simple' or 'complex' classification. however, the 2008 meeting in zurich abandoned the simple versus complex terminology, although the participants did agree to keep the concept that most (80–90%) concussions resolve in a short period (7–10 days) and although the recovery time frame may be longer in children and adolescents.