When it comes to the Principles of Neural Science, no one can hold a candle to Kandel, its sixth edition just published two months ago. I like this description of the book, as found on the bn.com website: “For more than 40 years, Principles of Neural Science has helped readers understand the link between the human brain and behavior. As the renowned text has shown, all behavior is an expression of neural activity and the future of both clinical neurology and psychiatry is dependent on the progress of neural science.”
The new edition of the now classic text is massive at 1646 pages, and the homeless gentleman who wanders into The Green Planet each morning marveled that someone would lug the book around to sift through its chapters. Though the connection of the book’s material to clinical neurology and psychiatry is readily apparent, the gems that pertain to vision are no less germane.
There are five chapters dedicated entirely to vision, but as is the case for vision being pervasive in the brain, it is pervasive throughout the entire textbook. Before sharing the flavor of those five chapters with you, consider this statement from the opening of Chapter 3 on Nerve Cells, Neural Circuitry, and Behavior: “The remarkable range of human behavior depends on a sophisticated array of sensory receptors connected to the brain, a highly flexible neural organ that selects from among the stream of sensory signals those events that are important for the individual. The brain actively organizes sensory information for perception, action, decision-making, aesthetic appreciation, and future reference – that is to say, memory. It also ignores and discards information judiciously, one hopes, and reports to other brains about some of these operations and their psychological manifestations.”
From Chapter 21, titled The Constructive Nature of Visual Processing, comes the following: “A potentially unifying insight reconciling the visual system’s remarkable ability to grasp the bigger picture with its inaccuracy regarding details of the input is that vision is a biological process that has evolved in step with our ecological needs. This insight helps explain why the visual system is so efficient at extracting useful information such as the identities of objects independent of lighting conditions, while giving less importance to aspects like the exact nature of the ambient light. Moreover, vision does so using previously learned rules about the structure of the world. Some of these rules appeared to have become wired into our neural circuits over the course of evolution. Others are more plastic and help the brain to guess at the scene presented to the eyes based on the individual’s past experiences. This complex, purposeful processing happens at all levels of the visual system. It starts even at the retina, which is specialized to pick out object boundaries rather than creating a point-by-point representation of uniform surfaces. This constructive nature of visual perception has only recently been fully appreciated … The modern view that perception is an active and creative process that involves more than just the information provided to the retina has its roots in the philosophy of Immanuel Kant.” Developmental, behavioral, and neuro-rehabilitative optometrists will recognize The writings of Al Sutton as pertinent to the constructive nature of vision in this regard.
Some additional gems from Kandel’s Chapter 21: “In vision, as in other cognitive operations, various features – motion, depth, form, and color – occur together in a unified percept. This unity is achieved not by one hierarchical neural system, but by multiple areas in the brain that are fed by parallel but interacting neural pathways. Because distributed processing is one of the main organizational principles in the neurobiology of vision, one must have a grasp of the anatomical pathways of the visual system to understand fully the phsyiological description of visual processing.”
From here, Charles Gilbert and Aniruddha Das, who co-authored the chapter, launch into a dissection of the dorsal and ventral visual pathways. Crucially they note: “The pathways are interconnected so that information is shared. For example, movement information in the dorsal pathway can contribute to object recognition through kinematic cues. Information about movements in space derived from areas in the dorsal pathway is therefore important for the perception of object shape and is fed into the ventral pathway. All connections between cortical areas are reciprocal – each area sends information back to the areas from which it receives input. These feedback connections provide information about cognitive functions, including spatial attention, and emotional content, to earlier levels of visual processing. The pulvinar of the thalamus serves as a relay between cortical areas.”
We’ll summarize the highlights of Chapter 21 in Part Two, but for now it’s time to shower and head back over to The Green Planet for the daily read.