i wish to point out a need for clarification concerning the methodology utilized in the study 
eye tracking detects disconjugate eye movements associated with structural traumatic brain injury and concussion  by samadani 
 et al . , 2015 
 .
the authors state that binocular eye movements were recorded using a single - camera infrared - based video - oculography technique ( eyelink 1000 , sr research , ontario , canada ) and that binocular disconjugate characteristics were analyzed without calibration of eye orientation .
the authors claim that their variance - based disconjugacy metric was sensitive to the severity of a concussive brain injury and to the status of recovery after the original injury .
the eyelink 1000 system is an excellent eye tracker with a single high resolution camera and an infrared light source affixed to the camera .
a typical recording setup consists of a computer monitor with which the visual stimuli are presented to the subject and the camera unit ( placed in front of the monitor base ) with which the eye movement is recorded monocularly or binocularly .
the system has an option of easily outputting image - based , uncalibrated eye coordinates with hundreds of units representing 1 of eye rotation .
the concern i would raise with samadani 
 et al.s paper is the unclearness of the relationship between their metric and binocular disconjugacy .
logically , for an identical amount of eye rotation , any asymmetry in the spatial relationship that the camera or the infrared light source has with the two fellow eyes would result in different extents of relocation of the images of the pupils or corneal reflections .
asymmetries exist because there is a physical separation between the two eyes as well as between the camera and the infrared light source .
in addition , although the biometric characteristics of eyes are highly symmetrical within individuals , they are not perfectly symmetrical 
  and a 12% non - conformity in corneal curvature or axial length is not uncommon , which further confounds the relationship between eye rotation and changes in pixel coordinates .
moreover , each of the two fellow eyes has its own function that maps pixel movement to the eye rotation , and this mapping is not linear .
thus , the arithmetic difference between the uncalibrated coordinates of the two eyes is quite removed from a physical representation of gaze misalignment . beyond the factors associated with the raw data , the analytic methods in the paper
it is puzzling why the disconjugacy metric is represented by the variance of the left - right differences after independently averaging for each eye the uncalibrated coordinates over several cycles for a given stimulus position , as opposed to the straightforward variance of the left - right differences at all sample points .
furthermore , the ranges of outcome values presented in the series of figures run from 0 to at most 0.25 , but how the value 0 could have been obtained is not clear .
the question arises because in the two eyes uncalibrated coordinates there must be a constant bias related to the interocular distance .
lastly , what the high end of the outcome range represents is not clear . since one unit in eyelink s uncalibrated data output is smaller than 0.01 of eye rotation , being able to report differences in 0.25 square units or less seems implausible .
if the raw data were numerically centered or scaled , the procedure should have been noted in the text .
however , these points can be directly addressed by implementing a calibration procedure under monocular viewing 
 3 , 
 4 .
a comparison between the results from thus calibrated and uncalibrated data , and a demonstration of test - retest reliability could have improved the paper . in summary ,
the reproducibility of the paper s findings may be challenged simply by the paucity of details in the methodological description .
more importantly , however , from the information supplied or cited in the paper it is difficult to evaluate the validity of the potentially interesting conclusion that deficits in conjugacy of eye movements may quantitate physiologic impact of brain injury .
 this correspondence points out a need for clarification concerning the methodology utilized in the study  
 eye tracking detects disconjugate eye movements associated with structural traumatic brain injury and concussion  
 , recently published in 
 journal of neurotrauma . 
 the authors of the paper state that binocular eye movements were recorded using a single - camera video - oculography technique and that binocular disconjugate characteristics were analyzed without calibration of eye orientation . 
 it is claimed that a variance - based disconjugacy metric was found to be sensitive to the severity of a concussive brain injury and to the status of recovery after the original injury . 
 however , the reproducibility of the paper s findings may be challenged simply by the paucity of details in the methodological description . 
 more importantly , from the information supplied or cited in the paper , it is difficult to evaluate the validity of the potentially interesting conclusions of the paper . 
Correspondence