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Sensorimotor systems Learning, memory & amnesia Chapters 8 and 11 Three principles of sensorimotor function hierarchical organization Two other organizing characteristics? motor output is guided by sensory input The case of G.O. – darts champion The exception? learning changes the nature and locus of sensorimotor control Posterior Parietal Association Cortex Function: Integrates of sensory information to plan and initiate voluntary movement and attention. Sensory system inputs: visual, auditory and somatosensory. Outputs: dorsolateral PFC, secondary motor cortex and frontal eye fields. Dorsolateral PFC Frontal eye field Auditory cortex Visual cortex Inputs to Posterior Parietal Association Cortex Dorsolateral PFC Frontal eye field Auditory cortex Visual cortex Outputs to Posterior Parietal Association Cortex Damage to the Posterior Parietal Association Cortex Can produce a variety of deficits Attention Perception and memory of spatial relationships Reaching and grasping Control of eye movements Damage to the Posterior Parietal Association Cortex Apraxia – a disorder of voluntary movement not attributable to a simple motor deficit (weakness or paralysis) or to a deficit in comprehension or motivation. Results from unilateral damage to the left posterior parietal cortex. Damage to the Posterior Parietal Association Cortex Contralateral neglect – a disturbance in a patient’s ability to respond to stimuli on the side of the body contralateral to a brain lesion (not a simple sensory or motor deficit). Often associated with large lesions of the right posterior parietal lobe. Dorsolateral Prefrontal Cortex Function: plays a role in the evaluation of external stimuli and initiation of voluntary responses to those stimuli. Main input: posterior parietal cortex Outputs: secondary motor cortex primary motor cortex frontal eye fields Dorsolateral Prefrontal connectivity Dorsolateral Prefrontal cortex Neurons in this area respond to the characteristics of objects (e.g., color/shape), the location of objects or to both. The activity of other neurons is related to the response itself. Secondary motor cortex Input: most from association cortex Output: primary motor cortex Two classic areas: 1) SMA 2) Premotor cortex Secondary Motor Cortex Current classifications suggest At least 7 different areas 3 supplementary motor areas SMA and preSMA and Supplementary eye field 2 premotor areas PMd and PMv 3 cingulate motor areas CMAr, CMAv and CMAd Secondary Motor Cortex Subject of ongoing research In general, may be involved in programming patterns of movements based on input from PFC Mirror neurons – in premotor cortex (also in posterior parietal cortex) are involved in social cognition, theory of mind and may contribute to autism if dysfunctional. Primary Motor Cortex Precentral gyrus of the frontal lobe Major point of convergence of cortical sensorimotor signals Major point of departure of signals from cortex Somatotopic – more cortex devoted to body parts which make many movements Motor homunculus Primary Motor Cortex Monkeys have two hand areas in each hemisphere, one receives feedback from receptors in skin. Stereognosis – recognizing by touch – requires interplay of sensory and motor systems Damage to primary motor cortex Movement of independent body parts (e.g., 1 finger) Astereognosia Speed. accuracy and force of movement Other sensorimotor structures outside of the hierarchy (sometimes called extrapyramidal systems) Cerebellum Basal ganglia both modulate and coordinate the activity of the pyramidal systems by interacting with different levels of the hierarchy. Cerebellum 10% of brain mass, > 50% of its neurons Converging signals from primary and secondary motor cortex brain stem motor nuclei (descending motor signals) Somatosensory and vestibular systems (motor feedback) Involved in motor learning, particularly sequences of movement Damage to cerebellum – disrupts direction, force, velocity and amplitude of movements; causes tremor and disturbances of balance, gait, speech, eye movement and motor sequence learning . Basal Ganglia A collection of nuclei Part of neural loops that receive cortical input and send output back via the thalamus (cortical-basal ganglia-thalamocortical loops) Modulate motor output and cognitive functions Cognitive functions of the basal ganglia Descending Motor Pathways Two dorsolateral Two ventromedial Corticospinal Corticorubrospinal Corticospinal Cortico-brainstem-spinal tract The corticospinal tracts are direct pathways Dorsolateral Vs Ventromedial Motor Pathways Dorsolateral one direct tract, one that synapses in the brain stem Terminate in one contralateral spinal segment Distal muscles Limb movements Ventromedial one direct tract, one that synapses in the brain stem More diffuse Bilateral innervation Proximal muscles Posture and whole body movement Experiments by Lawrence and Kuypers (1968) Experiment 1: bilateral transection of the Dorsolateral (DL) corticospinal tract Results: 1) monkeys could stand, walk and climb 2) difficulty reaching but improved over time 3) could not move fingers independently of each other or release objects from their grasp. Experiments by Lawrence and Kuypers (1968) Experiment 2: The same monkeys with DL corticospinal tract lesions received 1 of 2 additional lesions: 1) The other indirect DL tract was transected 2) Both ventromedial (VM) tracts were transected Experiments by Lawrence and Kuypers (1968) Experiment 2 Results: • The DL group could stand, walk and climb but limbs could only be used to ‘rake’ small objects of interest along the floor • VM group had severe postural abnormalities: great difficulty walking or sitting. Although they had some use of the arms they could not control their shoulders. Experiments by Lawrence and Kuypers (1968) Conclusions: • the VM tracts are involved in the control of posture and whole-body movements • the DL tracts control limb movements (only the direct tract controls independent movements of the digits. The case of H.M. Intractable epilepsy one generalized convulsion each week Several partial convulsions each day 1953 surgery: Bilateral medial temporal lobectomy temporal pole amygdala entorhinal cortex hippocampus Corkin et al. (1997) Corkin et al. (1997) Effects of Bilateral Medial Temporal Lobectomy Convulsions were dramatically reduced IQ increased from 104 to 118 Short-term memory (STM) intact Temporally-graded retrograde amnesia Severe anterograde amnesia Amnesia Retrograde (backward-acting) – unable to remember the past Anterograde (forward-acting) – unable to form new memories While H.M. was unable to form most types of new long-term memories, his STM was intact Mirror-drawing task H.M.’s performance improved over 3 days (10 trials/day) despite the fact that he could not consciously remember the task on days 2 and 3. Rotary-Pursuit Test H.M.’s performance improved over 9 daily practice sessions; again, with no recognition of the experience Explicit vs Implicit Memories Explicit memories – conscious memories Implicit memories – unconscious memories Repetition priming tests were developed to assess implicit memory performance; Incomplete pictures test Implications of H.M.’s amnesia Medial temporal lobes are involved in memory formation. STM and LTM are dissociable – H.M. is unable to consolidate certain kinds of explicit memory. the fact that he could form some memories suggests that there are multiple memory systems in the brain. Medial Temporal Lobe Amnesia Not all patients with this form of amnesia are unable form new explicit long-term memories, as was the case with H.M. Two kinds of explicit memory: Semantic memory (general information) may function normally while episodic memory (events that one has experienced) does not – they are able to learn facts, but do not remember doing so (the episode when it occurred) Vargha-Khadem et al., (1997) Studied three children that had bilateral temporal lobe damage early in life. Like H.M., the children could not form episodic memory, however they did acquire reasonable levels of factual knowledge and language ability in mainstream school. Effects of Cerebral Ischemia on the Hippocampus and Memory R.B. suffered damage to just one part of the hippocampus (CA1 pyramidal cell layer) and developed amnesia R.B.’s case suggests that hippocampal damage alone can produce amnesia H.M.’s damage and amnesia was more severe than R.B.’s Object-Recognition Memory Early animal models of amnesia involved implicit memory and assumed the hippocampus was key 1970’s – monkeys with bilateral medial temporal lobectomies showed LTM deficits in the delayed nonmatching-to-sample test Like H.M., performance was normal when memory needed to be held for only a few seconds (within the duration of STM) Delayed nonmatching-to-sample task pretend you’re the monkey Sample stimulus touch it and get a yummy treat 10 min delay during which other sample stimuli are presented Choice phase: pick the image that is new Darn, no food Another yummy treat Testing object-recognition memory Medial temporal lobe (MTL) Delayed non-match to sample results The Mumby Box Object recognition in rats Comparison of lesions in monkeys and rats Neuroanatomy of object recognition Bilateral removal of the rhinal cortex consistently results in object-recognition deficits. Bilateral removal of the hippocampus produces moderate deficits or none at all. Bilateral removal of the amygdala has no effect on object-recognition. Is the hippocampus involved in object recognition memory? The Case of R.B. suggests that the lesions of the CA1 region of the hippocampus (due to ischemia) can produce severe memory deficits Ischemia in animal models also produces deficits in object recognition Yet deficits in object recognition are only moderate to non-existent in other animal lesion models Why? Mumby et al. (1996) Bilateral hippocampectomy actually blocks the damage produced by ischemia! Explanation: Ischemia causes hippocampal neurons to release glutamate, which produces damage outside of the hippocampus (particularly in rhinal cortex), although standard histological techniques do not show the damage follow-up functional imaging studies have confirmed the dysfunction. The Hippocampus Rhinal cortex plays an important role in object recognition. Hippocampus plays a key role in memory for spatial location. Hippocampectomy produces deficits on Morris maze and radial arm maze (Chapter 5) Many hippocampal cells are place cells – responding when a subject is in a particular place Theories of Hippocampal Function O’Keefe & Nadel (1978) Cognitive map theory – constructs and stores allocentric maps of the world Rudy & Sutherland (1992) Configural association theory – involved in retaining the behavioral significance of combinations of stimuli Brown & Aggleton (1999) is involved in recognizing the spatial arrangements of objects Synaptic Mechanisms of Learning and Memory What is happening within the brain structures involved in memory? Hebb – changes in synaptic efficiency are the basis of LTM Long-term potentiation (LTP) – synapses are effectively made stronger by repeated stimulation Long Term Potentiation (LTP) Cross-section of the NMDA receptor complex Ca2+ NMDA Glutamate + AP-5 Na D-Cycloserine Glycine Polyamine HA-966 Zn 2+ PCP Mg MK-801 2+ Mg 2+ K+