Paul C. Knox

Does extraocular muscle proprioception influence oculomotor control

Disorders of ocular motility are encountered on a regular basis within ophthalmic practice. They include a wide variety of conditions from non-paralytic strabismus commonly seen in paediatric clinics to acquired restrictive and paralytic conditions, which may be indicative of more serious underlying pathology. For an accurate diagnosis to be made an understanding of the basic mechanisms involved in oculomotor control is desirable. Eye movements are mediated by a complex hierarchy of neuronal systems. While the final common pathway consists of the motor nuclei and associated structures in the brainstem, oculomotor behaviour is shaped by the cerebellum, the superior colliculus, the basal ganglia, and the cortical eye fields.1 In order to coordinate the movement of the eyes, a process vital for both vision and visually guided behaviour, these brain centres must “know” the direction in which the eyes are pointing. If the eyes were fixed within the orbits then retinal (that is, visual) information would be sufficient to tell us where we are looking. However, the eyes, as well as the visual world, can and do move, and under these circumstances extraretinal (that is, non-visual) information is required to determine gaze direction. There are two broad hypotheses that seek to explain the source of this extraretinal information; while not mutually exclusive, they are often presented as alternatives. The “inflow” hypothesis holds that afferent signals from the effector muscles in the oculomotor system, the extraocular muscles (EOM), provide the necessary information about the positions of the eyes in the orbits and about movements of the eyes. This view can be attributed to Sherrington,2 although it fell out of favour, particularly in the 1960s, when the role of muscle receptors in general came to be doubted (see Matthews3 for review). The outflow hypothesis, attributed to Helmholtz,4 holds that central …

Enhancement of blood vessels in digital fundus photographs via the application of multiscale line operators

We employed multiscale line operators (MSLO) in order to segment blood vessels in digital fundus images. Separately, a median filter technique was used in order to provide results that were compared to those of the MSLO. The green channel of the colour image was used, and both sets of results were further enhanced by subsequently employing a simple “randomly seeded” region-growing algorithm. We applied this approach to two sets of retinal images, namely, the ARIA (www.eyecharity.com/aria_online/) and STARE (www.ces.clemson.edu/∼ahoover/stare/) retinal image archives. The ARIA dataset contained colour fundus images from healthy subjects, diabetic subjects, and age-related macular degeneration (AMD) subjects. Similarly, the STARE dataset contained images from both “normal” (i.e., healthy) and “abnormal” (i.e., diseased) eyes. Manual segmentations of the blood-vessel structure for all images in the ARIA and STARE datasets were obtained by a retinal image interpretation expert. These images were then taken to be our gold standards. Receiver–operator characteristic (ROC) curves were determined and the areas under the ROC curve (AZ) were obtained. A large increase in efficiency for our MSLO algorithm was observed for the entire datasets (ARIA AZ=0.899; STARE AZ=0.953) compared to basic thresholding alone (ARIA AZ=0.608; STARE AZ=0.753). Interestingly, the simple median filter algorithm followed by region growing also performed well (ARIA AZ=0.888; STARE AZ=0.947). Our results compared favourably to those results of previous segmentation procedures for the STARE dataset. As expected, the best results were found for the healthy control group for ARIA and for the normal subjects for STARE.

The effect of the gap paradigm on the latency of human smooth pursuit of eye movement

WHEN a temporal gap is Introduced between the extinction of a central fixation target and the illumination of an eccentric target (the gap paradigm), normal human subjects initiate saccadic eye movements towards the eccentric target at lower latency than when there is no gap. The aim of this study was to examine the latency of human smooth pursuit eye movements using a modified gap paradigm. Smooth pursuit latency was reduced in gap tasks, and the magnitude of reduction was related to the duration of the gap. The distribution of smooth pursuit latencies was also altered. It thus appears that human smooth pursuit latency is modulated in a similar manner to saccade latency in gap tasks.

A metanalysis of the effect of the Müller-Lyer illusion on saccadic eye movements: No general support for a dissociation of perception and oculomotor action

Milner and Goodales (1995) proposal of a functional division of labor between vision-for-perception and vision-for-action is supported by neuropsychological, brain-imaging, and psychophysical evidence. However, there remains considerable debate as to whether, as their proposal would predict, the effect of contextual illusions on vision-for-action can be dissociated from that on vision-for-perception. Meta-analytical efforts examining the effect of the Müller-Lyer (ML) illusion on pointing (Bruno, Bernardis, & Gentilucci, 2008) or grasping (Bruno & Franz, 2009) have been conducted to resolve the controversy. To complement this work, here we re-analyzed 17 papers detailing 21 independent studies investigating primary saccades to target locations that were perceptually biased by the ML illusion. Using a corrected percent illusion effect measure to compare across different studies and across experimental conditions within studies, we find that saccadic eye movements are always strongly biased by the illusion although the size of this effect can be reduced by factors such as display duration and between-trials variability in display length and orientation, possibly due to a process of saccadic adaptation. In contrast to some reports, we find no general support for differences between voluntary and reflexive saccades or between saccades performed in conjunction with a pointing movement and saccades performed without pointing. We conclude that studies on the effect of the Müller-Lyer illusion do not provide evidence for a functional dissociation between primary saccades and perception.

How does action resist visual illusion? Uncorrected oculomotor information does not account for accurate pointing in peripersonal space

Using spatially identical displays (variants of the Müller–Lyer illusion), we compared the accuracy of spatial verbal judgments with that of saccadic (eye) and pointing (hand) movements. Verbal judgments showed a clear effect of the illusion. The amplitude of the primary saccade from one endpoint of the pattern (at fixation) to the other also showed an effect of the illusion. Conversely, movement amplitudes when pointing from one endpoint (initial finger position) to the other were significantly more accurate than both saccades and verbal responses. In a control experiment in which the viewing conditions between the saccade and pointing experiments were equalized, saccade amplitude was again affected by the illusion. In several studies, systematic biases in conscious spatial judgments have been contrasted with accurate open-loop pointing in peripersonal space. It has been proposed that such seeming dissociations between vision-for-action and vision-for-consciousness might in fact be because of a simple oculomotor strategy: saccade to the target before it disappears, then use the efference copy of the (accurate) saccadic movement to drive pointing. The present data do not support the hypothesis in this simple form.

Afferent Signals from Pigeon Extraocular Muscles Modify the Vestibular Responses of Units in the Abducens Nucleus

Although the extraocular muscles contain stretch receptors it is generally believed that their afferents exert no influence on the control of eye movement. However, we have shown previously that these afferent signals reach various brainstem centres concerned with eye movement, notably the vestibular nuclei, and that the decerebrate pigeon is a favourable preparation in which to study their effects. If the extraocular muscle afferents do influence oculomotor control from moment‒to‒moment they should exert a demonstrable effect on the oculomotor nuclei. We now present evidence that extraocular muscle afferent signals do, indeed, alter the responses of units in an oculomotor nucleus (the abducens, VI nerve nucleus, which supplies the lateral rectus muscle) to horizontal, vestibular stimulation induced by sinusoidal oscillation of the bird. Such stimuli evoke a vestibulo-ocular reflex in the intact bird. The extraocular stretch receptors were activated by passive eye movement within the pigeon’s saccadic range; such movements modified the vestibular responses of all 19 units studied which were all, histologically, in the abducens nucleus. The magnitude of the effects, purely inhibitory in 15 units, depended both on the amplitude and the velocity of the eye movement and most units showed selectivity for particular combinations of plane (e. g. horizontal versus vertical) and direction (e. g. rostral versus caudal) of eye movement. The results show that an afferent signal from the extraocular muscles influences vestibularly driven activity in the abducens nucleus to which it carries information related to amplitude, velocity, plane and direction of eye movement in the saccadic range. They thus strongly support the view that extraocular afferent signals are involved in the control of eye movement.

Afferent signals from the extraocular muscles of the pigeon modify the vestibulo-ocular reflex.

Although the extraocular muscles (EOM) contain stretch receptors it is generally thought that the afferent signals which they provide play no role in the control of eye movement. We have previously shown that these afferent signals do modify both the vestibular responses of single units in the oculomotor control system and the electromyographic responses of the EOM during the vestibulo-ocular reflex (VOR). We have now investigated the effect of EOM afferent signals on the VOR itself, by recording the electro-oculogram of one eye while imposing movements on the other eye during the VOR. Moving the eye in a manner which mimics the slow phase of the VOR, we have found that, as the peak velocity of the imposed eye movement increases, the amplitude of eye movement of the other eye decreases. These results confirm that the output of the VOR itself, expressed as movement of the globe, and not merely some of its component parts, is modified by EOM afferent signals.

Evidence for corrective effects of afferent signals from the extraocular muscles on single units in the pigeon vestibulo-oculomotor system

The role of extraocular muscle (EOM) afferent feedback signals in the control of eye movement is still controversial. We recorded from 106 single units in the vestibular nuclei, oculomotor nuclei and reticular formation of 80 decerebrate, paralysed pigeons. EOM afferents were stimulated by passive eye movement (PEM) during vestibular stimulation by sinusoidal oscillation in the horizontal plane. We found that EOM afferent signals profoundly modified the vestibular responses of 91 (86%) of the single units recorded. As well as using PEM to simulate eye movements similar to saccades, we moved the eye in a manner which mimicked the slow phase of the vestibulo-ocular reflex (artificial VOR, AVOR). We have found evidence that, as well as providing signals closely related to the parameters of eye movement, PEM alters the vestibular responses of cells during AVOR in a manner which suggests that EOM afferent signals may play a corrective role in the moment-to-moment control of eye movement in the vestibulo-ocular reflex.

Emulsification of Silicone Oil and Eye Movements

PURPOSE Emulsification is an inherent problem of silicone oil used in vitreoretinal surgery. It has been shown that silicone oil can be made more resistant to emulsification and easier to inject by adding high-molecular-weight components (5% or 10% 423-kDa polydimethylsiloxane [PDMS]) to normal 1000 mPa · s silicone oil. The authors hypothesize that this might also reduce the movement of oil within an eye. METHODS A model eye chamber made of surface-modified poly(methyl methacrylate) was driven by a computer and a stepper motor to mimic saccadic eye movement. Seven silicone oils with different shear and extensional viscosities were tested. Two sets of eye movements were used: (amplitude 9°, angular velocity 390°/s, duration 50 ms) and (amplitude 90 °, angular velocity 360°/s, duration 300 ms). The movements were captured and analyzed by video recording. RESULTS The angular velocity of an oil bubble relative to the eye chamber appears to form an exponential relationship with its shear viscosity. Depending on the thickness of the film of aqueous between the eye wall and the oil bubble, the shear rate was estimated to be between 6 and 14 × 10(4) s(-1). The addition of 10% of 423-kDa PDMS to 1000 mPa · s silicone oil significantly reduced the peak relative velocity compared with the base oil of 1000 mPa · s but not 5000 mPa · s. CONCLUSIONS The addition of high molecular components to a base oil increases its extensional and shear viscosity. Although the extensional viscosity affected the ease with which the oil could be injected, the results showed that it was the shear viscosity that determined the relative velocity between the oil and the wall of the vitreous cavity, and thus the propensity to emulsify.

The effect of scleral search coil lens wear on the eye

BACKGROUND/AIM Scleral search coils are used to measure eye movements. A recent abstract suggests that the coil can affect the eye by decreasing visual acuity, increasing intraocular pressure, and damaging the corneal and conjunctival surface. Such findings, if repeated in all subjects, would cast doubt on the credibility of the search coil as a reliable investigative technique. The aim of this study was to reassess the effect of the scleral search coil on visual function. METHODS Six volunteer subjects were selected to undergo coil wear and baseline measurements were taken of logMAR visual acuity, non-contact tonometry, keratometry, and slit lamp examination. Four drops of 0.4% benoxinate hydrochloride were instilled before insertion of the lens by an experienced clinician. The lens then remained on the eye for 30 minutes. Measurements of the four ocular health parameters were repeated after 15 and 30 minutes of lens wear. The lens was then removed and the health of the eye reassessed. RESULTS No obvious pattern of change was found in logMAR visual acuity, keratometry, or intraocular pressure. The lens did produce changes to the conjunctival and corneal surfaces, but this was not considered clinically significant. CONCLUSION Search coils do not appear to cause any significant effects on visual function. However, thorough prescreening of subjects and post-wear checks should be carried out on all coil wearers to ensure no adverse effects have been caused.