Finding Your Blind Spot and Perceptual Filling-in

Figure 3. Perceptual reports: What participants report seeing (shown in the right column) when the star in the left column images disappears in the blind spot. The reports reveal several principles of how the brain creates perceptual experience. Most broadly, in all cases, the brain fills-in the blind spot as the most likely content given the local context.

Figure 3 visualizes what people report seeing when shown the stimuli in Figure 2, that is, what they report seeing once the star disappears because it occupies their blind spot.

The reports revealed several principles of how the brain creates perceptual experience. The top stimulus revealed that the brain tends to expect uniformity in surface color, thus filling-in the blind spot to match the yellow background in the remainder of the image. The middle stimulus revealed that the brain looks for patterns, here causing perceptual experience to include a red circle in the blind spot in order to complete the prevailing pattern in the region. The bottom stimulus revealed that the brain implements expectations about object continuity. When the star occupies the blind spot, observers see a solid green rectangle, filling-in the missing middle portions of that object. Thus in general, the brain fills-in experience with the most likely stimulus to occupy the blind spot. Whatever is most likely given the context is what the brain causes people to perceive.

Because the brain fills-in perception within the blind spot, one application involves studies that seek to identify the brain areas involved in producing conscious experience. For example, for a long time, researchers were uncertain whether the earliest part of human visual cortex, called V1, was involved directly in the production of conscious experience. To address this issue, researchers used fMRI (Functional Magnetic Resonance Imaging) to measure neural responses in the monocular regions of V1 that mapped to the blind spot.¹ In other words, they looked at neural regions that only received input from one of the eyes, and by mapping the blind spots of their participants, they were able to look at neural responses specifically when direct stimulation was absent, because the stimulus was placed in the relevant eye's blind spot. What should happen? Would the neurons not fire because they were not receiving any signal from the outside world? Instead the neurons responded as though they were being stimulated by what the person perceived in that location-they responded as though the images that the brain filled-in were actually producing external stimulation. This suggested that these neurons in very early parts of the visual system are not only involved in transmitting signals received from the retina, but also in constructing perceptual experience itself.