Clinical Neuropsychology L5 THE OCCIPITAL LOBES

Primary (V1) and secondary (V2) visual projection areas in occipitalcortex

A portion of visual input ‘skips’ V1 and goes directly to V2 and subsequently to secondary areas• V3a (form) and V3 (form of movingobjects)• V4 (color, form)• V5 (motion detection)

• Facilitates fast object recognition (e.g., ripe fruit) but also perception of object motion and position (particularly in three-dimensional space: depth vision)• Color is an important property of visual perception in primates (man, apes)

Occipital visual areas (V1-V5) together enable elementary visualperception: location, form, color, and motion

visual guidance of movement (grasping, catching, avoiding objects)"vision for action"Egocentric orientation in space: determining spatial location ofobjects relative to self

visual recognition of objects like tools, hands, faces "vision for recognition"

visuospatial functions

• Spatial localization of objects• Spatial memory ("visual space")• Perceiving specific types ofmovements (e.g., body language)

STS has polysensory neurons combining information from dorsaland ventral stream for categorization

determining spatial location of objects relative to each other

(a.o. frontal eye fields for control of eye movements): active visual search behavior ("action for vision")

Selective visual attention for external world- with parietal cortex : for visual control of movements- with temporal cortex: for object recognition

"spotlight"-hypothesis

- Afferent system- V1- Secondary occipital cortex areas- Streams from the occipital lobe to other lobes

1) R half of the visual field falls on the L half of each retina, whichsends projections to the L hemisphere and vice-versa

Damage to V1 will typically affect ‘sight’ in BOTH eyes (visual field),While damage outside the CNS will affect one eye

2) Different parts of the visual field are topographically represented in parts of the V1

Damage to a specific V1 area will lead to loss of ‘sight’ in a specificpart of the visual world

Hemianopia

cortical blindness (= loss of subjective visual experience)

– Some submodalities may be preserved– "Blindsight": patient may be able to detect location, color, or motion of the stimulus without "seeing" the stimulus. How?– Some visual input to secondary or tertiary visual cortex bypassing V1 (via V2 or a pathway via superior colliculus and thalamus)

– V3 and V4: loss of form perception (e.g., not recognizing objects on the basis of line drawings)– V4: cortical color blindness and sometimes loss of ability to imagine or remember colors, even when dreaming (loss of color cognition)– V5: not seeing objects when moving (motion blindness)

Inactivation of V5 in healthy persons using transcranial magneticstimulation has a similar effect

Visual object agnosia

• Apperceptive agnosia
• Associative agnosia

Other agnosias

• Prosopagnosia
• Visuospatial agnosia
• Alexia

visual object agnosia

able to perceive elementary aspects of the stimulus (features such as lines or colors) but no integration into a meaningful image; mostly caused by large bilateral or R-sided lesions

not able to recognize or copy form of objectsnot able to recognize or copy form of objects

perceiving only one object at the same time (dorsal stimultagnosia) or unable to perceive unified picture (ventral simultagnosia)

able to perceive integrated image, also able to copy it, but not able to identify it (object remains meaningless; probably problems with semantic categorization); generally associated with L-sided lesions

prosopagnosia

• Generally proper recognition in case of typical facial features; alsoproper recognition of emotional expressions (but not of personalidentity)• Mostly in case of bilateral lesions but R-sided damage plays largerrole than L-sided damage

Lack of perception and recognition of spatial environment

mostly with R-sided lesions

Reading

recognition of letters OK, problems with- integrating letters into a word (type of apperceptive agnosia)- recognizing integrated word (type of associative agnosia)mostly with L-sided lesions

– Disturbances of visually guided movements, for example, grasping or avoiding objects: optic ataxia.Patient sees object correctly but cannot adjust movements to object– Problems with spatial orientation

visual perception, recognition, and imagination have certain cortical representations in common