Filled-in Blindspots Perceived As More Real Than Authentic Perception
To make sense of the world around us, we must combine information from multiple sources while taking into account how reliable they are. When crossing the street, for example, we usually rely more on input from our eyes than our ears.
However, we can reassess the reliability of the information: on a foggy day with poor visibility, we might prioritize listening for traffic instead.
But how do we assess the reliability of information generated within the brain itself?
We are able to see because the brain constructs an image based on the patterns of activity of light-sensitive proteins in a part of the eye called the retina. However, there is a point on the retina where the presence of the optic nerve leaves no space for light-sensitive receptors.
Blind Spot Perception
This means there is a corresponding point in our visual field where the brain receives no visual input from the outside world. To prevent us from perceiving this gap, known as the visual blind spot, the brain fills in the blank space based on the contents of the surrounding areas.
While this is usually accurate enough, it means that our perception in the blind spot is objectively unreliable.
To find out whether we are aware of the unreliable nature of stimuli in the blind spot, researchers at the University of Osnabrück led by Benedikt V Ehinger, presented volunteers with two striped stimuli, one on each side of the screen. The center of some of the stimuli were covered by a patch that broke up the stripes.
Summary and overview of blind spot effects. Posterior GLMM-effect estimates of all data combined (black) except experiment 4 (inversed task). Also shown for each experiment the 95% CI of bootstrapped means summary statistics of the data (yellow). Next, shown difference values between the blind spot and all other control locations (model dark, raw data pink). As discussed in the text, the control locations outward, inward and above do not differ (fourth last to second last row), and thus compare the blind spot effect to all locations combined (last row). DOI: http://dx.doi.org/10.7554/eLife.21761.010
The volunteers’ task was to select the stimulus with uninterrupted stripes.
The key to the experiment is that if the central patch appears in the blind spot, the brain will fill in the stripes so that they appear to be continuous. This means that the volunteers will have to choose between two stimuli that both appear to have continuous stripes.
If they have no awareness of their blind spot, we might expect them to simply guess. Alternatively, if they are subconsciously aware that the stimulus in the blind spot is unreliable, they should choose the other one.
In reality, exactly the opposite happened: the volunteers chose the blind spot stimulus more often than not. This suggests that information generated by the brain itself is sometimes treated as more reliable than sensory information from the outside world.
Future experiments should examine whether the tendency to favor information generated within the brain over external sensory inputs is unique to the visual blind spot, or whether it also occurs elsewhere.
The study’s authors write:
“Why do subjects choose the blind spot location when it is objectively the least reliable? Our interpretation takes the results at face value: subjects must possess at least implicit information about whether a percept originates from the blind spot in order to show a bias for or against it.
At the same time, the veridical information from the other stimulus is also available. This indicates that perceptual decision-making can rely more on inferred than veridical information, even when there is some knowledge about the reduced reliability of the inferred input available in the brain.”