Measurements of visual suppression during opening, closing and blinking of the eyes

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.

Blink- and saccade-related suppression effects in early visual areas of the human brain: Intracranial EEG investigations during natural viewing conditions

Blinks and saccades, both ubiquitous in natural viewing conditions, cause rapid changes of visual inputs that are hardly consciously perceived. The neural dynamics in early visual areas of the human brain underlying this remarkable visual stability are still incompletely understood. We used electrocorticography (ECoG) from electrodes directly implanted on the human early visual areas V1, V2, V3d/v, V4d/v and the fusiform gyrus to investigate blink- and saccade-related neuronal suppression effects during non-experimental, free viewing conditions. We found a characteristic, biphasic, broadband gamma power decrease-increase pattern in all investigated visual areas. During saccades, a decrease in gamma power clearly preceded eye movement onset, at least in V1. This may indicate that cortical information processing is actively suppressed in human early visual areas before and during saccades, which then possibly mediates perceptual visual suppression. The following eye movement offset-related increase in gamma power may indicate the recovery of visual perception and the resumption of visual processing.

Eye-blinks in choice response tasks uncover hidden aspects of information processing

Spontaneous eye-blinks occur much more often than it would be necessary to maintain the tear film on the eyes. Various factors like cognitive demand, task engagement, or fatigue are influencing spontaneous blink rate. During cognitive information processing there is evidence that blinks occur preferably at moments that can be assigned to input stream segmentation. We investigated blinking behavior in three different visual choice response experiments (Experiment 1: spatial Stimulus-Response correspondence, Experiment 2: Change Detection, Experiment 3: Continuous performance Test - AX version). Blinks during the experimental tasks were suppressed when new information was expected, as well as during cognitive processing until the response was executed. Blinks in go trials occurred within a short and relatively constant interval after manual responses. However, blinks were not a side effect of manual behavior, as they occurred in a similar manner in no-go trials in which no manual response was executed. In these trials, blinks were delayed when a prepared response had to be inhibited, compared to trials in which no response was intended. Additionally, time on task effects for no-go blinks mirrored those obtained in go trials. Thus, blinks seem to provide a reliable measure for cognitive processing beyond (or rather additional to) manual responses.

Where does attention go when you blink?

Many studies have shown that covert visual attention precedes saccadic eye movements to locations in space. The present research investigated whether the allocation of attention is similarly affected by eye blinks. Subjects completed a partial-report task under blink and no-blink conditions. Experiment 1 showed that blinking facilitated report of the bottom row of the stimulus array: Accuracy for the bottom row increased and mislocation errors decreased under blink, as compared with no-blink, conditions, indicating that blinking influenced the allocation of visual attention. Experiment 2 showed that this was true even when subjects were biased to attend elsewhere. These results indicate that attention moves downward before a blink in an involuntary fashion. The eyes also move downward during blinks, so attention may precede blink-induced eye movements just as it precedes saccades and other types of eye movements.

A data-driven algorithm for offline pupil signal preprocessing and eyeblink detection in low-speed eye-tracking protocols

Event detection is the conversion of raw eye-tracking data into events--such as fixations, saccades, glissades, blinks, and so forth--that are relevant for researchers. In eye-tracking studies, event detection algorithms can have a serious impact on higher level analyses, although most studies do not accurately report their settings. We developed a data-driven eyeblink detection algorithm (Identification-Artifact Correction [I-AC]) for 50-Hz eye-tracking protocols. I-AC works by first correcting blink-related artifacts within pupil diameter values and then estimating blink onset and offset. Artifact correction is achieved with data-driven thresholds, and more reliable pupil data are output. Blink parameters are defined according to previous studies on blink-related visual suppression. Blink detection performance was tested with experimental data by visually checking the actual correspondence between I-AC output and participants' eye images, recorded by the eyetracker simultaneously with gaze data. Results showed a 97% correct detection percentage.

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.

Alpha waves: a neural signature of visual suppression

Alpha waves are traditionally considered a passive consequence of the lack of stimulation of sensory areas. However, recent results have challenged this view by showing a modulation of alpha activity in cortical areas representing unattended information during active tasks. These data have led us to think that alpha waves would support a 'gating function' on sensorial stimulation that actively inhibits unattended information in attentional tasks. Visual suppression occurring during a saccade and blink entails an inhibition of incoming visual information, and it seems to occur at an early processing stage. In this study, we hypothesized that the neural mechanism through which the visual system exerts this inhibition is the active imposition of alpha oscillations in the occipital cortex, which in turn predicts an increment of alpha amplitude during a visual suppression phenomena. We measured visual suppression occurring during short closures of the eyelids, a situation well suited for EEG recordings and stimulated the retinae with an intra-oral light administered through the palate. In the behavioral experiment, detection thresholds were measured with eyes steady open and steady closed, showing a reduction of sensitivity in the latter case. In the EEG recordings performed under identical conditions we found stronger alpha activity with closed eyes. Since the stimulation does not depend on whether the eyes were open or closed, we reasoned that this should be a central effect, probably due to a functional role of alpha oscillation in agreement with the 'gating function' theory.

Spontaneous eye blinks are entrained by finger tapping

We studied the mutual cross-talk between spontaneous eye blinks and continuous, self-paced unimanual and bimanual tapping. Both types of motor activities were analyzed with regard to their time-structure in synchronization-continuation tapping tasks which involved different task instructions, namely "standard" finger tapping (Experiment 1), "strong" tapping (Experiment 2) requiring more forceful finger movements, and "impulse-like" tapping (Experiment 3) where upward-downward finger movements had to be very fast. In a further control condition (Experiment 4), tapping was omitted altogether. The results revealed a prominent entrainment of spontaneous blink behavior by the manual tapping, with bimanual tapping being more effective than unimanual tapping, and with the "strong" and "impulse-like" tapping showing the largest effects on blink timing. Conversely, we found no significant effects of the tapping on the timing of the eye blinks across all experiments. The findings suggest a functional overlap of the motor control structures responsible for voluntary, rhythmic finger movements and eye blinking behavior.

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.

Endogenous eye blinks in preadolescents: relationship to information processing and performance

Endogenous blinks--those occurring without apparent provocation--are regulated in adults with respect to the presentation, cognitive loading, and response demands of stimuli. This investigation determined the extent to which similar regulatory and response-related relationships were evident in preadolescents during a visual continuous performance task (CPT). As in adults, increased blink incidence on task, longer blink deferral following stimuli with greater cognitive loading, and blink-facilitated motor responses to imperative stimuli were observed. Reaction times significantly decreased when the button press (BP) occurred near (+/- 200 ms) blink onset and increased across the task period on blink-free but not blink-associated trials. More blinks occurred before motor responses in females, and a reaction time (RT) advantage for males on blink-free trials was maintained across blink-associated conditions. From these results, an interpretation is developed arguing that endogenous blinks are a meaningful and integral component of sensory-motor processing, indexing times of facilitated attentional and motor response capability.

Time on task and blink effects on saccade duration

Changes in saccade duration, saccade amplitude and slope of the regression line relating amplitude and duration were assessed during a 2 hour air traffic control simulating task. Mean duration significantly increased as a function of time on task (TOT). Saccade amplitude decreased during the beginning part of the two hour period, increased in the middle and decreased in the final part of task performance. Differences in saccade duration were also assessed for saccades occurring during and independent of eye blinks. When matched for amplitude, electrooculographically measured saccades occurring during a blink were significantly slower than those occurring independent of a blink. Our results suggest caution in interpreting saccade velocity change as an index of 'fatigue' since most of the reduction in average saccade velocity may be secondary to increases in blink frequency.

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.

Clinical and electromyographic features of levator palpebrae superioris muscle dysfunction in involuntary eyelid closure

We report on five patients with involuntary eyelid closure, diagnosed as blepharospasm and referred to use for treatment with botulinum A toxin. Synchronous electromyographic (EMG) recording was performed from the levator palpebrae superioris (LP) and the orbicularis oculi (OO) muscles. In the first two cases, EMG registration showed alternating, semirhythmic dystonic activities in both the LP and OO, clinically perceptible as "flickering" of the eyelids. While the eyelids were lowered, one of them also showed involuntary upper eyelid tractions due to dystonic activities of LP. In the third patient, EMG patterns were characterized by a gradual decrease in the level of LP activity, followed by the contraction of OO, which facilitated the return of LP to its tonic activity, termed "postinhibition potentiation" of LP. In the fourth patient, EMG recording showed involuntary inhibition of LP in combination with blepharospasm. Involuntary closure of the eyelids in the fifth patient was caused by short or prolonged periods of involuntary LP inhibition, whereas OO activity remained normal. Our results provide further evidence that LP muscle activities are regulated by burst-tonic motoneurons, and we suggest that these motoneurons, and/or the input signals regulating their activities, can be involved independently in a pathological process. Clinical symptoms are discussed that may be helpful to recognize those cases with LP motor dysfunction, whether or not accompanied by OO activity disorders. Because the term blepharospasm indicates an abnormal motor function of OO, we propose "blepharospasm-plus" to designate those cases with a combined motor dysfunction of LP and OO muscles.

Suppression of contrast sensitivity during eyelid blinks

Each blink of the eyelids is associated with a concurrent suppression of vision that lasts as long as 200 msec. Saccadic eye movements are also associated with a concurrent suppression of vision. Previous studies suggested that blink and saccadic suppression may be the result of a single mechanism. Volkmann, Riggs, White and Moore [(1978) Vision Research, 18, 1193-1199] demonstrated that saccadic suppression is most evident for low spatial frequency stimuli. However, the effect of stimulus spatial frequency on blink suppression has not been evaluated. If blink suppression and saccadic suppression result from a single mechanism, then blink suppression should also exhibit its greatest effect at low spatial frequencies. The purpose of this study was to determine the effect of stimulus spatial frequency on blink suppression. The stimulus was a sine-wave grating presented at different times after the blink. Psychometric functions were produced from the data for each post-blink, stimulus onset time and a Weibull function was fit to the data to determine threshold. The magnitude and duration of blink induced contrast sensitivity suppression was found to depend on the spatial frequency of the stimulus employed (similar to saccadic suppression). This is further evidence that a single mechanism may produce both blink induced visual suppression and saccadic suppression.

Blinks as an index of cognitive activity during reading

Horizontal and vertical EOG recordings of eye movements were analyzed to determine the spatial and temporal distribution of blinks and the patterns of eye movements (saccades and fixation pauses) exhibited by six subjects during the reading of stories presented in two formats (on paper and on a VDT). The frequency and placement of blinks was not affected by the presentation condition. Blinks were determined to be non-randomly distributed during reading. Significantly more blinks (36%) occurred in conjunction with saccades than the proportion of time consumed by saccades (12%) would predict. Significantly more blinks (36%) occurred in the vicinity of line change saccades, which accounts for 15% of reading time, and with fixation pauses associated with regressions (42%), which accounts for 26% of reading time, than with fixation pauses during normal reading (22%), which accounts for 60% of reading time. The results of the study suggest that blink behavior during reading is under perceptual and cognitive control.

Suppression of sensitivity to change in target disparity during vergence eye movements

It has been demonstrated recently in human psychophysical experiments that sensitivity to surround displacement is suppressed during convergence eye movements. To determine whether sensitivity to changes in target disparity is also reduced, responses to test disparities that were superimposed on standard 4 degrees step disparities were investigated. The test disparities consisted of brief (20 ms) positive and negative pulses as well as steps (in the range of +/- 0.6 degrees). A two-alternative forced-choice procedure was used in which the test disparity was presented in either the first or the second portion of a trial. The results showed that suppression of both test pulse and step disparities began before the start of the convergence movement and continued during the movement. Maximum suppression was about 0.50 to 0.85 log units and occurred between 150 ms before to 50 ms after convergence onset. The differences in sensitivity curves for pulse and step stimuli suggest the presence of different central and peripheral neural factors during vergence eye movements.

Dependence of visual suppression on the amplitudes of saccades and blinks

Visual suppression accompanying voluntary saccades and eyeblinks was measured for a range of amplitudes of both. Saccade amplitudes varied from 2 to 32 degrees and blink amplitudes varied from a slight movement to a full closure of the eye. In every case, thresholds for detecting full-field luminance decrements were determined with the method of constant stimuli and a two alternative forced choice procedure. Results from three subjects show a monotonic increase in the amount of suppression produced by saccades and blinks of increasing amplitude. Data are discussed with respect to theories about the origin of visual suppression.

Blinks, negative flashes, and visual responses

Evoked visual responses that were triggered by spontaneous blinking were recorded from the human subject. Although their waveform resembled that of conventionally recorded responses, two differences were noted: there was an early potential that was seen only with blink responses, and the late negative potential was smaller than that seen with conventional recording. Off- and on-components were seen in the blink responses, and these were compared with those produced by switching the stimulus off and on. The amplitudes of all of the components increased with stimulus intensity. The results suggest that blinking may play a role in vision, per se, as well as acting to protect the cornea.

Vergence eye movements and visual suppression

Visual sensitivity was measured during vergence eye movements in order to determine whether a suppression of vision similar to that associated with saccades is also present during vergence. Suppression was evaluated psychophysically by determining sensitivity to briefly presented, full-field decrements of light in a Ganzfeld. Subjects were rougly 0.5 log unit less sensitive when stimuli were presented at the beginning of a 2-3 deg convergent or divergent eye movement, than during steady fixation. Thus, the concept of saccadic suppression must be broadened to include visual suppression that also accompanies nonsaccadic eye movements. These results support the hypothesis that vision is affected by signals that accompany initiation of oculomotor activity.