Neuropsychology of Perception

The world surrounds us with an immense variety of sounds, colors, textures, and odors. In everyday life we need to receive and interpret this information to help us plan and adopt appropriate behaviors. Perception is the first phase of this interaction with the world. It progresses from initial encoding of data in a number of specialized sensory end organs for the different modalities (vision, hearing, taste, touch, and smell) through various stages of processing in subcortical and cortical structures in the brain, stages that elaborate, [Page 633]focus, interpret, and ultimately integrate this information to provide us with what we regard as our experience of the world.

Given the complexities of perceptual processing and the wide range of neural structures involved, it is not surprising to find that a large number of human diseases frequently impair perception. How these disorders—particularly those involving the brain—impact perception and what this can tell us about how perception works is the research domain of perceptual neuropsychology.

Neuropsychology is the scientific discipline that investigates the structure and function of the brain by assessing the effect of brain lesions in relationship to specific psychological processes and behaviors. This entry describes the basic procedures and goals of neuropsychology, neuropsychological studies of visual perception, and neuropsychology of other perceptual modalities.

The neuropsychological approach consists of assessing cognitive functions (i.e., memory, attention, perception, language, reasoning, etc.) by relying on tests specifically designed to assess each of those cognitive functions selectively. Neuro-psychological tests also provide an accurate assessment of a selective cognitive function according to different materials and sensory modalities. In the context of perception, for instance, a neuropsychological assessment involves the evaluation of perceptual skills in visual, auditory, or tactile modalities, confined to different types of information such as faces, objects, sounds, colors, or more complex stimuli. At each test, the patient's performance is compared to the performance of a group of healthy individuals matched for age and education, providing an accurate estimate of the degree to which a selective function is impaired in a brain-damaged patient.

For many decades, the neuropsychological approach was the only method available to study where functions were localized in the human brain, and such studies were often performed in tandem with studies using more precise recordings or artificial lesions in animals, particularly monkeys. In the modern era, functional neuroimaging techniques have added powerful new tools to the study of the relation between structure and function in the human brain. However, such approaches also have their limitations, and the neuropsychological approach still provides us with information that cannot be obtained by other methods.

The two main approaches used in neuropsychology are the case study and the group study. Both have their advantages and disadvantages. Single-case studies with well-controlled experiments can provide important information about cognitive operations, but conclusions about the anatomic bases of the deficits in single cases is difficult. Natural lesions tend to be large, affecting a number of brain regions, limiting deductions about which region is the critical one. Group studies can show which lesions are associated with a specific functional deficit and which are not. On the other hand, group studies can suffer from the fact that any collection of patients is heterogeneous in many respects, regarding for example age, premorbid functioning, and lesion size and location. Careful selection, matching, and exclusion criteria can help, but heterogeneity can never be eliminated totally. Also, the assumption that all patients with a certain syndrome have the same functional deficit is not always correct, and group studies may inadvertently blur this distinction, leading to false conclusions about the anatomic correlates of function. To address some of the limitations previously described, neuropsychologists make use of an experimental technique named dissociation, which helps to identify the neural substrates of a selective cognitive function more reliably. In a single dissociation, the neuro-psychologist is able to demonstrate that a specific brain lesion in region A is affecting the cognitive function X but not the cognitive function Y, which demonstrates that the two functions are independent and rely on different brain regions (a typical single dissociation is the inability to name an object when the patient can only see it, whereas he or she is able to name the same object by touching it). A single dissociation is strengthened when the neuropsychologist is able to demonstrate that a brain lesion in region A is affecting the cognitive function X but not Y and that a brain lesion in region B is impairing the cognitive function Y but not X. This is called a double dissociation and allows researchers to make conclusions about the independency of different neural substrates related to different cognitive functions (a typical example is a patient with a selective lesion in region A showing an impairment in visual memory but not [Page 634]auditory memory, and a patient with a selective lesion in region B showing an impairment in auditory memory but not in visual memory).

...