From a subjective standpoint, there seems to be little to explain about perception. Our perception of the world is direct, immediate, and effortless, and there is no hint of any intervening operations taking place in the brain. The apparent simplicity of perception is reinforced by the fact that our perceptions are almost always accurate. We rarely make mistakes when identifying people by their face or voice, or in judging how hot a cup of tea is, or in navigating a flight of steps. Moreover, our own perceptions nearly always agree with those of other people. Sounds, sights, and smells seem to be "out there" in the world, not constructed in our head.
Despite appearances to the contrary, our perceptual world is constructed in the brain by a huge mass of neurons performing complex, but hidden, operations. Three observations hint at the complexity of the brain processes involved in perception. First, a large proportion of the brain's most highly developed structure, the cerebral cortex, is devoted entirely to perception. Vision alone consumes over half of the neurons in the cortex. Second, despite the complexity and power of modern computer
technology, computer scientists have not yet succeeded in building general-purpose systems with the perceptual proficiency of even an infant. Relatively confined problems, such as detecting abnormalities in medical images, identifying a face or a voice, or guiding an autonomous vehicle, have proven to be formidable problems to solve by computer (see Huang, 2005). Third, as a result of brain damage through injury or disease, a small number of unfortunate individuals suffer deficits in their perceptual capabilities. These deficits can be very specific and debilitating, but also dramatic and perplexing to other people. It seems difficult to believe that someone can fail to recognize their own face reflected in a mirror (prosopagnosia), or cannot judge the position of their limbs without looking directly at them. Such people remind us of the sophisticated brain processes serving perceptual abilities that most of us take for granted.
A clinical condition resulting from brain damage, in which a patient is unable to recognize familiar faces.
Spectator sports provide a very clear example of the reliability, and occasional fallibility, of the information extracted by our perceptual systems. Everyone involved—participants, referees/umpires, and spectators—must make perceptual judgments in order to interpret events on the sports field, and to decide what should happen next. Did the tennis ball bounce out of court? Did the football cross the goal line and enter the goal? All those involved nearly always agree on what happened, because their perceptual systems arrive at the same decisions. Sporting activities would not be viable either for participants or for spectators without reliable perceptual systems. Certain critical judgments do require special skills and observation conditions. For instance, net-cord judges were once used in professional tennis tournaments to decide whether the ball delivered by the serving player had struck the top edge of the net (the net-cord) on its way across the court. They relied
Think of other reasons for disagreements between spectators about the same sporting incident.
on a combination of three senses—sight (deflection of the ball in flight), sound (the impact of the ball on the net), and touch (vibration of the net). As a result, the net-cord judge could detect the slightest of contacts between ball and net that were missed by most or all of the spectators. Professional courts now use an electronic net-cord sensor rather than a human observer; the sensor picks up the tiny vibrations caused by the ball (the first version of the sensor, introduced in the 1970s was the pick-up from an electric guitar).
Fine sensory discriminations during sporting activities probe the limits of our perceptual abilities. Disagreements can arise from the inherent variability of sensory signals. Copyright © Brian Snyder/Reuters/Corbis.
Perceptual judgments are not infallible, as demonstrated by the disagreements between participants or observers that can and do arise in many sports. Such disagreements can offer hints about the nature of the underlying perceptual processes (as well as providing additional entertainment; see Figure 1.1
). Common sources of disagreement involve decisions about whether a ball crossed a line on the sports field, such as whether a tennis ball bounced inside a court line. Participants often reach contradictory decisions in "close" calls. This disagreement is not simply a reflection of differences in skill or concentration level, or gamesmanship, but a natural consequence of the inherent variability in our perceptual decisions. In optimal conditions, perceptual responses are highly reliable, both within and between observers. When a ball bounces some distance to one side of a line, there is no disagreement as to where it bounced. However, psychophysical
research has taught us that in marginal conditions when stimuli are very close together or indistinct, perceptual responses are probabilistic. When a ball bounces slightly to the left of a line, the response of the perceptual system itself will sometimes lead to a "left" response, and other times lead to a "right" response. As a result, different observers are likely to disagree a certain proportion of the time. Perceptual research aims to estimate the precise degree of uncertainty attached to perceptual judgments, and to identify its likely causes (see Mather, 2008).
Sensations are simple conscious experiences generated by stimulation of a sense organ such as the eye: Your awareness of the brightness of a camera flash, or the sourness of a lemon, or the sharpness of a pinprick are all sensations. Perceptions are complex, meaningful experiences of objects and events. Sensations are immediate and automatic, whereas perceptions can take time to reach a stable state, and may require effort. For instance, at one of the sports events described earlier you may sense the green color of the grass, or the loudness of the public address system, but you perceive the identities of the players on the court or pitch, and the way their actions are interconnected.
Philosophers and psychologists call simple sensations qualia because they relate to the qualities of conscious experiences. By their very nature, qualia are private, and accessible only to the person who has them. Most researchers believe that qualia map onto specific brain states or functions of brain states. For example, there is a specific brain state associated with the sensation of the color red. If your sensation of color changed to, say, green, there would be a corresponding change in brain state. The assumed link between sensations and brain states lies at the very foundation of modern theories of perception, as will become clear below. However, an "explanatory gap" (Levine, 1983, 1999) remains between the physical world (brain states) and the mental world (sensations). No one has been able to explain precisely how the qualitative nature of sensation can be explained by reference to neural activity.
The scientific study of the relationship between physical stimulation and perceptual experience.
Primitive mental states, such as sensory impressions induced by stimulation of a sense organ (e.g., loudness, brightness, heat).
A mode of sensation that is qualitatively different from other modes, so its sensations cannot be confused with those of other modalities: for example, the experience of seeing is qualitatively different from that of hearing or touching.
Qualia divide up the sensory world into qualitatively different modes of sensation, also known as sensory modalities. The sensations evoked by light, for example, are qualitatively different from those evoked by sounds, touches, or smells. There is no possibility of us confusing a visual sensation with an auditory one, but it is possible to confuse one auditory sensation with another (for instance, within limits you cannot discriminate between two sounds of different intensities). One of the most fundamental questions one can ask about sensation is: "How many different sensory modalities do we have?" The answer is important because it determines the divisions which perceptual science should respect when seeking explanations. For instance, we cannot investigate how the different senses interact (as we do later in the book) without first distinguishing between them as separate modalities.
Since the time of Aristotle, common sense has divided the sensory world into five modalities—seeing (vision), hearing (audition), touch (somatosensation), smell (olfaction), and taste (gustation)—on the basis of visible sense organs (eye, ear, skin,
nose, tongue). However we must consider another four candidate modalities for this list, none of which have visible sense organs. The sense of balance (vestibular sense) is now recognized as a distinct modality, for reasons that will become clear below, and three more modalities relate to sensory awareness of the body: The sense of body position (proprioception), the sense of body motion (kinesthesis), and the sense of pain (nociception).
The criteria for distinguishing between the different sensory modalities are summarized in Table 1.1
. Seeing, hearing, and balance are easy to distinguish in physical terms, because they detect different forms of energy (electromagnetic radiation, air pressure waves, and motive force, respectively). Although smell and taste both detect chemical contact, they qualify as different modalities in the sense that they have different neural pathways. Touch, body position, and body motion do not really qualify as distinct modalities because they share a neural pathway and cortical destination. The status of pain as a distinct sensory modality is not clear-cut because it does have separate recep...