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Original Investigation |

Infrared-Based Blink-Detecting Glasses for Facial Pacing:  Toward a Bionic Blink

Alice Frigerio, MD, PhD1,2; Tessa A. Hadlock, MD2; Elizabeth H. Murray, MS3; James T. Heaton, PhD3,4
[+] Author Affiliations
1Human Physiology Section, Department of Pathophysiology and Transplantation, Università degli Studi di Milano, Milan, Italy
2Facial Nerve Center, Carolyn and Peter Lynch Center for Laser and Reconstructive Surgery, Division of Facial Plastic and Reconstructive Surgery, Department of Otology and Laryngology, Massachusetts Eye and Ear Infirmary, Harvard Medical School, Boston
3Department of Communication Sciences and Disorders, MGH Institute of Health Professions, Boston, Massachusetts
4Department of Surgery, Harvard Medical School, Massachusetts General Hospital, Boston
JAMA Facial Plast Surg. 2014;16(3):211-218. doi:10.1001/jamafacial.2014.1.
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Importance  Facial paralysis remains one of the most challenging conditions to effectively manage, often causing life-altering deficits in both function and appearance. Facial rehabilitation via pacing and robotic technology has great yet unmet potential. A critical first step toward reanimating symmetrical facial movement in cases of unilateral paralysis is the detection of healthy movement to use as a trigger for stimulated movement.

Objective  To test a blink detection system that can be attached to standard eyeglasses and used as part of a closed-loop facial pacing system.

Design, Setting, and Participants  Standard safety glasses were equipped with an infrared (IR) emitter-detector unit, oriented horizontally across the palpebral fissure, creating a monitored IR beam that became interrupted when the eyelids closed, and were tested in 24 healthy volunteers from a tertiary care facial nerve center community.

Main Outcomes and Measures  Video-quantified blinking was compared with both IR sensor signal magnitude and rate of change in healthy participants with their gaze in repose, while they shifted their gaze from central to far-peripheral positions, and during the production of particular facial expressions.

Results  Blink detection based on signal magnitude achieved 100% sensitivity in forward gaze but generated false detections on downward gaze. Calculations of peak rate of signal change (first derivative) typically distinguished blinks from gaze-related eyelid movements. During forward gaze, 87% of detected blink events were true positives, 11% were false positives, and 2% were false negatives. Of the 11% false positives, 6% were associated with partial eyelid closures. During gaze changes, false blink detection occurred 6% of the time during lateral eye movements, 10% of the time during upward movements, 47% of the time during downward movements, and 6% of the time for movements from an upward or downward gaze back to the primary gaze. Facial expressions disrupted sensor output if they caused substantial squinting or shifted the glasses.

Conclusions and Relevance  Our blink detection system provides a reliable, noninvasive indication of eyelid closure using an invisible light beam passing in front of the eye. Future versions will aim to mitigate detection errors by using multiple IR emitter-detector units mounted on glasses, and alternative frame designs may reduce shifting of the sensors relative to the eye during facial movements.

Level of Evidence  NA.

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Figure 1.
Prototype Blink Detection Pair of Glasses

Frontal (A) and lateral (B) views of a prototype blink detection pair of glasses with an arrow demonstrating the path in infrared (IR) light traveling horizontally across the corneal surface from the IR LED (nasal side) toward the IR detector (temporal side).

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Figure 2.
Flow Diagram of the Eye Blink Detection System

The diagram shows the (1) infrared (IR) emitter and detector units in relation to the eye, (2) variable gain of the preamplifier receiving the phototransistor output, (3) signal filtering, and (4) final output of the circuit as a time-varying analogue voltage ranging from 0 to 5 V. An unbroken beam generates a 5-V output, whereas a completely obstructed beam generates a 0-V output, with partial obstruction producing voltages in between these extremes. LED indicates light-emitting diode.aFairchild Semiconductor Corporation.

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Figure 3.
Blink Detection System Testing and Data Collection

A, A participant is shown standing in position for the 6-minute blink detection recording session, photographed during the initial 102-second instructional video. B, An adjustable chin rest helped participants maintain a forward head position, and a forward gaze was held through the instructional video (during which spontaneous blinks were detected). C, Photograph of a participant assuming an upper right gaze position as she looked at a target (blue dot). Targets appeared in the 4 corners of the smart board screen, in addition to center, upper center, and lower center (see Methods section for presentation sequence).

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Figure 4.
Examples of the Blink Detection Circuit Output for a Blink and Downward Gaze

The detector receives a relatively unobstructed beam during forward gaze, producing an output in the 4- to 5-V range (A, time zero), but drops in voltage when the beam is broken by the lowering upper eyelid during a blink or looking downward. This drop in voltage is typically more rapid for blinking than gaze-related eyelid movement, as shown by the slope (downward pointing arrows) and by the first derivative of the output signals (B). The difference in peak derivative for blink vs downward gaze is highlighted by the dotted lines and double-headed arrow.

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Video 1.

Infrared (IR) Voltage Output During Forward Gaze and Gaze Movements

An unbroken light path between the IR emitter and detector causes a relatively high voltage output from the blink detection circuit. In forward gaze, at each blink event the IR beam is broken by eyelids/lashes and the circuit output voltage drops toward a low state. During gaze movements, the IR beam is occasionally broken (mainly during downward gaze and 1 leftward gaze event in this video).

Video 2.

Blink vs Downward Gaze

High-speed video recording of the circuit output during a blink event followed by downward gaze. Drops in the circuit voltage caused by interruption of the infrared light path during blinking are generally faster than when light path obstruction occurs during downward gaze. Thresholding on peak rate of signal change (first derivative) can help distinguishing blinks from slower, gaze-related eyelid movements.

Video 3.

Infrared Signal of a Complete Eye Blink

High-speed video capture of the circuit voltage output during a complete blink. The blink closing phase causes gradual interruption of the infrared light path, with proportional drop of the circuit voltage toward the lowest state (complete beam interruption), followed by a gradual increase of the voltage during the eye opening.

Video 4.

Infrared Signal of an Eye Twitch

High-speed video capture of the circuit voltage output during an eye twitch. The onset of signal change is synchronous with visible eyelid movement. The shape of the signal reflects that the light path is only partially interrupted by the twitch movement. This drop in the voltage output is not detected as a blink event because it is under the threshold.

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