Visual Neuropsychology

Characteristics Visual neuropsychology is the sub-discipline of neuropsychology dedicated to understanding how visual information is processed by cerebro-cortical and subcortical mechanisms and how that information gives rise to perceptions and other behaviors. The discipline has grown in conjunction with knowledge of the anatomical connections and tuning properties of visual cortical neurons in▶occipital cortex and extra-occipital cortex. In particular, the discovery of streams of processing arising within the ▶primary visual cortex (Brodmann’s area 17; striate cortex; ▶area V1) and extending into other cortical areas (see Fig. 1) (▶Visual processing streams in primates; Extrastriate visual cortex), and the elucidation of the contribution of those processing streams to visual behaviors through scientific and clinical observations forms the basis of visual neuropsychology. Thus, visual neuropsychologists explore cortical functioning using psychophysical and neuroimagining techniques in healthy and braindamaged humans to understand the nature of visually mediated behaviors. Interestingly, while visual perceptions seem coherent and unitary, striking clinical dissociations illustrate that each stream can to some extent operate independently of the other (▶Visual perception). Behavioral dissociations manifest as processing difficulties in one domain in the relative absence of difficulties in another, so suggesting independence of the two domains. An example of a single-dissociation within visual neuropsychology has been the observation of a patient who has impaired motion perception (▶Motion blindness or ▶akinetopsia) but almost normal form perception (see for example [2]). Double dissociations occur when two single dissociations with complementary deficit/ ability profiles due to comparable size of brain damage are observed. Thus, while clinical participants are more heterogeneous in terms of their neural functioning than healthy participants, case studies illuminate sometimes unpredicted relationships between visual processes. Indeed neuropsychological case observations have often provided first clues on functional specialization within the human brain. It now seems clear, then, that visual perceptions and visually guided behaviors arise from simultaneous and largely parallel processing in both streams (▶Visual perception).