Investigating the Effect of Mild TBI on the Global Processing of Form and Motion

Abstract

Purpose: Individuals with mild traumatic brain injury (TBI) often experience a range of visual symptoms. The cortical processing of visual information involves two parallel streams; the ventral stream (form perception) and the dorsal stream (motion perception). Our purpose was to assess whether processing in the dorsal and/or ventral stream is affected by mild TBI. A range of basic visual functions were also assessed and we investigated the association between each vision measure and Post-Concussion Symptom Inventory PCSI scores.

Methods: Eleven adults with mild TBI (mean age 25.5 yrs, range: 19 – 42, 17 ± 5.2 months post injury), and 25 controls (mean age 27.9 yrs, range: 19 - 38) participated. Global processing of form and motion, as an index of dorsal and ventral stream function, were assessed psychophysically using Glass patterns and random dot kinematograms respectively. Contrast thresholds for coherent motion direction discrimination were also measured. Thresholds were measured twice for each test. Contrast sensitivity (Freiburg test), stereo acuity, near point of convergence, accommodative facility and amplitude, negative and positive fusional vergence and vergence facility were also measured. Mild TBI symptoms were assessed using the Post-Concussion Symptom Inventory PCSI.

Results: Patients with mild TBI demonstrated higher (worse) global form and motion coherence thresholds than controls (p=0.01). Global form coherence thresholds in the mild TBI was 25.07%, SD: 5.91 versus the normal 21.23%, SD: 5.37 while global motion coherence thresholds in the mild TBI was 14.38%, SD: 6.67 versus the normal 10.79%, SD: 3.37. Threshold elevations were not due to either the reduced contrast sensitivity or the accommodation and vergence anomalies. The magnitude of the processing deficit did not differ significantly between the global form and motion tasks (p>0.05). Contrast thresholds for motion discrimination did not differ significantly between groups, but there was more between-subject variability in the mild TBI group (mild TBI (1.13%, SD: 0.47) versus the normal (1.20%, SD: 0.038)). There was a significant correlation between contrast thresholds for motion discrimination and PCSI score (R2= 0.51. p=0.01) in the mild TBI group. PCSI scores were not significantly correlated with global form or motion coherence thresholds. As expected, the mild TBI group were worse that controls for almost all clinical measurements of vision.

Conclusion: Mild TBI affects processing in both the dorsal and ventral cortical processing streams. In addition, our results suggest that mild TBI impairs spatiotemporal contrast sensitivity and that this impairment may contribute to the symptoms of mild TBI.