Make your brain hurt

 Which horizontal line is the shorter one? 

    Ehh, wrong! Go ahead, line them up and you’ll see they’re the same size. This is the Muller-Lyer illusion.

    Now try this one. Focus your eyes on the center dot and move your head closer to the screen or away from it without taking your eyes off the center. Why are the circles counter-rotating? The speed at which the circles rotate also depends on your motion. Perhaps it's the angled shapes or the black and white edges that's confusing our brains. But we know that there is no real motion, so what is going on? 

This image is called the Pinna-Brelstaff Illusion and it’s a great example of rotation perception in the absence of real motion. The visual areas mediating false perceptions in illusions are not well identified, but there are associations of both primary visual systems and higher-level processing (Tabei et al., 2015). Complex motion patterns such as rotation, expansion and contraction are encoded by a class of neurons in the dorsal region of the medial superior temporal area (MSTd) and receive signals from the medial temporal region (MT) which is sensitive to global motions (Tabei et al., 2015). An fMRI study done on macaque monkeys shows that rotation-sensitive neurons in the MSTd also encode illusionary motion, but there is a delay of about 15 milliseconds to distinguish the illusion reliably (Luo et al., 2019). 

Let's go back to the image. The earliest cortical stage of visual processing occurs in the primary visual cortex (V1) and these neurons are stimulated by their preferred orientation through its receptive field (Pinna, 2008) . In the Pinna figure, they’re stimulated by the implicit orientation of the rhombuses in each concentric layer, giving it “direction.” Although this seems to explain the basis of this illusion, the rotational motion is processed by higher cortical areas such as MT and MSTd (Pinna, 2008). 

There is obviously a disconnect and in this case our brain fails to re-create the physical world. Here are more illusions for funsies. 

Tabei, K.-ichi, Satoh, M., Kida, H., Kizaki, M., Sakuma, H., Sakuma, H., & Tomimoto, H. (2015). Involvement of the extrageniculate system in the perception of optical illusions: A functional magnetic resonance imaging study. PLOS ONE, 10(6). https://doi.org/10.1371/journal.pone.0128750 

Luo, J., He, K., Andolina, I. M., Li, X., Yin, J., Chen, Z., Gu, Y., & Wang, W. (2019). Going with the flow: The neural mechanisms underlying illusions of complex-flow motion. The Journal of Neuroscience, 39(14), 2664–2685. https://doi.org/10.1523/jneurosci.2112-18.2019 

Pinna, B. (2009). Pinna illusion. Scholarpedia, 4(2), 6656. https://doi.org/10.4249/scholarpedia.6656 

Alvarez, B., & Laliberte, M. (2022, September 30). 30 optical illusions that will make your brain hurt. Reader's Digest. Retrieved October 17, 2022, from https://www.rd.com/article/optical-illusions/