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.