Hallucinations are a cue word for schizophrenia and severe mental illness. Yet, in themselves, hallucinations are neither unique to schizophrenia, nor are they uniquely related to ill people. Instead, they are common in other diseases, such as Parkinson’s, and even occur in some people without any mental illnesses. Have you ever heard your name in a place where you knew nobody could have said it? Have you ever had a creepy-crawly feeling without anything bugging you? Or have you ever thought you’ve seen someone you know at first glance, yet realised later it was someone else? Chances are high that you’ve had one of these experiences because hallucinations lie on a spectrum. On one end of the spectrum, people suffering from severe diseases like schizophrenia might hear vicious voices they can’t turn off, that are terrifying and aggressive, while, on the other end, there may be such common everyday misperceptions. In fact, if you regarded my very minor examples as hallucinations, we all might be susceptible to hallucinating to an extent and a recent study published in the renowned journal Science does indeed suggest that this is the case. The study led by researchers Powers, Mathys, and Corlett has looked closely at how hallucinations come to life and suggests they may be failures in how we integrate our previous knowledge with the actual perceptual input.
Our visual system is mesmerising. It is our predominant means of corresponding with the outside world, allowing us to navigate safely through traffic, spotting the mosquito bugging us at night, or reading blogs like this on our phone, tablet or computer. To make things visible, the very first process going on inside our bodies is the transformation of light into neuronal signals on the retinas of our eyes. To do this, our retina is equipped with so-called rods and cones, photoreceptors that turn light photons into electrical signals. Rods are good at sensing light-dark differences while cones come in three different types, each responding best to different wavelengths or colours. Fun side note: Some women have superhuman vision because they carry two recessive genes on each X chromosome resulting in the expression of a fourth type of cone, thus enabling them to perceive additional colours! Overall, each of us has about 90 million rods (mostly in the periphery of our eyes) and 5 million cones (mostly in the fovea, or the centre, of our eyes). Simplifying this, our eyes have an initial resolution of 95 million “pixels”, which is far better than the full HD resolution of 1920×1080 (~2 million) pixels on our TVs.