The human visual system is a remarkably complicated thing, although in spite of – or perhaps because of – its complexity, it is capable of making some pretty major mistakes. This fallibility is playfully exposed by optical illusions, which are often very simple in design yet completely deceptive to our brains, causing us to see things that aren’t there. According to a new study in the Journal of Neuroscience, this occurs because the neural pathways that feed into the visual cortex get themselves tied up in knots, creating “feedback” loops that supplement the external stimuli seen by the eyes.

The visual cortex is the part of the brain responsible for processing visual information, and is stimulated by a pathway of neurons originating in the eyes themselves. When light hits the retina, signals are sent along the optical nerve, through the thalamus and into the visual cortex.

This type of sensory stimulation generates what is known as “bottom-up” processing, meaning the chain of events begins with the external stimulus before progressing upwards through the various stages of mental processing, culminating in a visual experience.

However, the visual cortex also receives “feedback” from other areas of the brain when we see something familiar. This creates “top-down” processing, whereby information about what we are looking at travels down to the visual cortex from higher cortical regions, generating a visual experience that is internally created. In other words, it’s all in your head, and is not a reflection of the real world.

The advantage of top-down cognition is that it allows the brain to fill in the gaps in our visual field by drawing on previous experiences and understandings of what the world should look like, generating the visual image that it expects to see.

^{The upside-down triangle you see in this illusion is an example of top-down cognition in action. Carnegie Mellon University}

Normally, these two processes complement each other seamlessly, although optical illusions reveal how easy it is to trick the brain into seeing something simply because it expects to see it, even if it isn’t actually there.

While this information is not new, researchers from Carnegie Mellon University have now taken our understanding of this phenomenon a step further by determining exactly how much of what we see comes from internal feedback, and how much is a reflection of external visual stimuli.

To do this, they used a technique called optogenetics to silence the transmission of information from a brain region called the lateral medial area (LM) to the visual cortex in mice as they looked at lines moving across a screen. This pathway has previously been identified as one of the major feedback routes affecting what we see, as information about what we are looking at travels from the LM to the visual cortex.

Measuring the activity of neurons in the mice’s visual cortices, the researchers found that silencing this pathway reduced their stimulation by about 20 percent, suggesting that roughly a fifth of what the mice were “seeing” actually came from within their brains rather than the external environment.

Commenting on this finding, lead researcher Sandra Kuhlman explained that learning how bottom-up and top-down processing combine to create an overall experience of reality “represents a new way to study visual perception and neural computation.”