AS WE HAVE SEEN, LOW-LEVEL visual processing is responsible for detecting various types of contrasts in the patterns of light projected onto the retina. Intermediate-level processing is concerned with the identification of so-called visual primitives, such as contours and fields of motion, and the segregation of surfaces. High-level visual processing integrates information from a variety of sources and is the final stage in the visual pathway leading to visual perception.
High-level visual processing is concerned with identifying behaviorally meaningful features of the environment and thus depends on descending signals that convey information from short-term working memory, long-term memory, and executive areas of cerebral cortex.
High-Level Visual Processing Is Concerned With Object Recognition
Our visual experience of the world is fundamentally object-centered. We can recognize the same object even when the patterns of light it casts onto the retina vary greatly with viewing conditions, such as lighting, angle, position, and distance. And this is the case even for visually complex objects, those that include a large number of conjoined visual features.
Moreover, objects are not mere visual entities, but are commonly associated with specific experiences, other remembered objects, and sensations—such as the hum of the coffee grinder or the aroma of a lover’s perfume—and a variety of emotions. It is the behavioral significance of objects that guides our action based on visual information. In short, object recognition establishes a nexus between vision and cognition (Figure 24–1).
Representation of entire objects is central to high-level visual processing. Representation of entire objects involves integration of visual features extracted at earlier stages in the visual pathways. This integration is a generalization of the numerous retinal images generated by the same object and of different members of an object category. The representation also incorporates information from other sensory modalities, attaches emotional valence, and associates the object with the memory of other objects or events. Object representations can be stored in working memory and recalled in association with other memories.
The Inferior Temporal Cortex Is the Primary Center for Object Recognition
Primate studies implicate neocortical regions of the temporal lobe, principally the inferior temporal cortex, in object perception. Because the hierarchy of synaptic relays in the cortical visual system extends from the primary visual cortex to the temporal lobe, the temporal lobe is a site of convergence of many types of visual information.
Neuropsychological studies have found that damage to the inferior temporal cortex can produce specific failures of object recognition. Neurophysiological and functional imaging studies have, in turn, yielded remarkable insights into the ways in which the activity of inferior temporal neurons represents objects, how these representations relate to perceptual and cognitive events, and how they are modified by experience.