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1.8. Visual System
Fovea: center of the retina. Vision is clearest in the fovea and it has the highest concentration of rods and cones (photoreceptors). The retinais a heterogeneous structure where cells get larger and less dense with greater distance from the fovea (also why our periphery is not as good). Rods: respond to dim light. Good at differentiating in black and white. Not very receptive to color. More concentrated around the periphery.
Cones:respond to color, and are concentrated in fovea.
Color is not a fundamental property of things around us. It is a trick of the visual system that understands the absorption properties of the object.
Light conditions can change the perception of color cones with different opsins are sensitive to different wavelengths of color.
Primary Projection Pathways
Optic Chiasm: crossing of optic nerves in the brain.
90% of nerves originate in Lateral Geniculate Nucleus (LGN). This is the primary relay center for visual information received by the retina. It is found in the thalamus so it makes sense why it would be a relay. Remaining 10% terminate in the thalamus & superior colliculus.
Eye>Optic Nerve→ Optic Chiasm→ LGN>(slingshots over to) V1. ** Don't forget about the age old question of What is the household life cycle?
Coding in Ganglion & LGN Neurons
Visual neurons have receptive fields too so the stimulus must be in a particular region of space. Ganglion & LGN cells have a center/surround organization that makes them very sensitive to contrast.
Primary Visual Cortex (V1): Info from LGN comes here. This is where visual information makes the first cortical synapse. The cells here are simple, they care about orientation of a particular region in space due to LGN cells.
Organization of V1
● simple cells code for orientation We also discuss several other topics like What stage of piaget’s theory of cognitive development is associated with early childhood?
● Distribution of simple cells follows gradient change in concentration.
● information from left and right eyes are kept separate in alternating OcularDominance
Columns (stripes in the brain containing neurons that respond preferentially to info coming from each eye).
● Location, color & size are also represented here.
The visual system is not explicitly hierarchical. Processing is divided and specialized.
Brett Article: Functional Localization in the Brain
A review of methods used for reporting location in functional imaging, and discuss the problems that arise from the variation in brain anatomy between individuals. Don't forget about the age old question of What happened in the judgement of paris?
PET and fMRI= Awesome spatial res. Not as good temporal res.
Coregistration: mapping fMRI image onto structural MRI.
Voxel:Volumetric pixel. “Imagining the brain as a lego cube.” Allows researchers to choose the resolution we view the brain with.
Spatial Normalization:Taking individual brains and warping it to the template brain. Stereotaxic Coordinates:A coordinate system in 3D (x,y,z)
3 Types of Labelling
Microanatomical Labels:Cytoarchitecture, neurotransmitters, neurons. Macroanatomical Labels: Gyri, sulci & main structures (deep brain nuclei) Functional Labels: Labelling according to function. Broca’s area, Wernicke’s area.(general areas) If you want to learn more check out What are examples of polyatomic ions?
Talairach & Torneau: Created awesome coordinate system. they align the axis with the anterior and posterior commissures. They also prepared a template brain prepared with Brodmann Areas.
Montreal Neural Institute (MNI) Coordinate System: Took a lot of brains and averaged them together to make a template brain. This is another way of trying to create a template.
Warping is supposed to make all of the brains as similar as possible. But a template brain can cause slight differences in where activation is. Could mistakenly conclude a functional difference, not an anatomical difference.
Tong Article: Binocular Rivalry
Region of Interest: Region of the brain that is the interest of the experiment (FFA/PPA in this case).
Counterbalancing:In any systematic setup (left vs right) most have alternating things to reduce bias. ex.5 subjects with red image in right eye is counterbalanced with 5 subjects with red image in left eye.
Hypothesis:Has binocular rivalry been resolved by the time it gets to the FFA/PPA? Yes, somewhere along the pathway it is resolved.
First localizer scans were performed in each subject to locate their FFA and PPA. They used nonrivalrous images. If you want to learn more check out Is smoking marijuana in a city park in fort collins a deviant act?
Then rivalry scanswere performed where the subjects were shown a house in one eye and a face in the other. The images were actually superimposed on each other but there were red and green filters over each eye so that only one eye was receiving each picture. Subjects perceived an alternation between the pictures. Don't forget about the age old question of What is opsonization?
The non rivalry scans were done by showing the subjects one picture at a time for the same length of time that they perceived them during the rivalry scan.
FFA responses are strongly modulated by voluntary selective attention. Also the FFA and PPA are activated during imagining houses and faces as well.
Sternberg Memory Task:Participants are given a set of letters to memorize and then they are shown a new set and asked to say if the new letters were in the first set.
Mental Chronometry: The study of the time course of information processing in the human nervous system
Associative Priming: Reaction time to a stimulus is faster if that stimulus is preceded by a stimulus of similar meaning (semantic priming). Associative priming also interacts with stimulus degradation and with how distinctive a face is.
Visual cortical magnification:Things in the fovea are disproportionately magnified. What we see in the outside world gets mapped directly onto the brain. As you advance in the the hierarchy the processes become more complex.
Specialization of Function: (MT in monkeys, V5 in humans) Neuron only fires when the stimulus is in its receptive field. Neuron only fires when the stimulus is moved in a certain direction. Rate of fire is correlated with speed of motion.
Imaging Visual Areas: Sensitivity to color vs motion. V4 is more attuned to deciphering color whereas V5 is attuned to motion not color.