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3.2 - Vision 2.pdf

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Joe Kim

Arnav Agarwal 2011 Vision 2 Module 1: Visual Pathways Introduction to Visual Pathways - Although some initial processing of visual info is done in the retina, visual perception comes together in the brain - Assembly-line system: areas along the visual pathways process parts of the visual input before sending those partially-processed bits of info on to the next set of areas down the line for further processing Visual Fields and Hemispheres - Right and left halves of our visual fields are actually processed by the contralateral side of our brain o Visual input from right visual field -> optical nerve -> left hemisphere o Visual input from left visual field -> optic nerve -> right hemisphere - Each visual field receives information from both eyes; therefore, each hemisphere receives information from both eyes - Axons from the inner region of the retina (closest to the nose) have to cross over to the opposite hemisphere before reaching their respective hemispheres - Point at which the optic nerves from the inside half of each eye cross over to the opposite hemisphere is the “optic chiasm” Arnav Agarwal 2011 Two Visual Pathways - After optic chiasm, info from each visual field arrives in the opposite hemisphere - Optic nerve fibers split and travel along two pathways - Pathway 1: main path o Most of the retinal or ganglion cell axons travel along the main pathway and synapse in the lateral geniculate nucleus (LGN)  LGN: part of the thalamus that receives visual information o Visual signals processed in LGN -> sent to areas in the occipital lobe that make up the primary visual cortex - Pathway 2: detour o Smaller portion of the axons from the retinas take a detour to the superior colliculus (located in midbrain) -> information sent upwards to thalamus -> sent to occipital lobe or downward to structures in the brainstem o This smaller, secondary pathway seems to deal with:  coordinating visual input with info coming in from other senses  localizing objects in space through head and eye movements and helping to guide those movements Arnav Agarwal 2011 Main Pathway: Two Subdivisions - Two subdivisions of specialization that are able to process specific information in parallel - Magnocellular pathway: movement information - Parvocellular pathway: color and form information Arnav Agarwal 2011 Main Pathway: LGN - Lateral geniculate nucleus (LGN): first stop for info sent from retinal ganglion cells - LGN cells have receptive fields (like retinal ganglion cells do); these have many photoreceptors and are made up of a combination of many ganglion cells - Info from many smaller bits -> one overall neural signal Arnav Agarwal 2011 - LGN is made up of 6 layers; info from each eye projects to different layers - One layer receives input from a specific eye - Each layer also receives input from a specific subpathway - Magnocellular info (movement): runs to 2 of the LGN layers - Parvocellular info: runs to the other 4 LGN layers Arnav Agarwal 2011 Main Pathway: Occipital Lobe - Info pathway: retinal ganglion cells -> LGN -> occipital lobe (further processing) Arnav Agarwal 2011 - Over 20 cortical areas that process visual info, but area v1 of occipital lobe (primary visual cortex) is the biggest research focus - Extrastriate cortex: occipital lobe’s visual processing areas outside the striate cortex Primary Visual Cortex - Receptive field of a single V1 cell is a combination of the receptive fields of many LGN cells o Info from many sources processed down into a single target Arnav Agarwal 2011 - Receptive fields from retina are arranged in a topographical map in the primary visual cortex - Neighboring locations in the retina project to neighboring locations in the visual cortex - Primary visual cortex -> 6 layers - LGN (lateral geniculate nucleus) projects directly onto layer IV neurons -> info carried to neurons in other five layers Arnav Agarwal 2011 - Layers organized into cortical columns, made up of 0.5 mm square of cortex that are perpendicular to the cortical surface - Vast majority of neurons in visual cortex can respond to visual stimuli presented to either eye - Most still have a stronger response to one eye than the other, though o Info from each eye -> LGN -> projects more strongly to some cortical neurons and less strongly to others - Eye preference maintained within the individual cortical column o All neurons within a given cortical column respond more strongly to input from the same eye - Info from each eye is still being processed separately in the primary visual cortex - Cortical neuron is also the first site of binocular processing, though Dorsal & Ventral Streams of Extrastriate Cortex - Processed visual info (color, form, movement, etc.): primary visual cortex -> extrastraite cortex -> gets separated into dorsal and ventral streams - Dorsal stream (“where pathway”): processes where objects are (including depth and motion in the field) o From the extrastriate cortex to the parietal lobe - Ventral stream (“what pathway”): processes what the object is (color, form) o From extrastriate cortex to temporal lobe Arnav Agarwal 2011 Conclusions: Visual Pathway - Each brain region has a specific task in visual processing before passing information on to next region; certain regions perform certain tasks - Information is compressed as it is processed - Input from many retinal cells -> fewer ganglion cells -> fewer LGN cells -> fewer primary visual cortical cells - Information is processed through separate channels to some extent - Most neurons respond most strongly to a specific attribute, despite responding to more than one attribute in general Concept Check: 1) This is the area in the brain where information from the two visual fields crosses to the contralateral side: a. Optic chiasm 2) This area of the brain is involved in integrating vision with the other senses: a. Parvocellular pathway b. Superior colliculus c. V1 d. Lateral Geniculate Nucleus e. Magnocellular pathway Arnav Agarwal 2011 3) There are ____ layers in the LGN. ____ layers process magnocellular information and _____ process parvocellular information. a. 8;4;4 b. 10;6;4 c. 6;6;6 d. 6;2;4 i. 6 layers in the LGN ii. 2 layers process magnocellular information iii. 4 layers process parvocellular information e. 6;4;2 4) The concept of a topographic map in V1 describes how each single retinal ganglion cell corresponds to a single cell in visual cortex. a. True i. Incorrect. There is extensive combination of information, such that a large cluster of retinal ganglion cells feeds into one V1 cell. b. False 5) Match the descriptions to either being for the dorsal or the ventral stream: a. Processes color Ventral b. Processes motion Dorsal c. Processes form Ventral d. Leads to the temporal lobe Ventral e. Leads to the parietal lobe Dorsal f. Processes depth Dorsal Module 2: Evolution of the Eye Introduction to Eye Evolution - How does the eye develop its structure in order to turn physical stimulus of light into a signal that the brain can read and produce complex imagery through? Light-Sensitive Patch - Eyes originally began as light-sensitive patches - Eg: jellyfish, worms Curved “Cup” Eye - At some point, individuals developed light-sensitive patch with a slight depression, allowing light direction to be sensed -> survival advantage - Eg: modern clams Crude Lens Arnav Agarwal 2011 - Some later developed crude lens -> able to process visual input at different distances - Further improvements -> better focusing and accommodation (eg: more transparent, curved lens) - Adaptations layered on top of each other to facilitate complex vertebrate eye evolution - Eg: modern fish, reptiles Cumulative Selection - Small changes were made to existing eye, then new small changes were made to modified eye -> result: increasingly sophisticated eye -> through process of cumulative selection The Costs and Benefits of Eyes - Changes happened relatively quickly in evolutionary time, but didn’t happen at once - For modifications to evolve: advantages must outweigh energetic costs of sustaining them - Advantages of eyes: o Survival advantages  Easier to find food and shelter  Easier to avoid predators and other dangers o Reproductive advantages  Easier to find a mate - Cost: energy Predation: The Driving Force of Eye Evolution Arnav Agarwal 2011 - Precambrian period: o Animals likely only had crude light-sensors, if at all o Tracks indicate slow, small movements -> poor mobility o Main concern was mobility; no point seeing a predator if one can’t escape -> therefore, eyesight not a major concern at this time - Early Cambrian period: o Animals were larger and more mobile o Eyes would have been useful for:  Hunting prey  Escaping predators o Result: arms race between predators and prey to develop superior vision and locomotion skills  Led to adaptive selection for increasingly improved eyes Concept Check: 1) Place in the correct order the evolutionary development of the human eye: a. Light-sensitive patch b. Curved-cup eye c. Crude lens Arnav Agarwal 2011 2) The fact that animals such as worms and clams have simpler version of a human eye suggests that their eyes are not as good. a. True b. False 3) This is the process of new adaptations being subsequently layered upon older adaptations to slowly produce a complex structure: a. Cumulative selection 4) Consider this hypothetical animal, which we will call Intropsychipus. The Intropsychipus is a very small animal, and cannot move very quickly. The natural predator of Intropsychipus is very slow as well, and so the Intropsychipus needs to know the predator is there, but it never needs to mount a rapid escape. What might we expect about the Intropsychipus’ eye? a. It should be relatively small b. It should be relatively large c. It should contain a simple lens d. It should be a light-sensitive patch or curved cup e. Intropsychipus doesn’t need eyes Module 3: Different Eye Designs for Different Environments Different Eye Designs for Different Environments - Eyes vary a lot across different species depending on what the species regularly deals with o Amount of light in living area o Food coming from above or below o Which characteristics of predator
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