Over the years, the applications of sensation and perception have grown considerably. Today, some of the areas where knowledge of sensation and perception is applied include human factors, ecological psychology, industrial/organizational psychology, and neuropsychology.We consider the role of sensation and perception in two of these fields: human factors and neuropsychology.
One of the fundamentally important outcomes of sensory research is the development of an important set of applications based upon a functional knowledge of the operation of our sensory systems. Human Factors is one applied field that has made great use of diverse areas of psychology including sensation and perception. Human Factors refers to the application of scientific knowledge of human capabilities to the development of equipment (Proctor & Van Zandt, 1994). Basically, the idea behind human factors is to make the equipment fit the human user instead of the other way around. In other words, human factors is where we make basic psychology work in the real world, which is a very exciting task.
One vital area of application of visual knowledge in human factors has been the development of displays such as televisions and computer monitors. Historically, the development of color televisions and monitors owes much to the trichromatic theory of color vision. Beginning in 1931, the Commission Internationale de líEclairage (CIE) had formalized human color matching behavior using a quantitative model of the trichromatic theory, with major updates in 1960 and 1976. See Figure 18 for a depiction of the 1931 CIE diagram. Even in the years prior to the measurement of the responses of the cones, engineers were able to make a precise prediction of the human color response. Engineers need only to give a coordinate for a color in this space and, depending on the reliability of the equipment, the color will be precisely duplicated on any television and monitor or even printer.
Figure 18. The 1931 CIE color system. Colors are specified by their chormaticity coordinates (x, y). The spectrum is on the outside with example wavelengths given. The triangle represents the range of colors that can be reproduced on a typical television (Silverstein and Merrifield, 1985). [Figure 18 description]
Because the visual system has a functionally trichromatic receptor layer, there are other important engineering simplifications in developing color television and monitors that have been made possible. In essence the trichromatic theory specifies that three primaries should be sufficient for reproducing most colors we see. Thus, all modern color monitors and televisions use three color elements for color reproduction. These three elements are placed close enough together so that the colors are blended by the spatial summation mechanisms of the eye. To show how this is a real engineering blessing, consider what might happen if we had more than three color systems in our visual system. For each additional color mechanism, the engineer would need another color element in the system to be able to match the color of the scene being caught on film or tape. If we had even octachromatic vision (eight color mechanisms or cones), then there would need to be eight guns on our color television and not three. It might be very hard to fit all eight dots inside the limits of human spatial summation, that is, close enough together so that they are blended together by the visual system.
More recent efforts in display development have been in two primary areas. The first area is in assisting the development of new display technologies so that they have the same visual excellence as the CRT (cathode ray tube, the technology behind the standard monitor). The second area is in developing displays for harsh visual environments such as the airplane cockpit.
When the electron gun of a CRT strikes the surface of the display it stimulates several elements. The elements in the middle are bright and the ones at the edge are dim. This pattern of stimulation can be described as bell shaped or gaussian. Think of the lines on the CRT as being drawn with a bright center and dim slightly fuzzy edges. Thus the dots in the middle of the line are brightest and the dots on the edge are much dimmer. Since the jagged edges of the lines are made by dim dots, they are not as noticeable. If the lines are drawn directly like a pencil on paper as they are on some CRTs, then the jagged edges are all but unnoticeable (Silverstein & Merrifield, 1985). Liquid Crystal Displays (LCDís) and other technologies do not have these bell shaped beams but access each element directly in a digital fashion. Thus, the jagged edges of lines are easily noticeable. Thus, research efforts have determined how to take advantage of the way the visual system operates to most effectively and efficiently minimize the visibility of these jagged edges (Silverstein, Krantz, Gomer, Yeh, & Monty, 1990).
The airplane cockpit is a harsh environment in which to place an electronic display such as a CRT or LCD. If the sun is in front of the pilot, then it might take the pilot a relatively long time to be able to see the much dimmer display (i.e., dark adaptation). In some flight situations even a few seconds might be critical. If the sun enters the cockpit from the side window onto the display, the surface is washed out making it difficult or impossible to read and distinguish the colors of the elements. In the evening, it is important that the display not be too bright that the pilot can not see important but possibly dim targets in the sky or on the land. Visually oriented human factors research has focused on the stimulus intensity needed on these displays so that pilots can read them accurately and quickly under the wide range of conditions. An important goal of these efforts is to develop an automatic control mechanism that senses the ambient light visible out the front cockpit windows and the light falling on the display surface and automatically adjusts the intensity of the display for the pilot (Silverstein & Merrifield, 1985; Krantz, Silverstein, & Yeh, 1992).
One important future direction for sensation and perception's application to human factors will be developing a good general model of visual system-display interaction. With a single TV at home, this was not a big issue, but with scanners and digital cameras, printers, computer monitors, and TVs all at home the difference in color from one display to the next can be very great and disappointing. Obviously the model that is developed should not be tied to any single display type. There are also a plethora of new display technologies coming out. The venerable CRT will still be around for many years, the LCD is found on laptop computers and may be on our walls if HDTV takes off, color printers are coming into the home, and there are many other new display technologies besides.Two difficulties arise with these new technologies. First, the research effort to develop good images on CRTs and LCDs were very expensive and it would be nice if what has been learned from one technology could be applied to another. Secondly, the image reproduced on different displays can look very different, especially the colors. Colors will change and some times so dramatically that we would describe it as a different color on different displays. A good general model of visual system display interaction would allow new technologies to benefit from previous research and to develop images that have the same color from screen to screen and screen to printer.
Whereas studies of sensation and perception comprise a substantial portion of the field of human factors, studies of sensory and perceptual function have also recently made important contributions to the field of neuropsychology. This work has helped us to better understand and characterize a diverse group of neuropathologies and neurodegenerative disorders.
The field of neuropsychology examines the relationship between central nervous system function and behavior, often in cases of pathology. Scientists have long speculated on the links between cortical function and impaired behavior, and in the past have relied on studies of gross brain abnormality (e.g., the case of Phineas Gage) in order to gain insight into this relationship. Studies of specific visual and auditory processes now contribute considerably to our understanding of diseases and disorders that involve central nervous system pathology. These disorders include dyslexia, mental retardation, schizophrenia, and autism, as well as progressive neurodegenerative disorders such as Alzheimerís disease, Parkinsonís disease, and multiple sclerosis.
One challenge confronting researchers in these areas is that many disorders show multiple behavioral impairments. Most neurological disorders have a characteristic profile of spared and impaired behavioral abilities: some behaviors are relatively unaffected; others may be profoundly affected. For many syndromes, scientists still seek to identify the range of different behaviors that show impairments, the relative magnitude of these impairments, and the cortical and subcortical areas of the brain that may be affected.
Without the answers to the questions above science cannot begin to understand and explain these disabilities. Obviously there is a great interest to society, as well. If we know how and to what extent different behaviors are affected there may be ways to help compensate or reduce the impact of an impairment. Perhaps much the same way large-print books help the visually impaired. Also, we may discover that specialized training may be able to reduce or eliminate some perceptual impairments. There is evidence that suggests that this may be true with dyslexia (Merzenich et al., 1996).
Sensory psychologists have contributed greatly to our understanding of these disorders. Because such a large proportion of cerebral cortex is devoted to processing sensory information, studies of sensation and perception offer a ìwindowî into the brain. The information gathered from these studies helps identify specific sensory or perceptual impairments, helps infer the cortical locations of the impairments, and helps move us towards thorough causal explanations. As examples, we consider the cases of developmental dyslexia and Alzheimerís disease.
Dyslexia. Dyslexia refers to a broad family of impairments. Generally speaking, dyslexia is a term used to define a severe learning problem that is unrelated to intellectual ability, emotional disturbance, gross sensory or physical handicaps, sociocultural status, or insufficient schooling. The most common type of dyslexa, specific reading disability (SRD) involves difficulties in learning to read and write. At present we lack clear agreement about the nature of dyslexia, including its causes and symptomology. In fact, a minority of investigators even propose that dyslexia simply represents the lowest portion of the normal distribution of these abilities in the general population, rather than a distinctive disorder (Shaywitz, Escobar, Shaywitz, Fletcher, & Makuch, 1992).
However, most researchers agree that dyslexia probably results from abnormal neurodevelopment. Psychophysical studies suggest problems that are neural in origin as do anatomical studies of brain structure and physiological studies of neural activity. Stein and Walsh (1997) provide a succinct and recent review.
SRD is a reading impairment and children diagnosed with SRD are impaired in their abilities to auditorially distinguish the small differences among phonemes, the smallest units of speech (Brunswick & Rippon, 1994; Merzenich et al., 1996). Other verbal skills such as the rapid naming of objects and the ability to break words down into smaller segments (i.e., cowboy to ìcowî and ìboyî) are also impaired (Eden, Stein, Wood, & Wood; 1995).
A recent, systematic study of the nature of phonological processing in dyslexia was conducted by Shaywitz et al. (1998). Shaywitz et al. (1998) required dyslexic readers and control participants to perform a series of tasks, some of which required extensive phonological processing and others that required very little. While the participants engaged in these tasks, Shaywitz et al. used a brain imaging technique called fMRI (functional magnetic resonance imaging) to measure and map the pattern of neural activity in cerebral cortex. Shaywitz et al. observed what they refer to as a ìneural signatureî of dyslexia: A pattern of neural overactivation in some areas of cortex and underactivation in others. Some of the affected areas include traditional language areas and traditional visual ones. The authors interpret their findings as evidence that dyslexia is primarily a phonological impairment, and one that may involve a functional disruption of the mapping of the visual image (the printed words) to phonology during reading. These findings and this theory of dyslexia explain a large set of the research findings in this area and may describe the most proximal cause of SRD.
There is considerable evidence that children with SRD exhibit a set of visual perceptual problems that precede this stage, however, and researchers are working to characterize their relationship to SRD. Lovegrove, Garzia, & Nicholson (1990) provide a good discussion of the early work in this area. A large literature now documents impairments in motion perception (Cornelissen, Richardson, Mason, Fowler, & Stein, 1995; Eden et al., 1996) contrast sensitivity (Borsting et al., 1996; Cornelissen et al., 1995; Evans, Drasdo, & Richards, 1994), flicker sensitivity (Evans et al., 1994), as well as other tasks that preferentially involve the magnocellular pathway (Edwards, Hogben, Clark, & Pratt, 1996). Further support for visual perceptual correlates of SRD comes from a recent study revealing a correlation between motion detection thresholds and word reading performance in children without SRD (Cornelissen, Hansen, Hutton, Evangelinou, & Stein, 1998).
Anatomical and physiological abnormalities in the brain are associated with these behavioral abnormalities. In individuals with dyslexia the magnocellular layers of the LGN have been reported to be disordered and the cells themselves much smaller than normal (Livingston, Rosen, Drislane, & Galaburda, 1991). Eden et al. (1996) used fMRI to reveal abnormal neural activity during visual motion processing in adults with dyslexia.
In an attempt to explain the range of perceptual and anatomical impairments that are associated with SRD, investigators have recently suggested that dyslexia may be linked to a general sensory temporal processing impairment (Farmer & Klein, 1995; Stein & Walsh, 1997). That is, individuals with SRD have difficulty in processing sensory information that is brief or that changes rapidly over time. Studies of visual perceptual abilities of children with SRD would be consistent with this explanation, as well as studies that show that children with SRD have difficulties with some non-verbal auditory perceptual tasks as well as verbal ones (Tallal, 1980).
Again, the role that psychological research will play in understanding SRD will be large. Psychological research will help identify the cause(s) of SRD by helping to detail all the behaviors that are affected. A thorough description of SRD is very helpful because it permits researchers to eliminate alternative explanations of dyslexia that cannot explain the profile of impairments. Psychological research may also help find ways to compensate of help minimize the affect of the impairment, for example, with specialized training.Merzenich et al. (1996) report that children with learning disability can improve their abilities to perceive speech and nonspeech auditory stimuli with relatively little (8 to 16 hours) training.
Sensory psychologists have typically investigated these impairments in contrast sensitivity using specialized stimuli (e.g. sine wave gratings). Several undergraduate students working in one of our laboratories decided to investigate how well children with SRD would perform with more real-world stimuli, for example a visual acuity chart made up of high and low contrast letters. These students used a computer program designed in-house to test a group of children with and without dyslexia on their abilities to read low and high contrast Tumbling-E's. The Figures below show representations of the stimuli used.
Figure 19. The panel on the left shows E's that are black on a white background-they have high luminance contrast. The panel on the right shows E's that are gray on a white background-they have low luminance contrast. [Figure 19 description]
The student's hypothesis, based on reports in the literature, was that the children with SRD would perform no differently than the children without SRD on the high-contrast E's but have difficulty in correctly identifying the low-contrast E's. The data collected are shown in the Figures below.
Figure 20. Each graph shows proportion correct for the different size E's, shown as the Snellen acuity equivalent (the way your optometrist describes the size of the letters). The panel on the left shows the data for both groups of children when the E's were high contrast; the right panel for the low contrast stimuli. [Figure 20 description]
Notice that there was no difference in performance between groups when the letter contrast was high. Both groups of children could see the large to small E's equally well. In the right panel, however, you can see that when the contrast between the letters and the background was reduced the performance for the children with dyslexia was affected to a much larger extent. Our students had demonstrated that the effects of reduced contrast apply to the critical stimuli present in reading: individual letters (Ballew, Brooks, & Annacelli, 2001). More importantly, special stimuli and conditions were not required to observe the effects (see also Woods & Oross, 1998).
Studies of sensation and perception have also contributed to our understanding of progressive neurodegenerative disorders, such as Alzheimerís disease.
Alzheimerís Disease. Alzheimerís disease is characterized by neuropathology (neurofibrillary tangles and amyloid plaques in cerebral cortex) and behavioral impairment, primarily progressive cognitive decline. To the general public, Alzheimerís disease is a ìmemory disorder.î Alzheimerís disease involves memory impairments, but it involves other behavioral deficits as well. In fact, according to the Diagnostic and Statistical Manual of Mental Disorders (American Psychiatric Association DSM-IV, 1994) a diagnosis of Alzheimerís disease (Dementia of the Alzheimerís type) requires the presence of other non-memory related symptoms.
Early work suggested that the sensory systems and general perceptual abilities were spared in Alzheimerís disease. The initial observation that primary visual cortex was relatively free of the plaques and tangles that are characteristic of the disease and because visual acuity in Alzheimerís patients was not significantly different from that typically observed in normal aging supported this conclusion. Science, in its thoroughness, continued to investigate and assess other cognitive and noncognitive behaviors. As research in the area continued and specific perceptual skills were assessed, however, several visual and perceptual changes associated with Alzheimerís disease were revealed.
In addition to visual changes that are a consequence of normal aging, Alzheimerís disease is now associated with ganglion cell death in the retina (Blanks, Hinton, Sadun, & Miller; 1989), a substantial degeneration of optic nerve fibers (Hinton, Sadun, Blanks, & Miller, 1986), and cell loss in primary visual cortex (Hof & Morrison, 1990).
The association of visual perceptual problems with Alzheimerís disease is now well documented. Relative to age-matched controls, individuals with Alzheimerís disease have been reported to have impairments in contrast sensitivity (Bassi, Solomon, & Young, 1993; Gilmore & Whitehouse, 1995), blue-yellow color discrimination (Cronin-Golomb, Suguira, Corkin, & Growdon, 1993), depth perception (Mittenburg, Malloy, Petrick, & Knee, 1993) and motion perception (Gilmore, Wenk, Naylor, & Koss, 1994). The extent of the visual problems in Alzheimerís disease has led many investigators to consider the visual sequelae one of the hallmarks of the disease. In fact, some investigators have reported that Alzheimerís disease may have its origin in the visual sensory pathways (Gilmore & Whitehouse, 1995).
Importantly, research in visual perception may have helped identify a set of new diagnostic tools. There is evidence that in Alzheimerís disease the perceptual impairments may present before many of the cognitive impairments. Some individuals with Alzheimerís disease report consulting their optometrist or ophthalmologist concerning a vision problem before memory or cognitive impairments became evident (Kiyosawa et al., 1989). Researchers are currently investigating the extent to which visual perceptual tests may be used clinically to help identify Alzheimerís disease and other progressive neuropathologies in their earliest stages.