Occipital Lobes

Occipital Lobes

1. Subdivisions

   • Area V1
     • Laminar organization most distinct of all cortical areas
   • Striate cortex
     • Also visual cortex - striate = striped appearance
   • Color vision
     • Primary job of V4, but distributed through the occipital cortex

2. Connections of the visual cortex

   • Primary visual cortex (V1)
     • Input from LGN
     • Output to all other levels
   • Secondary visual cortex (V2)
     • Output to all other levels
   • After V2
     • Output to the parietal lobe - dorsal stream
     • Output to the inferior temporal lobe - ventral stream
     • Output to the superior temporal sulcus (STS) - STS stream
   • Visual pathways
     • Dorsal stream
       • Visual guidance of movements
     • Ventral stream
       • Object perception
     • STS
       • Visuospatial functions (body oriented)

3. A theory of occipital lobe function

   • Vision begins in V1 that is heterogenous, and then travels to more specialized cortical zones
   • Selective lesions up the hierarchy produce specific visual deficits
   • Visual functions beyond the occipital lobe
     • Vision related areas in the brain make up 55% of the total cortex
       • Multiple visual regions in the temporal, parietal and frontal lobes
     • Vision
       • Not unitary, composed of many specific forms of processing
       • Vision for action
         • Parietal visual areas in the dorsal stream
         • Reaching, ducking and catching
       • Visual recognition
         • Temporal lobes
         • Object recognition
       • Visual space
         • Parietal and temporal lobes
         • Spatial location
           • Location of an object relative to person (egocentric space)
           • Location of an object relative to another (allocentric space)
       • Disorders of visual pathways
         • Monocular blindness
           • Loss of sight in one eye
           • Results from destruction of the retina or optic nerve
         • Bitemporal hemianopia
           • Loss of vision from both temporal fields
           • Results from a lesion to the optic chiasm
         • Nasal hemianopia
           • Loss of vision of one nasal field
           • Results from a lesion of the lateral chiasm
         • Homohemianopia
           • Blindness of one entire visual field
           • Results from a complete cut of the optic tract, LGN or V1
         • Macular sparing
           • Sparing of the central or macular region of the visual field
           • Results from a lesion to the occipital lobe
         • Quadrantoanopia/hemianopia
           • Complete loss of vision in 1/4 of the fovea or in 1/2 of the fovea.
           • Results from a lesion to the occipital lobe
         • Visual agnosia
           • Object agnosia
             • Apperceptive agnosia
               • Deficit in the ability to develop a percept of the structure of an object
               • Simultagnosia
                 • Unable to perceive more than one object at a time
               • Results from bilateral damage to the lateral parts of the occipital lobes
             • Associative agnosia
               • Can perceive objects, but cannot identify them
               • Results from lesions to the anterior temporal lobes
           • Other agnosias
             • Prosopagnosia
               • Cannot recognize faces
               • Can recognize facial features, facial expressions and tell human form from non human faces
             • Alexia
               • Inability to read
               • Form of object agnosia
                 • Inability to construct perceptual wholes from parts
               • Form of associative agnosia
                 • Word memory is damaged or inaccessible
               • Results from damage to the left fusiform and lingual areas

4. Imaging studies of dorsal and ventral streams

   • Haxby and colleagues
     • PET study
     • Found activation for facial stimuli in the temporal region
       • Also activation during a location task in the posterior parietal regions and frontal lobes
     • Detection of motion activated V5
       • Detection of shape activated STS
     • Colour perception activated area V4

5. Visual imagery

   • RH superiority in mental rotation
   • Left temporal-occipital region is responsible for image generation