Ed. Note: Some of the content of this blog appeared previously in the "Illusions" column that we write for Scientific American Mind.
Your retinal photoreceptors communicate visual signals to the brain via nerve fibers—more than one million individual wires—bundled in a structure called the optic nerve that exits the back of your eyeball to connect the retina to the brain. If your photoreceptors pointed toward the front of your eye, as any sensible sensory cell should, the nerve fibers would gather at the back of the eye—behind the photoreceptors—and the optic nerve could leave the eye without disturbing your visual field. Yet in the human retina, the circuits connecting the photoreceptors to the brain congregate toward the middle of the eye, so nerve fibers must dive back down through the retina to form the optic nerve. The result is a spot where no visual image can exist. Even though this spot lies surprisingly near to the center of your retina, you cannot see a hole in your vision, because your brain fills it in.
To prove this to yourself, first read this paragraph completely and then try this exercise: Hold out your hands at arm’s length with elbows straight, thumbs touching, and point your two index fingers straight up [see accompanying illustration]. Close your right eye and look at your right fingertip with your left eye. At the same time, pay close attention to your left fingertip. You will notice that it has disappeared into the blind spot of your left retina (if not, rotate your left wrist up and down while maintaining contact between your two thumbs to see your left fingertip disappear). Once your fingertip is gone, notice that you can still see what is behind it! Now be honest with yourself, do you have x-ray vision in your blind spot, or are you blind in your blind spot? Assuming that you agree that you are indeed blind in your blind spot, we can now analyze how the brain fills in the hole to understand how filling in works. Notice that the filled-in area looks like the area immediately surrounding your blind spot: your brain fills in the hole with the nearest visual information available. Yet the algorithm is not smart enough to fill in your finger.
New research, published in the journal Current Biology last August, has now shown something that many vision scientists would have thought previously impossible: that a mere 8 days of training can reduce the functional size of the physiological blind spot.
The authors of this study, led by Paul A. Miller of the University of Queensland, Australia, trained 10 experimental subjects to indicate the direction of moving stimuli shown in the blind spot’s periphery. These visual stimuli consisted on drifting sinusoidal waveforms that were presented in a ring centered on the physiological blind spot of one of the eyes.
The training, which took place over 20 consecutive weekdays, increased the subjects’ sensitivity to both direction and color, therefore demonstrating generalization to visual domains (color) outside the immediate training (motion direction). The improvement did not extend to the blind spot of the untrained eye, however, indicating that that the results were not due to a general practice effect.
The estimated extent of the blind spot, previous to the training, was 24 degrees of visual angle (think of 1 degree of visual angle as the size of your thumbnail at arm’s length). By the end of the training, the blind spot had shrunk to 21 degrees of visual angle. The benefit manifested very rapidly, with 88% of the improvements requiring only 8 days of training.
Visual neurons have localized “windows” onto the visual world called “receptive fields.” The scientists speculated that the training must have increased the responses of neurons with receptive fields that abutted or partially overlapped the blind spot, thereby enhancing weak signals originating primarily from within the region of functional blindness. They also proposed that similar training regimes may prove useful in cases of localized blindness resulting from damage or injuries to the visual system.