## 4 Discussion  
In this pilot study, we used a comprehensive range of rapid, non-invasive, and objective cognitive, neuropsychological, and sensorimotor functional measures, along with blood 5-mC% level, to compare and identify functional deficits and underlying biological factors in symptom expression and complications resulting from mTBI among a group of college students. Even several years after the injury (7.1 years, on average, elapsed since the last injury), the 11 students with mTBI histories (mean age of 28.7 years, \(SD\) = 5.92) showed significantly poorer episodic memory, more severe anxiety, and more sleep disturbance problems than 14 healthy control students (mean age of 22.0 years; \(SD\) = 4.13). Higher peripheral global DNA methylation ratio (5-mC%) was also found in mTBI group, compared with control group. The mTBI history effect became insignificant on sleep problems only when controlling for age. Our results of post-mTBI cognitive deficits highlight that college students who reported mTBI histories showed poorer episodic memory, than their counterpart controls. We employed six different modules of the NIH toolbox to examine the college students' executive function, attention, episodic and working memories, vocabulary knowledge, and processing speed. Those with mTBI histories showed significantly lower scores on the Picture Sequence Memory Test for episodic memory only, both age-corrected and -uncorrected, but not the other test modules nor any of the cognitive composite scores. Episodic memory, which is involved in storage of distinctive events or experiences encoded in a time-specific manner [44], is one of the key indices to determine premature aging in several neurodegenerative disorders, such as Alzheimer's disease [24]. Our findings on poorer episodic memory post-mTBI confirm the previous findings from MRI and CT studies with clinical TBI patients [31; 50]. As shown in a recent study comparing over 80 young and older individuals with mTBI [51], declines in episodic memory may serve as an early indicator of post-mTBI premature aging among young adults like college students. We further compared multiple to single injury within the mTBI cases and found significantly lower scores of the age-corrected Pattern Comparison Processing Speed Test for processing speed in the multiple-injury group. Lower processing speed is one of the other main indices of brain aging along with declined executive function. In a national study with over 200 professional fighters, lower processing speed, and lower brain volumes were associated with multiple or repetitive head trauma [52], which supports our findings among college students although the sample size in the present study was small. Both episodic memory and processing speed are fundamental elements of human cognitive function, and deficits in either or both may assist more effective and sensitive screening for disrupted brain functions following mTBI. We also found that, post mTBI, college students suffered from psychosocial complications long after the injury, particularly anxiety and sleep disturbances based on PROMIS measures. There was the effect of age on the post-mTBI sleep disturbance problems in this study, but different formats of sleep  
Figure 1: Comparison of peripheral blood global methylation ratio (5-mC%) between mTBI and healthy control college student participants. Note: Biological samples that failed to pass the quality of global DNA methylation ratio (5 mC%) assay were not included in the analysis. problems (e.g., unable to fall asleep, daytime drowsiness, or sleeping too much or little) have been found in larger samples of young individuals with mTBI [53; 54]. In a recent qualitative study, a quarter of the participants, part-time or full-time undergraduate and graduate students enrolled at a large state university who have suffered one or more mTBI, reported sleep cycle disturbances [55].