Altered pattern of 1^st and 2^nd order visual processing after mild traumatic brain injury in humans

Abstract

Purpose : Traumatic brain injury (TBI) is one of the most common causes of disability among the North American population. One of the most often complaints of TBI patients are visual deficits, including blurry vision and increased motion and light sensitivity. We have assessed visual function in TBI patients by estimating the full contrast sensitivity function (CSF) for both static and dynamic 1^st and 2^nd order stimuli. Our approach—normalizing the 2^nd order stimuli by the first order input—allowed us to accurately measure alterations in 2^nd order contrast perception that are independent of 1^st order performance. Our study provides a unique dataset describing the effects of TBI on fundamental aspects of visual processing.

Methods : A group of 26 mild TBI patients (mean age 34.69 years, 9 males) was recruited for the study. The participants were tested with the modified quick CSF (qCSF) method on five conditions: 1^st order static and motion stimuli, and 2^nd order orientation-defined, motion-defined, and contrast-defined stimuli. The outcome variables were estimates of qCSF parameters for each condition, namely CSF peak sensitivity (maximum gain), peak spatial frequency, bandwidth, and cut-off spatial frequency. These estimates were compared with a normative dataset of 102 healthy participants.

Results : The three most notable results emerged: (1) we found a significantly higher sensitivity for the 1^st order motion stimuli, (2) TBI patients’ sensitivity to 2^nd order orientation- and contrast-modulated stimuli was lower, and (3) TBI patients’ sensitivity was shifted towards higher spatial frequencies for 1^st order motion and orientation, and 2^nd order contrast-modulated stimuli, as assessed by the peak spatial frequency estimates.

Conclusions : In general, our findings are in agreement with the real-life visual complaints of TBI patients, in particular the increased motion sensitivity and blurry vision. We discuss these findings in terms of an altered pattern of cortical excitation and inhibition after TBI. The shift of contrast sensitivity functions towards larger spatial frequencies is intriguing, however, it is consistent with previous reports indicating that cortical lesions may predominantly affect processing of lower spatial frequencies. Our results expand the growing body of information about cortically-based visual deficits after mild TBI.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.