The fastest sense on record is proprioception (Fadiga and Pupilli 1964; Fuchs and Kornhuber 1969; Schubert and Minor 2004; Snider and Stowell 1944) but the vestibular sense is comparable (Miles and Lisberger 1981; Shinoda and Yoshima 1975). It takes 3 ms for a proprioceptive signal to reach the central nervous system at the cerebellum (Fuchs and Kornhuber 1969). We propose that the proprioceptive-visual illusion caused by a perturbation of the ocular proprioceptors, which abruptly shifts the position of a visual image (Quinet and Chen 2011; Roll and Roll 1987; Roll et al. 1991; Valey et al. 1994, 1995, 1997), is supported by cerebellar oculo-skeletal postural circuits that store the efference copy of all body positions per context via the Purkinje neurons (Cullen 2015; De Zeeuw 2021; Fukutomi and Carlson 2020; Keller et al. 1983; Loyola et al. 2019; Miles and Lisberger 1981; Noda et al. 1991; Shadmehr 2020; Tehovnik et al. 2021; Wang et al. 2023). Once overtrained on a task, the neocortex immediately relays the context of a situation to the cerebellum.

Many visual illusions are dependent on where the fovea falls on a visual image (Schiller and Tehovnik 2015). When viewing an image depicting Escher’s flock of birds (Figure 1), where one looks will establish the direction of bird flight, left for leftward flight and right for rightward flight. A similar principle applies to the next image (Figure 2): depending on how one fixates the image will determine whether the image is a pair of faces or a vase. The next image is more difficult to discern (Figure 3): it is easier to view the 3-dimensional cube than the 2-dimensional pinwheel. As well it is easier to view the young girl than the elderly woman (Figure 4): to view the young woman fixate the central part of the image and to view the elderly woman fixate the lower part of the image. With all the images, a visual cortex is required for processing the visual details, but where one puts the eyes will establish the percept that dominates. Given that the cerebellum encodes eye position it would only follow that this part of the brain must be employed to establish a rapid transition between the images. Also, if while viewing the images one is unsuccessful at transitioning smoothly between them, this means one will need to work on the weak image by updating their efference copy representation for the weak image through further viewing.

If anyone has ever had too much alcohol to drink one may have experienced vertigo such that the entire room is made to spin around. This illusion is caused by the alcohol diluting the fluid within the vestibular canals, thereby changing the nature of the vestibular signal being sent to the central nervous system, including the cerebellum. This experience is not that different from the vertigo experienced by returning astronauts who must re-adapt to the earth’s gravitational field (Carriot et al. 2021; Demontis et al. 2017; Lawson et al. 2016). In both cases, the cerebellum is receiving an altered signal from the vestibular apparatus which causes vestibular illusions (Carriot et al. 2021). On this point, it is believed that John F. Kennedy Jr. crashed his airplane killing everyone on board because he misread his vestibular signals that the plane was increasing in elevation which compelled him to fly his plane into the ground. It is for this reason that pilots always need to pay attention to a plane’s instrument panel that provides the speed, altitude, and orientation of a plane.

Hebb (1969) observed that if one dons a prism that curves a line, over time that line becomes straight through adaptation—and according to him this is because the motor system has the final say in how we experience the world (also see: Held and Hein 1963; Yarbus 1967). That the cerebellum is central to prism adaptation is well accepted (Thach et al. 1992). It is the cerebellum that provides an estimate of how the neocortex should ultimately interact with the physical world through the motor system by synchronizing all the senses to conform to a common motor output. So, if a line appears curved as specified by a prism, but all the other senses register that the line is in fact straight (see Footnotes 1&2), as when one feels the trajectory of the line with one’s hands, for example, then the cerebellum must update the impression of straightness experienced by the visual cortex. In short, the function of the cerebellum is to keep the neocortex calibrated to all the sensory-motor realities of the world. This surely will be a challenge for those such as Elon Musk and Jeff Bezos planning to live on other planets, whose gravitational and chemical environments are nothing like that of earth.

Footnote 1: That the mammalian brain contains a preponderance of visual neurons that are tuned to straight lines is well-established (Dräger 1975; Hubel and Wiesel 1977), an idea that was anticipated by studies of the fortification illusion—which is composed of whitish intersecting lines—that are experienced when an epileptic event starts at the foveal representation of the occipital cortex and spreading rostrally (Grüsser 1992; Lashley 1941). The encoding of straight lines is furthermore a characteristic of somatosensory neurons (Bensmaia et al. 2008; Yau et al. 2013). And computing the shortest distance to a sound source also requires an appreciation of straightness by the auditory system (Middlebrooks et al. 2002).

Footnote 2: The property of straightness already exists at the single-cell level of amoeboid organisms (Reid and Beekman 2013).

Figure 1: The Escher flock-of-birds illusion. (auto_245.gif)

Figure 2: The face-vase illusion. (auto_247.jpg)

Figure 3: The cube-pinwheel illusion. (auto_246.jpg)

Figure 4: The young-vs.-old-woman illusion. (auto_248.jpg)