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A Robust Method of Eye Torsion Measurement for Medical Applications Nan Jin, Sébastien Mavromatis, Jean Sequeira, Stéphane Curcio

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A Robust Method of Eye Torsion Measurement for Medical Applications Nan Jin, Sébastien Mavromatis, Jean Sequeira, Stéphane Curcio A Robust Method of Eye Torsion Measurement for Medical Applications Nan Jin, Sébastien Mavromatis, Jean Sequeira, Stéphane Curcio To cite this version: Nan Jin, Sébastien Mavromatis, Jean Sequeira, Stéphane Curcio. A Robust Method of Eye Torsion Measurement for Medical Applications. Information, MDPI, 2020, 11 (9), pp.408. 10.3390/info11090408. hal-02919784 HAL Id: hal-02919784 https://hal-amu.archives-ouvertes.fr/hal-02919784 Submitted on 24 Aug 2020 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Distributed under a Creative Commons Attribution| 4.0 International License information Article A Robust Method of Eye Torsion Measurement for Medical Applications Nan Jin 1,2,* ,Sébastien Mavromatis 1 , Jean Sequeira 1 and Stéphane Curcio 2 1 Aix Marseille Univ, Université de Toulon, CNRS, LIS, 13000 Marseille, France; [email protected] (S.M.); [email protected] (J.S.) 2 SYNAPSYS, 13013 Marseille, France; [email protected] * Correspondence: [email protected]; Tel.: +33-4-9111-7682 Received: 8 July 2020; Accepted: 19 August 2020; Published: 21 August 2020 Abstract: The detection of eye torsion is an important element for diagnosis of balance disorders, although it is rarely available in existing eye tracking systems. A novel method is proposed in this paper to provide robust measurement of torsional eye movements. A numerical approach is presented to estimate the iris boundary only according to the gaze direction, so the segmentation of the iris is more robust against occlusions and ambiguities. The perspective distortion of the iris pattern at eccentric eye positions is also corrected, benefiting from the transformation relation that is established for the iris estimation. The angle of the eye torsion is next measured on the unrolled iris patterns via a TM (Template Matching) technique. The principle of the proposed method is validated and its robustness in practice is assessed. A very low mean FPR (False Positive Rate) is reported (i.e., 3.3%) in a gaze test when testing on five participants with very different eye morphologies. The present method always gave correct measurement on the iris patterns with simulated eye torsions and rarely provided mistaken detections in the absence of eye torsion in practical conditions. Therefore, it shows a good potential to be further applied in medical applications. Keywords: eye tracking; torsional eye movement; balance disorders 1. Introduction 1.1. Background and Motivation Eye torsion is an involuntary eye movement that aims to stabilize the vision during a head movement (i.e., head tilt). It is defined as a rotation of the eye around the LoS (Light of Sight). The LoS is the line that connects the point of regard and the center of the entrance pupil and it is used to indicate the gaze direction [1]. Torsional eye movement is an important element for diagnosis of balance disorders. Head tilt test is a simple method to stimulate eye torsion, in which the subject is asked to tilt his/her head and keep gazing at zero position (i.e., looking straight forward). It aims to provoke a compensatory torsional eye movement in the inverse direction of the head motion to keep a more stable image on the retina. It is a physiological ocular reflex and should be present in every healthy subject. Otherwise, the subject may report unstable vision or dizziness, so further examinations will be needed. Eye torsion is especially interesting for diagnosis of BPPV (Benign Paroxysmal Positional Vertigo), the most common problem among the vestibular syndromes. The patient’s head is placed into different positions to trigger nystagmus (Table A1) and vertigo during the examination. The presence of eye torsion will allow practitioners to distinguish if either the anterior or posterior semicircular canal (Table A1) is affected, because it only occurs in case of a posterior BPPV in addition to a high-speed Information 2020, 11, 408; doi:10.3390/info11090408 www.mdpi.com/journal/information Information 2020, 11, 408 2 of 15 vertical nystagmus. It is one of the most challenging medical applications, and none of the existing eye tracking systems is able to provide reliable eye torsion measurement in such conditions. The measurement of eye torsion is rarely available in existing eye tracking systems due to its complexity in practice. A sub-degree precision in eye torsion measurement is generally reported on artificial and simulated eye images in related works [2,3], whereas it is difficult to maintain the reported precision in practice due to some inherent problems (e.g., occlusions, iris distortion, various morphology). Otero-Millan et al. compared their method of eye torsion measurement to the scleral search coil (Table A1) on human subjects, but they did not conclude on either the precision or robustness of their method [4]. The robustness of eye torsion measurement in practice is especially focused on in the present work. The goal is to provide robust torsional eye movement tracking with sufficient precision regarding the requirements of relevant medical applications. 1.2. Related Works Eye torsion is detected in 2D eye images by measuring the angular shift of the iris pattern around the pupil center. It corresponds to measuring the rotation of the iris around the LoS in the 3D pupil-iris plane. Such an approach is commonly adopted in related works, because it requires neither invasive markers [5] nor additional illumination [6]. The boundaries of the pupil and iris are firstly detected in eye images. Several image-based methods are proposed in related works, such as binarization [7,8], edge detection [9,10], ray projection [11, 12], etc. The detected candidate boundaries are generally refined to reduce the impact of noises (e.g., occlusions, mistaken points). The eyelids are detected and excluded in some related works to prevent occlusions [4,13]. The CRs (Corneal Reflections) are often filled to recover the connectivity of the detected pupil [14,15]. Next, the candidate boundary points are fitted to a regular shape (i.e., a circle or an ellipse) to provide smooth eye segmentation [4,8,12], i.e., segmentation of the pupil and iris. In general, the detected iris is unrolled into a rectangular shape via polar transformation, so the linear displacement of a rectangular pattern is measured rather than the angular shift of an annular iris. Of course, the angular shift of the iris can be directly measured along a curved path. However, this will increase the complexity of the calculation (e.g., application of TM algorithm along a non-linear path) and the processing will be consequently slowed. For these reasons, the iris unroll is quite a mandatory step for eye torsion measurement as proposed in most related works [4,9,16]. Several techniques can be used to measure the angular shift of the iris pattern, such as cross-correlation, feature tracking and template matching. The displacement of the gray-level distribution along several samples (e.g., narrow arcs, arc-shaped windows) are measured in cross-correlation based systems [2,16]. The feature tracking method consists of measuring the displacements of some stable features of the iris over time [3,17]. However, both methods suffer from the occlusion and distortion of the iris in practice. Template Matching (TM) is widely used in recent eye tracking systems to measure torsional eye movement [4,9]. A referent iris is firstly unrolled and defined as a unique texture, which is further compared with the current iris. This approach is more robust against occlusions in comparison with other methods, since a unique and global texture is considered rather than some pieces of iris (i.e., arc or window samples, individual features). Nevertheless, it is more sensitive to perspective distortion of the iris pattern at eccentric eye positions [4]. Therefore, it is important to rectify the iris before the measurement of eye torsion in a TM approach. 1.3. Paper Organisation A robust method of eye torsion measurement is presented in this paper (Figure1a). The iris is firstly detected at a referent eye position (i.e., zero gaze direction) using a traditional image-based approach. It is then unrolled into a rectangular pattern of fixed size. The location of the iris boundary is estimated in subsequent images by transforming the initial iris according to the current gaze direction (Figure1b). This transformation is also used to map each pixel of current iris pattern onto the referent Information 2020, 11, 408 3 of 15 one. This allows the system to always unroll the iris in the same way and to correct the perspective distortion of the iris pattern at eccentric eye positions. The contrast of the unrolled iris is next enhanced and the noises (e.g., reflections, intruded eyelids) are removed. A TM algorithm is then applied on the unrolled iris pattern to measure the shift distance between its referent position to its current position (Figure1c). The angle of the eye torsion is finally obtained by converting the measured distance from pixel to degree. Figure 1. (a) Main steps of the proposed method of eye torsion measurement; (b) the initial iris is transformed according to the gaze direction to estimate current iris; (c) the TM algorithm is applied to measure the shift of current unrolled iris pattern in respect to the referent one. The robustness of present method in eye torsion measurement is assessed in a gaze test.
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