Cortical Visual Processing
VENTRAL STREAM
Temporal lobe & Occipital lobe
- Comprises the what, object recognition pathway (or P pathway).
- Cone photoreceptors are responsible for color detection and excite parvocellular ganglion cells of the what pathway.
- Within this stream, components of objects are integrated to allow for cohesive object identification.
DORSAL STREAM
Parietal lobe & Occipital lobe
- Comprises the where spatial localization (or M pathway).
- Rod photoreceptors are responsible for motion detection and excite magnocellular ganglion cells of the where pathway.
- Within this stream, numerous visuospatial processing centers exist.
FRONTAL EYE FIELDS
Posterior Middle frontal gyrus
- Responsible for the cortical initiation of many different classes of eye movements.
PRIMARY VISUAL CORTEX, V1, BRODMANN AREA 17
Lies along the calcarine sulcus of the medial face of the occipital lobe at the tip of the lateral occipital pole.
- Called V1 because visual cortical stimuli first collect in this area and called striate cortex because of the heavy myelination of its fourth cytoarchitectural layer, which produces a white stripe called the stria of Gennari.
- V1 processes the most basic visual properties (eg, line orientation, motion direction, luminance, orientation, and color).
- Encodes the visual field from the opposite half of the world - right V1 encodes the left visual field and left V1 encodes the right visual field.
- The cortical representation of central, or macular, vision lies in the posterior calcarine sulcus and occupies a large cortical area relative to its small retinal expanse, whereas representation of the peripheral retina lies in the anterior calcarine sulcus and encompasses a small cortical area relative to its broad retinal expanse.
- The upper bank of the calcarine sulcus encodes the lower half of the visual field and the lower bank encodes the upper half of the visual field.
SECONDARY AND TERTIARY VISUAL CORTICES, V2 AND V3, BRODMANN AREAS 18 AND 19:
- Secondary visual cortex is V2 and corresponds to Brodmann area 18.
- Tertiary visual cortex is V3 and corresponds to Brodmann area 19.
- On the medial face of the cerebral hemisphere, V2 lies above and below V1, and V3 lies above and below V2.
- On the lateral surface of the hemisphere, V2 comprises a small strip of the posterior occipital lobe, just anterior to V1; V3 lies in front of V2.
- The secondary and tertiary visual cortices process simple visual properties akin to those processed in V1.
- V2 also processes illusory boundaries - contours that cannot actually be visualized but that are implied by the context of a larger scene.
COLOR-PROCESSING AREA, V4
- Lies on the medial and lateral surfaces of the cerebral hemisphere, in the ventral-occipital lobe, inferior to V3.
- One important aspect of color processing found within area V4 is that of color constancy, which is the property of color vision wherein regardless of the illumination cast on an object, the object maintains its perceived color.
- Clinical corollary: ventral occipito-temporal injury causes a color processing deficit, called achromatopsia, which results in the visual world appearing drained of color.
- Injury to area V4, alone, may or may not be sufficient to cause achromatopsia.
LATERAL OCCIPITAL COMPLEX
- Lies in the lateral aspect of the hemisphere, anterior to V4.
- Responds disproportionately strongly for object recognition.
- Displays perceptual constancy, meaning an object can be recognized equally well regardless of such properties as object viewpoint, size, or illumination; it also displays form-cue invariance, meaning, for example, that an object is equally identifiable whether it is viewed in the form of a drawing or a photograph.
FUSIFORM FACE AREA
Lies within the right, lateral posterior fusiform gyrus.
- The fusiform face area is the most well-studied area for facial processing.
- Debate exists whether the fusiform face area is specific for the recognition of human faces or whether it responds to any overtrained visual stimulus (eg, cars in car salesmen or birds in bird-watchers).
- Clinical corollary; injury to the fusiform face area can result in prosopagnosia - a deficit for the recognition of familiar faces (ie, friends and family).
EXTRASTRIATE BODY AREA
Key terms: Temporal lobe & Occipital lobe
- Non-face body parts are processed separate from faces in the lateral occipitotemporal cortex (most notably).
- For higher-level visual processing, at least in part, the body is separated from the face and is processed near the V5 motion-sensitive area, presumably because of the relationship between body parts and movement.
PARAHIPPOCAMPAL PLACE AREA
Processing of places lies within the parahippocampal place area, which lies within the posterior parahippocampal (parahippocampal gyrus) and anterior lingual cortices (lingual gyrus).
- The parahippocampal place area responds to environmental scenes and buildings.
- Injury to the parahippocampal place area can potentially cause landmark agnosia, which is the inability to recognize fundamental navigation landmarks.
MOTION-PROCESSING CENTER, V5
Lies at the occipital-temporal-parietal junction.
- Is often referred to as hMT because it represents the human correlate to the macaque middle temporal visual area.
- Visual perception of motion is processed in this region; it plays a role in smooth pursuit eye movements.
- Clinical corollary - patients with injury to V5 demonstrate akinetopsia, which is an inability to visualize moving objects despite a preserved ability to accurately visualize stationary objects.
PARIETAL DORSAL STREAM AREA
- Contains numerous visual cortical areas for the processing of spatial awareness, which comprise the where pathway.
- Some of the specific visual areas that are identified within the parietal dorsal stream area are the parietal reach region in the intraparietal sulcus, which activates during reach and pointing movements.
- The anterior parietal area in the anterior intraparietal sulcus, which activates during fine motor movements.
- The ventral parietal area in the depths of the intraparietal sulcus, which activates during multimodal motion detection of moving stimuli.
- The lateral intraparietal area in the posterior intraparietal sulcus, which activates during visually guided saccadic eye movements.
- The superior parietal lobule and inferior parietal lobule (supramarginal gyrus, angular gyrus), which function in attention control and spatial awareness.
- The parietal dorsal stream area instructs the motor cortex where to move.
DEPTH PERCEPTION
- A product of stereoscopic vision (the reconciliation of binocular disparity) and also due to high-level cortical processing of numerous visual cues.
- Allows for the preservation of depth perception in the setting of monocular (one-eye) vision.