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BHS 140.2 – Vision Science II
Notetaker: Elisabeth Anderson
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Date: 4/16/2013, 2nd hour
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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
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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
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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