W. W. Norton & Company
A Book Review by: John Abbondanza, O.D., FCOVD
Does a textbook on visual cognition have relevance for the behavioral optometrist? My answer is: that depends on the book. James Enns is a professor of psychology and neuroscience at the University of British Columbia and his book The Thinking Eye, The Seeing Brain – Explorations in Visual Cognition truly is relevant to a behavioral understanding of vision. In my opinion, this is the textbook that every optometry school should be using in their visual perception curriculum. It is well written, easy to understand, and has many examples of ‘experiments’ that one can perform at home with a minimum of equipment that make the visual phenomenon discussed readily observable (and therefore understandable) to the reader. He has the obligatory chapters on eye and brain, color vision, edge detection, and object recognition, as you would expect. But what really captured my interest were the chapters on time, space, imagination, and consciousness.
As the title might suggest, Enns conceives of vision and cognition as intimately linked. He writes,”… seeing is much more dependent on a healthy brain than on an optically correct set of eyes. In fact, no amount of seeing is actually accomplished by the eyes at all.” Pg 2 And “…much of what we call thinking relies heavily on the same parts of the brain that are used when we see the world around us.” Pg 3 It’s almost like he was a behavioral optometrist! The connection between vision and thinking is explored further in his chapter on ‘Imagination’. Visual thinking relies on visual images that we generate for ourselves. These images form the basis of imagination. Our imagination “enables us to form mental images of scenes, objects, and events we have never witnessed. We can use our imagination to play out scenarios to test the possible outcomes of our actions and decisions.” Pg 312 He continues: “A vast portion of our everyday thinking makes use of visual images. Yet these images do not bear any direct relationship to the pattern of light bombarding our eyes. They are images we manufacture for ourselves in order to think clearly”. p 311 In this way, vision is used to guide and direct action.
What is vision for? Enns has the audacity to ask that quintessential behavioral optometric question. His answer: to aid in survival. For this, vision must accurately portray the world around us AND do it rapidly. If we have to think before we act while we look at the lion charging at us, we are less likely to survive long enough to reproduce. His point is well taken. This leads directly to the prominent role of vision in humans. He writes “…more than half of the brain regions that researchers have probed in humans’ closest biological relative, the monkey, are responsive to visual input.” That’s correct. Vision is so crucial to survival that half of the cortex is directly responsive to visual input (not to mention how many other areas play an indirect role, such as in eye movements). We are visual creatures by design.
Vision as a construction project – Throughout the book, Enns references the fact that vision is a ‘construction project’ rather than a simple pictorial representation of the world as you would find in a camera. Many optical illusions illustrate this point. From the illusions of size and distance to the illusions of motion, he clearly indicates how vision is an active process where critical features of the environment and of objects of interest are extracted and manipulated for further mental use. For example, he discusses visual memory experiments that suggest that long term storage of visual memory scenes reflect a type of structural description, hierarchical in nature, rather than pictorial storage you would get in a camera or a computer.
Enns discusses how prior visual experiences form the basis of novel visual images. Visual images can be combined and manipulated in different ways, and thus we can build visual images of objects and events we have never experienced. We ‘construct’ the image based on the representation of past experiences. In this way, we can use our past to guide our actions in the present toward a desired (imagined) end in the future. I am reminded of that basic behavioral optometric tenet – the main purpose of the visual process is to guide and direct action.
Enns writes “Vision must be far less photographic than subjective experience suggests it to be…. Vision involves the construction of a model of the external world, not a detailed registration of the images that arise from such objects.” p 167
Space and Time – The chapters on time and space are fascinating and thought provoking. We all know that the visual system must develop an internal representation of space that accurately reflects the three dimensional world around us. To a large extent, it does this remarkably well, even dominating over our other senses. For example, Enns mentions how subjects placed in a rotating room feel disoriented when their visual and vestibular information is not congruent, as when they see themselves to be tilted because the room is tilted but they are upright. As a testimony to the dominance of vision, he reports how some subjects feel that they are upright if both the room and the subjects are upside down, even though they can feel the straps pulling on them and their hair standing on end! Remarkable! Obviously, Skeffington’s ‘Where am I’ circle is dominated by vision.
Much of the chapter on space deals with monocular cues to depth perception and stereopsis, as we all remember from optometry school. However, the shadow paradox is particularly interesting. Shadows are simultaneously the most powerful cues to depth in pictures and the least noticed of all cues. Surely, not all visual processing occurs at the level of consciousness, as this paradox suggests.
The problem of time is a tricky one for the visual system to resolve. Unlike edges and borders, there are no known neurological receptors responsible for recording the timing of a visual event. For example, it takes about 200 milliseconds for a signal to reach the visual cortex from the retina, yet our perception is as if there is no delay. If you are driving 65 miles per hour, your car would drive about 1 car length in that 200 millisconds, so your actions need to account for the fact that since the image hit your retina, your position changed. Enns concludes that our conscious experience is delayed and that we visually live in the ‘near past’. Yet our visual experience is as if it is in the present. Further, it takes time for object recognition (what is it) to occur. The brain solves timing problems through temporal binding and temporal integration. Events that fire together are perceived as occurring at the same time, and integrated into a single, unitary visual experience. This is yet another example of vision being a construction project.
Will every optometrist rush right out and buy The Thinking Eye, The Seeing Brain and add it to their collection of reference books? Probably not, and that would be a shame. I feel it is a must read for optometrists interested in understanding how the brain puts together visual information and constructs a representation of the external world, which is a remarkable task for a brain and a remarkable feat for an author.