Ocular microtremor laser speckle metrology

Abstract

Ocular Microtremor (OMT) is a continual, high frequency physiological tremor of the eye present in all subjects even when the eye is apparently at rest. OMT causes a peak to peak displacement of around 150nm-2500nm with a broadband frequency spectrum between 30Hz to 120Hz; with a peak at about 83Hz. OMT carries useful clinical information on depth of consciousness and on some neurological disorders. Nearly all quantitative clinical investigations have been based on OMT measurements using an eye contacting piezoelectric probe which has low clinical acceptability. Laser speckle metrology is a candidate for a high resolution, non-contacting, compact, portable OMT measurement technique. However, tear flow and biospeckle might be expected to interfere with the displacement information carried by the speckle. The paper investigates the properties of the scattered speckle of laser light (λ = 632.8nm) from the eye sclera to assess the feasibility of using speckle techniques to measure OMT such as the speckle correlation. The investigation is carried using a high speed CMOS video camera adequate to capture the high frequency of the tremor. The investigation is supported by studies using an eye movement simulator (a bovine sclera driven by piezoelectric bimorphs). The speckle contrast and the frame to frame spatiotemporal variations are analyzed to determine if the OMT characteristics are detectable within speckle changes induced by the biospeckle or other movements.

Simultaneous recordings of ocular microtremor and microsaccades with a piezoelectric sensor and a video-oculography system

“…However, a previous study limited to one subject (Al-Kalbani, 2010), using the same basic probe design as described here, found no significant difference between mean peak OMT frequency measured in both eyes, where the probe mass on one eye was increased by ∼30% (14 g to 18.5 g), compared to the probe used in the other eye. Neither was there any difference observed in the mean RMS OMT amplitude of the eyes with the heavier and lighter probes.…”

“…Non-contact laser speckle interferometry offers the possibility of future OMT measurements without interfering with eye dynamics (Al-Kalbani et al, 2009). …”

Refractory neural nets and vision

“…We ask is there a biological process that does this movement. And there is indeed; it is the ocular microtremor (OMT) and it is 30 Hz to 120 Hz peaking at around 83Hz [1]. This would equate to about 12 msec per microtremor.…”

Biospeckle in the human sclera and impact on laser speckle correlation measurement of eye tremor

“…To simulate the effect of eye movement on the speckle pattern, a 5-s long fixational eye movement trace that had been previously acquired in vivo using a piezoelectric (PZT) probe system 39,40 was used to drive the speckle pattern displacement in the simulation. The trace was the raw output of the PZT probes and included drift and microsaccades.…”