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Chapter 13

PSY295 CHapter 13 + Lecture 6.docx

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PSY 434
Jian Guan

Lecture 6 + Chapter 13 Vision and Agnosia Vision -~50% of cortex devoted to visual perception -Stimulus in the visual system is light (electromagnetic energy) The Eye -In retina there are 1. Photorecepter ; there are 2 types -Rods  sensitive to dim light, are used mainly for night vision -cones  transducer bright light and are used for daytime vision. There are 3 types 2.Bipolar cells  use action pot. in ganglion cells. 3. Ganglion cells  sends axons into the brain proper. From the Eye to the CNS Pg 206-208. -Signal processing of visual information is performed -Photorecepter  bipolar cels  Ganglion cells -Axons of the ganglion cells form a bundle = The optic nerve -Visual information is transmitted to the CNS via optic nerve. -optic nerve splits into two parts before entering brain -temporal branch = ipsilateral -nasal branch = contralateral -point of crossover = optic chaism -Once inside the brain, each optic nerve divides into pathways that differ with respect to where they terminate *IN summary -Each eye is divided into two identical halves -within each eye, one half receives stimulation from the left visual field and the other from the right visual field -Optic nerve -Lateral or temporal branch stays on the same side -Medial or nasal branch crosses over -optic chiasm  before entering the nerve, two optic nerves (one from each eye) meet and form this Geniculo- Cortical pathway - 90% of the visual information -consists of axons that exit from the LGN(Lateral geniculate nuclueus) and ascends to the cortex  LGN is the primary relay center for visual information receives from the retina of the eye; found in thalamus. LGN has 6 layers: layer 2,3,5 receive fibers from ipsilateral eye. Layer 1,4,6 from contralateral eye -Almost all the fibers terminate in the primary visual area of the occipital lobe. Geniculo-striate pathway pg 208-209 -Major visual pathway from the retina to the LGN to the striate cortex is the geniculostriate pathway - 10% of visual information -Consists of axons that exit from the LGN and go to subcorital structures ( pulvinar and superior colluculus)  10% of optic tract constitutes more fibers that the entire auditory system  Big role in visual attention  Sometimes viewed as a more primitive visual system -takes part in pattern recognition and conscious visual functions -second main takes part in visual pathway is detecting and orienting to visual stimulation. -So by the time infor reaches primary visual cortex  Visual information in the cortex has been processed by at least five distinct neurons: Photoreceptors, bipolar cells, ganglion cells , LGN cells, and cortical cells summary THUS FAR -Primary visual cortex (90%) -Pulvinar and Superior colliculus (10%) -Superior collicus(10% - projects back to thalamus and then to cortex)- tectopulvinar pathway  tectopulvinar pathway : relays from the eye to the superior coliculus in the midbrain tectum and reaches the visual areas in the temporal and parietal lobes through relays in the lateral posterior- pulvinar complex of the thalamus. Visual Cortex – Primary visual cortex -Different names for primary visual cortex -Brodmann’s area 17 -V1 (the striate cortex)- Primary vision area: receives the largest input from the LGN and project to all other occipital regions.  also the first processing level in the hierarchy Retinal Topography and Cortical Blindness pg 354 -Damage to LGN causes blindness ( but rare) -Damage of the primary visual cortex causes blindness ( common consequence of stroke) -Hemianopia – loss of vision in either the left or right visual field (ex. [ |x]; x indicates loss of vision) -Quadrantanopia- blindness in one quadrant of the visual field (ex. * | ‘+; ‘ indicates loss of vision) -Damage to the optic tract, LGN or V1. Cortical Blindness and Consciousness -Case D.B  Area around the right calcarine fissure was removed  Reported not seeing anything in the left visual field  Able to point to light in the left visual field Blindsight – Residual visual abilities within a field defect in the absence of acknowledged awareness  Importance of subcortical visual pathways Visual Association areas - ~30 cortical visual areas with distinct functions -V1 is initial projection region of LGN - Do not synapse sequentially to V2, V3, V4, etc -How do we define a visual area? Based on how areas respond to visual stimuli Vision for action -ex. When reaching for a particular object such as a cup, the fingers form a specific pattern that enables a person to grasp the cup. -Movement is guided by vision -Vision for action must be sensitive to movement of the garget. Ex. Catching a ball requires specific information about the location, trajectory speed and shape of the object. -Vision for action is a function of the parietal visual areas in the dorsal stream -the shaping of the hand corresponding to different object is due to dorsal stream. Area MT (aka V5 Motion) - Cells in area MT respond to movement but no colour -ex. Visual pathways beyond the occipital lobe -vision evolved first for motion , not for recognition -The system of knowing what an object is includes the flow of visual information from area V1 to the temporal lobe in the ventral stream. - The system controlling the visual guidance of movements includes the flow of information from area V1 to the parietal lobe in the dorsal stream -those with dorsal stream damage can consciously report seeing objects but cannot reach accurately or shape the hand appropriately when reaching. Motion – area v5( MT) Colour – Area V4. Visual cortex – functional dissocations - Double dissociation -MT is activated by motion ,but v4 is not. -V4 is activated by colour, but MT is not Disorder of Visual Pathways -monocular blindness – loss of sight in the eye with destruction of the retina or optic nerve -bitemporal hemianopita –occurs when a lesion of the medial region of the optic chiasm severs the crossing fiber.  leads to loss of vision of both temporal fields Basically, you lose both the outer peripheral field of vision. -nasal heminanopia – Lesion of the lateral chiasm results in a loss of vision of one nasal field. Basically, you lose one of the inner field of vision. If its binasal then you its both. -Homonymous hemianopia – occurs from a complete cut of the optic tract, l
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