Our title is the subtile of a new book authored by vision researcher Richard Masland published posthumously in March 2020, several months after his passing.
Born in Philadelphia in 1942, Masland made his way to McGill University to study under D.O. Hebb and earn a Ph.D. in psychology. Hebb of course is well-known for the eponymous Hebbian synapse, the neuron doctrine, and the concept that “cells that wire together fire together” (or vice-versa). Maslsand’s career launched with a solo publication in the journal Science in 1969, showing that prolonged exposure to a rotating stimulus persists up to 20 hours in the visual system, detectable as a persistent illusion of counter rotation. As noted his obituary in the journal Neuron, fifty years later we still don’t know where or how this counter rotation occurs, but Dr. Masland’s cycles back in this book to reconsider what he discovered fifty years ago. One of last decade’s internet sensations, the spinning ballerina, taps into the Masland phenomenon.
Consider these key sets of concepts from Masland’s introduction, and from which the book’s title is derived: “Plasticity is a general rule in the brain – not just in sensory systems. It helps the brain recover from injury, and allows it to allocate extra brain resources to tasks that are particularly important. In vision, the nerve nets of the brain can learn to anticipate the identity of an object in the world – to supplement the raw information coming from the retina with its knowledge of what it has seen before. Boiled down, this means that much of perception is not just a fixed response to the visual scene but it is learned. The brain’s nerve nets recognize certain combinations of features when they see them.”
One of the intriguing elements of Masland’s book is his emphasis on how much visual processing occurs at the retinal level. Before the 2000s, the retina was thought of as a simple neural system, with only a few major cell types. It was quite a revelation for neurobiologists to discover that there were 29 kinds of amacrine cells and 13 types of bipolar cells. Why the need for all this processing power? Because, as Masland notes, the world that you think you see is not the world that actually exists. It has been altered by your retina, fragmented into dozens of different signals for transmission to the brain. The retina parses the visual image into various components sending separate streams of signals about each of them to the brain. Essentially the retina is a microprocessor, like the one contained in your cell phone, but with many different types of neurons doing the processing. The retina’s output is converted by one million retinal ganglion cells, comprised of different types such as sustained and transient, operating through lateral inhibition, whose long axons constitute the optic nerve. The signals sent by the retinal ganglion cells work a lot like the touch signals sent by skin sensory neurons, justification for Merleau-Ponty’s famous quote that vision is the brain’s way of touching.
One of the more enjoyable features of Masland’s book, if you’re into personal vignettes, is the way his portrayals bring notable figures in vision research to life. An example is Adelbert “Del” Ames Jr., who uncovered some of the vagaries in visual perception.
But it was Adelbert Ames III, with whom Masland comported, who provided him with his greatest insights about the retina. Masland writes: “Non-scientists rarely recognize that that the central nervous system includes not only the brain itself but also the spinal cord and the retina. These three structures have the same embryological origin and the same kind of neurons and supporting cells. Most important, all three are behind the blood-brain barrier, a system of coverings that creates a privileged chemical environment, separate from the rest of the body. Most of the neurons of the retina and the spinal cord are bona fide brain cells.
Masland never does get around to explaining the basis for his motion after-effect illusion. But his conjectures on these and other visual topics, such as whether your perception of the color red is the same as mine, provide ample food for thought.