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Lecture

Chapter 4: Visual Cortex and Beyond

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Department
Psychology
Course
Psychology 2115A/B
Professor
Christine Tsang
Semester
Winter

Description
Chapter 4 Review: The Visual Cortex and Beyond By the end of this chapter, you should know about: - Basic visual pathway to the cortex - LGN organization and processing - V1 organization and processing - Processing after V1 Retino-Geniculo-Cortical Pathway  Optic nerve o Bundle of retinal ganglion cells that leaves the eye at the retina  Optic chiasm o The location where the different optic nerves from each eye meet o The axons from the different portions of each retina segregate – kept in a precise order o The axons representing the axons closest to your nose cross o Crossing of certain axons from the retina  Optic tract o A bundle of axons that runs from the chiasm to the lateral geniculate nuclei  Lateral geniculate nuclei o The lateral part of the thalamus (sensory relay) o Thalamus relays information to the lateral geniculate for vision o Majority of axons from the retinal ganglion cells terminate in the LGN o Made up of 6 layers of cells o The inputs form each eye remain segregate – keeps left and right eye information separate o Keeps a retinotopic map of the visual world  Visual cortex (striate cortex/V1)  Ventral (what) / dorsal (where) pathways Visual Fields  Visual field: what you actually see, not what is projected into/out of the eye  Visual field is the view seen by the 2 eyes when looking straight ahead o Requires input from both eyes – binocular point of view o The images to your two eyes are slightly different from the different visual fields, but they both overlap  Right visual field  Left visual field  Each retina has a nasal section and a temporal section  The right visual field goes to the right nasal retina and the left temporal retina, vice versa for left visual field  We have information coming out of nasal retinal ganglion axons and temporal retinal ganglion axons  They are segregated as they leave the eye – they all leave via the optic nerve, but the nasal and temporal axons leave separately  Temporal information stays to the same side of the brain (ipsolateral)  Nasal information crosses to the opposite side of the brain via the optic chiasm (contraleral)  Information from the right visual field gets routed to the left visual cortex, information from the left visual field gets routed to the right visual cortex  Things viewed by the left/right visual field get transferred to the opposite side of the brain where it is processed  Everything that is on the right in the physical world ends up in the left via the crossing at the optic chiasm, vice versa for the left side The Lateral Geniculate Nucleus  Making decisions about what gets passed on to area V1  Controls the flow of information between retina and cortex  LGN cells have center-surround receptive fields  Major function of LGN is to regulate neural information from the retina to the visual cortex o Signals are received from the retina, the cortex, the brain stem, and the thalamus  Receives information from subcortical cortex called superior colliculus, which goes its own processing and sends its information to the lateral geniculate  All visual signals relayed to the LGN of the thalamus o Signals are organized by the eye, receptor type, and type of environmental information  Relation to the LGN (ipsolateral, contralateral), which receptor sent the information (rod, cone) and the type of information it is (movement-based, fine detail, etc.)  LGN is deconstructing the visual world into its component pieces – how it is received, etc. so the visual cortex doesn‟t have to in the end  Retinal ganglion neurons sends neurons out to the brain, but LGN receives information from all sorts of sources and also sends sorted information out to other cortex and receiving information back from Retinotopic Organization  Keeps a representation of how information is mapped on the retina Anatomy of Visual Cortex  Primary visual cortex = Striate Cortex = V1 o Striate cortex contains 6 layers o Mostly concerned with neurons in layer 4 of visual cortex because they receive information from the LGN  Located in the calcarine fissure at the occipital pole of the cortex  Contains an orderly retinotopic map of the opposite visual hemi-retina Visual Field Defects  The actual perception of the observer  White = vision, black = blocked vision  Lesions of the visual pathways and which part of the eye is effected  If the right optic nerve is cut – blindness in right eye, normal vision in left  If the optic chiasm is cut – blinding in sections that cross over (nasal parts cut, temporal parts of visual field blinded)  Right optic tract cut – only effects the contralateral visual field  Right LGN cut – partial blindness in contralateral visual field Receptive Fields  Area of the retina in which light alters the firing rate of a cell  Excitation vs. inhibition  ON responses: burst of neural impulses when a stimulus comes on in the environment  OFF responses: burst of energy/impulses following the termination/end of a stimulus  ON-OFF responses: neurons that fire rapidly and then are silent as long as the stimulus is present, and then when it is no longer present they fire again Retinal Ganglion and LGN Receptive Fields  RGC have high spontaneous activity  Their activity is modified (increased or decrease) by light stimulation  Circular center-surround organization Function ON-/OFF-center Receptive Fields  RGC show maximal response when the light in the centre and the surround are very different  Mainly signal contrast, rather than absolute intensity levels Visual Cortical Receptive Fields  Elongated, orientation specific receptive fields o Somewhat rectangular in shape o Do not respond well to small spots/rings of light o Respond best to bars/lines of light  First site in the brain where information from the two eyes combine o Everything from the retina through to the LGN through to the LG tracts are all segregating information from the two eyes – keeping it all separate o The cortex is the first place where all the information gets pieced back together again – visual cortex is the first point where this begins o While they respond to both eye stimulations, they have a preference for one or the other – have an ocular dominance preference Simple Cells  Excitatory in the middle and inhibitory on the sides – means that if you shine a bar of light that encompasses the entire center of the cell, there will be a series of firing/neural impulses indicating that it has sensed a bar of light in its receptive zone  If a spot of light is shined, it will not fire  If a bar of light is shined on the inhibitory sides, it will stop firing  Simple cells respond to the orientation of light  These cells are detecting edges of lights/features of shapes  Measuring static bars of light Complex Cells  Measures moving bars of light – movement in all directions  Much larger receptive fields than simple cells  Orientation-specific – specific orientations of lines that they like the best  Have on and off zones mixed within the cells – allows them to be direction-sensitive (sensitive to movement)  Their on/off zones are set up to detect the direction of movement  Allow for a detailed way of seeing the visual environment Another Class: End Stopped Cells  Also often have movement preferences – prefer a certain direc
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