The near response: modeling, instrumentation, and clinical applications

Soft, wireless periocular wearable electronics for real-time detection of eye vergence in a virtual reality toward mobile eye therapies

Recent advancements in electronic packaging and image processing techniques have opened the possibility for optics-based portable eye tracking approaches, but technical and safety hurdles limit safe implementation toward wearable applications. Here, we introduce a fully wearable, wireless soft electronic system that offers a portable, highly sensitive tracking of eye movements (vergence) via the combination of skin-conformal sensors and a virtual reality system. Advancement of material processing and printing technologies based on aerosol jet printing enables reliable manufacturing of skin-like sensors, while the flexible hybrid circuit based on elastomer and chip integration allows comfortable integration with a user's head. Analytical and computational study of a data classification algorithm provides a highly accurate tool for real-time detection and classification of ocular motions. In vivo demonstration with 14 human subjects captures the potential of the wearable electronics as a portable therapy system, whose minimized form factor facilitates seamless interplay with traditional wearable hardware.

Scaling of compensatory eye movements during translations: virtual versus real depth

Vestibulo-ocular reflexes are the fastest compensatory reflex systems. One of these is the translational vestibulo-ocular reflex (TVOR) which stabilizes the gaze at a given fixation point during whole body translations. For a proper response of the TVOR the eyes have to counter rotate in the head with a velocity that is inversely scaled to viewing distance of the target. It is generally assumed that scaling of the TVOR is automatically coupled to vergence angle at the brainstem level. However, different lines of evidence also argue that in humans scaling of the TVOR also depends on a mechanism that pre-sets gain on a priori knowledge of target distance. To discriminate between these two possibilities we used a real target paradigm with vergence angle coupled to distance and a virtual target paradigm with vergence angle dissociated from target distance. We compared TVOR responses in six subjects who underwent lateral sinusoidal whole-body translations at 1 and 2 Hz. Real targets varied between distance of 50 and 22.4 cm in front of the subjects, whereas the virtual targets consisting of a green and red light emitting diode (LED) were physically located at 50 cm from the subject. Red and green LED's were dichoptically viewed. By shifting the red LED relative to the green LED we created a range of virtual viewing distances where vergence angle changed but the ideal kinematic eye velocity was always the same. Eye velocity data recorded with virtual targets were compared to eye velocity data recorded with real targets. We also used flashing targets (flash frequency 1 Hz, duration 5 ms). During the real, continuous visible targets condition scaling of compensatory eye velocity with vergence angle was nearly perfect. During viewing of virtual targets, and with flashed targets compensatory eye velocity only weakly correlated to vergence angle, indicating that vergence angle is only partially coupled to compensatory eye velocity during translation. Our data suggest that in humans vergence angle as a measure of target distance estimation has only limited use for automatic TVOR scaling.

Short exposure to telestereoscope affects the oculomotor system

Under natural viewing conditions, the accommodation and vergence systems adjust the focus and the binocular alignment of the eyes in response to changes in viewing distance. The two responses are linked via cross-coupling and proceed almost simultaneously. Some optical devices, such as virtual reality or helmet mounted displays, create an oculomotor conflict by modifying demands on both vergence and accommodation. Previous studies extensively investigated the effect of such a conflict on the cross-coupling between vergence and accommodation, but little is known about the plasticity of the whole oculomotor system. In the present study, an oculomotor conflict was induced by a telestereoscope which magnified the standard inter-pupillary separation threefold and thus increased the convergence demand while accommodation remained almost unchanged. The effect of a 10 min exposure was assessed via a series of optometric parameters selected on the basis of existing oculomotor models. Associated with subject's visual complaints, most of the oculomotor parameters tested were modified: there was (1) deterioration of stereoscopic threshold; (2) increase in AC/A ratio; (3) increase in near and far phorias; and (4) shift of the zone of clear and single binocular vision towards convergence. These results showed a change in gain of accommodative vergence and a shift of vergence reserves towards convergence in response to telestereoscopic viewing. The subject's binocular behaviour tended towards esophoria with convergence excess as confirmed by Sheard's and Percival's criteria. Such changes in oculomotor parameters support adaptive behaviour linked with telestereoscopic viewing.

Sensitivity analysis of the stimulus-response function of a static nonlinear accommodation model

The effect of parameter variation of a nonlinear static feedback control model of the accommodation system was investigated. Simulations of a MATLAB/SIMULINK model showed a nonlinear relationship between stimulus and response in which the response curve was above the 1:1 line in the region to the left of the crossover and below the 1:1 line in the region to the right of the crossover. At the crossover, the response curve exhibited an inflection that was constant and equal to the tonic accommodation value (ABIAS). Sensitivity analysis showed that increasing depth of focus (deadspace range between +/- DSP) increased the separation between the boundary lines of the deadspace region, with a larger separation associated with late-onset myopia and congenital nystagmus. Increasing accommodative controller gain (ACG) increased the slope of the function on either side of the deadspace, with lower ACG values corresponding to lower slopes that indicated an amblyopic deficit. Increasing ABIAS increased the accommodative level at the inflection region. In addition, the saturation level of the accommodative response decreased with increasing age, while the slope remained the same, which was consistent with the Hess-Gullstrand theory of presbyopia. The accuracy and relative simplicity of the model indicated that it could serve as a basis for further comprehensive investigation of the basic and clinical aspects of the accommodation system.

Tonic accommodation: a review. II. Accommodative adaptation and clinical aspects

Part I of this review considered basic aspects of tonic accommodation (TA), i.e. the accommodative response observed under degraded stimulus conditions. Part II considers accommodative adaptation, i.e. the apparent change in TA following periods of sustained fixation, and clinical aspects of both baseline TA and accommodative adaptation. It is suggested that the apparent post-task shift in TA reflects the slow rate of decay of the stimulus-mediated adaptive accommodative response, while the actual level of tonic innervation to the ciliary muscle remains relatively constant. The clinical implications of both TA and accommodative adaptation are discussed with regard to night, space and instrument myopia and refractive error development, notably nearwork-induced myopia. It is concluded that the evidence for any association between this form of myopia and either TA or accommodative adaptation is equivocal, and furthermore it seems likely that TA plays only a minor role in influencing the closed-loop steady-state accommodative response.

Tonic accommodation: a review. I. Basic aspects

In the absence of an adequate visual stimulus, accommodation adopts an intermediate position of approximately 1 D. Since this position was believed to reflect the level of tonic innervation to the ciliary muscle, this response has been termed tonic accommodation (TA). Part I of this review will consider various aspects of this parameter, including its reference to closed-loop accommodative function and autonomic physiology. In addition, both the methods of measurement and appropriate terminology for this function will be discussed. It is concluded that the response, which becomes apparent under so-called 'stimulus-free' conditions, in fact probably represents an aggregate response resulting from multiple, non-optical stimuli. Thus the designation tonic accommodation may not be appropriate, since it fails to describe accurately the heterogeneous composition of the stimulus-free accommodative response. An associated paper (to be published as part II of this review) will examine accommodative adaptation and both clinical aspects of TA and adaptation of TA.

Absence of pupil response to blur-driven accommodation

The drive to the pupil constriction associated with near fixation has generally been attributed to accommodation with convergence and fusional convergence having secondary roles. However, our previous investigations have shown that significant changes in accommodation can take place without concomitant pupil response. To investigate further, the present study recorded pupil and accommodation responses to a blur-only accommodative stimulus using a target moved sinusoidally at a range of temporal frequencies. Care was taken to minimise target size change and apparent lateral or vertical target displacement. Results show that pupil response could be very much reduced or absent irrespective of stimulus temporal frequency and despite maintained accommodation response. The results suggest that blur-driven accommodation alone is not sufficient to drive pupil near response and that the presence of cues such as size change and lateral or vertical displacement of an approaching object may be necessary to elicit a response.

Accommodation without pupillary constriction

In this paper it will be shown that the association of pupillary contraction with even substantial accommodation breaks down under rigorous control of alignment. Conditions exclude other than "accommodative" (blur induced) pupillary responses by eliminating light reflexes due to temporal and spatial luminance differences, colour and polarization differences. The visual solid angles of the targets were also precisely equalized. A subtle procedure is described to obtain accurate alignment along a single line of sight of the stimulated eye. The pupillary system appears to be quite sensitive to alignment errors, but the "accommodative pupillary reflex" is totally absent under rigorous alignment. Pupillometry used an infrared television camera to monitor the non-stimulated eye in total darkness and silence. Occurrence of correct accommodation for every stimulus event was confirmed by observing accommodative vergence. Judging by the size of the accommodative vergence response and, occasionally, the size of Purkinje-Sanson images, the accommodative response was found to be essentially equal to the stimulation.