April 13, 2018

Ventral visual pathway


  1. Overview of the ventral visual pathway

 I.1. Localization (neuroanatomy/structure) of the ventral visual pathway:

 I.1.1. Localization of the ventral visual pathway in humans

Kravitz (2011) defines the ventral visual pathways as an occiptotemporal route from the V1 area to other cortical and subcortical neural structures, involved in learning and memory via visual information processing.

 Ventral pathway travels along the route previously ascribed to the so called ‘central route’, running from the V1 to the V2 visual area, further connecting them to the inferior temporal cortex and further towards the temporal pole (Baizer, 1991; Unterleider, 1982).

I.1.2. Primate neuroanatomy of the ventral visual pathway

In primates, the ventral stream also consists of extrastriate cortical areas in the temporal cortex downstream of the V4 visual area. More precisely, in primates (macaque monkey studies), cortex ventral to MT area and anteria to V4 are considered to be a part of the ventral processing pathway (alternatively called ‘ventral processing stream’). The MT (motion sensitive) has recently been attributed to the ventral pathway (previously being considered a part of the dorsal pathway only), based on new research findings pointing out MT’s input into both pathways (Milner, 2006; Schenk, 2000; Schenk, 2005).

The ventral visual pathway also involves an alternative route passing through the ventral superior temporal sulcus (or STS) which contains polysensory areas as well as areas participating in form and motion processing (Oram, 1996; Saleem, 2000). Areas adjacent to V4 and V8 are also part of the ventral stream, including the lateral occipital complex or LOC (Grill-Spector, 2001; Kourtzi, 2001), the so called fusiform face area or FFA (Gauthier, 1999; Kanwisher, 1997), the parahippocampal place area or PPA (Epstein, 1998), along with several other overlapping areas reacting to (or sensitive to) different object categories (Ishai, 1999).

I.2. Function of the ventral visual pathway (comparison to the dorsal visual pathway):

1.2.1. General function: the ventral stream transforms or translates visual input into particular representations. In other words: the ventral pathways takes care of the object’s identity (or features) whereas the dorsal pathway deals with object location and call for action while overseeing global perception. I.e., the ventral route is designed for detailed examination and recognition of patterns and objects, whereas the dorsal one works with action and overall perception (Sheth, 2016). It is rather feature than object vision being taken care of by the ventral pathway, i.e. discrimination of such features as colour, shape, brightness and shades thereof (Buckley, 2006: Lehky, 2016). Excluding motion, however, which is better dealt with by the dorsal pathway.

1.2.2. Role in perception: Overall, the ventral visual pathway is responsible for perception whereas the dorsal stream takes care of action, according to the perception-action model (Schenk, 2010). I.e., it is ‘vision for perception’ (ventral pathway) vs. ‘vision for action’ (dorsal pathway). Perception involves pattern recognition, analysis of form/shape of objects (Tanaka, 1996; Tanaka, 2001); for instance, face recogintion and spatial layouts’ perception are handled by the ventral pathway ( Gross, 1992; Logothetis, 2000; Kreiman, 2000; Perrett, 1982; Tanaka, 1996; Tanaka, 2001).

1.2.3. Local (vs. global) processing: On the other hand, based on evidence from lesion studies, it can be concluded that the ventral visual stream may be mostly involved in local processing (recognizing details) whereas the dorsal pathway would be more likely responsible for global perception (Merigan, 1989; Schiller, 1991). A classical example of recognizing the general structure and missing details (which would then correspond to perceiving visual input globally but failing to recognize local details) would be seeing (and conciously perceiving) a forest but failing to recognize that it consists of trees (lacking perception of details).

1.2.4. Multisensory integration: Multisensory integration is often considered to be rather a task for the dorsal pathway than the ventral one (Pisella, 2009). However, it seems that both pathways are rather complementary of each other and participate, to a degree, in many multisensory tasks which we illustrate below on the example of haptic information processing. Haptic signals (relating to the sense of touch) involve both pathways (ventral and dorsal), with three distinct theories being postulated. The first model claims that haptic signals relating oject features are dealt by the ventral patway, whereas those signals coding spatial location are done via the dorsal route. Another model suggests that object recognition and perception are handled by the ventral pathway, whereas motion and action-related cues are identified by the dorsal pathway. Finally, the third model suggests that both streams (ventral and dorsal) converge with haptic pathways in different points, the ventral convergence specializing for processing multisensory shape cues for object perception, whereas the dorsal convergence deals with object.directed motor actions. All the three models are reflecting different aspects of perception and action, and are supported by research findings (James, 2010). This confirms that multisensory integration is happening in both pathways, the division depending on the level and specificity of processing required.

1.2.5. Abstract knowledge base: Based on experimental findings, Milner and Goodale (1998) suggest that the ventral visual pathway is linked to some kind of abstract visual knowledge or memory about different objects and spatial relationships between them. The authors also postulate that selective attention modulates visual information processing occuring in the ventral stream, thus allowing the above-mentioned abstract visual database to be selectively accessed whenever necessary. They furthermore hypothesize that object recognition and perception is being done on both conscious and subconscious levels, processing being capable of activating certain semantic representations. It is logical to presume that far not all unconscious perceptions of the sort reach conscious awareness, which is the next level in perception. One explanation for certain perceptions not being transferred to the conscious stage would be neuronal activations not reaching a certain threshold in such cases.

2. Function of the ventral pathway: additional examples

 2.1. Categorical specificity and selectivity of the ventral visual pathway:

 Neurons along the ventral visual pathway demonstrate categorical specificity, reacting selectively to particular features of observed objects. This selectivity is maintained regardless change of viewpoint, colour of the object or retinal image size. Selectivity is however affected by the frequency of presentation of object type (how often it is being seen or presented), as well as the presence of reward (or absence thereof).

Functional studies revealed that ventral areas are maximally responsive to some particular object features or categories, including faces (Kanwisher, 1997: Puce, 1995), places (Aguirre, 1998; Epstein, 1998), body parts (Downing, 2001), letter strings (Puce, 1996), tools (Martin, 1996), and animals (Martin, 1996; Chao, 1999).

2.2. Lesion studies

2.2.1. Lesion studies: When the ventral visual pathway is impaired, orientation as well as complex shape discrimination being compromised, and perceptial invariance affected. Lesions to the ventral stream however do not disrupt any functions of the dorsal stream. Impaired ventral pathway can be demonstrated in agnostic patients.

Milner et al. (1991) performed a series of case studies on agnostic patients who clearly showed misjugement of object orientation (perception or ventral stream task) however had no difficulty when reaching out for the same object (action or dorsal stream task).

2.2.2. Function compensation case: Emotional face recognition in blindsight patients: De Gelder and colleagues (1999) performed a case study on an amnestic patient (medial temporal lobe amnesia case), with a result of the patient being able to discriminate better than chance emotional facial expressions on forced-choice tasks. Named ‘cover recognition’, this functional compensation correlating to activity in an extra-geniculo-striate colliculo-thalamo-amygdala pathway as determined by fMRI recordings. This finding suggests that even in some blind subjects, the ventral visual pathway may contribute to unconscious perception of visual stimuli.



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Key principles underpinning information processing along the ventral visual pathway
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