Illusions of Motion
A new study has found that people will hallucinate that objects are moving when a certain area of their visual cortex is stimulated.
Though previous research has shown that visuocortical stimulation can cause people to see lights and colors, this study – published in the journal PLoS ONE – is the first to induce the illusion of motion in human brains, then directly detect that illusory perception with brain scanning technology:
Our results show, for the first time, that altering neural activity … by electrical charge delivery is sufficient for producing complex positive illusions of visual motion.
I’ve gotta admit that I have a real soft spot for these “Let’s Poke People’s Brains to Make Them Hallucinate” studies. Mainly, I’m fascinated by how easy it is to hack the visual pathway – just try looking as far to the left as you can, while gently pressing and wiggling your fingertip against the right side of your left eye. Instant hallucination! Isn’t neuroscience fun? Well, anyway, let’s look at the data.
A team led by Dr. Josef Parvizi of Stanford University’s Department of Neurology and Neurological Sciences began by using fMRI and intracranial electrocorticography (ECoG) to study patterns of activation related to moving (as opposed to static) visual stimuli in three subjects. The reason for the small sample group is that patients with electrodes implanted in area MT are extremely rare.
The researchers found that responses to movement were mainly correlated with activity in the posterior inferior temporal sulcus, a.k.a. area MT. (When scientists are talking about humans as opposed to other animals, area MT is called area hMT+ or the human MT complex.) Area hMT+ sits right on the border between the visual cortex and the temporal lobe – which is why it’s described both as “temporal” and as “layer V5 of the visual cortex.”
There’s plenty of previous research demonstrating that area MT responds to visual motion and helps coordinate the rapid eye movements known as saccades. The Stanford team went a little further, though, and mapped patterns of hMT+ activity associated with frequency of movement, inward vs. outward movement, and a few other factors. This way, they could correlate the subjective reports of their volunteers with objective fMRI data.
And correlate they did. By applying electrical stimulation to area hMT+ in patients, the scientists were able to induce a variety of “illusory percepts” (i.e., hallucinations):
Electrical stimulation of right hMT+ in subject A caused displacement and transposition of the entire visual field to the left … While looking at the experimenter’s face, [another] subject reported that “the top right corner of the face is vibrating” … The subjects volunteered their descriptions readily and seemed to be completely captivated by the intensity of the experience.
In other words, these hallucinations seemed as real as anything else the subjects could see.
The scientists also confirmed these subjective reports with several types of hard neurological data – they checked that telltale hMT+ activation patterns were present via fMRI and ECoG. This enabled them to confirm, for the first time in the history of science, that a little hMT+ stimulation is all that’s needed to make people see illusory motion:
In the context of previous research, our results show that the hMT+ network circuitry is both necessary and sufficient for producing conscious motion percepts.
This is just one more way of showing that – even when it comes to our perceptions of external reality itself – what you see isn’t necessarily what you get.