Lecture 19_ Cognitive Functions and the Organization of the cerebral cortex.docx

Lecture 19: Cognitive Functions and the Organization of the cerebral cortex 16 through 19.5 on exam Something that you studied for in the exam, but didn’t actually show up on the exam Association cortex Association cortex integrates information from multiple modalities - sensory, motor, emo...

Lecture 19: Cognitive Functions and the Organization of the cerebral cortex 16 through 19.5 on exam Something that you studied for in the exam, but didn’t actually show up on the exam Association cortex Association cortex integrates information from multiple modalities - sensory, motor, emotional Parietal association cortex analyzes space, generates attention, and transmits sensory information to the motor system Temporal cortex (hippocampus) organizes declarative memory, also involved in higher order visual and auditory processing, language Frontal association cortex governs “executive functions” - plans behavior, facilitates working memory, inhibits responses Cortical Lamination Humans have 3 to 6 corticallayers, depending on the area of cortex Cortical layer 4 typically rich instellate neurons Primary sensory cortex: receives input from the thalamus (the major sensory relay from theperiphery) Layer 5-6 contains pyramidal neurons whose axons typically leave the cortex Layer 2-3 contains smaller pyramidal neurons that primarily have corticocortical connections Hippocampal cortex has 3-4 laminae Cortical Module The circuitry of all cortical regions has common features: Each cortical layer has a primary source of inputs and a primary output target Each area has connections in the vertical axis (columnar or radial) and connections in the horizontal axis (lateral or horizontal) Cells with similar functions tend to be arrayed in radially aligned groups that span all cortical layers Interneurons within specific cortical layers give rise to extensive local axons that extend horizontally in the cortex, linking functionally similar groups of cells. Cortical lesions leading to left hemispatial neglect syndrome None of the lesions spare the parietal lobe in every case the parietal is affected Thalamic connections to association cortex Thalamic inputs to association cortices come from Mediodorsal thalamus Pulvinar Ventral anterior nucleus These thalamic nuclei are not primary recipients of sensory or motor inputs Instead, they receive inputs from other regions of cortex Association cortex connectivity Does not imply everything is connected with everything: each association area is defined by a distinct, if overlapping, set of thalamaic, cortical and subcortical connections Limitation: most of the evidence comes from animal research or post-mortem, lesion studies Synesthesia Synesthesia is a perceptual phenomenon in which stimulation of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway How do we define cortical subregion functionality? Lesions studies: Induced damage to tissue disrupts function. Damage can be caused in multiple ways (destruction of cytoarchitecture leads to circuit failure) Function of damaged area is inferred by observing behavioral output Messy technique Lasts long term (possible for adaptations to arise) Lacks fine spatial resolution May not be directly replacatable patient to patient (as in the case of injuries) Can exist with other comorbidities in patients Functional studies Direct probing of tissue in living patients can provide real-time assessment of function Surgical techniques/probes Transcranial Magnetic Stimulation Deep brain stimulating electrodes Transcranial Magnetic Stimulation A TMS device generates a perpendicular magnetic field that penetrates the skull and brain tissues, induced by two coils of metal wire that carry electrical current A computer program rapidly switches the magnetic field off and on, inducing another electric current that stimulates neurons By moving the device, scientists and clinicians can focus the stimulation on brain regions that are involved in depression Association cortices (Posterior) Parietal cortex Function of the posterior parietal cortex Parietal cortex participates in the construction of spatial representations that guide both attention and movement Spatial location: where things are (relative to own body) Body image (“where are my own parts?”) Attention Transfer of sensory information to the motor system Flow of visual information in the cortex - The parietal “where” and the temporal “what” pathway Dorsal (spatial vision) pathway - where? Spatial information needed to guide eye, hand, or limb movements Ventral (object recognition) pathway - what? Details of object characteristics (color, shape, size) Relational properties of objects (independent of the observer) The parietal association cortex mediates attention Lesions in parietal cortex: Contralateral neglect syndrome Inability to attend to objects, or even one’s own body, in a portion of space, despite visual acuity, somatic sensation, and motor ability remaining intact Cortical lesions leading to left hemispatial neglect syndome Injury to the right inferior parietal lobe is relatively common Deficits in attentio to the left side of personal & extrapersonal space (the side contralateral to the lesion) Right parietal, right superior temporal, or right frontal brain damage can all lead to difficulty attending to the difficulty attending to the left side of visual space Contralateral neglect syndrome Some play from the left, primarily from the right Typically due to right-side lesion Right hemisphere dominates attention for both sides (→ lesions on left side can be compensated) Left hemisphere is occupied with language Hemispatial neglect syndrome “Attention neurons” in parietal cortex Objects can be made silent; e.g., objects change their color or luminance (bottom-up processing), or instructions make objects more important (top-down processing) Parietal neurons respond more intensely to attended objects than to unattended ones Balint’s Syndrome Three characteristics Simultanagnosia: the inability to attend to and/or perceive more than one visual object at a time Optic ataxia: the impaired ability to reach for or point to an object in space under visual guidance Oculomotor apraxia: difficulty voluntarily directing the eye gaze toward objects in the visual field with a saccade Apraxia Ideomotor apraxia: Inability to copy movements when asked to do so. For example, an apraxic patient patient may have difficulty demonstrating hammering movements when asked, but cna be perfectly capable of spontaneously hammering a nail Almost always associated with left hemisphere damage, but its symptoms are always bilateral Constructional apraxia: Spatial organization is disordered. Patients cannot assemble a puzzle, draw a picture or copy a series of facial movements Summary of major symptoms of parietal damage Temporal cortex “Recognition neurons Ventral visual pathway/object recognition (“what”) pathway Face-specific neurons, also specific for orientation of face Language: Wernicke and Broca Wernike’s area: Understanding of written and spoken language Located in the posterior section of the superior temporal gyrus in the dominant cerebral hemisphere (left side in ~97% of people) Wernicke’s aphasia - Receptive/Fluent aphasia: Impairment of language comprehension where speech is phonetically and grammatically normal, but meaningliess Broca’s area: language/speech production Broca’s aphasia - Excessive/Nonfluent aphasia: Patients know what they want to say, they just cannot get it out Typically able to comprehend words, and sentences with a simple syntactic structure, but are more or less unable to generate fluent speech (Pre-) Frontal cortex Prefrontal cortex = dorsolateral and orbital frontal cortex Phineas Gage In 1848, Mr. Phineas Gage’s left frontal lobe was destroyed when a metal rod passed through his head after a freak explosion First documented personality change due to frontal lobe damage Before the accident he was described as hard-working, responsible and popular with his coworkers; after the accident he lost all social skills, was impatient, obstinate and unfocused. Psychosurgery Neurosurgical destruction of the frontal lobe Frontal lobotomy or leukotomy More than 20,000 procedures performed, mostly in the US Frontal lobes important in personality structure and behavior Interfering with frontal lobe function might alter the course of disorders such as schizophrenia and other chronic psychiatric disorders Overview - Functions of PFC Effects of PFC lesions in primates by sub-region Orbital Inability to suppress distracting stimuli Impaired decision making Flattened affect Impaired social behavior Medial Inability to focus attention Apathy, loss of attention to surroundings Dorsalateral Inability to sustain attention Lack of motivation and awareness Deficits in working memory Deficits in (motor) planning (temporal organization of behavior) Dorsolateral PFC executive functions: Attention Working memory Planning Response inhibition “Planning neurons” Delayed response task Monkey watches experimenter place food in 1 or 2 wells Followed by a delay period Monkey gets one chance to choose well containing food Bilaterally lesion prefrontal cortex Diminished/abolished ability to locate food Neurons in the prefrontal cortex specifically fire to delayed response task Maximally active during short deay period (red area) As if their firing represented information about the location of the food morsel maintained from the presentation part of the trial Same neurons have reduced activity during the motor response Indicates neurons are active in short-term memory and planning, rather than the actual movement itself