- 1 Lecture Notes
- 1.1 Cortex overview
- 1.2 Cortex histology
- 1.3 Brodmann's areas
- 1.4 Location of function in the neocortex
- 1.5 Functional localisation
- 1.6 Motor areas of the cortex
- 1.7 Primary cortical areas
- 1.8 Primary visual cortex
- 1.9 Auditory area
- 1.10 Tonotopic organisation in the auditory cortex
- 1.11 Unimodal association areas
- 1.12 Multimodal association areas
- 1.13 Arcuate fasciculus
- 1.14 Aphasia
- 1.15 Phineas gage
- 1.16 Prefrontal cortex
- 2 Official Notes
- Lobes, gyri and sulci; note also the limbic lobe (functional division)
- Sup, mid and inf frontal and temporal gyri. Sulci between them are just sup and inf
- Recall precentral and postcentral gyrus
- Intraparietal sulcus = groove separating the parietal lobe into superior and inferior parietal lobule
- Cingulate gyrus surrounds the corpus callosum
- Medial ends of pre and postcentral gyri wrap themselves around the central suclus. This area is called the paracentral lobule.
- Parietooccipital sulcus
- Calcarine sulcus
- Superior frontal gyrus wraps around onto the medial side
- Medial side of temporal lobe, the most medial gyrus is called parahippocampal gyrus. A little bit protruding forwards from that is the uncus. (Uncus = olfactory area of cortex)
- Occipitotemporal gyrus
- Orbital gyri exist underneath the front of the frontal lobe (sit on the orbital plate of the frontal bone)
- Cortex is divided histologically into neocortex (6 layers; new and fancy) and allocortex (3-4 layers; primitive)
- Archicortex (limbic system - behaviours associated with survival e.g. learning and memory and instincts; hippocampus)
- Paleocotrex (olfactory areas)
- In the human, archicortex and paleocortex take up a relatively small part of the cortex (neocortex evolved over it)
- See other lectures
- She wants to focus on neocortex
- Pyramidal cells are very large, in layer 5 and are well developed in motor cortex. Sensory areas of cortex: thinner and not as many pyramidal cells. There are therefore structural differences in the sensory and motor parts of the cortex.
- Histological mapping of the brain
Location of function in the neocortex
- Patients who had difficulty in speech post-strokes have the same area of the brain affected (Broca's area)
- Can map the areas with electrodes
- fMRI measures bloodflow; PET scanning uses radioactively labelled glucose
- Neurosurgical studies (split brain) also tell us about function
- Note that functional areas in the cortex reflect that a particular area plays a key role in that function, but there area ctually other areas involved (don't be retarded and oversimplify it)
- Cortex = higher thinking part; decisions made, initiate responses, etc
- Huge in the human; this guy has a huge one.
- PET scans can be used to diagnose Alzheimer's disease
- Substance binds to Alzheimer's plaques (amyloid plaques)
- Flatten out the cortex = 1800cm^2
- Break down areas of cortex into functional groups
- There are areas of cortex associated with motor output: if you run current into these areas, you will get a movement occurring: cortical control areas for movement (two red areas)
- Primary sensory areas - receive specific inputs from sensory pathways, and relay first in the thalamus then on to the cortex. We learned about two of these; one for touch (medial lemniscus) and one for pain and temperature (spinothalamic), both of which end in the thalamus. From the thalamus, they project up to the somatosensory areas of the cortex.
- Inputs from the eyes go back to the thalamus and project onto the visual areas of the cortex. Similarly for area inputs.
Motor areas of the cortex
- Precentral gyrus = primary motor area; very sensitive (even with a small stimulus you get a contraction of a group of muscles)
- Can then map the motor functions onto the motor cortex (homunculus map on precentral gyrus)
- Paracentral lobule = primary motor for leg and foot (medial side of brain). Then you map the rest of the body in sequence
- Large representation for the hand, face tongue and lips (for very fine motor control)
- These motor fibres give rise to about 1/3 of the corticospinal fibres. Note also that it crosses over (hence stroke on one side causes paralysis on the other side).
- Leg and foot area has a different blood supply to the other parts of the motor cortex
- Premotor and supplementary motor areas lie nearby. These motor areas get big inputs from other parts of the motor cortex (higher thinking, for planning etc in complex movements). Premotor area gets inputs from sensory areas. Patients with lesions involving premotor area = apraxia (can't undergo complex tasks e.g. reaching for an object requires seeing where the object is).
- Supplementary area = for imagining a movement in your mind (used for doing learned movements).
- PMA and SMA also contribute fibres to the corticospinal tract
Primary cortical areas
- Postcentral gyrus = primary somatosensory area (touch, pain, proprioception etc)
- Primary visual area surrounds calcarine sulcus in occipital lobe
- Primary auditory area is mostly buried (inside the lateral fissure)
- Smell area is on the medial side of the temporal lobe
- In each of these areas, we have a map of the sensory world; these areas allow us to consciously perceive a stimulus and determine where it is
- You get a slightly different homonculus somatosensory area to the motor one
- Medial = leg/foot/genitals, etc (see slides)
- Areas with a very strong sensory innervation have a large area (lips and tongue are the most sensitive). Fingers are also very sensitive.
- Inputs from the thalamus will travel up the ascending pathway in a specific location in the pathway and land in a specific part of cortex (therefore know the localisation of sensory innervation).
- Somatosensory stuff is also mapped to contralateral part of brain
Primary visual cortex
- Surrounds the calcarine sulcus
- One half of visual field maps to the opposite side
- Right half of the visual field goes to the left side of the brain (see diagram from CCS)
- Transverse temporal gyri
Tonotopic organisation in the auditory cortex
- In the auditory cortex, sound frequencies are mapped precisely onto the auditory cortex
Unimodal association areas
- Next to each primary area is an association area
- E.g. visual association cortex, needed for processing visual information at a higher area
- Somatosensory association area = recognise an object based on shape or touch, or weight, connecting to past experience etc
- Damaging the association areas = agnosia (inability to recognise a stimulus)
- Visual areas = enables you to determine shape, texture and movement in the visual field (gives the object some meaning)
- Thalamus information comes in, goes to visual area, then is relayed out to the rest of the occipital lobe and into the lower part of the temporal lobe. There's also an area allowing you to see movement (strobe light effect if it's damage)
- Auditory association area overlaps closely with language area
Multimodal association areas
- Anything left over that wasn't the above or limbic is multimodal association
- Broca's area and Wenicke's area
- Broca's area = motor speech area (on the inferior frontal gyrus) - note in patients with impaired speech. Sits right next to head and neck area of motor cortex. Contains the computer programs for enunciating specific words, and relays it to the muscles we need nearby.
- Wernicke's area is the receptive speech area. It overlaps the auditory association area (that we saw before). Speech is only located in one hemisphere. Your dominance is determined by which side your speech is in (nearly all right handed people have speech only in their left hemisphere). 2/3 of Lt handers also have speech in this side. Some of them have in both.
- Wernicke's area is involved in interpretation of speech and also in formulating speech and constructing sentences.
- The arcuate fasciculus interconnects our speech areas; we decide what we want to say in Wernicke's area, but then need to get the speech to Broca's area to say it
- Broca's area: inability to use language; can't produce words any more, but they can make sense
- They use minimal words to produce speech; they have difficulty producing the words, so it's slow and slurred
- If you damage Wernicke's area: can't understand speech and can't produce speech.
- Broca's area is able to function, but not getting input from Wernicke's area (might sound normal but they don't make sense).
- Conductive aphasia
- Lesion of arcuate fasciculus: know what you want to say, but the words that come out aren't what they want
- Global aphasia = everything is gone
- Multimodal in the parietal area gets input from somatosensory, auditory and visual association areas. This helps with spatial orientation, and mediating attention. On left side, this area is largely for Wernicke's area. On right side, it's for integrating that information and determining where we are in the world (damage = neglect: don't think one side of your body is yours, and one side of your world is ignored e.g. only eat food on one side of their plate)
- If they are drawing something, they will only draw one half, and also males might only shave one side
- Frontal lobotomy, changing his personality; became labile, unstable, irresponsible, behaved in socially inappropriate way.
- Executive brain; higher level functions, working memory, decision making, problem solving, abstract thinking
- Inferior part is for personality and emotions, and social behaviours (Tourette's)
Major sulci and gyri of the cerebral cortex
- Lateral surface:
- Frontal lobe – superior, middle and inferior frontal gyri, precentral gyrus
- Parietal lobe – postcentral gyrus, superior and inferior parietal lobules separated by the intraparietal sulcus.
- Temporal lobe – superior, middle an inferior temporal gyri
- Occipital lobe – lateral occipital gyri
- Medial and Inferior surfaces
- Cingulate gyrus – surrounding the corpus callosum
- Paracentral lobule – formed by medial ends of re- and postcentral gyri. Represents leg and foot of primary motor and somatosensory areas.
- Parieto-occipital sulcus; calcarine sulcus (in occipital lobe).
- Parahippocampal gyrus and uncus – on medial side of temporal lobe
Structure of the cerebral cortex
- Cerebral cortex - approx 40% total brain weight. If flattened out would cover approximately 1800 cm2. Ranges from 1.5 - 4 mm in thickness and contains 10 - 20 billion neurons. Based on pattern of cellular lamination it can be divided into:
- allocortex ranges from 3-5 layers - makes up 10% of human cortex. Subdivided into
- archicortex - most primitive - hippocampal formation
- paleocortex - olfactory areas, including uncus (primary olfactory).
- neocortex (also known as isocortex) - remaining 90% cortex - 6 layers.
- allocortex ranges from 3-5 layers - makes up 10% of human cortex. Subdivided into
Functional Localisation Within the Cerebral Neocortex
- Our current knowledge of functional localisation in the cerebral cortex is based on evidence from early clinical studies, physiological studies and more recently functional MRI (fMRI) and PET scanning. Broadly speaking the cerebral neocortex can be divided up into:
- Motor areas – give rise to descending motor tracts, and if stimulated electrically, result in movement.
- Primary sensory areas - receive discrete inputs from sensory pathways via relays in the thalamus. Each contains a map of its sensory ‘world’ and is involved in the initial perception and localisation of particular sensory stimuli.
- Unimodal association areas – located adjacent to primary sensory areas. Receive presorted info from the adjacent sensory area and are concerned with higher level processing relevant to that specific sensorymodality. Lesions result in various types of agnosias – inability to recognize stimulus based on sensory cues.
- Multimodal association areas – integrate information from a multiple areas of the cortex and are concerned with high-level perceptual and intellectual functions.
- Limbic areas - concerned with behaviours associated with survival, in particular the formation and retrieval of memories. Will be covered in later lectures.
- Primary Motor Cortex (M1) - Precentral gyrus (area 4) low threshold to stimulation -> simple movements. Strong input from Cb via thalamus. Somatotopically organized (motor homunculus). Regions with fne motor control have larger areas of cortex devoted to them. M1 gives rise to 30% of fibres in pyramidal tracts. Lesion -> contralateral flaccid paralysis that becomes spastic after several days. Some recovery of trunk and proximal limb movements but fine precision movements of hands and feet remain impaired (movements clumsy).
- Premotor (PMA) and supplementary motor (SMA) areas – higher motor areas (they in part control what happens in M1). Located anterior to precentral gyrus and form Brodmann’s area 6 (PMA lat surface, SMA medial). Their output cells project to the M1 as well as giving rise to bout one third of pyramidal fibres. Electrical stimulation needs to be higher than M1 but produces produces more complex movements, usually involving the trunk and proximal limbs. PMA important for movements involving sensory cues. Lesion results in a form of apraxia, in which the patient is unable perform to complex movements which are guided by sensory cues, such as dressing oneself. SAM important for movements retrieved from memory - active when mentally rehearsing movements.
Primary Sensory Areas
- Primary Somatosensory Area (S1) – postcentral gyrus. Corresponds to Brodmann’s areas 1, 2 and 3. Inputs (eg. touch, proprioception, pain) from opposite side of body via thalamus - somatotopically organised – areas with richer sensory innervation have larger areas of cortex – sensory homunculus (Homunculus – latin for ‘little man’). Lesion causes loss of ability to perceive and localize a stimulus eg. pin prick.
- Primary Visual area (V1) - area 17 - inputs from opposite half half visual field both eyes via thalamus. Inputs organised retinotopically --> map of contralateral (opposite) half of visual field. Lesion causes loss conscious awareness visual stimuli in contra hemifield (homonymous hemianopia).
- Primary Auditory area - A1 - area 41 - tonotopically organised (low freq sounds anterolateral, high freq posteromedial). Inputs from both ears, so lesion on one side does NOT cause significant hearing loss.
Unimodal Association Areas
- Somatosensory Association area
- Located in superior parietal lobule behind S1. Integration of somatosensory info with other areas of cortex recognition of objects. Lesion: can't recognise object based on touch (eg. astereognosis)
- Visual association area (V2-5) - areas 18, 19, surrounding primary visual area in occipital lobe, and extending into inferolateral part of temporal lobe (areas 20, 37). Important for recognition of form, colour, movement as well as attentional aspects vision. Lesion various types visual agnosias eg. can’t perceive colour or movement, or can’t recognize faces (prosopagnosia).
- Auditory Association area - area 22 - recognition of sounds – overlaps with Wernicke’s area (speech -dominant side; environmental, musical - non dominant)
Multimodal Association Areas
- Language Areas
- Motor Speech (Broca’s) area – in dominant hemisphere only - inf. frontal g. Controls articulation (production) of words. Lesion motor aphasia - speech slow, slurred, comprehension normal.
- Receptive Speech (Wernicke’s) area - on dominant side, includes planum temporale, posterior part of sup. temporal (post part) and part of supramarginal g. Lesion -> receptive aphasia - lack comprehension of speech, can’t formulate speech.Two areas linked by arcuate fasciculus. Lesion -> conduction aphasia.
- Parieto-temporal association cortex
- Integrates somatosensory, visual and auditory information. Relays information to premotor area and prefrontal areas. Important in mediating spatial orientation and also attention. Lesions parietal lobe can cause unilateral neglect syndrome (Rt hemisphere); problems with movements guided by sensory cues (apraxia).Inferior temporal - visual memory (usually symbols in L, complex shapes, scenes in R)
- Prefrontal Association Area
- Makes up majority of frontal lobe anterior to motor areas. Concerned with problem-solving, judgement (dorsolateral part) and emotional/social behaviour (orbitofrontal part). Lesion > inappropriate social behaviour, emotionally labile, poor concentration, motivation and abstract reasoning.
- Lateralisation of function
- Dominant (usually left) hemisphere - language, mathematical, computational skills, intellectual functions involving rational and symbolic thought processes.
- Non-dominant hemisphere - limited or nil language capacity, recognition of complex three-dimensional structures and patterns, faces and images; creative artistic abilities, including music; intellectual functions involving more non-verbal, intuitive processes.