Blinks, negative flashes, and visual responses

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.

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.

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.