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BHS 140.2 – Vision Science II Notetaker: Elisabeth Anderson Date: 4/16/2013, 2nd hour Page1 Neural Basis of Visual Perception o The visual system can only start to process information from the retinal image o Retinal image info is very ambiguous o The visual system makes a best guess o Visual perception is really a series of hypotheses of what is out there Role of Retina o Dark adaptation, light adaptation, extraction of edges, detection of contrast, contrast constancy occurs in the retina o Retina is important for these tasks as well as others o Retina is not just simply a relay station Role of LGN o A mystery that is just beginning to get solved o The receptive field organization is like RGCs Circularly organized Excitatory center and inhibitory surround Inhibitory centers and excitatory surrounds also o If those LGN cells have the same receptive field properties as RGCs then why have them? o The main role of the LGN is not to extract info from RGCs but to regulate the neural signal and where it goes o More of a control center than a place of analysis o All LGNs can be classified as either a relay cell or an interneuron Relay cells synapse in cortex Interneurons synapse with other neurons in LGN o Output of LGN is only through relay cells Can be inhibited from 2 sources Interneuron can make inhibitory synapse with relay neuron Thalamic regulatory neuron can make inhibitory synapse with relay neuron (Black squares on diagram on pg. 3) o Excitatory input comes from the retina RGCs make excitatory synapse on LGN relay cells (Green circles on diagram on pg. 3) o Triad synapse 1 LGN relay cell 2 excitatory synapses with RGC from retina 1 inhibitory synapse with interneuron o Relay axons synapse in V1 If this pathway is damaged blindness or scotomas may occur V1 can also influence and regulate the output of the LGN o LGN acts as a regulator o People are wondering if the LGN is part of the mechanism of saccadic suppression We don’t see a smear caused by the eye moving between two points of fixation Vision is suppressed during eye movements There is a depression in LGN activity slightly before, during, and after eye movement Eye movement center may be sending signals back to LGN to turn it off during saccadic eye movement Info is not getting through LGN during these movements so this is a hypothesis of what may be happening Role of Cortical Area of V1 o V1 is often called primary visual cortex/striate cortex/ Area 17 o Extends out from posterior pole in a lateral direction o Extra striate areas are areas beyond V1 (post V1) Visual area 2,4, V7 May be at least 30 areas in the brain devoted to vision o V1 can start to encode info that is different from the retina and LGN BHS 140.2 – Vision Science II Notetaker: Elisabeth Anderson o Date: 4/16/2013, 2nd hour Page2 Can get info about size of object, direction of motion, depth perception, and the beginnings of color vision processing o In the retina and LGN there are neurons that are classified as M-like, P-like, or K-like with different anatomical features and functions o In V1 the info from P, K, and M cells is recombined, it does not stay segregated o The recombined info is sent to extra striate areas for even more processing o There are very intricate synapses in the V1 V1 projects to V2 V2 sends feedback to V1 o 6 principal layers in V1, about 2 mm thick o If you ablate/destroy V1 in a mouse or rat they still have good acuity This does not work in humans, we need V1 to see, we would be blind without V1 o Neurons are retinotopic 2 regions in visual field that are next to each other will be processed by neurons next to each other True for retina, LGN, and V1 Input to V1 o Relay cells of LGN synapse in layer IV (4) of V1 o Layer IV is divided into IVA ,IVB, IVC Then further subdivided into Layer IVCα, Layer IVCβ o Superficial layers are 1-3, deep layers are 5 and 6 o P-cells synapse in IVCbeta o M-cells synapse in IVCalpha o K-cells synapse in 3 areas Layer 1 Layer IVA Layer 3 o Once the input has a arrived in V1 from 3 types of cells the information is no longer segregated it gets all mixed together o Input from the two eyes are segregated at the first synapse o Every other neuron gets input from both eyes o Some neurons can be right eye dominant or left eye dominant but neurons get input from both eyes o There is input to V1 from the brainstem, thalamus (pulvinar), basal ganglia nuclei o There is extensive input from visual areas 2,3,4,and 5 in the form of feedback o If V1 sends info to a nucleus that nucleus will send feedback back to V1 Receptive Field Properties of V1 cells o Properties of individual neurons that you haven’t seen before Selectivity of direction of motion RGCs have center surround receptive field but the response to motion is the same no matter what direction the stimulus is moving Not selective to a particular direction of motion o V1 neurons are tuned to a particular direction of movement Only rightward, upward, etc. o V1 cells are sensitive to stimuli oriented in one direction or the other Concentric receptive fields are not made to be sensitive to orientation differences so RGCs are not sensitive to orientation of stimulus o V1 has the combination of input from both eyes which allows us to have binocularity o Receptive field organization is critical for the ability to extract these new kinds of details V1 Output o General rule of thumb is that Layer IV only receives input o Neural layers superficial to Layer IV (Layers I-III) send output to extra striate places (V2,V3,V4) o Neural layers deeper to Layer IV (Layers V and VI) send output to subcortical levels of the brain o Layer VI is unique because it sends its output back to the LGN o Layer V sends a major projection of its axons to nuclei in the brain responsible for controlling eye movements BHS 140.2 – Vision Science II Notetaker: Elisabeth Anderson o Date: 4/16/2013, 2nd hour Page3 M,P, and K cells are so intermingled that the segregated pathways that leave V1 are not based on these characteristics Turning Point Questions o Q: Which of the following is an inhibitory synapse? A: LGN interneuron onto LGN relay cell o Q: Which neural area is thought to be suppressed during a saccadic eye movement? A: LGN o Q: Which of the following is not unique to V1 neurons? A: Response to motion within the receptive field o Q: Which cortical layer projects back to the LGN? A: VI (6) o Q: Post V1 pathways are differentiated on the basis of M, P, and K cell characteristics. A: False