Detail of a self-portrait by Chuck Close. Viewed from a short distance, this painting appears to be an abstract grid of vividly colored squares and ovals. But, when viewed from farther away, the local colors blend and we begin to perceive a spectacle-framed eye. The interplay between these local and global features, which are conveyed by discrete visual pathways, gives the portrait its particular dynamism.
Chuck Close has prosopagnosia, or difficulty in recognizing faces; his technique of flattening and subdividing an image into manageable elements enhances his ability to both perceive and portray the face. The complete painting is shown above. (Reproduced, with permission, from digital image: copyright the Museum of Modern Art/licensed by SCALA/Art Resource, NY; Copyright Chuck Close, courtesy of The Pace Gallery.)
… one day in winter, on my return home, my mother, seeing that I was cold, offered me some tea, a thing I did not ordinarily take. I declined at first, and then, for no particular reason, changed my mind. She sent for one of these squat, plump little cakes called "petites madeleines," which look as though they had been moulded in the fluted valve of a scallop shell. And soon, mechanically, dispirited after a dreary day with the prospect of a dreary morrow, I raised to my lips a spoonful of the tea in which I had soaked a morsel of the cake. No sooner had the warm liquid mixed with the crumbs touched my palate than a shudder ran through me and I stopped, intent upon the extraordinary thing that was happening to me. An exquisite pleasure had invaded my senses, something isolated, detached, with no suggestion of its origin. And at once the vicissitudes of life had become indifferent to me, its disasters innocuous, its brevity illusory—this new sensation having had on me the effect which love has of filling me with a precious essence; or rather this essence was not in me, it was me.*
The taste of the madeleine dipped in tea is one of the most famous evocations of sensory experience in literature. Proust's description of the conscious nature of sensation and memory provides profound insights into some of the subjects that we shall explore in the next few chapters. His description of the shape of the pastries on the plate, the warmth of the tea, and the mingled flavors of tea and cake remind us that knowledge of the world arises through the senses.
Perceptions begin in receptor cells that are sensitive to one or another kind of stimulus energy. Most sensations are identified with a particular type of stimulus. Thus, light of short wavelength falling on the eye is seen as blue, and sugar on the tongue tastes sweet. How the quantitative aspects of physical stimuli correlate with the sensations they evoke is the subject of psychophysics. Additional information about perception can be obtained from studying the various sensory receptors and the stimuli to which they respond as well as the sensory pathways that carry information from these receptors to the cerebral cortex. Specific cells in the sensory system, both peripheral receptors and central neurons, encode certain critical attributes of sensations, such as location and intensity. Other attributes of sensation are represented by the pattern of activity in a population of sensory neurons. We know, for example, that taste depends greatly on receptor specificity. In contrast, the differentiation of sounds depends in large part on pattern coding. Determining the extent to which receptor specificity and patterns of neural activity are used in different sensory systems to encode information is a major task of current research in sensory physiology.
Each sensory modality is mediated by a distinct neural system with multiple components that contribute to perception. Sensory pathways include neurons that link the receptors at the periphery with the spinal cord, brain stem, thalamus, and cerebral cortex. The perception of a touch on the hand begins when cutaneous mechanoreceptors cause a population of afferent fibers to discharge action potentials, thus setting up a propagated response in the dorsal column nuclei and then in the thalamus. From the thalamus sensory information flows to several areas of the cerebral cortex, each of which analyzes particular aspects of the original stimulus. This cortical representation is closely correlated with our conscious perception. For example, an illusion of sensation in the hand, albeit a slightly blunted one, can be elicited by electrical stimulation of the cortical area that represents the hand.
In this part of the book, we examine the principles essential for understanding how perception occurs in the brain. Contrary to our intuitive understanding based on personal experience, perceptions are not direct copies of the world around us. The information available to sensory systems at any instant in time is imperfect and incomplete. So perceptual systems are not built like physical devices for making measurements, but instead are built to perform inferences about the world. Sensory data should not be thought of as giving answers, but as providing clues.
The brain, for example, is where seeing happens; it is the brain that figures out what the clues mean. Thus visual perception is a creation of the brain. It is based on the input extracted from the retinal image. But what is seen in the "mind's eye" goes far beyond what is presented in the input. The brain uses information it has extracted previously as the basis for educated guesses—perceptual inferences about the state of the world.
Sensory systems contain many representations that each specialize in different kinds of sensory information processing. Throughout each sensory system, from the peripheral receptors to the cerebral cortex, information about physical stimuli is transformed in stages according to computational rules that reflect the functional properties of the neurons and their interconnections at each stage.
The visual system, for example, transforms the stimulus energy that the retinal receptors receive into a neural code of action potentials like the dots and dashes of a Morse code. The brain solves the problem of computation by performing relatively simple operations in parallel in massive numbers of neurons, and by repeating these operations at multiple hierarchical stages. The great mystery of vision is how we respond to trains of action potentials in different neurons of the visual system by seeing an image—like a face.
A major goal of cognitive neural science is to determine how the information that reaches the cerebral cortex by means of parallel afferent pathways is bound together to form a unified conscious perception. Indeed, one of the hopes driving cognitive neural science is that progress in understanding the binding problem will yield our first insights into the biological basis of attention and ultimately consciousness.