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PSYB65H3 (479)


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University of Toronto Scarborough
Zachariah Campbell

CHAPTER SIXTEEN Subdivisions of the Frontal Cortex Frontal lobe = all tissue anterior to the central sulcus = 20% of the neocortex 3 general areas:  Motor  Premotor  Prefrontal Prefrontal cortex = 3 general regions  Dorsolateral prefrontal cortex  Inferior (ventral) prefrontal cortex o Sometimes called the orbitofrontal cortex due to relation to eye socket  Medial frontal cortex Many areas in frontal cortex are multimodal – deal with many different kinds of sensory information A Theory of Frontal Lobe Function People with frontal-lobe injury cannot handle a situation with many different details. The main areas of difficulty are:  Planning in advance and selecting from many options  Ignoring extraneous stimuli and focusing on the task at hand  Keep track of past occurrences and chains of events The frontal lobe contains systems that are implement different behavioural strategies for dealing with external and internal cues. Functions of the Premotor Cortex Motor cortex is the mechanisms for executing individual movements. The premotor cortex selects the motions to be executed.  This is mainly in response to external cues  The supplementary premotor regions contributes internal cues when there are no external cues available The supplementary motor region is involved in the selection and direction of motor sequences. These are self-paced/internally driven. Motor acts are driven by cues but can also be associated with cues:  Red means stop. Eye movement:  Can be made towards specific targets or made on basis of internal cues o Looking at specific objects or just looking around randomly Functions of the Prefrontal Cortex Prefrontal cortex handles cognitive processes so that correct movements are chosen at the right time/place  Premotor cortex selects movements  Motor cortex is responsible for executing movements. Internal cues Internalized information contains a set of ‘rules’ that guides thoughts and actions. Temporal memory: neural record of recent events and their order. Can be related to:  Things  Object-recognition stream of processing  Movements  Motor stream of processing o Both of these streams project to the frontal cortex (in different places) which means that there is temporal memory of both streams somewhere in the cortex External cues People whose temporal memory is defective depend on external cues on how to act. Behaviour is controlled completely by external cues.  People with frontal-lobe injuries have a problem inhibiting their actions towards external cues One type of external cue: feedback about the rewarding properties of stimuli  See a picture of mother. Know mother will give you love and affection. Picture of mother = feelings of love and affection  The orbitofrontal cortex is essential to learning by association Context cues Behaviour is context-driven  behaviour that is appropriate in one situation may very quickly become inappropriate if some of the context changes even slightly Frontal lobe is very large in social primates Choice of behaviour is dependent on sensory information:  Comes from temporal lobe to inferior frontal cortex It can also depend on affective context – comes from the amygdala Orbitofrontal injuries (lesions, traumatic brain injury) have difficulty with context, especially in social situations. Autonoetic Awareness Def.: autobiographical knowledge that allows us to decide on behaviour after experiences throughout life.  Allows a person to bind themselves as a continuous entity through time Impairment in autonoetic awareness leads to a deficit in self-regulation.  Medial and ventral frontal injury patients show this Asymmetry of Frontal Lobe Function Left frontal lobe: preferential role in language related movements (speech) Right frontal lobe: greater role in nonverbal movements (facial expression)  This asymmetry is relative not absolute – people with damage in one lobe don’t lose all functions related to that lobe. They retain some, which shows that the other lobe still holds some of the processes. Bifrontal lesions cannot be duplicated in lesions to either singular hemispheres.  Patients with bifrontal lesions may have problems with proverbs and telling the time of day, for example Heterogeneity of Frontal Lobe Functions Correlations among tasks specific to frontal lobe are very low. Why?  Tasks often require different processes, all linked to different areas of the frontal lobe  Patients will all have different lesions in different areas, meaning that performance will be affected to different degrees, depending on the location of the lesion Symptoms of Frontal Lobe Regions Disturbances of Motor Function Fine Movement, Speed, and Strength Damage to primary motor cortex  chronic loss of ability to make fine, independent finger movements  Maybe due to loss of direct corticospinal projections onto motor neurons Can also lead to loss of speed and strength in limb and hand movements  This can also occur in damage to the prefrontal cortex Movement Programming Damage to the supplementary motor cortex  disruption to all voluntary movements but this is usually quickly recovered. Only lasting symptom is damage to the performance of rapidly alternating movements in the fingers or hands Large lesions in the supplementary motor cortex may only lead to relatively minor symptoms  This is due to the fact that both the left and right premotor cortices participate in movement Patients with frontal-lobe injuries make more errors in sequence  Components are recalled correctly but in the wrong order Can also lead to issues in copying facial movements = frontal lobe may have role in controlling the face Voluntary Gaze Damage to frontal lobe  issues with voluntary gaze Study: recording patients’ eye movements while looking at complex image  Control – when asked about people’s clothing, would look at a person’s body in an image. When asked about age, would look at the person’s face.  Person with frontal-lobe damage – when asked same questions, would just look around image randomly. Change is question about the image didn’t change the pattern of eye gaze. Study: Part 1: ability of patients to make voluntary eye motions towards or away from a briefly appearing target to the left or right of a fixation point.  Normally, a person would make a quick eye movement (saccade) towards a stimulus  Person with damage: no issues with this part Part 2: Patients had to move eyes to same spot as the target had appeared on an opposite visual field. This meant – limiting normal voluntary saccade and making a voluntary saccade in opposite direction  Patients with damage had two issues with this: o Couldn’t limit normal voluntary saccade o Couldn’t make eye movements in contralateral direction to the damaged hemisphere Corollary Discharge Pushing on eyeball = world moves. Move eyes = world stays stable. Corollary discharge (reafference): a neural signal is needed to make the eyes move and other is needed to warn that the movement is going to happen.  Moves eyes mechanically: there is no signal and world moves  Moves eyes normally: signals and the world stays still Voluntary eye movements need two sets of signals: 1. Movement command (through the motor system) effects the movement 2. Signal (corollary discharge) goes from frontal lobe to parietal and temporal association cortex prepares the sensory system for the movement This means that a person’s sensory system can interpret changes in the world due to information about movement. Frontal lobe lesion  can disturb movement of voluntary eye gaze and the signal to the rest of the brain that a movement is about to take place Evidence of frontal lobe participation in corollary discharge:  Some cells in frontal eye fields fire simultaneously with eye movement  They seem to monitor ongoing movement, a process very similar to corollary discharge Speech Frontal lobe has two speech zones:  Broca’s area – selects words on basis of cues  Supplementary speech area – required to find words with no external cues Damage to Broca’s area  agrammatism: impairment in ability to use verbs and produce appropriate grammar Damage to supplementary area (occurs with strokes)  may result in becoming mute  Unilateral damage: ability to speak may come back after a few weeks  Bilateral damage: ability will not come back Loss of Divergent Thinking Posterior region lesions = decrease in IQ scores. Frontal lobe lesions do not but mistakes are still made. Theory that IQ tests measure convergent thinking: the idea that there is only one answer to every question.  Examples: definitions, puzzles, etc. Other tests measure divergent thinking: where number and variety of answers to a single question is emphasized.  Lesions to the frontal lobe interfere with this kind of thinking Behavioural Spontaneity Frontal-lobe damage  general loss of spontaneity Thurstone Word Fluency test: write as many words as possible starting with this letter in 5 minutes. Write as many four-letter words as possible starting with another letter in 4 minutes.  People with lesions often have a very low output in this test This seems to occur when there is damage to the left orbitofrontal region. However, lesions to the right orbitofrontal region may result in loss of verbal fluency Test with Mrs. P – damage to right frontal lobe. 4 features of frontal lobe damage when she was given the test: 1. Low output of words 2. Rule breaking – didn’t write four-letter words on second part of test. She understood the instructions but couldn’t organize her behaviour to follow them 3. Writing was very jerky 4. Kept talking and looking around the room during the test – diminished attention = loss of verbal spontaneity Study: verbal-fluency deficit
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