Veterans with chronic TBI demonstrate diminished Implicit Sequence Learning

Veterans with chronic TBI demonstrate diminished Implicit Probabilistic Sequence Learning

Kiser, S., Chanpimol, S., Fioriti, C., & Schwartz, B.

Background:

The goal of this study was to conduct the first experiment to examine implicit probabilistic sequence learning (IPSL) in Veterans with chronic TBI. Considerable published work has investigated the impact of various neurologic disorders on IPSL, a cognitive process which plays an integral role in motor-skill acquisition and inhibitory control. These are important to rehabilitation/recovery after physical impairment. This line of research is particularly important to our Veterans due to the significant risk for TBI. Presently, there is neither an existing standard measure of IPSL for use in clinical populations nor a clear understanding of what role nonconscious cognitive processes play in recovery and functional outcomes.

Methods:

Veterans, 18-60 years of age, were recruited from the Washington DC VA Medical Center. IPSL was assessed using the Triplets Learning Task (TLT). In the TLT, learning is measured by calculating the relative reaction-time between low and high frequency task conditions. Greater implicit learning is demonstrated by higher reaction time difference between the task conditions. Participants were unaware of the probabilistic nature of the TLT and probed afterwards using a recognition test to confirm lack of awareness. A mixed-factorial design was used to compare IPSL in Veterans with chronic TBI to a Veteran control group (i.e. no diagnosis of TBI). Exclusion criteria included a diagnosis of substance abuse within 3 months of enrollment, pre-morbid history of neurologic disease (e.g., stroke), and prior history of bipolar disorder, schizophrenia, or severe psychiatric illness as determined by the 22-item Modified Mini Screen and the medical record. Data were analyzed using repeated-measures ANOVA and, when necessary, corrected for sphericity and multiple comparisons using Greenhouse-Geisser and Bonferroni adjustments, respectively.

Results:

In this preliminary analysis, data were collected from 9 Veterans, 5 with a diagnosis of chronic TBI. Results from the mixed model ANOVA demonstrated robust effect sizes between overall reaction-time to high and low frequency events (F(1,7)=22.96; p=.003; η²_p=.793) and reaction-times with practice (F(5,35)=11.90; p<.001; η²_p=.665). These results show that Veterans in both the experimental and control groups demonstrate IPSL and inhibitory control in the experimental task. However, metrics of implicit learning between groups showed marginal significance (F(1,8)=4.57, p=0.07), where Veterans with TBI performed worse than the control group. Statistical analyses of the recognition test revealed that Veterans were unable to identify probabilistic event structures beyond chance. These finding shows that IPSL is diminished in Veterans with chronic TBI, adversely affecting motor-skill learning.

Discussion:

To our knowledge, the present study is the first to investigate IPSL in Veterans with chronic TBI. The novelty of our findings is twofold. First, both Veteran groups appear to learn probabilistic regularities in the TLT, though those with chronic TBI perform worse than controls. Second, these preliminary results serve as a proof of concept demonstrating the ability to identify sensitive metrics of IPSL, motor learning, and inhibitory control in Veterans with chronic TBI. Furthermore, these findings suggest cognitive deficits not presently accounted for in neuropsychological assessment that likely contribute to functional outcomes in TBI. Understanding the relationship between TBI and IPSL should prove useful for informing rehabilitation techniques, particularly when motor-skill acquisition is required. Additionally, IPSL is a marker of striatal-dependent learning and could potentially characterize disease progression following TBI. Previous studies have linked IPSL performance to poor dopamine regulation in the caudate nucleus of the striatum, which is shown to decrease into old age (65+ years) and in chronic diseases such as Parkinson’s disease and Huntington’s disease. The pattern of learning exhibited by Veterans with chronic TBI suggests untimely deterioration of striatal function and could indicate dopamine dysregulation or depletion. This is significant because pathophysiology of TBI is expected to impair this kind of function. Further research is needed to understand why Veterans with chronic TBI demonstrate reduced IPSL and whether TBI directly or indirectly affects striatal-dependent learning.