Cognition and measuring brain activity

What are higher cortical functions with examples

Higher cortical functions are complex processes that involve integrating information from different brain regions to produce meaningful thoughts, decisions and behaviours

eg executive functions like planning, reasoning, learning and memory, language, emotional processing, self-awareness

Positron Emission Tomography (PET)

How does it work

• short radioactive isotope with an extra proton is injected into the body

• the radioisotope circulates aorund the blood stream and gives off a positron leaving a neutron

• Positron is annihilated when it encounters an electron producing 2 gama photons in opposite directions

• simultaneous detections detectors collect the gamma photons and compute where they came from

• poor temporal resolution as it has a slow response time

PET and glucose metabolism

• use a glucose analogue as a radioactive tracer

• Brain regions with greater metabolic activity may take up more of the tracer.

• PET can therefore identify differences in regional glucose metabolism.

• These differences can be compared across mental states or cognitive tasks.

Magnetic resonance imaging (MRI)

what does it use, different between structural MRI and fMRI, based on what and advantages + disadvantages

• uses a magnetic field and radio waves to create cross sectional images of the body

• based on magnetic resonance properties of oxygenated and deoxygenated blood (BOLD signal)

• non invasive and rapid but its loud and can be clustraphobic

• Structural MRI provides anatomical images, while fMRI measures changes associated with brain activity.

Blood Oxygen Level Dependent (BOLD) Signal

how is it generated, what two things does it compare

The BOLD signal relies on differences in the magnetic properties of oxygenated and deoxygenated haemoglobin.

• Oxygenated haemoglobin is diamagnetic.

• Deoxygenated haemoglobin is paramagnetic.

• These differences affect the magnetic resonance signal.

BOLD signal versus neuronal activity

• BOLD signal is not a direct measure of APs

• Electrical neural response happens quickly but the haemodynamic response is delayed by several seconds.

• While its delayed, the BOLD signal lasts longer after neural response has gone back to resting state

fNIR (functional near infrared spectroscopy)

how does it work, advantages + disadvantages

• shine near infrared light through the skull which reaches the cortex

• oxygenated and deoxygenated blood absorb this light different

• non invasive and portable but expensive

• better spatial resolution than EEG as infrared light can penetrate deeper

Mechanism of MRIs

• Neuronal activity increases in a particular brain region.

• Local metabolic demand increases.

• Blood flow to that region increases.

• The balance of oxygenated and deoxygenated haemoglobin changes.

• fMRI detects the resulting change in the BOLD signal.

Electroencephalography (EEG)

what it is how it works

• multi channel non invasive electrodes placed on the scalp which detect underlying neural activity

• detect summed electrical activity (net excitatory and inhibitory activity) generated by populations of neurons in ms so rapid

• EEG rhythms depend on synchrony of firing of population of cells

• EEG patterns vary with level of arousal

• Signals are small and need amplification

• reference electrode used to measure voltage differences

• electrode jelly used to help conduct current

Advantages and disadvantages of EEG

• poor spatial resolution (has to go through skin and hair so signals get attenuated as they go through the structures and other things like blinking can add noise to the signals)

synchronous vs asynchronus activity

Synchronous activity

• Populations of neurons exhibit coordinated electrical activity.

• Their contributions summate to produce larger rhythmic EEG signals.

• Often produces relatively high-amplitude, regular oscillations.

Asynchronous activity

• Neuronal populations exhibit less coordinated activity.

• Their electrical signals summate less consistently.

• EEG tends to show smaller-amplitude, less regular fluctuations.

Characteristics and cortical state

Role of the Thalamus in Generating EEG Rhythms

contributes to rhythmic activity in the cerebral cortex

What two things does rhythmic activity arise from

• Oscillatory activity in corticothalamic circuits.

• Intrinsic burst firing of thalamic neurons.

Corticothalamic Oscillatory Loops

basic mechanism

Mechanism

• Excitatory neurons activate inhibitory neurons.

• Inhibitory neurons suppress the excitatory neurons.

• When inhibition decreases, excitatory neurons can become active again.

• Repetition of this process generates rhythmic activity.

Intrinsic Thalamic Bursting

what causes it

• neurons in non specific thalamic nuclei can switch from bursting to asynchronous firing

• bursting due to t type (low threshold) calcium channels

• burst is usually during sleep related states

• these channels are inactivated at normal MP and become available at hyperpolarization -75mV

• oppoiste of burst is tonic

What is the association cortex

A large region of the cortex that processes and integrates info from sensory and motor pathways

manages higher order mental functions like thinking

75% of the brain is the AC

What is cognition

The ability to pay attention t, identify and plan meaningful responses to external stimuli or internal motivations

What is the simplified information flow in association cortex

Sensory information → primary sensory cortices → association cortex → integration with memory and motivation → planning and decision-making → motor output or other behavioural response

What are the 3 major division of the AC and their functions briefly

Prefrontal: Executive functions, motor planning, attention, working memory

Parietal-Occipitotemporal: Polysensory integration and language

Limbic/Temporal: Memory, emotion, motivation

Prefrontal cortex PFC

important for what and deteroriation

• important for planning, motivation, personality, social behaviour, working memory etc

• deterioration in working memory as we age which correlates with loss of neurons in specific regions of the PFC like area 8A near arcuate sulcus

Briefly explain an experiment with monkey testing working memory in the PFC

Experimental procedure

1. A monkey observes food being placed in one of two wells.

2. A screen is lowered, hiding the wells.

3. A delay occurs.

4. The screen is removed and the monkey must remember which well contains food.

Neurons in PFC show task related activity therefore PFC needed for working memory

Limbic/temporal AC

contains what and function, 2 types of memory

• contains the hippocampus and is important for memory and emotional responsiveness

• Declarative (explicit) eg explaining to someone how to walk home

• Procedural (implicit or motor) eg walking home

Short vs long term memory

how long term memory is stored

• Short-term memory involves retaining information over a brief period.

• Long-term memory allows information to be stored and retrieved over extended periods.

• consolidation process required to convert short to long term which involves interaction between hippocampus and other structures

  • new info comes in, hippocampus encodes and organizes info

  • hippocampus interacts with other brain regions and over time info is reorganized across cortical networks so that info stored becomes independent of the hippocampus

Role of sleep in memory consolidation

• Declarative memory transfer from the hippocampus to neocortex needs sleep and low Ach (some areas like CA3 needs lots of ACH)

• These interactions contribute to the reorganisation and stabilisation of memories.

Limbic/temporal AC- emotional responsiveness

• connects with amygdala and other parts of the limbic system

• important for emotional responsiveness and expression

Parietal–Occipitotemporal AC

role, what damage to this region can cause and why is it more common on one side

• contributes to spatial awareness and body representation

• damage to this region can cause hemi spatial neglect where a person fails to respond to stimuli on one side of space eh right hemisphere damage resulted in left side neglect

• Left side neglect more common because right hemisphere association regions contribute attention to both left and right sides of space but left hemisphere regions are more associated with right side attention

• therefore lesions to right hemisphere can disrupt attention towards left side and left hemisphere is limited in compensating for lost attention to the left side

• left hemisphere lesion can impair right side attention but right hemisphere can support attention on both sides so right side neglect less common