Synonyms

Ventral stream; “What system”

Structure

The ventral visual pathway is a functional stream involved in the visual recognition of objects. The anatomical substrates to the ventral visual pathway were initially identified in macaque monkeys by Mishkin and Ungerleider (1982). They observed that visual input from primary visual cortex is projected to the inferior temporal cortex (areas TEO and TE) via prestriate cortex (Mishkin et al. 1982; Mishkin et al. 1983). An analogous pathway is present in the human brain. This pathway consists of visual input from primary visual cortex V1 relayed through areas V2 and V4, and ultimately projected into the inferior temporal cortex. While areas V1, V2, and V4 are involved in the processing of basic-level visual features such as edges, contours, and color, the inferior temporal cortex is suggested to process complex shapes (Ungerleider and Haxby 1994).

Function

Since the 1960s, researchers had suggested that the visual system could be divided into two separate components, one responsible for object recognition and one responsible for the localization of objects in space (Held 1968; Schneider 1969). Mishkin and Ungerleider (1982) provided the first empirical evidence for this intuition, by showing that monkeys with inferior temporal cortex lesions had problems in recognizing objects by their shape (what), while monkeys with parietal lobe lesions had problems processing the location of objects in space (where).

Since the seminal study by Mishkin and Ungerleider, much research has been devoted to understand the function of the ventral visual pathway in the human brain. Lesion studies were the first to suggest that the what/where dissociation observed in monkeys could also be found in brain injury patients, suggesting that the human visual system is organized in a similar manner. Patients with occipitotemporal lesions had difficulties in recognizing a variety of objects (what), while patients with parietal lesions had difficulties in spatial tasks (where), including reaching for objects (Milner and Goodale 1991; Newcomb et al. 1969, 1987). These findings confirmed the hypothesis that the ventral temporal pathway is involved in the processing and recognition of objects.

While the function of the ventral visual pathway is widely accepted, there is much debate in the literature regarding the functional organization of this pathway. Case studies of patients with different occipitotemporal lesions have hinted that different groups of neurons within the occipitotemporal cortex may be selectively active for specific classes of objects. This idea was motivated by reports of patients showing deficits primarily in the recognition of faces and not other objects, or in the recognition of letters and not faces, or even in the recognition of one or more specific categories of objects (see De Renzi and Saetti 1997). These case studies motivated a series of investigations of the functional organization of the ventral stream using techniques such as PET and fMRI. Recently, several proposals regarding the functional organization of object representations in occipitotemporal cortex have been put forth (see Grill-Spector 2004 for a review).

All these proposals agree on the presence of generic-object selective cortex bilaterally in the lateral portion of the occipital lobe (area LO, Malach et al. 1995). These regions are the first portions of cortex activated more strongly by objects as wholes when compared with a large range of texture patterns (Malach et al. 1995). Malach and colleagues have suggested that LO represents an intermediate processing stage that leads to object recognition, but that these areas are not capable of recognizing specific exemplars.

A divergence among opinions emerges with regard to the organization of ventral temporal cortex, considered to be the site where exemplar recognition takes place. Based on the studies of patients with selective recognition impairments, researchers have proposed that ventral temporal cortex may contain object representations that are separable on the basis of the object category; however, the functional principles underlying the separation of category-specific information is a matter of active debate. Some researchers have suggested that there might be module-like subregions within the ventral portion of the temporal cortex that respond selectively to specific object categories such as faces, places, and tools (Aguirre et al. 1998; Epstein and Kanwisher 1998; Kanwisher et al. 1997; Martin et al. 1996). The modules may be organized based on their ecological importance (often argued in the case of faces) (Kanwisher et al.) or possibly the organization of semantic knowledge related to the class (Martin et al. 1996). A second explanation has focused on the manner of processing that is applied to different object categories. This proposal argues that the functional segregation among object classes in ventral temporal cortex is determined by the computations required to recognize them, such as specialized processing for expert categories where the specific exemplars are recognized quickly and efficiently (Gauthier et al. 1999). Lastly, it has been proposed that the ventral temporal cortex may contain feature-based object topographies. That is, the object categories could be represented by a distributed network of neurons that code for its different visual attributes, with category organization based on the visual similarity among the object categories (Haxby et al. 2000).

Illness

Damage to the ventral visual pathway can produce a set of deficits termed visual agnosias. Visual agnosias are impairments in the perception and/or recognition of objects. These patients have spared basic perceptual abilities, such as acuity, motion detection, and contrast sensitivity, and as such are not cortically blind, but they are greatly impaired in matching, recognizing, and discriminating objects and shapes visually. Visual agnosias have been divided into two types (Farah 1990; Lissauer 1890).

The first type has been termed apperceptive, and it is described as primarily a disorder of perception. Apperceptive agnosias have difficulty in creating an accurate and unified percept of what they see. They can perceive parts of an object, but cannot group the parts together to recognize a full object. These patients will usually show very extensive brain damage to the occipital lobes bilaterally (Farah 1990).

The second type of visual agnosia has been termed associative, and it is primarily a deficit in the association of a visual percept with its stored representation and its meaning. These patients can form a perceptually “normal” representation of what they see, and they are usually able to copy and match objects and shapes; however, they cannot access any stored object representation that is connected to semantic knowledge of the object. These patients have usually damage either bilaterally or unilaterally to more anterior portions of the ventral stream, such as the lateral occipital cortex and the inferior temporal lobes (Farah 1990). The severity of the impairment is usually correlated with the extent of the lesion. Patients with smaller lesions will have the most trouble in recognizing specific classes of objects (i.e., faces, letters), while they might be able to recognize some object types. Patients with larger lesions will be impaired on the recognition of most types of objects or shapes.

Cross-References