What does it mean for something to be visible? Light hits the retina and you see it, right? Well, no, actually. Any number of illusory effects can result in failed detection of an otherwise visible retinal image. That image might also look different from reality. Or you may see something that is not even there. That’s why we usually describe illusions as perceived events that do not match the physical reality. But lately, Susana Martinez-Conde and I have been thinking that the definition may be too restrictive. Perception may not be the right word for non-sensory illusions. Examples of these include illusions of memory: you remember someone’s name incorrectly. Or illusions of logic: if you play the same slot machine over and over, your chances of hitting a jackpot on the next pull should increase, right? Wrong. Neuroscientists call these and other similar effect cognitive illusions. Can we really say that the physical reality doesn’t match perception in those cases? Or is it more accurate that the brain chose (term used lightly here without necessarily connoting intention) to bring certain ideas to our conscious awareness in lieu of others?

In addition, a given sensory experience can lead to one or more very different perceptions (at different times or in different contexts) due to the way that we (for example) attend to specific aspects of the sensorium. Our specific memories or cultural contexts can color how we see the same exact information (example: the young woman who pulled down the Confederate Flag in South Carolina last weekend is perceived by some as a hero, and by others as a criminal).

What about inputs to the brain that challenge our idea of what constitutes a sensory information?

A recent field of endeavor has expanded neuroscience to the lumen of the gut. The microorganisms eating our poop somehow affect our brain through a mechanism that Mark Lyte of Texas Tech Health Science Center (Abilene, TX) refers to as psychobiotics. Peter Andrey Smith wrote an article last week in the New York Times, which I recommend, on the topic of Lyte’s long-term research program to discover the role of our microbiome in behavior and disease. To paraphrase, the microbes inside your gut can affect your thinking processes—which means that we are all shitheads at least a little bit. In all seriousness, our perception and cognition depend, at least to some extent, on what the mindless bacteria in our intestines tell our brain how to feel. 

So is the output of the critters in your gut the physical reality or the perception in our illusion equation? They can affect your mood and so change the way you perceive a number of cognitive events (that’s what “mood” means), so they modulate your perception of the physical reality, thereby either deepening or lessening our illusory perception, or perhaps changing it with affecting the degree of accuracy of the perception.