Suppression of contrast sensitivity during eyelid blinks

The interplay of perceptual processing demands and practice in modulating voluntary and involuntary motor responses

Understanding how sensory processing demands affect the ability to ignore task-irrelevant, loud auditory stimuli (LAS) during a task is key to performance in dynamic environments. For example, tennis players must ignore crowd noise to perform optimally. We investigated how practice affects this ability by examining the effects of delivering LASs during preparatory phase of an anticipatory timing (AT) task on the voluntary and reflexive responses in two conditions: lower and higher visual processing loads. Twenty-four participants (mean age = 23.1, 11 females) completed the experiment. The AT task involved synchronizing a finger abduction response with the last visual stimulus item in a sequence of four Gabor grating patches briefly flashed on screen. The lower demand condition involved only this task, and the higher demand condition required processing the orientations of the patches to report changes in the final stimulus item. Our results showed that higher visual processing demands affected the release of voluntary actions, particularly in the first block of trials. When the perceptual load was lower, responses were released earlier by the LAS compared to the high-load condition. Practice reduced these effects largely, but high perceptual load still led to earlier action release in the second block. In contrast, practice led to more apparent facilitation of eyeblink latency in the second block. These findings indicate that a simple perceptual load manipulation can impact the execution of voluntary motor actions, particularly for inexperienced participants. They also suggest distinct movement preparation influences on voluntary and involuntary actions triggered by acoustic stimuli.

Eye blinks as a visual processing stage

Humans blink their eyes frequently during normal viewing, more often than it seems necessary for keeping the cornea well lubricated. Since the closure of the eyelid disrupts the image on the retina, eye blinks are commonly assumed to be detrimental to visual processing. However, blinks also provide luminance transients rich in spatial information to neural pathways highly sensitive to temporal changes. Here, we report that the luminance modulations from blinks enhance visual sensitivity. By coupling high-resolution eye tracking in human observers with modeling of blink transients and spectral analysis of visual input signals, we show that blinking increases the power of retinal stimulation and that this effect significantly enhances visibility despite the time lost in exposure to the external scene. We further show that, as predicted from the spectral content of input signals, this enhancement is selective for stimuli at low spatial frequencies and occurs irrespective of whether the luminance transients are actively generated or passively experienced. These findings indicate that, like eye movements, blinking acts as a computational component of a visual processing strategy that uses motor behavior to reformat spatial information into the temporal domain.

The perceptual consequences and neurophysiology of eye blinks

A hand passing in front of a camera produces a large and obvious disruption of a video. Yet the closure of the eyelid during a blink, which lasts for hundreds of milliseconds and occurs thousands of times per day, typically goes unnoticed. What are the neural mechanisms that mediate our uninterrupted visual experience despite frequent occlusion of the eyes? Here, we review the existing literature on the neurophysiology, perceptual consequences, and behavioral dynamics of blinks. We begin by detailing the kinematics of the eyelid that define a blink. We next discuss the ways in which blinks alter visual function by occluding the pupil, decreasing visual sensitivity, and moving the eyes. Then, to anchor our understanding, we review the similarities between blinks and other actions that lead to reductions in visual sensitivity, such as saccadic eye movements. The similarity between these two actions has led to suggestions that they share a common neural substrate. We consider the extent of overlap in their neural circuits and go on to explain how recent findings regarding saccade suppression cast doubt on the strong version of the shared mechanism hypothesis. We also evaluate alternative explanations of how blink-related processes modulate neural activity to maintain visual stability: a reverberating corticothalamic loop to maintain information in the face of lid closure; and a suppression of visual transients related to lid closure. Next, we survey the many areas throughout the brain that contribute to the execution of, regulation of, or response to blinks. Regardless of the underlying mechanisms, blinks drastically attenuate our visual abilities, yet these perturbations fail to reach awareness. We conclude by outlining opportunities for future work to better understand how the brain maintains visual perception in the face of eye blinks. Future work will likely benefit from incorporating theories of perceptual stability, neurophysiology, and novel behavior paradigms to address issues central to our understanding of natural visual behavior and for the clinical rehabilitation of active vision.

Learning to silence saccadic suppression

Sensory systems often suppress self-generated sensations in order to discriminate them from those arising in the environment. The suppression of visual sensitivity during rapid eye movements is well established, and although functionally beneficial most of the time, it can limit the performance of certain tasks. Here, we show that with repeated practice, mechanisms that suppress visual signals during eye movements can be modified. People trained to detect brief visual patterns learn to turn off suppression around the expected time of the target. These findings demonstrate an elegant form of plasticity, capable of improving the visibility of behaviorally relevant stimuli without compromising the wider functional benefits of suppression.

Perceptual stability is facilitated by a decrease in visual sensitivity during rapid eye movements, called saccadic suppression. While a large body of evidence demonstrates that saccadic programming is plastic, little is known about whether the perceptual consequences of saccades can be modified. Here, we demonstrate that saccadic suppression is attenuated during learning on a standard visual detection-in-noise task, to the point that it is effectively silenced. Across a period of 7 days, 44 participants were trained to detect brief, low-contrast stimuli embedded within dynamic noise, while eye position was tracked. Although instructed to fixate, participants regularly made small fixational saccades. Data were accumulated over a large number of trials, allowing us to assess changes in performance as a function of the temporal proximity of stimuli and saccades. This analysis revealed that improvements in sensitivity over the training period were accompanied by a systematic change in the impact of saccades on performance—robust saccadic suppression on day 1 declined gradually over subsequent days until its magnitude became indistinguishable from zero. This silencing of suppression was not explained by learning-related changes in saccade characteristics and generalized to an untrained retinal location and stimulus orientation. Suppression was restored when learned stimulus timing was perturbed, consistent with the operation of a mechanism that temporarily reduces or eliminates saccadic suppression, but only when it is behaviorally advantageous to do so. Our results indicate that learning can circumvent saccadic suppression to improve performance, without compromising its functional benefits in other viewing contexts.

Faster visual reaction times in elite athletes are not linked to better gaze stability

The issue of whether visually-mediated, simple reaction time (VRT) is faster in elite athletes is contentious. Here, we examined if and how VRT is affected by gaze stability in groups of international cricketers (16 females, 28 males), professional rugby-league players (21 males), and non-sporting controls (20 females, 30 males). VRT was recorded via a button-press response to the sudden appearance of a stimulus (circular target-diameter 0.8°), that was presented centrally, or 7.5° to the left or right of fixation. The incidence and timing of saccades and blinks occurring from 450 ms before stimulus onset to 225 ms after onset were measured to quantify gaze stability. Our results show that (1) cricketers have faster VRT than controls; (2) blinks and, in particular, saccades are associated with slower VRT regardless of the level of sporting ability; (3) elite female cricketers had steadier gaze (fewer saccades and blinks) compared to female controls; (4) when we accounted for the presence of blinks and saccades, our group comparisons of VRT were virtually unchanged. The stability of gaze is not a factor that explains the difference between elite and control groups in VRT. Thus we conclude that better gaze stability cannot explain faster VRT in elite sports players.

Target Displacements during Eye Blinks Trigger Automatic Recalibration of Gaze Direction

Eye blinks cause disruptions to visual input and are accompanied by rotations of the eyeball [1]. Like every motor action, these eye movements are subject to noise and introduce instabilities in gaze direction across blinks [2]. Accumulating errors across repeated blinks would be debilitating for visual performance. Here, we show that the oculomotor system constantly recalibrates gaze direction during blinks to counteract gaze instability. Observers were instructed to fixate a visual target while gaze direction was recorded and blinks were detected in real time. With every spontaneous blink-while eyelids were closed-the target was displaced laterally by 0.5° (or 1.0°). Most observers reported being unaware of displacements during blinks. After adapting for ∼35 blinks, gaze positions after blinks showed significant biases toward the new target position. Automatic eye movements accompanied each blink, and an aftereffect persisted for a few blinks after target displacements were eliminated. No adaptive gaze shift occurred when blinks were simulated with shutter glasses at random time points or actively triggered by observers, or when target displacements were masked by a distracting stimulus. Visual signals during blinks are suppressed by inhibitory mechanisms [3-6], so that small changes across blinks are generally not noticed [7, 8]. Additionally, target displacements during blinks can trigger automatic gaze recalibration, similar to the well-known saccadic adaptation effect [9-11]. This novel mechanism might be specific to the maintenance of gaze direction across blinks or might depend on a more general oculomotor recalibration mechanism adapting gaze position during intrinsically generated disruptions to visual input.

Reading with a filtered fovea: the influence of visual quality at the point of fixation during reading

Reading relies critically on processing text in foveal vision during brief fixational pauses, and high-quality visual input from foveal text is fundamental to theories of reading. However, the quality of visual input from foveal text that is actually functional for reading and the effects of this input on reading performance are unclear. To investigate these issues, a moving, gaze-contingent foveal filtering technique was developed to display areas of text within foveal vision that provided only coarse, medium, or fine scale visual input during each fixational pause during reading. Normal reading times were unaffected when foveal text up to three characters wide at the point of fixation provided any one visual input (coarse, medium, or fine). Wider areas of coarse visual input lengthened reading times, but reading still occurred, and normal reading times were completely unaffected when only medium or fine visual input extended across the entire fovea. Further analyses revealed that each visual input had no effect on the number of fixations made when normal text was read, that adjusting fixation durations helped preserve reading efficiency for different visual inputs, and that each visual input had virtually no effect on normal saccades. These findings indicate that, despite the resolving power of foveal vision and the emphasis placed on high-quality foveal visual input by theories of reading, normal reading functions with similar success using a range of restricted visual inputs from foveal text, even at the point of fixation. Some implications of these findings for theories of reading are discussed.

Precisely timed oculomotor and parietal EEG activity in perceptual switching

Blinks and saccades cause transient interruptions of visual input. To investigate how such effects influence our perceptual state, we analyzed the time courses of blink and saccade rates in relation to perceptual switching in the Necker cube. Both time courses of blink and saccade rates showed peaks at different moments along the switching process. A peak in blinking rate appeared 1,000 ms prior to the switching responses. Blinks occurring around this peak were associated with subsequent switching to the preferred interpretation of the Necker cube. Saccade rates showed a peak 150 ms prior to the switching response. The direction of saccades around this peak was predictive of the perceived orientation of the Necker cube afterwards. Peak blinks were followed and peak saccades were preceded by transient parietal theta band activity indicating the changing of the perceptual interpretation. Precisely-timed blinks, therefore, can initiate perceptual switching, and precisely-timed saccades can facilitate an ongoing change of interpretation.

Spontaneous eyeblink activity

Spontaneous blinking is essential for maintaining a healthy ocular surface and clarity of vision. The spontaneous blink rate (SBR) is believed to reflect a complex interaction between peripheral influences mediated by the eye surface and the central dopaminergic activity. The SBR is thus extremely variable and dependent on a variety of psychological and medical conditions. Many different methods have been employed to measure the SBR and the upper eyelid kinematics during a blink movement. Each has its own merits and drawbacks, and the choice of a specific method should be tailored to the specific needs of the investigation. Although the sequence of muscle events that leads to a blink has been fully described, knowledge about the neural control of spontaneous blinking activity is not complete. The tear film is dynamically modified between blinks, and abnormalities of the blink rate have an obvious influence on the ocular surface.

Temporal characteristics of global motion processing revealed by transcranial magnetic stimulation

The ability to detect the motion of objects is critical to survival, and understanding the cortical mechanisms involved in this process remains a key challenge in sensory neuroscience. A relatively new approach to this problem is to temporarily disrupt processing at specific cortical sites and measure the behavioural consequences. Several previous studies have shown that transcranial magnetic stimulation (TMS) of human visual area V5/MT disrupts global motion perception, but reports vary widely in the timescale of this effect. To resolve this issue we employed psychophysical techniques to investigate how discrimination of translational, rotational and radial global motion is affected by TMS. Prior to applying TMS we established baseline coherence thresholds for global motion perception. Adopting each observer's coherence level at threshold we examined how TMS delivered to V5/MT modulated performance. Importantly, we measured the influence of single-pulse TMS over a broad temporal range to reveal the fine temporal structure of the disruption profile for global motion perception. Results show that the disruption profile consisted of two distinct epochs during which global direction judgments were reliably impaired, separated by an interval in which performance was unaffected. The bimodal nature of the distribution profiles is consistent with feedforward and feedback processing between visual areas mediating global motion processing. We present a novel quantitative model that characterizes the contribution of each process to visual motion perception.

Synchronization of spontaneous eyeblinks while viewing video stories

Blinks are generally suppressed during a task that requires visual attention and tend to occur immediately before or after the task when the timing of its onset and offset are explicitly given. During the viewing of video stories, blinks are expected to occur at explicit breaks such as scene changes. However, given that the scene length is unpredictable, there should also be appropriate timing for blinking within a scene to prevent temporal loss of critical visual information. Here, we show that spontaneous blinks were highly synchronized between and within subjects when they viewed the same short video stories, but were not explicitly tied to the scene breaks. Synchronized blinks occurred during scenes that required less attention such as at the conclusion of an action, during the absence of the main character, during a long shot and during repeated presentations of a similar scene. In contrast, blink synchronization was not observed when subjects viewed a background video or when they listened to a story read aloud. The results suggest that humans share a mechanism for controlling the timing of blinks that searches for an implicit timing that is appropriate to minimize the chance of losing critical information while viewing a stream of visual events.

Optical Quality of the Eye Degraded by Time-Varying Wavefront Aberrations with Tear Film Dynamics

PURPOSE

Wavefront aberrations (WFAs) of the eye vary with time because of the tear film dynamics. We investigated, using a simulation method, the variation of optical quality with time-varying wavefront measurements of 13 eyes with different refractions.

METHODS

WFAs of 13 normal eyes of 13 subjects were measured every second for 10 s. First, we simulated WFAs with conventional corneal laser refractive surgery by subtracting the second-order aberrations of the least aberrated measurement from measured consecutive WFAs. Second, we simulated customized refractive surgery by subtracting the second- to sixth-order aberrations of the least aberrated measurement from measured consecutive WFAs. We calculated Strehl ratios and retinal images from these corrected consecutive WFAs.

RESULTS

In one eye, the root mean square (RMS) values of WFAs with a second-order correction were sometimes smaller than those of WFAs with a second- to sixth-order correction, when these were compared at the same time point after a blink. However, in the other 12 eyes, the RMS values with second- to sixth-order corrections were smaller than those with only a second-order correction. In eight eyes, the Strehl ratios with second- to sixth-order corrections were larger than those with second-order corrections. In the remaining five eyes, Strehl ratios with second- to sixth-order corrections were sometimes smaller than those with second-order corrections.

CONCLUSIONS

In a simulation, the correction of time-invariant higher order aberrations usually reduced RMS values, but it did not always result in higher Strehl ratios than those obtained with only second-order corrections.

Vision: in the blink of an eye

Although we blink every 4 to 6 seconds, we notice neither the act of blinking nor the mini-blackouts they cause. A new study using imaging techniques identifies the neural structures in humans involved in suppressing vision processing and visual awareness during blinking.

Saccadic eye movements cause compression of time as well as space

There is now considerable evidence that space is compressed when stimuli are flashed shortly before or after the onset of a saccadic eye movement. Here we report that short intervals of time between two successive perisaccadic visual (but not auditory) stimuli are also underestimated, indicating a compression of perceived time. We were even more surprised that in a critical interval before saccades, perceived temporal order is consistently reversed. The very similar time courses of spatial and temporal compression suggest that both are mediated by a common neural mechanism, probably related to the predictive shifts that occur in receptive fields of many visual areas at the time of saccades.

Effect of Artificial Tears on Visual Performance in Subjects With Dry Eye

PURPOSE

Disruption of the anterior refracting surface of the eye (i.e., the tear layer) reduces visual performance. Tear layer breakup occurs soon after a blink in contact lens wearers and patients with dry eye. This study determined whether artificial tears stabilize the tear film and improve visual performance in contact lens wearers who also exhibit a dry eye.

METHODS

Five subjects with mild to moderate dry eye (probably as a result of an evaporative dry eye) during spectacle and contact lens wear were fitted with a Focus Night & Day hydrogel lens for this study. A temporal, two-alternative, forced-choice paradigm was used to measure contrast sensitivity. The stimuli were vertically oriented sine wave gratings (between 0.5 and 14 cpd) presented for 16.67 msec. The stimuli were presented at two different times after blink detection: 2 sec after blink detection (i.e., before tear layer breakup) or 4 sec after tear film breakup. Four conditions were investigated at 4 sec after tear layer breakup: 1) without artificial tears added, 2) with Clerz2 (Alcon, Fort Worth, TX) instilled, 3) with Sensitive Eyes (Bausch & Lomb, Rochester, NY), and 4) with GenTeal (Novartis, Basel, Switzerland) applied. The artificial tears were instilled at 10-min intervals during the data collection. The short-term visual effects of drop instillation were also investigated by continually monitoring contrast sensitivity for a 14-cpd grating after a single-drop administration.

RESULTS

High spatial frequency contrast sensitivity and visual acuity were found to be reduced after tear film breakup in the absence of supplementation with artificial tears. For the group data (and four of five subjects), the instillation of Sensitive Eyes improved the contrast sensitivity and visual acuity to the level attained before tear breakup, thus prolonging visual performance. Clerz2 and GenTeal did not produce any enhancement in visual performance. A short-term decrease in contrast sensitivity was also observed with a single administration of Clerz2 and GenTeal.

CONCLUSIONS

This study indicates that there was a benefit of Bausch & Lomb Sensitive Eyes tear supplementation on visual performance in subjects with an evaporative dry eye. This may be the result of 1) aqueous supplementation in these subjects and/or 2) the minimal tear layer disruption found with Sensitive Eyes drop administration. The results suggest that practitioners need to identify those patients who can benefit from the use of appropriate artificial tear supplements.

Two distinct neural effects of blinking on human visual processing

Humans blink every few seconds, yet the changes in retinal illumination during a blink are rarely noticed, perhaps because visual sensitivity is suppressed. Furthermore, despite the loss of visual input, visual experience remains continuous across blinks. The neural mechanisms in humans underlying these two phenomena of blink suppression and visual continuity are unknown. We investigated the neural basis of these two complementary behavioural effects using functional magnetic resonance imaging to measure how voluntary blinking affected cortical responses to visual stimulation. Two factors were independently manipulated in a blocked design; the presence/absence of voluntary blinking, and the presence/absence of visual stimulation. To control for the simple loss of visual input caused by eyelid closure, we created a fifth condition where external darkenings were dynamically matched to each subjects' own blinks. Areas of lateral occipital cortex, including area V5/MT, showed suppression of responses to visual stimulation during blinking, consistent with the known loss in visual sensitivity. In contrast, a medial parieto-occipital region, homologous to macaque area V6A, showed responses to blinks that increased when visual stimulation was present. Our data are consistent with a role for this region in the active maintenance of visual continuity across blinks. Moreover, both suppression in lateral occipital and activation in medial parieto-occipital cortex were greater during blinks than during matched external darkenings of the visual scene, suggesting that they result from an extra-retinal signal associated with the blink motor command. Our findings therefore suggest two distinct neural correlates of blinking on human visual processing.

Short-term Effects of Artificial Tears on Visual Performance in Normal Subjects

PURPOSE

Temporal changes in tear film structure can distort the optical wavefront as it passes through the tear layer and reduce contrast sensitivity. Theoretically, any substance applied to the tear layer that alters its structure could affect contrast sensitivity. The purpose of this study is to investigate how different formulations of carboxymethylcellulose sodium (CMC) applied to the tear layer affect contrast sensitivity over time. Additionally, the visual effect of these drops applied over soft and rigid, gas-permeable contact lenses was also investigated.

METHODS

Twenty normal subjects took part in this project. Refresh Celluvisc (Allergan, Irvine, CA, 1.0% high-viscosity CMC) was compared with Refresh Liquigel (Allergan, 1.0% total CMC made by blending 0.35% high-viscosity with 0.65% medium viscosity CMC). Ten of the subjects were habitual soft contact lens wearers and 10 were habitual gas-permeable lens wearers. The stimulus, viewed monocularly, was a stationary, vertically oriented, sine wave grating (14 CPD). A temporal, two-alternative, forced-choice paradigm combined with a self-paced method of limits was used to monitor threshold over time. After baseline data collection, a drop of the artificial tear was applied to the tear layer and the procedure continued for 30 min. This allowed continual tracking of the threshold. Data were collected while viewing the stimulus with the subject's contact lens or with their spectacle prescription.

RESULTS

One drop of Liquigel or Celluvisc decreased contrast sensitivity for a 14 CPD sine wave grating (all p values < 0.005). This decrease in contrast sensitivity was observed during spectacle, soft contact lens, and gas-permeable contact lens wear. Soft contact lens wear resulted in a greater decrease in contrast sensitivity than spectacles when Liquigel was applied to the tear layer. None of the other conditions were different between contact lens and spectacle wear. The return to baseline contrast sensitivity was not significantly different between soft or gas-permeable contact lens wear and spectacles for either Liquigel or Celluvisc. In general, Celluvisc had a greater effect on visual performance than Liquigel.

CONCLUSIONS

These results suggest that Liquigel and Celluvisc alter the tear layer and affect contrast sensitivity. The results agree with patient observations that Celluvisc causes a moderate amount of blur that gradually subsides. In such patients, the shorter duration of blur with Liquigel (about half that of Celluvisc) may be more acceptable. The technique of blending various viscosity CMC materials while maintaining the total CMC concentration of 1.0% may be beneficial in dry eye therapy without causing excessive blur to patients.

Different effects of eyelid blinks and target blanking on saccadic suppression of displacement

Displacements of visual stimuli during saccadic eye movements are often not noticed. We have demonstrated that saccadic suppression of image displacement can be eliminated by blanking the stimulus for a short period during and after the saccade (Deubel, Schneider, & Bridgeman, 1996). Here we report an experiment in which target visibility was interrupted after the saccade, either by distal target blanking or by voluntary eyeblink. The data show that the effect of blinking is different from blanking; interruption of vision due to a blink did not enable subjects to detect target displacements any better than they had done in the no-blank condition. The results provide evidence for an extraretinal signal that distinguishes between endogenous and exogenous sources of temporary object disappearance after the saccade.

Blink effects on ongoing smooth pursuit eye movements in humans

Blinks are known to affect eye movements, e.g., saccades, slow and fast vergence, and saccade-vergence interaction, in two ways: by superimposition of blink-associated eye movements and changes of the central premotor activity in the brainstem. The goal of this study was to determine, for the first time, the effects of trigeminal evoked blinks on ongoing smooth pursuit eye movements which could be related to visual sensory or premotor neuronal changes. This was compared to the effect of a target disappearing for 100-300 ms duration during ongoing smooth pursuit (blank paradigm) in order to control for the visual sensory effects of a blink. Eye and blink movements were recorded in eight healthy subjects with the scleral search coil technique. Blink-associated eye movements during the first 50% of the blink duration were non-linearly superimposed on the smooth pursuit eye movements. Immediately after the blink-associated eye movements, the pursuit velocity slowly decreased by an average of 3.2+/-2.1 degrees /s. This decrease was not dependent on the stimulus direction. The pursuit velocity decrease caused by blinks which occluded the pupil more than 50% could be explained mostly by blanking the visual target. However, small blinks that did not occlude the pupil (<10% of lid closure) also decreased smooth pursuit velocity. Thus, this blink effect on pursuit velocity cannot be explained by blink-associated eye movements or by the blink having blanked the visual input. We propose that part of this effect might either be caused by incomplete visual suppression during blinks and/or a change in the activity of omnipause neurons.

The Effect of Artificial Tears on Visual Performance in Normal Subjects Wearing Contact Lenses

PURPOSE

Disruption of the anterior refracting surface of the eye reduces visual performance. In the case of a contact lens wearer, this surface is the prelens tear film. This study determined whether artificial tears stabilize the tear film in contact lens wearers and maintained optimal visual performance for a prolonged period.

METHODS

Five normal subjects all in good general and ocular health and adapted to contact lens wear were fitted with Acuvue lenses. A temporal, two-alternative, forced-choice paradigm was used to measure contrast sensitivity. The stimuli were vertically oriented sine-wave gratings (between 0.5 and 14 cpd) presented for 16.67 ms. The stimuli were presented at two different times after blink detection: 2 s after blink detection (i.e., before tear layer break-up) or 4 s after tear film break-up. Three conditions were investigated at 4 s after tear layer break-up: (1) without artificial tears added, (2) with Clerz2 (Ciba Vision) instilled, and (3) with Sensitive Eyes (Bausch & Lomb) applied. The artificial tears were instilled at 10-min intervals during the experimental run.

RESULTS

High spatial frequency contrast sensitivity was found to be reduced after tear film break-up and was not enhanced by either tear solution.

CONCLUSIONS

This study indicates that there is no effect of Clerz2 or Sensitive Eyes in maintaining or improving visual performance beyond the normal prelens tear film break-up time in normal subjects wearing Acuvue lenses.

Contact lens drying and visual performance: the vision cycle with contact lenses

BACKGROUND AND PURPOSE

The purpose of this study was to measure the effect of precontact lens tear film break-up on visual performance.

METHODS

Four asymptomatic soft contact lens wearers had contrast sensitivity measured by a temporal, two-alternative, force choice paradigm combined with a self-paced methods of limits. Stimuli were vertically orientated sine wave gratings (0.5 to 14 cycles per degree [cpd] presented for 16.67 ms. Contrast sensitivity was measured before precontact lens tear break-up by a stimuli presented 2 s after the blink. A post-tear layer break-up measurement taken with the stimuli presented after break-up had been observed by the use of a video camera attached to a Tearscope.

RESULTS

Contrast sensitivity was found to be reduced following precontact lens tear film break-up for stimuli of 4, 6, and 10 cpd; the data approached significance at 14 cpd. Further reductions in contrast sensitivity were observed for one subject when measurements were continued for 4 s following break-up.

CONCLUSIONS

Contrast sensitivity is significantly reduced for middle to high spatial frequencies when the precontact lens tear film dries and breaks up. The combination of observations of visual performance immediately following the blink (from earlier experiments) and measurements following tear film break-up in this experiment allows description of a "vision cycle" for contact lens wearers in the interval between blinks. It is suggested that break-up of the precontact lens tear film could account for the complaints of intermittent blurred vision in some contact lens wearers and may provide a stimulus to blinking in these individuals.

Comparison of human ocular torsion patterns during natural and galvanic vestibular stimulation

Galvanic vestibular stimulation (GVS) is reported to induce interindividually variable tonic ocular torsion (OT) and superimposed torsional nystagmus. It has been proposed that the tonic component results from the activation of otolith afferents. We tested our hypothesis that both the tonic and the phasic OT are mainly due to semicircular canal (SCC) stimulation by examining whether the OT patterns elicited by GVS can be reproduced by pure SCC stimulations. Using videooculography we measured the OT of six healthy subjects while two different stimuli with a duration of 20 s were applied: 1) transmastoidal GVS steps of 2 mA with the head in a pitched nose-down position and 2) angular head rotations around a combined roll-yaw axis parallel to the gravity vector with the head in the same position. The stimulation profile was individually scaled to match the nystagmus properties from GVS and consisted of a sustained velocity step of 4-12 degrees /s on which a velocity ramp of 0.67-2 degrees /s(2) was superimposed. Since blinks were reported to induce transient torsional eye movements, the subjects were also asked to blink once 10 s after stimulus onset. Analysis of torsional eye movements under both conditions revealed no significant differences. Thus we conclude that both the tonic and the phasic OT responses to GVS can be reproduced by pure rotational stimulations and that the OT-related effects of GVS on SCC afferents are similar to natural stimulations at small amplitudes.

Activity of primate V1 cortical neurons during blinks

Every time we blink our eyes, the image on the retina goes almost completely dark. And yet we hardly notice these interruptions, even though an external darkening is startling. Intuitively it would seem that if our perception is continuous, then the neuronal activity on which our perceptions are based should also be continuous. To explore this issue, we compared the responses of 63 supragranular V1 neurons recorded from two awake monkeys for four conditions: 1) constant stimulus, 2) during a reflex blink, 3) during a gap in the visual stimulus, and 4) during an external darkening when an electrooptical shutter occluded the entire scene. We show here that the activity of neurons in visual cortical area V1 is essentially shut off during a blink. In the 100-ms epoch starting 70 ms after the stimulus was interrupted, the firing rate was 27.2 +/- 2.7 spikes/s (SE) for a constant stimulus, 8.2 +/- 0.9 spikes/s for a reflex blink, 17.3 +/- 1.9 spikes/s for a gap, and 12.7 +/- 1.4 spikes/s for an external darkening. The responses during a blink are less than during an external darkening (P < 0.05, t-test). However, many of these neurons responded with a transient burst of activity to the onset of an external darkening and not to a blink, suggesting that it is the suppression of this transient which causes us to ignore blinks. This is consistent with other studies where the presence of transient bursts of activity correlates with the perceived visibility of a stimulus.

A comparison of saccadic and blink suppression in normal observers

Recent research suggests that blink and saccadic suppression are produced by the same mechanism (Volkmann, 1986; Uchikawa & Sato, 1995; Ridder & Tomlinson, 1993, 1995). These studies demonstrated that blink and saccadic suppression have the same effect on various visual functions. However, none of these studies made a comparison of blink and saccadic suppression in the same individual. The purpose of this study was to compare the effects of blink and saccadic suppression on contrast sensitivity functions in the same subject. The effect of saccadic suppression on the contrast sensitivity function in three normal observers was determined. Employing a two-alternative, forced-choice technique, thresholds were measured for seven spatial frequencies. At each spatial frequency, the threshold was determined immediately following detection of a voluntary saccade. The magnitude of suppression was taken as the log ratio of the contrast sensitivities obtained while foveating the stimulus and those obtained during saccades. The magnitude of saccadic suppression was found to increase as the saccade amplitude increased and to be spatial-frequency dependent. Low spatial frequencies were suppressed more than high spatial frequencies. The blink suppression data have been measured previously (Ridder & Tomlinson, 1993). Saccadic and blink suppression were qualitatively similar. A vertical shift of the data brought the saccadic and blink suppression data into register. These results suggest that blink and saccadic suppression are produced by the same or similar mechanisms.

Lack of visual suppression in the rabbit lateral geniculate nucleus during blink reflex

Stimulation of the supraorbital branch of the trigeminal nerve (SO) elicited eye blinks in the rabbit, but did not decrease the amplitude of visual cortical evoked potential from stimulation of the optic chiasm (OX). In addition, the SO stimulation neither induced an inhibitory postsynaptic potential (IPSP) in LGN cells, nor activated inhibitory interneurons in the thalamic reticular nucleus (TRN), which proved to mediate both recurrent inhibition and saccadic suppression in the dorsal lateral geniculate nucleus (LGN). All these indicate that there is no visual suppression in the rabbit LGN during blink reflex.

Very short-term visual memory via reverberation: a role for the cortico-thalamic excitatory circuit in temporal filling-in during blinks and saccades?

There is a large projection of neurons from Layer VI of V1 that makes excitatory connections on LGN relay cells. It has been proposed that this circuit is involved in signal processing and thalamic sensitivity regulation. Alternatively, Crick has suggested that the circuit could be a reverberatory loop-a site for very short-term (iconic) visual memory. This hypothesis is shown to be plausible if the reverberation is keyed to the onset of neurally initiated visual disruptions such as blinks and saccades. Neural mechanisms suppress perception during these events but little is known about temporal filling-in processes analogous to the mechanisms that fill-in spatial scotomas. Crick's reverberatory loop could provide a process for filling-in temporal scotomas with information acquired just before the disruption, thus maintaining the continuity of visual experience.

Spectral characteristics of blink suppression in normal observers

Previous studies of the characteristics of suppression occurring under various visual conditions show similarities and differences which may indicative of the mechanism of suppression. The primary purpose of this study was to determine if the suppression that occurs in response to an eyelid blink (blink suppression) is similar to that which occurs during a saccade (saccadic suppression). In addition, the characteristics of blink suppression and other forms of suppression (i.e. permanent and binocular rivalry suppression) are compared. A test probe paradigm was utilized to determine the effect of blink suppression on the spectral sensitivity function in three normal observers. Employing a two alternative forced choice technique, thresholds were determined for wavelengths from 420 to 680 nm in 20 nm steps. At each wavelength, the threshold was determined at 0 and 400 msec after the onset of a voluntary blink. The magnitude of suppression was taken as the difference between the 0 and 400 msec thresholds. Similar to saccadic suppression, the magnitude of blink suppression increased as the stimuli biased detection towards the luminance channel. These results suggest that blink suppression and saccadic suppression are the result of a single mechanism. Similarities between blink suppression and other forms of visual suppression are also considered.