Cancelling planned actions following mild traumatic brain injury

Clinical testing of mild traumatic brain injury using computerised eye-tracking tests

Traumatic brain injury (TBI) refers to the alteration of typical brain function that occurs following a blow to the head. Even a mild case of traumatic brain injury (mTBI) can lead to long-term impairment, so accurate and timely detection is vital. Visual symptoms are common following mTBI, so while it may seem to fall outside their typical scope of practice, optometrists are ideally qualified to assess the visual impacts and help with the diagnosis of mTBI. Given that mTBI is challenging to objectively diagnose and has no universally accepted diagnostic criteria, clinicians can lack confidence in diagnosing mTBI, and be hesitant in becoming involved in the management of such patients. The development of easily quantifiable techniques using eye tracking as an objective diagnostic tool provides practitioners with an easier pathway into the field, assigning numerical values to parameters which are difficult to assess using conventional optometric tests. As this evolving technology becomes increasingly integrated into optometric clinical settings, the potential for it to identify deficits accurately and reliably in patients following mTBI, and to monitor both their recovery and the effectiveness of potential treatments will increase. This paper provides an overview of clinical tests, relevant to optometrists, that can uncover oculomotor, attentional, and exteroceptive deficits following a mTBI, so that an optometrist with an interest in eye tracking can play a role in the detection and monitoring of mTBI symptoms.

A Preliminary High-Definition Fiber Tracking Study of the Executive Control Network in Blast-Induced Traumatic Brain Injury

Blast-induced traumatic brain injury (bTBI) is common in veterans of the Iraq- and Afghanistan-era conflicts. However, the typical subtlety of neural alterations and absence of definitive biomarkers impede clinical detection on conventional imaging. This preliminary study examined the structure and functional correlates of executive control network (ECN) white matter in veterans to investigate the clinical utility of using high-definition fiber tracking (HDFT) to detect chronic bTBI. Demographically similar male veterans (N = 38) with and without bTBI (ages 24 to 50 years) completed standardized neuropsychological testing and magnetic resonance imaging. Quantitative HDFT metrics of subcortical-dorsolateral prefrontal cortex (DLPFC) tracts were derived. Moderate-to-large group effects were observed on HDFT metrics. Relative to comparisons, bTBI demonstrated elevated quantitative anisotropy (QA) and reduced right hemisphere volume of all examined tracts, and reduced fiber count and increased generalized fractional anisotropy in the right DLPFC-putamen tract and DLPFC-thalamus, respectively. The Group × Age interaction effect on DLPFC-caudate tract volume was large; age negatively related to volume in the bTBI group, but not comparison group. Groups performed similarly on the response inhibition measure. Performance (reaction time and commission errors) robustly correlated with HDFT tract metrics (QA and tract volume) in the comparison group, but not bTBI group. Results support anomalous density and integrity of ECN connectivity, particularly of the right DLPFC-putamen pathway, in bTBI. Results also support exacerbated aging in veterans with bTBI. Similar ECN function despite anomalous microstructure could reflect functional compensation in bTBI, although alternate interpretations are explored.

Comparing three portable, tablet-based visuomotor tasks to laboratory versions: An assessment of test validity

The assessment of visuomotor function can provide important information about neurological status. Many tasks exist for testing visuomotor function in the laboratory, but the availability of portable, easy-to-use versions that allow reliable, accurate, and precise measurement of movement timing and accuracy has been limited. We developed a tablet application that uses three laboratory visuomotor tests: the double-step task, interception task, and stop-signal task. We asked the participants to perform both the lab and tablet versions of each task and compared their response patterns across equipment types to assess the validity of the tablet versions. On the double-step task, the participants adjusted to the displaced target adequately in both the lab and tablet versions. On the interception task, the participants intercepted nonaccelerating targets and performed worse on accelerating targets in both versions of the task. On the stop-signal task, the participants successfully inhibited their reaching movements on short stop-signal delays (50–150 ms) more frequently than on long stop-signal delays (200 ms) in both versions of the task. Our findings suggest that the tablet version of each task assesses visuomotor processing in the same way as their respective laboratory version, thus providing the research community with a new tool to assess visuomotor function.

Deficits in saccades and smooth-pursuit eye movements in adults with traumatic brain injury: a systematic review and meta-analysis

PURPOSE

To conduct a review of literature and quantify the effect that traumatic brain injury (TBI) has on oculomotor functions (OM).

METHODS

A systematic review and meta-analysis was conducted from papers that objectively measured saccades and smooth-pursuit eye movements in mild and severe TBI.

RESULTS

The overall impact of TBI on OM functions was moderate and significant with an effect size of 0.42 from 181 OM case-control comparisons. The heterogeneity, determined using the random effect model, was found to be significant (Q (180) = 367, p < 0.0001, I² = 51) owing to the variety of OM functions (reflexive saccades, antisaccades, memory-guided saccades, self-paced saccades and pursuits) measured and varying post-injury periods.The overall effect on OM functions were similar in mild and severe TBI despite differences in combined effect size of various OM functions. OM functions involving complex cognitive skills such as antisaccades (in mild and severe TBI) and memory-guided saccades (in mild TBI) were the most adversely affected, suggesting that OM deficits may be associated with cognitive deficits in TBI.

CONCLUSION

TBI often results in long-standing OM deficits. Experimental measures of OM assessment reflect neural integrity and may provide a sensitive and objective biomarker to detect OM deficits following TBI.

Comparing fMRI activation during smooth pursuit eye movements among contact sport athletes, non-contact sport athletes, and non-athletes

Objectives

Though sub-concussive impacts are common during contact sports, there is little consensus whether repeat blows affect brain function. Using a "lifetime exposure" rather than acute exposure approach, we examined oculomotor performance and brain activation among collegiate football players and two control groups. Our analysis examined whether there are group differences in eye movement behavioral performance and in brain activation during smooth pursuit.

Methods

Data from 21 off-season Division I football "starters" were compared with a) 19 collegiate cross-country runners, and b) 11 non-athlete college students who were SES matched to the football player group (total N = 51). Visual smooth pursuit was performed while undergoing fMRI imaging via a 3 Tesla scanner. Smooth pursuit eye movements to three stimulus difficulty levels were measured with regard to RMS error, gain, and lag.

Results

No meaningful differences were found for any of the standard analyses used to assess smooth pursuit eye movements. For fMRI, greater activation was seen in the oculomotor region of the cerebellar vermis and areas of the FEF for football players as compared to either control group, who did not differ on any measure.

Conclusion

Greater cerebellar activity among football players while performing an oculomotor task could indicate that they are working harder to compensate for some subtle, long-term subconcussive deficits. Alternatively, top athletes in a sport requiring high visual motor skill could have more of their cerebellum and FEF devoted to oculomotor task performance regardless of subconcussive history. Overall, these results provide little firm support for an effect of accumulated subconcussion exposure on brain function.

Lasting deficit in inhibitory control with mild traumatic brain injury

Being able to focus on a complex task and inhibit unwanted actions or interfering information (i.e., inhibitory control) are essential human cognitive abilities. However, it remains unknown the extent to which mild traumatic brain injury (mTBI) may impact these critical functions. In this study, seventeen patients and age-matched healthy controls (HC) performed a variant of the Stroop task and attention-demanding 4-choice response tasks (4CRT) with identical stimuli but two contexts: one required only routine responses and the other with occasional response conflicts. The results showed that mTBI patients performed equally well as the HC when the 4CRT required only routine responses. However, when the task conditions included occasional response conflicts, mTBI patients with even a single concussion showed a significant slow-down in all responses and higher error rates relative to the HC. Results from event-related functional magnetic resonance imaging (efMRI) revealed altered neural activity in the mTBI patients in the cerebellum-thalamo-cortical and the fronto-basal-ganglia networks regulating inhibitory control. These results suggest that even without apparent difficulties in performing complex attention-demanding but routine tasks, patients with mTBI may experience long-lasting deficits in regulating inhibitory control when situations call for rapid conflict resolutions.

Antisaccadic Eye Movements Are Correlated with Corpus Callosum White Matter Mean Diffusivity, Stroop Performance, and Symptom Burden in Mild Traumatic Brain Injury and Concussion

Antisaccades are thought to involve higher level inputs from neural centers involved in rapid eye movement inhibition and control. Previous work has demonstrated that performance on the antisaccade task can help in the assessment of injury in acute and/or chronic mild traumatic brain injury (mTBI). In this exploratory study, we performed cross-sectional and longitudinal comparisons of rapid eye movement, followed by correlations of antisaccade performance with assessments of symptom burden, diffusion tensor imaging, and a neuropsychological test of response inhibition. Significant deficits in antisaccade median latency, F(2, 31) = 3.65, p = 0.04 and prosaccade error mean duration, F(2, 31) = 3.63, p = 0.04 were found between patient groups and controls: the former was correlated with loss of white matter integrity in the splenium of the corpus callosum in acute mTBI, rho = 0.90, p = 0.0005. Furthermore, increased antisaccade median latency was also correlated with poor performance on an executive functioning task, r² = 0.439, p = 0.03, and greater symptom burden, r² = 0.480, p = 0.02 in the acute mTBI patients. Our preliminary research suggests that the antisaccade task could be useful as a neurological marker for mTBI and concussion, but more work is required.

Tonic and phasic alertness training: a novel treatment for executive control dysfunction following mild traumatic brain injury

Many individuals with traumatic brain injury (TBI) suffer difficulty regulating fundamental aspects of attention (focus, sustained attention) and may also exhibit hypo- or hyper-states of alertness. Deficits in the state of attention may underlie or exacerbate higher order executive dysfunction. Recent studies indicate that computerized cognitive training targeting attentional control and alertness can ameliorate attention deficits evident in patients with TBI or acquired brain injury. The current study examined whether improvements in attentional state following training can also influence performance on higher-order executive function and mood in individuals with mild TBI (mTBI). The current study examined five patients with executive control deficits as a result of mTBI, with or without persistent anxiety. Three patients engaged in ~5 hours of an executive control training task targeting inhibitory control and sustained attention; two additional patients were re-tested following the same period of time. Performance on standard neuropsychological measures of attention, executive function, and mood were evaluated pre- and post-training. The results indicate that tonic and phasic alertness training may improve higher-order executive function and mood regulation in individuals with TBI.

Combat veterans with comorbid PTSD and mild TBI exhibit a greater inhibitory processing ERP from the dorsal anterior cingulate cortex

Posttraumatic stress disorder (PTSD) is common among combat personnel with mild traumatic brain injury (mTBI). While patients with either PTSD or mTBI share abnormal activation of multiple frontal brain areas, anterior cingulate cortex (ACC) activity during inhibitory processing may be particularly affected by PTSD. To further test this hypothesis, we recorded electroencephalography from 32 combat veterans with mTBI-17 of whom were also comorbid for PTSD (mTBI+PTSD) and 15 without PTSD (mTBI-only). Subjects performed the Stop Task, a validated inhibitory control task requiring inhibition of initiated motor responses. We observed a larger inhibitory processing eventrelated potential (ERP) in veterans with mTBI+PTSD, including greater N200 negativity. Furthermore, greater N200 negativity correlated with greater PTSD severity. This correlation was most dependent on contributions from the dorsal ACC. Support vector machine analysis demonstrated that N200 and P300 amplitudes objectively classified veterans into mTBI-only or mTBI+PTSD groups with 79.4% accuracy. Our results support a model where, in combat veterans with mTBI, larger ERPs from cingulate areas are associated with greater PTSD severity and likely related to difficulty controlling ongoing brain processes, including trauma-related thoughts and feelings.

Executive dysfunction assessed with a task-switching task following concussion

Concussion frequently results in executive function deficits that can be specifically probed using task-switching tasks. The current study examined in detail the influence of concussion on task switching performance using both spatial and numerical stimuli. Individuals with concussion (n = 16) were tested within 48 hours of injury and 7, 14, and 28 days later. Healthy sex-, age-, height-, weight- and activity-matched controls (n = 16) were also tested at the same intervals. Switch costs were significantly greater in the participants with concussion than in the controls for both types of stimuli. By contrast, the global costs on non-switching trials were unaffected by concussion. We conclude that concussion has pronounced negative effects on the ability to switch task sets that generalize across task combinations (spatial or numerical) and that persist across at least a month after injury.

The countermanding task revisited: fast stimulus detection is a key determinant of psychophysical performance

The countermanding task is a standard method for assessing cognitive/inhibitory control over action and for investigating its neural correlates. In it, the subject plans a movement and either executes it, if no further instruction is given, or attempts to prevent it, if a stop signal is shown. Through various experimental manipulations, many studies have sought to characterize the inhibitory mechanisms thought to be at work in the task, typically using an inferred, model-dependent metric called the stop-signal reaction time. This approach has consistently overlooked the impact of perceptual evaluation on performance. Through analytical work and computer simulations, here we show that psychophysical performance in the task can be easily understood as the result of an ongoing motor plan that is modified (decelerated) by the outcome of a rapid sensory detection process. Notably, no specific assumptions about hypothetical inhibitory mechanisms are needed. This modeling framework achieves four things: (1) it replicates and reconciles behavioral results in numerous variants of the countermanding task; (2) it provides a new, objective metric for characterizing task performance that is more effective than the stop-signal reaction time; (3) it shows that the time window over which detection of a high-visibility stimulus effectively occurs is extremely short (∼20 ms); and (4) it indicates that modulating neuronal latencies and the buildup rates of developing motor plans are two key neural mechanisms for controlling action. The results suggest that manipulations of the countermanding task often cause changes in perceptual detection processes, and not necessarily in inhibition.

Reduced contribution of executive functions in impaired working memory performance in mild traumatic brain injury patients

AIM

Mild traumatic brain injury (MTBI) is associated with often selective impairment of both working memory (WM) and the executive functions (EFs). Research indicates that one of the commonest deficits present in MTBI patients falls in the domain of WM. We aimed to investigate the role of EFs in WM impairment following MTBI.

METHODS

Performance on the tests of EFs and the verbal and visuo-spatial WM of 30 consecutive MTBI patients were compared with age/education/IQ matched 30 normal healthy control participants. Correlation between EFs and WM was studied separately for the MTBI and the control group.

RESULTS

The MTBI and control group were tested on a range of EF tests and WM. The MTBI group was demonstrated impairment on verbal and visuo-spatial WM and category fluency tests only. Furthermore, the MTBI group had fewer significant correlations between the WM and EFs (5 out of 54 possible correlations) than in the control group (13 out of 54 possible correlations).

CONCLUSIONS

We suggest that MTBI may lead to WM deficits as the contribution of executive processes to support the WM is diminished following MTBI. Such an understanding of the poor WM performance in MTBI patients will be helpful when planning appropriate strategies for cognitive rehabilitation.

Measurement of the extraocular spike potential during saccade countermanding

The stop signal task is used to investigate motor inhibition. Several groups have reported partial electromyogram (EMG) activation when subjects successfully withhold manual responses and have used this finding to define the nature of response inhibition properties in the spinal motor system. It is unknown whether subthreshold EMG activation from extraocular muscles can be detected in the saccadic response version of the stop signal task. The saccadic spike potential provides a way to examine extraocular EMG activation associated with eye movements in electroencephalogram (EEG) recordings. We used several techniques to isolate extraocular EMG activation from anterior electrode locations of EEG recorded from macaque monkeys. Robust EMG activation was present when eye movements were made, but no activation was detected when saccades were deemed canceled. This work highlights a key difference between the spinal motor system and the saccade system.