Lecture 5 The Cerebral Association Cortex

imissyou419's version from 2017-10-27 05:38


Question Answer
What are the 4 anatomical subdivisions?frontal, temporal, occipital, parietal
Primary sensory/somatosensory, motor areasSomatosensory - where sensory info first arrives. Primary motor - send commands to the muscle. In rat, primary sensory + motor occupy nearly all cortex, in humans - 20%
Higher order (secondary) sensory and premotor areasHigher order visual, somatosensory, auditory lie near the respective primary areas. Where each modality of sensory info is further processed without influence of the other modalities. Premotor area sends command to the motor area.
Prefrontal and Parietal-Temporal-Occipital association areasmake up over 1/2 of cortex. It is where: a) different modalities combine, b) attention is shifted, c) planning and decision making are made, d) things are remembered
How are the different areas of cortex interconnected?motor response to sensory input, a reflex. Short loop reflex -> rapid but simple responses such as swatting a mosquito. Processing power of association regions -> complex reflexes such as writing down name of seen object
Grey matterneurons, extensively folded sheet to maximize area, contains tons of synapses, requires more oxygen than white matter, blood is directed preferentially to areas of grey matter that are in use (basis for fMRI - measures delivery of oxygen)
White matterfiber tracts that connect all areas of cortex, extensive interconnection predisposes cortex to epilepsy -> abnormal activity in 1 area spreads to other regions. Interconnections are where our memory is.
Each neuron connects on average to1000 other neurons. In turn, each neuron's output is dependent on the input from a large number of other neurons. Connectivity allows for fast processing speed. Diffusion tensor imaging (DTI) - another new MRI technique that maps axons
Grey matter connects of how many layers?6 anatomically distinct layers
Information arrives in which layer and then what happens? Which layer is thickest?layer 4 (via the thalamus) - thickest in primary sensory region and primary visual cortex region. Spreads to more superficial and deep layers, integrated by output cells whose bodies are located in layers 3 and 5 to other cortical regions, brain stem, spinal cord. Layer 3 and 5 are thickest in primary motor cortex.
How many areas was the cortex divided in?43 areas based on layer thickness
Lashley's equipotential theory(cortical areas not unique) information on a particular function is spread out over the entire cortex.
Evidence for: small lesion -> minimal impairment.
Evidence against: cortex is not uniform. Different regions serve different functions. e.g. primary sensory and motor areas
Grandmother cell theory(cortical areas super unique) simple cells connect to complex cells, complex connect to hyper-complex cells, and so on until there is 1 unique cell that fires when you see your grandmother. If you lose that cell, you can no longer recognize your grandmother, but have no problem recognizing grandfather.
Evidence for: lesion of the FFA do impair recognition of faces selectively. Some cells are activated only by a particular face.
Evidence against: brain cell death is common, yet memory loss observed is a general fuzziness in remembering faces, not an absolute loss of 1 face and not another
Is the function of a particular cortical area identical in different people? No, cortex is plastic especially in early life. If a particular sensory input is missing, other areas push into it. Loss of both eyes allow auditory and somatosensory areas to expand. If a particular cortical area is damaged the same function may be organized in a new cortical area e.g. damaged left cortex in early life, right cortex develops stronger language function (language normally represented strongly on left)
Dominant cortexleft, excels in sequential or serial tasks such as language (reading, writing, speaking, signing) and math (algebra); series of words and series of tasks
Non-dominant cortexright, excels in tasks requiring parallel processing i.e. face recognition, geometry. excels in tasks requiring spatial or intuitive, music. FFA is larger on the right side
Is there other cortex also activated for a particular function?Yes, although 1 side may dominant for a particular function, other side also activated
When patient with a cut corpus callosum is shown an apple in his left visual field, what happens?can visually recognize apple, would not be able to name apple, can reach for apple with left arm
What are the advantages and disadvantages of lateralization?Advantage: corpus callosum pathway are long. Pathways within the same hemisphere that interconnect related regions are shorter and faster. Duplications = waste.
Disadvantage: less redundancy not good if one part breaks down
Prefrontal cortex evolutionPrefrontal cortex has become larger, as a percentage of total brain size. The most important factor in our brain's growth was learning to cook food. The brain is only 2% of body weight uses up 20% of its oxygen and metabolic consumption.
Prefrontal association area functionsplanning and working memory, decision-making
Lesions of prefrontal association areaproduces deficits in tasks that are spatial + delayed (hold location in mind over time) = no working memory.
Little frustrations is displayed when the patient makes mistakes in everyday decisions = no planning & decision-making
Neurons in the prefrontal cortex activitya) show activity, which starts when a stimulus appears in a particular location.
b) unlike neurons in V1, here activity continues even when stimulus disappears. Tonic activity holds the object location in working memory. Different cells hold the memory of objects in different target locations.
What impairs working memory?depletion of dopamine in the prefrontal cortex. Prefrontal cortex is interconnected with the basal ganglia so Parkinson's patients have difficulty initiating movements to targets in working memory. They need an actual external stimulus to initiate movement
Parietal-Occipital-Temporal (PTO) area functions1. Polimodal convergence of senses: this area is near visual, somatosensory, auditory areas and thus ideal for tasks that may require these senses. Locating objects can be done by touch, sight, sound. Language involves sound, sight, touch.
2. Attention: allows us to focus in on a specific stimuli and neglect others e.g. a specific voice in a crowd.
3. Memory: inferior and medial portions of temporal lobe involved in long term memory. Right side - pictorial memory (e.g. faces) Left side - verbal memory (e.g. names of people)
Attentionan analogy is a flashlight that casts light on particular features. Acts like a bottleneck - limits what enters the brain. Retina and visual cortex see both the person and bike but attention limits what gets further down. Bottle neck required because you would faint due to high metabolic cost of APs. Attention is another method the brain uses to limit the number of active brain cells. Attention directs activity to particular brain areas that are best suited to process that info
Visual perception 2 stage process1. an early involuntary stage that automatically performs rapid low level processing of the visual world.
2. A voluntary and attention-demanding capacity-limited bottle neck that regulates what enters working memory, awareness, and consciousness.
Attention blinkwhile attention is processing 1 visual object, blind to the presence of other objects even those at the location you had been attending
How can attention be drawncan be drawn below by objects that pop out from the background. In early visual areas V1, V2, V3, similar objects are inhibited and different are accentuated. Areas in the parietal dorsal stream can also direct attention voluntarily, as can areas that direct them.
Why does it take time to find an object in a crowded scene?1. object does not automatically draw your attention.
2. takes time to voluntarily shift your attention w/o moving your eyes (covert shift), or make an eye movement (overt shift),
3. takes time to process image at each location
Attention vs. arousalarousal is general activation of brain while attention is specific. Arousal is mediated by 1 of the 7 diffuse systems arising from brain stem - locus coeruleus whose neurons release NE projecting to cortical neurons to increase one's alertness.
Attention at specific location, different features, color and orientationDifferent features - causes activity in different cortical regions best suited for processing that feature e.g. color - early visual areas V1, V2, V3 are activated; form - LOC; motion - MT+;

Color & orientation - early visual areas automatially cause the odd line to pop out. Attention can selectively enhance the activity of neurons that code a particular color or orientation. Selective action is guided by feedback from higher areas to early visual areas.
3 types of attention1. Spatial attention: voluntarily direct the spotlight of attention to a particular location.
2. Feature-based: voluntarily focus your attention onto a particular attribute such as orientation, colour, etc,
3. Object-based attention: objects which stand out from background automatically attract attention
A lesion of the right parietal cortex vs lesion in right V1right parietal cortex - neglect of object's left side independent of where the patient looked;
right V1 - blind to everything to the left of where one's eyes look
A lesion of the left partial cortexno neglect because both object sides are represented in the right parietal cortex. Right parietal attends to both
Allocentric coordinatesgives an object's location with respect to some other object e.g. a table in terms of its location in a room, a feature of a face in terms of its location on the face. Object centered.
Egocentric coordinateswhere the object is relative to you (your body, your head, or your eyes/fovea), "north" is allocentric as opposed to "right" is egocentric
Nested coordinates examplenested allocentric frame - apple sitting on a table located in corner of a room
Where stream coordinateencode locations in egocentric coordinates. Walk to the restaurant your left.
What stream coordinateencode locations in allocentric coordinates. Features of a face are encoded in an allocentric frame and you can attend to a feature independent of where the face is on the retina