Abstracts

Rationale:
Temporal lobe epilepsy (TLE) is the most prevalent form of epilepsy. Surgical removal of the ictogenic tissues may be curative. Toward this end, distinguishing mesial TLE (mTLE) from neocortical (nTLE) is crucial. A reliable marker that aids the localization of the epileptic network would be highly desirable in the presurgical evaluation. Facilitation and suppression between groups of neurons are well studied in the visual system and have perceptual consequences. One example is the ability to perceive motion direction accurately. It has been shown that, compared to neurotypical subjects, the threshold to perceive motion is altered in several conditions, (e.g., schizophrenia, depression, migraine, autistic spectrum disorder, and dementia). Importantly, these perceptual changes are linked to facilitation and suppression of neural activity measured in the visual cortex. Our hypotheses are that neural suppression in visual paradigms may be impacted differently in mTLE and nTLE, and that this relationship would correlate with the hemisphere of the seizure onset zone.

Methods:
We conducted a prospective, controlled, visual psychophysics study measuring the motion duration threshold using a well-established spatial suppression paradigm (Tadin et al, Nature, 424, 312–315, 2003). Visual stimuli (grating patterns) were presented in two hemifields (7 deg from center), two contrasts (5% and 98%), and three stimulus sizes (0.5, 1, and 6 deg radius). Thresholds for perceiving the direction of motion were determined from an adaptive, two alternative forced choice paradigm. Trials and subjects with data that failed predetermined quality measures were discarded. Twenty-six patients diagnosed with TLE based on clinical, imaging, and EEG findings at a tertiary referral center were enrolled. Seven, three, and two were excluded due to poor data, bilateral disease, unknown localization (mTLE vs nTLE), respectively. Six patients with nTLE and eight with mTLE were included for analysis. The relationships between perceptual thresholds and stimulus parameters and patient characteristics were analyzed using linear mixed effect modeling (R version 3.6.3).

Results:
Measured thresholds were reliable and qualitatively consistent with published reports (Fig. 1). Mixed effect modeling showed that motion duration thresholds depended on the stimulus contrast, stimulus size, and the interaction between the localization of TLE and the hemifield of the stimulus (Table 1). Specifically, in nTLE, but not mTLE, thresholds were higher in the hemifield contralateral to the hemisphere of seizure onset than the ipsilateral.

Conclusions:
This pilot study showed that changes in the motion perception threshold manifested differently in unilateral nTLE and mTLE and correlated with the hemisphere of seizure onset zone. If confirmed, these findings may lead to a low-cost, noninvasive test that could help guide presurgical evaluation of TLE patients. A strength of this study is the within subject control, which obviates potential confounds due to medication use, age, and other between subject variables.

Funding:
Supported by the ITHS under NIH grant UL1TR002319.