Cerebral Cortex: Association Areas and Memory Tutis Vilis

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1 97 Cerebral Cortex: Association Areas and Memory Tutis Vilis a) Name the 5 main subdivisions of the cerebral cortex. Frontal, temporal, occipital, parietal, and limbic (on the medial side) b) Locate the primary sensory and motor areas. This is where most of the sensory information first arrives and motor commands to the muscles originate. Note that in the rat these areas occupy nearly all the cortex. In humans they occupy only about 20%. Frontal c) Locate the higher order olfaction? (secondary) sensory and motor areas. Premotor Higher order visual, somatosensory, and c auditory lie near the respective primary area. This is where sensory information is further processed. Premotor lies in front of motor. a b Parietal Temporal motor motor auditory central sulcus Occipital vestibular taste somatosensory somatosensory visual P226 case 21 Higher order somatosensory Higher order visual d) Locate the 3 association areas. Prefrontal, Limbic, and Parietal-Temporal-Occipital areas Prefrontal This is where: association a) different modalities combine. b) attention is shifted c) planning occurs d) memories are stored d Limbic association Higher order auditory Parietal-temporal-occipital association 1 revised

2 How are the different areas of cortex interconnected? F26.4 a) What is the general pattern of connections between primary, secondary, and association areas? Primary Secondary Higher order Association motor Premotor Prefrontal working memory Short loop reflexes sensory Sensory P-T-O attention Limbic long term memory Short loop reflexes mediate rapid, but simple, responses eg swatting a mosquito. Long loop reflexes, eg writing down the name of a seen object, require the complex processing power of the association regions. long loop reflexes b) At the cellular level, which are the input and output layers? Layer IV receives input from the thalamus and other cortical regions. It is thickest in primary sensory regions. The striate cortex (primary visual) is so called because of it's thick layer IV. Layers III and V send output to other cortical regions, the brain stem, and spinal cord. These layers are thickest in primary motor cortex. output All areas have columns. The bulk of the output from one column is to another column forming a vast interconnective network. P569 Columns um primary sensory primary motor I pyramidal cell small III IV V large VI input to other cortical regions to sub-cortical structures eg thalamus and spinal cord This feature also predisposes the cortex to epilepsy. A locus of abnormal activity in one area quickly spreads to other regions leading to a seizure. These interconnections are, as we will see later, where our memory is. 2

3 Prefrontal association Limbic association Parietal-temporal-occipital association c) List the main functions of each association area and the experimental supporting evidence. Prefrontal: : P225 Planning and working memory ~Lesion of the principal sulcus produces deficits in motor tasks that are spatial and delayed (remembering which of two boxes contains food). This is called working memory, a form of short term memory. For example: close your eyes, wait, and point a particular object in the room. This area is also involved in more cognitive planning such as the consequences of social behavior. Limbic: i) Orbito frontal: Emotion: After lesions, no anger displayed when subject makes mistakes. Has a calming effect. Frontal lobotomies used to be a popular (among some physicians) cure for aggression. Unfortunately it also destroyed all initiative. ii)temporal lobe: Long term memory. The right side is more involved with pictorial memory (eg the recognition of faces) while left side in verbal memory (eg: names of people). Parietal- Occipital-Temporal: Polimodal convergence of senses and attention. ~where information from our various senses first converge. ~right side: manipulation of objects in space by touch, sight, sound etc. ~left side: language: sound of words, written words (sight), or Braille (touch) Attention allows one to focus in on one object and neglect all others. 3 97

4 How and why do the two sides of the cortex differ in function? a) In what tasks does each hemisphere excel? Dominant (usually left) - sequential or serial tasks eg: language (reading writing speaking signing), analytic (math A=B, B=C, therefore A=C) Non-dominant (usually right)- tasks requiring parallel processing eg: spatial tasks, intuitive (C resembles O as I resembles L), geometry, music. F27.3 b Patients with section of the corpus callosum. P594 First tested by Sperry who received a Nobel Prize for his studies. Subject cannot name the apple shown on the left because it is not seen by the language center on the left. The subject can visually recognize an apple and pick it out from a group of other objects with his left arm (the one controlled by the right side of the brain). Two independent brains function in one person (eg patient would hug wife with one arm and push away with the other). to left arm Front c) What are the advantages and disadvantages of lateralization? Advantage: Shorter paths to interconnect related regions. Eg In recognizing a face, the connections involved in binding the features of a face are shorter if both sides of a face are represented on the same side. motor area left arm Disadvantage: less redundancy. 4

5 Neglect P571 We saw that the right PTO is involved in both the spatial organization of objects in space and in focusing our attention to a particular object. A lesion of the right PTO causes neglect of things on the left. When asked to draw a flower, will draw only the right side. You might suppose the a lesion of the left PTO would result in neglect of things on the right. Strangely it does not. Functional imaging shows this is because the right superior parietal area contains a bilateral representation (of things on the left and right) while the left contains only a representation of things on the right. Thus after a lesion on the left, the right side still attends to things on the right (as well as left). After a lesion on the right, the representation of things on the left is lost. Front Front Front R L R normal left lesion right lesion F26.7 5

6 Learning & Memory P666 Definitions: Memory > information that is stored Learning > the storage process (e.g. any form of synaptic plasticity) Remembering > the retrieval process Types of Memory Short term / Working memory - a sort of scratch pad which allows for temporary storage of information - eg: stores the location of objects in the absence of external cues such as when you close your eyes and point to remembered objects - involves tonic activity of neurons in such areas as the frontal lobe Long term Declarative (knowing that) - representations of objects and events e.g. face of a friend - involves associations e.g. name with face - often established in one trial - conscious - starts only after the age of 2 yrs - affected by amnesia - learning involves limbic association areas (hippocampus in medial temporal lobe) Reflexive / Procedural (knowing how) - Skills e.g. riding a bike - established by practice - not conscious - start to develop from birth - not affected in amnesia - learning involves all of the CNS, for example, the tuning of binocular V1 cells during the critical period for stereopsis & the cerebellum for motor skills. 6

7 Declarative memory is divided into two parts. Declarative memory Episodic Particular times and places in one s personal past. Involves the medial temporal lobe Semantic Knowledge of facts and concepts Involves the anterior and lateral temporal lobe. 7

8 Memories of Faces and Places Recall that the visual perception of objects involves ventral stream. Images of faces and places are first decoded by common areas of primary visual (V1), higher order visual (V2, V3/VP), and color and form areas. Familiar places are then decoded and recognized in more medial areas of the temporal lobe while familiar faces in more lateral areas (fusiform face area). primary visual (V1) higher order visual areas color and form areas places medial areas of the temporal lobe faces lateral areas of the temporal lobe 8

9 The amygdala is also involved face recognition. When we encounter someone we know two things happen: A) the conscious identification of who that person is. B) an automatic concurrent glow of familiarity. This glow can occur without the conscious recognition of the person. This glow is accompanied by autonomic responses such as sweating. These responses can be entirely unconscious. It is these autonomic responses that are form the basis of lie detector tests which measure the changes in skin conductance. A fusiform face area ventral what stream B limbic amygdala These two aspects of recognition are mediated by two parallel pathways A) the right inferior temporal cortex (fusiform face area). B) the limbic amygdala conscious identification unconscious autonomic resposes A lesion of A but not B produces sense of familiarity without being able to identify who that person is (prosopagnosia). A lesion of B but not A produces the converse. The patient can identify who the person is but has no sense of familiarity. One young man, after a car accident which affected the path through the amygdala, thought that his parents had been replaced by aliens. 9

10 Brenda Milner's famous patient H.M. P674 ~ the medial temporal lobe (entorhinal cortex) and parts of the hippocampus were lesioned bilaterally to relieve epilepsy. coronal section hippocampus medial temporal lobe lesion Short term OK remembers names for as long as not distracted Declarative (what) Reflexive (how) OK e.g. language old memories OK recognizes his mother new memories Lost cannot remember new acquaintances old new retrograde anterograde damage patient H.M. 10

11 A model for memory processes The circuit of Papez is involved in consolidating short term into long term memory. It consists of a loop from cerebral cortex, through the hippocampus, and back to cerebral cortex. Short term memory ~ involves reverberating circuits ~ activity continues long after input/sensation ends ~this prolonged activity allows gradual consolidation in long term declarative memory after only a single input. input sensations 1 short term working memory limited capacity frontal lobe P676 output actions Long term memory limbic emotional tags (amygdala) 2 consolidation hippocampus 4 remembering Semi permanent changes in synaptic strength between assemblies of neurons. ~ e.g. rats raised in a rich environment have a thicker cortex with larger and more synapses ~ in the case of reflexive memory the changes are produced gradually by repeated exposure to the stimulus. long term memory huge capacity 3 distributed in limbic association areas 11`

12 Amnesia affects declarative memory only a) anterograde b) retrograde - failure in consolidation (forgetting) - failure in retrieval (not - cannot store new memories remembering) old memories - e.g. patient HM: rereads a story - lesions of thalamus or cortex always as if new especially frontal (Alzheimer s - old memories intact disease) - lesions of hippocampus, temporal - or after long term memory trace is cortex, or amygdala lost eg after head trauma to the temporal lobe. input sensations short term working memory limited capacity frontal lobe output actions limbic emotional tags consolidation hippocampus remembering long term memory hugh capacity distributed in association areas 12

13 Most areas of cortex remain plastic throughout life example remapping of homunculus in primary somato sensory cortex of adults. post central gyrus initial representation of digits After long term selective use of digits 2 & 3 (e.g. key boarding with 2 fingers) the representation of these two digits expand and the representation of other digits become smaller 1 After the nerve from digit #3 is cut, the representation of #3 is taken over by digits 2 & Both these plastic changes can occur even in adults. The processes involved are relevant to recovery of function, e.g. after stroke. 13

14 Summary Encoding Long Term Memories The ventral stream extracts the visual features, encodes them in a object centered reference frame, and stores them temporally in working memory. Features from various association areas are funnel into the hippocampus. The hippocampus in turn re-activates these and other association areas. Features extracted by the other senses, e.g. hearing, are associated with the visual features and visa versa. The activation somehow binds together/associates various features combination into a rich multi modal memory. The memory is long term and requires the changes in the structure of synapses. These structural changes involve the expression of genes and the synthesis of proteins. Patients like HM suggest that once this long term memory is formed, seeing the same object, e.g grandmother s face, will activated the same associations directly, without the need of activating the hippocampus. 14

15 Take Home Problems 1) Suppose you had a lesion of the right PTO and you were asked to imagine you are standing at one end of your bedroom facing the room. What objects would you remember and what would you not remember? Now suppose that you were asked to imagine yourself standing at the opposite end of the room. How would your memory of things change? When asked to point forward, where will you point? Do you think there might be something odd about the way you dressed yourself? 2) How is neglect different from agnosia? 3) What type of memory does the ability to read involve? 4) What other things do you think H.M. can and cannot remember? 5) Is the hippocampus used in long term memory retrieval? Support your answer with what you know of patient H.M. 6) What might be different from normal in the primary somato sensory cortex of a professional string player? 15