Have you ever anxiously looked at a clock on the wall repeatedly and noticed that the second hand seemed to be frozen? Maybe you were nervously awaiting the poll closing time on Tuesday. Well, there is a scientific reason that hand refuses to move. I noticed this a long time ago when I used to make my eyes dart up to the clock in class, but I thought it was just my imagination that the second hand froze for a few moments before resuming its movement. Not until recently did I learn that this is an actual thing everybody experiences called chronostasis, or the stopped-clock illusion. Yes, it even works with digital clocks--in fact, it's not limited to clocks at all. Whether or not you notice it, you are "losing" many minutes per day to chronostasis.
Long story short, chronostasis happens because our brains block out the blur that occurs when our eyes make rapid movements (called "saccades"). This process is called saccadic masking. To replace the moments that were "lost" to unprocessable blur, the brain fills in the gap with the very next image you see (such as the second hand). This results in the illusion that the second hand is frozen in place for a few moments. The process:
1. The eyes receive information from the environment regarding one particular focus. This sensory input is sent directly to the visual cortex to be processed. After visual processing, we consciously perceive this object of focus. In the context of a student in a classroom, the student’s eyes focus on a paper on their desk. After his eyes collect light reflected off of the paper and this information is processed in his visual cortex, the student consciously perceives the paper in front of him.
2. Following either a conscious decision or an involuntary perception of a stimulus in the periphery of the visual field, the eyes intend to move to a second target of interest. For the student described above, this may occur as he decides that he wishes to check the clock at the front of the classroom.
3. The muscles of the eye contract and it begins to quickly move towards the second object of interest through an action known as a saccade. As soon as this saccade begins, a signal is sent from the eye back to the brain. This signal, known as an efferent cortical trigger or efference copy, communicates to the brain that a saccade is about to begin. During saccades, the sensitivity of visual information collected by the eyes is greatly reduced and, thus, any image collected during this saccade is very blurry. In order to prevent the visual cortex from processing blurred sensory information, visual information collected by the eyes during a saccade is suppressed through a process known as saccadic masking. This is also the same mechanism used to prevent the experience of motion blur.
4. Following the completion of the saccade, the eyes now focus on the second object of interest. As soon as the saccade concludes, another efferent cortical trigger is sent from the eyes back to the brain. This signal communicates to the brain that the saccade has concluded. Prompted by this signal, the visual cortex once again resumes processing visual information. For the student, his eyes have now reached the clock and his brain’s visual cortex begins to process information from his eyes. However, this second efferent trigger also communicates to the brain that a period of time has been missing from perception. To fill this gap in perception, visual information is processed in a manner known as neural antedating or backdating. In this visual processing, the gap in perception is “filled in” with information gathered after the saccade. For the student, the gap of time that occurred during the saccade is substituted with the processed image of the clock. Thus, immediately following the saccade, the second hand of the clock appears to stop in place before moving.
It is estimated that about 40 minutes of every day is "lost" due to saccadic masking. We're essentially blind for 40 minutes per day, and we don't even notice. Pretty cool.
Now, keep your eyes still and tell us what you want to kibitz about tonight.
