Cerebral cortex

Question 1

Cerebral white matter: explain the function of association, commissural and projection fibres in the cerebral white matter

Answer 1

The cerebral white matter has THREE types of fibre:

Association Fibres = connect with areas in the SAME hemisphere (short pathways)

Commissural Fibres = connect the two hemispheres (make up the corpus callosum) another small posterior commissural

Projection Fibres = connect the cortex with lower brain structures (e.g. thalamus), brainstem and spinal cord

Question 2

Cortical cellular organization: identify the vertical and horizontal organization of the cells of the neocortex

Answer 2

Grey Matter: Cortical Layers: 6 layer neocortex (layer 1:outside)

• The grey matter is organised into layers (laminar structure) with 3-­‐6 cortical layers which are usually designated by roman numerals, with letters for laminar subdivisions
• Each of the cortical laminae in the neocortex (which covers the bulk of the cerebral hemispheres and is defined by six layers) has characteristic functional and anatomical features
• NEOCORTEX = a part of the cerebral cortex concerned with sight and hearing in mammals, regarded as the most recently evolved part of the cortex
• The different layers of cortex have different connections e.g. layer 4 has input from the cortex – thalamus (LOOK AT THE DIAGRAM)
• The generally smaller pyramidal neurones in layers 2 and 3 (which are not as distinct as their roman numeral assignment suggests) have primarily corticocortical connections
• Layer 1 contains mainly neuropil (area composed mostly of unmyelinated axons, dendrites and glial cell processes that forms a synaptically dense region containing a relatively low number of cell bodies)
• E.g. cortical layer 4 is typically rich in stellate neurones with locally ramifying axons, in the primary sensory cortices
• These neurones receive an input from the thalamus
• Layer 5 and to a lesser degree layer 4, contain pyramidal neurones whose axons typically leave the cortex
• Neocortex is arranged in layers (lamina structure) and columns
• More dense vertical connections – basis for topographical organization
• Neurons with similar properties are connected in the same column
• The neurones are also locally organised into columns
• Along the column of neurones, there are dense vertical connections
• These neurones are all talking to each other and they have a particular function
• This is the basis for topographical organisation

Question 3

Association cortex: list the functions of the association cortex in each cortical lobe, explain the types of deficits caused by lesions there, and compare the attributes of the primary and association cortex

Answer 3

Primary cortices

• function predictable
• organised topographically
• left-right symmetry

Auditory cortex, taste and smell

Association cortices

• function less predictable
• not organised topographically
• left-right symmetry weak or absent

Auditory association cortex, visual cortex

Cortex is divided into 4 lobes

Visual association cortex analyses different attributes of visual image in different places

Form and Colour are analysed along the ventral pathway

Spatial relationships and movements are analysed along the dorsal pathway

Posterior parietal association cortex creates a spatial map of the body in its surroundings from multi-­‐modality information

Injury may cause disorientation, inability to read a map or understand spatial relationships, apraxia, hemispatial neglect

DEFINITION OF APRAXIA = inability to make skilled movements with accuracy

Object recognition o Memory

o Emotion

Injury leads to:

Agnosia = disorder of the brain whereby the patient cannot interpret sensations correctly although the sense organs and nerves conducting sensation to the brain are functioning normally

E.g. auditory agnosia = patient can hear but can’t interpret sounds

Tactile agnosia = retains normal sensation in the hands but cannot recognise three dimensional objects by touch alone

Visual agnosia = patient can see but cannot interpret symbols, including letters

Receptive Aphasia = the patient is unable to understand language in its spoken or written form

Judgement o Foresight o Personality

Appreciation of self in relation to the world

Injury leads to deficits in planning and inappropriate behaviour

Primary Cortices:

• Their function is predictable
• Organised topographically
• Left-­‐right symmetry

Association Cortices:

Function is less predictable

• It is NOT organised topographically
• Left-­‐right symmetry is weak or absent

Question 4

Cortical function: recognise the inter-hemispheric differences in cortical function, and recognise ways to assess cortical function, including lesions

Answer 4

E.g. fMRI has decent spatial resolution but it takes a long time to measure a response e.g. a blood flow response will take about 5-­‐7 seconds after onset of stimulus

Testing Function: Lesions

Image attributes are processed separately:

What (colour, form)

Where (spatial relationships)

Visual Association Cortex Lesions

Ventral (what, form) and dorsal (localization, the where pathway)

• Lesions of the visual posterior association area can result in the inability to recognise faces or learn new faces
• Other parts of visual recognition will be intact
• This is called PROSOPAGNOSIA

NOTE: Oliver Sacks has prosopagnosia

Frontal Cortex Lesions

Phineas Gage -­‐ the rod went through his frontal lobe destroying many parts of his prefrontal cortex but he survived

His personality changed -­‐ he became unreliable and impulsive with little regard for consequences, he became an alcoholic

With unilateral or bilateral prefrontal lobotomy there is a lack of ability to remember and relate things over time

Attention span and ability to concentrate are diminished

Abstract reasoning largely disappears

The prefrontal cortex receives massive inputs from the sensory association cortex (somatosensory, visual and auditory) and from the dorsomedial nucleus of the thalamus

Lesions of the dorsomedial nucleus of the thalamus can produced many of the same symptoms as prefrontal lobotomy

In short, people with these lesions can’t really control their impulses and act inappropriately

Parietal Cortex Lesions

• More subtle, has to do with how you construct images around you posterior parietal association cortex creates a spatial map of the body in surroundings, from multi-modality information injury may cause disorientation, inability to read maps or understand spatial relationships, apraxia, hemispatial neglect
• Right parietal lobe strokes are MORE COMMON than left parietal lobe strokes
• These patients have no problem with vision
• But if you give them something to draw then they will draw half of it (the right side) and then stop half way
• This is a deficit of attention
• If you remind them to draw the left side they will end up drifting back to the right

Temporal Cortex Lesions

Language, object recognition, memory, emotion. Injury leads to agnosia, receptive aphasia

Patient HM. Bilateral resection of anterior medial temporal lobe structures to cure epilepsy. HM was left with dense anterograde amnesia.

The temporal lobe connects emotions, memory and language

Lesions of the temporal lobe will impair short-­‐term memory

They are effectively trapped in a 30 second window of memory

Patient H.M. had his anterior medial temporal lobes resected to cure him of his epilepsy but this resulted in terrible antero

Question 5

Hemispheric Specialization

Answer 5

Hemispheric Specialisation

There is some degree of specialisation within hemispheres

Right Hemisphere = artistic + creative

Left Hemisphere = logical + scientific

Callosotomy = palliative surgical procedure for the treatment of seizures because the corpus callosum is key for the interhemispheric spread of epileptic activity.

If you perform a callosotomy, then you can show things to one eye or another and you’ll know that that image is only being processed by one hemisphere

E.g. if you show the word face to the left hemisphere you will be able to read it but if you show it to the right hemisphere you will be able to draw it but not read it

Question 6

, functional imaging and brain stimulation

Answer 6

Testing Function

• We can look at electrical activities EEGs, electrograms, fMRI
• SPect and Pect ligaments binding in the brain
• Intracellular and extra recording + microscopes
• There are loads of different techniques and some have advantages over others

E.g. fMRI has decent spatial resolution but it takes a long time to measure a response e.g. a blood flow response will take about 5-­‐7 seconds after onset of stimulus

Diffusion Tensor Imaging -­‐ Tractography

• The movement of water molecules in the brain can be used to infer the underlying structure of the white matter
• This information is used to estimate the location and connections between different white matter pathways
• For patients with traumatic brain injury or concussion injuries in sports such as boxing, it is though that the white matter connections becomes disrupted

Testing Function: Brain Stimulation

Transcranial magnetic stimulation (TMS)

The magnetic field induces an electric current in the cortex, causing neurons to fire.

This can be used to test whether a specific brain area is responsible for a function, e.g. speech

Transcranial Magnetic Stimulation (TMS)

This is a method of focally stimulating different areas of the cortex

Putting a current through the wire coil induces a current through your brain

This means that you can briefly activate a brain area and hence test which specific brain area is responsible for a certain function

Transcranial Direct Current Stimulation (TDCS)

• Changes the local excitability of neurones, increasing or decreasing the firing rate
• This does NOT directly induce neuronal firing
• The anode will INCREASE excitability
• The cathode will DECREASE excitability
• TDCS could be used to reduce motion sickness by suppressing the area of the cortex associated with processing vestibular information

Measuring and Imaging

Positron Emission Tomography (PET)

PET uses a radioactive tracer attached to a molecule to locate brain areas where that particular molecule (e.g. dopamine) is being absorbed

It is expensive but has good spatial resolution and specificity in terms of underlying biology (it is the only way to identify brain regions absorbing particular substances)

Image on the right shows a PET scan following administration of 18F-­‐FDOPA to label dopaminergic terminals in the striatum

Left scan shows a normal state where dopamine innervation is homogenous throughout the striatum

In Parkinson’s Disease, there is profound loss in the posterolateral putamen with relative preservation of the caudate

Magnetoencephalogrpahy (MEG) and Electroencephalography (EEG)

MEG = measures magnetic fields

EEG = measures electric fields

MEG maps brain activity by recording magnetic fields produced by electrical currents occurring naturally in the brain

EEG records the electrical activity in the brain

It is typically non-­‐invasive with the electrodes placed along the scalp

EEG measures voltage fluctuations resulting from ionic current within the neurons of the brain

EEG is a bit less elaborate than the MEG

EEG and MEG signals are noisy

So participants perform a large number of trials so that an average can be found

Once the average is known, you can take that away from the captured signal to identify the underlying activity

Functional Magnetic Resonance Imaging (fMRI)This measures brain activity by detecting changes associated with blood flow

This relies on the fact that cerebral blood flow and neuronal activation are coupled

When an area of the brain is in use, the blood flow to that region increases

The red blobs seen on an fMRI show areas that are slightly more active than the surrounding brain regions

Measuring Optimism

When participants imagined positive events in the future or the past, the amygdala and rostral anterior cingulate cortex were MORE ACTIVE than when they imagined negative events