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.

INTRODUCTION

By shining a light through the roof of the mouth of human subjects to permit consistent retinal illumination independent of eyelid position, it has been determined that there is a reduced sensitivity to light around the time of blink onset (Volkmann et al. 1980). However, it is not clear how a reduced sensitivity to light during a blink correlates with the apparent continuity of a detailed perception in the absence of direct visual stimulation. Because blinking occurs so frequently, and because the continuity of perception across a blink is so robust, investigating this mechanism may be of particular utility in determining perceptual mechanisms.

It has been proposed that reverberating activity in the geniculo-cortical feedback loop could fill in the temporal gap that occurs during a blink (Billock 1997). 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, and4) during an external darkening [the “negative flash” (Armington and McCarthy 1984)] when an electrooptical shutter occluded the entire scene.

METHODS

All experimental procedures and care of the animals were carried out in compliance with guidelines established by the National Institutes of Health and were approved by the UAB Animal Care and Use Committee.

Single-unit recordings were made from V1 cortical supragranular complex cells in two rhesus monkeys (Macaca mulatta) (Gawne et al. 1996). The animals were trained to fixate on a spot of light on a computer screen, and stationary stimuli were flashed on one at a time centered in the receptive fields of the neurons. Each stimulus condition was presented once in random order, then the order was randomly shuffled, and the stimuli were again presented once each, and this was repeated for a minimum of 40 cycles. Eye position was monitored with a video eye tracking system (ISCAN Inc.) running at 240 Hz that could detect the onset of a blink to within a 4-ms period. The ISCAN system had a fixed processing delay of 12 ms, which was subtracted out before any analysis was performed. Stimuli were optimally oriented, either a static white bar or a static square-wave grating on a gray background (background luminance 33.1 cd/m2, bar and grating luminance 213.0 cd/m2, contrast 75%). They were presented either by turning them on and leaving them on for either 10 or 50 video frames (at 67 Hz, 150 or 750 ms), by turning them on and briefly replacing them with the background gray for 10 video frames (150 ms), by turning them on and then inducing a reflex blink with a very gentle air puff, or by using a pair of electrooptical shutters (Displaytech) to completely darken the entire scene for 200 ms [negative flash (Armington and McCarthy 1984)]. The shutters were transparent to the near-IR light used by the video tracker, so that changes in eye position or unwanted blinks could be detected even when it was opaqued to visible light. Because the three different methods of interrupting the visual stimulus (gap, blink, and external darkening) were so different in kind, it was not practical to get them to occur at precisely the same delays after stimulus onset; they all fell within 140 to 360 ms after stimulus onset, a period when the average firing rate of these neurons was relatively constant.

The end of a blink was often associated with a small (<1°) downwards eye movement that caused the bar stimulus to be mispositioned on the receptive field. Therefore, for 23 neurons we used a grating stimulus that covered a circular 3° patch composed of parallel white lines 0.06° wide spaced 0.4° apart. For each trial the grating was presented in the same location but at a different spatial phase (lines offset in the direction perpendicular to their orientation). For these complex cells with receptive fields of this eccentricity (range: center from 4 to 5° from the point of fixation), equally strong responses were elicited for all spatial phases. This served to ensure that the visual stimulus at the end of a blink was approximately the same as that at the onset of a blink.

Trials where the animal did not blink in response to the air puff, or did blink during any other condition, were discarded. Only recordings where there were at least 30 valid trials for each condition were included in the analysis. Smoothed poststimulus histograms were constructed by convolving the individual spikes in a response with a Gaussian pulse ς = 3 ms, and then averaging across trials.

RESULTS

When a stimulus is flashed on the receptive fields of these neurons, the average firing activity exhibits a brief burst of activity which then drops to a roughly constant sustained rate and decreases when the stimulus is turned off (Fig. 1,A and B). When the animal blinks, the neuronal activity decreases (Fig. 1C); aligning the responses to blink onset shows that, after a brief delay, neuronal activity decays rapidly to a relatively low level (Fig. 1D). Aligning the responses around the end of a blink shows that when the eyes reopen, the average activity rapidly increases back to a relatively high level (Fig. 1E). Mean blink duration was 192 ± 54 standard deviation (SD) ms, minimum 101 ms and maximum 338 ms. Interrupting the presentation of the stimulus with a gap (Fig. 1F) or with the darkening of an electrooptical shutter (Fig. 1G) shows declines in activity similar to that seen with blinks, although the rate of decay of the average activity is slower for gaps or external darkenings than it is with blinks. Time for average activity in response to the grating stimuli to decline to half-sustained level is 52 ms after blink onset, 92 ms after gap onset, and 88 ms after onset of the external darkening. For the 40 neurons where the single-bar stimulus was used, the time for the average activity to decline to half-sustained was 53 ms after blink onset, 81 ms after gap onset, and 97 ms after onset of the external darkening. This delayed activity for the external darkening seems similar to that seen previously in human-evoked potential recordings (Armington and McCarthy 1984). Fig. 1.Average time course of the responses of the 23 neurons for which the grating stimuli were used. Stimulus timings are represented by horizontal bars under the abscissa. Shaded gray regions around the black line are the standard error of the means for individual neurons about the overall mean. A: stimulus turned on for 10 video frames (67 Hz display, 150 ms), then turned off.B: stimulus turned on for 50 video frames (750 ms)C: condition identical to B, but with an elicited reflex blink. Timing and duration of blinks was variable: horizontal black bars represent the ranges of blink onsets and offsets.D: data from C, aligned about the onset of a blink (vertical bar). E: data fromC, aligned about the end of a blink (vertical bar).F: stimulus interrupted for 10 video frames (150 ms) with uniform gray background. G: external darkening via electrooptical shutters which occluded the entire scene for 200 ms (“Negative Flash”). There was some weak modulation of the averaged response at the 67 Hz display rate.

The responses for the 40 neurons stimulated with a single white bar were similar to those shown in Fig. 1. However, we could not accurately determine the time course of the response as we could when using the wide-area grating stimuli for the end of a blink because of small shifts in eye position. Figure 2 shows the average firing activity for all 63 neurons (grating and bar) for a 100-ms period starting 70 ms after the stimulus was interrupted. During a blink, the average activity declines even more than during a gap or an external darkening (t-test, P < 0.05). Not only is there no active interpolation of neuronal activity during a blink, but the responses appear to be actually suppressed. Presumably this suppression corresponds with the decreased sensitivity to diffuse light during a blink seen previously (Volkmann et al. 1980), but if a blink results in even more of a decrement of neuronal activity than an external darkening, then how is it that the blink is not more, rather than less, noticeable? Fig. 2.Average firing rate for all 63 neurons (combined bar and grating stimuli) for (left to right) the sustained baseline from 300 to 400 ms after stimulus onset, the 100-ms period starting 70 ms after blink onset, the 100-ms period starting 70 ms after the onset of the gap where the stimulus was turned to gray, and the 100-ms period starting 70 ms after the electrooptical shutter occluded the display.

There was some diversity in the responses of individual neurons that was not clearly evident in the averaged activity. Figure3A shows a scatterplot of the strength of the responses in the 100-ms interval starting 70 ms after stimulus interruption for the blink versus external darkening condition. For this epoch, 24 of the 63 neurons had significantly different responses for the blink versus external darkening (t-test, P < 0.05). As expected from Fig.2, the responses after blink onset are generally less than those for the external darkening. Panels B–D show three examples of the responses of individual neurons where the external darkening resulted in strong transient off-responses that were not present when the visual scene was interrupted by a blink. This was a common pattern, with 14 of the 63 neurons showing this clear modulation of transient off-responses. (The transients are not always reflected in the spike counts because of the width of the integration interval.) However, we found one neuron that had a strong off-response to the blink but not to the external darkening (Fig. 3E). This neuron had a transient pulse of activity in response to a blink at the time where other neurons often had a pulse in response to the external darkening (see C immediately above), but it also had an even larger pulse that occurred very early, almost immediately after the onset of the blink. Fig. 3.A: plot of the mean spike rate for the individual 63 neurons in this study, blink versus external darkening, for the time interval used in Fig. 2. B–E: examples of the responses of single neurons to (top of each subpanel) the onset of a blink and (bottom of each subpanel) the onset of an external darkening. The shaded gray areas are the standard error of the mean for the data of each neuron. Time of onset of the blink and the external darkening were aligned. In panelsB–D, there is a strong transient response for the start of an external darkening that is suppressed in the onset of a blink. In panel E there is a strong transient response to blink onset and none for an external darkening.

DISCUSSION

A blink is extremely rapid, reaching velocities of 2400°/s in macaques (Fuchs et al. 1992). Because of the Stiles-Crawford effect (Stiles and Crawford 1933), it is primarily the time that it takes for the lid to transit the central region of the pupil that is important. It can take <4 ms for the lid to cover the central 4 mm of the pupil! Additionally, because the lid is so far removed from the plane of focus, it does not cause a sharp dark shadow to fall across the retina as it closes, but instead causes a relatively uniform darkening. Finally, the visual system is not very sensitive to short (<15 ms or so) changes in the detailed dynamics of the onset of a visual stimulus. For example, while in this study we saw a weak modulation of averaged firing with the 67 Hz video display (15-ms time between light pulses from the phosphor), such effects are generally never seen for higher rates. Thus while an external darkening via electrooptical shutter does not create precisely the same pattern of darkening as a natural blink, the differences between the two conditions are small and seem unlikely to have significantly affected our results.

It has been proposed that the presence or absence of transient on- and off-responses in V1 cortical neurons correlates with the psychophysical phenomena of forward- and backward-masking (Macknik and Livingstone 1998). [More generally, it has been proposed that transient bursts of activity by neurons could be of particular significance for the functioning of the nervous system (DeBusk et al. 1997; Lisman 1997; Martinez-Conde et al. 2000).] Conceivably a similar situation occurs during blinks, where a suppression of visually evoked neuronal activity also suppresses those specific transient responses that are normally generated by a sudden darkening of a visual scene.

It is reasonable to assume that accurately representing the detail of a complex visual scene requires the activity of a large number of neurons. However, detecting gross changes in a visual scene (e.g., darkening) in principle requires only a relatively small number of neurons. Therefore, it may be that there are relatively few (although still large in absolute number) neurons specialized to detect gross changes and to signal whether or not the change is important. Neurons of different classes, such as those in Fig. 3, B–D, and Fig. 3E, may play complementary roles in this process; just because they are not common does not mean that they do not play a critical role in perception.

The suppression of sensitivity to saccadic eye movements has similar psychophysical properties to the suppression seen in blinks, suggesting that the same mechanism might be involved in both cases (Ridder and Tomlinson 1993, 1995, 1997). In one recent study it was found that, while microsaccades cause depression of activity in V1, they cause an excitation in areas V2 and V4 and no appreciable change in IT cortex (Leopold and Logothetis 1998). This suggests that the pattern of response for blinks that we found in V1 might not hold for the extrastriate cortical visual areas. [However,Martinez-Conde et al. (2000) found modest excitation in V1 in response to microsaccades, so this is not clear.] Nevertheless, the results of this paper demonstrate that the apparent continuity of visual perception during a blink does not depend on continuity of neuronal activity in all of the neurons in visual cortical area V1, but may instead rely on a suppression of those specific transient pulses which signal that something has changed.

The authors acknowledge the technical assistance of M. Bolding, D. Crossman, and J. Millican and the advice of A. Dobbins and P. Gamlin. This work was supported by McDonnell-Pew Program in Cognitive Neuroscience Grant 96-27 and National Eye Institute Grant EY-11552-01.

FOOTNOTES