Lecture19

The Prefrontal Cortex Intro Subdivisions Connections Functions Volition Working Memory cat lemur macaque dog chimp human Proportion frontal cortex Claim: The frontal lobes mediate those abilities that make us uniquely human. Herein lies the riddle... What makes us uniquely human? Frontal Lobes William James (1890) The uniqueness of the human mind ? the ability to pursue goals. Frontal Lobes Frontal Lobes Fixed vs. Flexible The Frontal Lobes 3 natural boundaries posterior: central sulcus inferior: Sylvian fissure (lateral fissure) medial/inferior: corpus callosum Frontal Lobes Major Subdivisions Precentral Premotor Prefrontal Anterior Cingulate (medial) Subdivisions 46 10 9 45 45 Premotor & Motor Areas Premotor areas (BA 6) ? input from prefrontal regions and parietal association areas (5,7). Selection of actions and intention to move Primary motor cortex (BA 4) input from premotor area (6) and area 44 sends output to spinal cord, and other motor structures (basal ganglia) Frontal network for stimulus directed and self-initiated voluntary, planned actions. Subdivisions Electrophysiological recordings VISUAL CUE (prepare to move) MOVE CUE (Make the move) Onset of premotor activity signaling intention to move Onset of motor activity commanding movement Prefrontal cortex ~ 1/3 of cortical surface Most recently evolved Well developed only in primates the advent of the human species: "age of the frontal lobe" develops late in ontogeny differentiation through age 1 maturation through age 6 Subdivisions Prefrontal Regions Ventromedial PFC Orbito PFC DorsoLateralPFC Ventrolateral PFC Subdivisions Prefrontal Afferents Three major classes of inputs Receives processed information from the 5 sensory systems (not primary sensory regions). Receives information from the hippocampus (long-term memory). Receives information about the internal physiological and motivational state via the limbic system. Connections Three major classes of outputs Sends connections back to all sensory areas from which it receives input (control of attentional processes). Sends multiple connections to motor structures (premotor, supplementary motor, basal ganglia, and superior colliculus). Influences the initiation & regulation of motor behavior. Sends direct connections to limbic structures. Provides a mechanism for modulating memory, influencing autonomic & endocrine function & for the regulation of emotional behavior. Prefrontal Efferents Connections The Prefrontal Cortex Connections Functions Volition Working Memory Frontal Lobe Function Overview Executive Control Functions Inhibition of stimulus bound behavior Voluntary control of behavior by contextually appropriate representations What is context? Task-based rules Self-generated plans Social surroundings Internal environment Prefrontal Regions Ventromedial PFC Orbito PFC DorsoLateralPFC Ventrolateral PFC Subdivisions Each region contributes to different aspects of behavioral ?control? Re-emergence of "primitive" reflexes following frontal damage. Sucking/rooting reflex: elicited by touching the lip/cheek groping reflex: involuntary following with hand /eyes of moving object grasp reflex: forceful grasping of an object that contacts palm or sole of foot stimulus capture: utilization behaviors and visual grasp reflex. Frontal Release signs Volition Utilization Behaviors Patients become reactive to nearby objects, touching them, using them; impulsive tendency to use objects. Volition Without frontal control behavior becomes stimulus bound ANTI-saccade task saccade AWAY from an eccentric target + Release of visual grasp reflex? Volition ANTI-saccade task patients w/ prefrontal damage including FEF (Area 8) + Reflexively saccade to the target. Cannot correct errors in order to generate an anti-saccade left lesion patients can?t generate rightward anti-saccades right lesion patients can?t generate leftward anti-saccades Volition Release of visual grasp reflex? Two aspects of impaired anti-saccade performance More reflexive saccades. Why? Superior colliculus- control rapids, stimulus-driven eye movements. Disinhibited by frontal lobe damage, "releasing" reflexive glances Inability to generate anti-saccades. Why? Difficulty forming representation of goal to control voluntary behavior. Volition Without goal representation... Behavior is determined by reflex habit past-reward (perseveration) immediate stimulus conditions NOT by representations that integrate the relevant current spatial and temporal context. Working memory: On-line representations for guiding behavior Working Memory The Prefrontal Cortex Connections Functions Volition Working Memory Frontal Lobes & Working memory... Goldman-Rakic: Working memory allows behavior to be determined by ?cognitive goals?. A system for maintaining and manipulating information to perform complex cognitive activities (Baddeley, 1992). Working Memory Working memory Model Task: Delayed Response Task Correct response requires keeping baited well in mind. Working Memory Monkeys and humans w/lesions of LPFC fail these tasks. Infants younger than 12 months also fail versions of these tasks. Delayed Response Task Working Memory Delayed Saccade Task ( Goldman-Rakic) Method: Single unit recordings from principal sulcus (BA46). Task Cue one of 8 locations: time 1 3 sec. Delay: time 2 fixation removed signaling GO: time 3 Monkey saccades to remembered location Working Memory Cognitive Role of DLPFC Delay activity location specific reduced when monkeys make errors. Lesions of DLPFC impair performance on this task. Interpretation: Neural activity corresponds to representation of a GOAL The goal is maintained "on-line" available for use. This is working memory. Working Memory Human Working Memory on-line store short-term retention (approx. 10 sec) rehearsal processes executive processes material specific buffers verbal (phonological loop) left hem. spatial (visuo-spatial sketchpad) right hem. EXECUTIVE phono- logical loop visuo- spatial Scratch pad Baddeley?s Model (1992/1995) Working Memory Working Memory Tasks ? 500 msec Verbal Memory ? 3000 msec ? 500 msec M K R D 1500 msec m + 500 msec 500 msec 3000 msec 1500 msec Spatial Memory + + + Regions of Significant Activation Working Memory Reuter-Lorenz, Jonides et al., 2000