Chapter 7 - Imaging the Brain's activity

what was the idea of Mosso?

That changes in the blood flow might provide a way to assess brain func during mental activity (behavior)

what are fontanelles?

* the soft areas on a newborn baby’s head where the bones of the skull are not yet fused
* they pulsate with the rhythm of the heartbeat

what happened when subjects with head injuries that left them with skull defects when the subjects engaged in mental activity?

increase in the magnitude of those pulsations

What did he find about brain blood flow when problem solving math question?

* blood pressure in brain increased but not pressure in arm
* mental activity was accompanied by selective increase in brain blood flow

what does increase in blood flow in brain region suggests?

active brain regions require more oxygenated blood to perform ongoing funcs

what does need for O2 suggests?

active brain areas signal the circulatory system to increase blood flow to those areas.

what are the 2 types of elec activity neurons produce?

1. graded potentials
2. action potentials

what are techniques for recording brain’s graded potentials?

1. electroencephalographic (EEG)
2. event-related potential (ERP)
3. magnetoencephalography (MEG)

what are techniques for action potentials?

single-cell recording

what do single-cell recordings address?

what each neuron in the brain is doing at any given moment

what technique was used in squid axons?

single-cell recording

how does single recording work?

* an electrode is inserted directly into the brain adjacent to a single neuron
* the info is recorded on computer
* then activity is correlated with ongoing behavior

Are all regions able to be accessed for single cell recordings?

* PNS accessible
* a few areas in CNS

what do action potentials in brain represent?

sights, sounds, smells, tastes, pain and temperature sensations, desire and emotion

how does an action potential in one neuron represent vision, whereas a similar action potential represents a diff sense?

* not fully sure
* but suggest that neurons tune themselves to the sensory input they receive: if they sense light, they; if they sense touch, they feel.

what do bright areas in the visual image might be represented by?

neurons firing more rapidly

what do reduced or absent firing show?

darkness across visual cortex

what happens when relaying info in visual cortex?

* there is an initial decrease in num of neurons in visual cortex, then within the cortical association areas, the num of cells increase
* changing num suggests that visual info must be transmitted as a code

what do single-cell recordings at diff lvls of the visual relay reveal?

visual code’s features

what do cells along the visual pathway respond to?

* only to dots of light → so then cells in the primary visual cortex respond only to bars of light at specific orientations

what do cells in higher visual areas respond to?

more-complex stimuli, including position and movement of obj and facial features

what is the overall information processing path for visual cortex?

info encoded as dots → transfers info into bars → transfers bars to more complex representations to tell us what world looks like

how do cortical neurons fire?

* at a relatively low rate
* fewer than 3 discharges per min
* can increase to 10 when neurons become more active

what kind of behavioural repertoire do neurons have?

* narrow
* responds to only 1 kind of sensory event or behaviour

what is a code expressed in low-lvl activity useful?

since neuronal activity requires energy

what do the neighboring neurons behavioural repertoires look like?

* vv diff
* suggests that in the brain’s association areas, networks subserving diff behaviours interact closely
* e.g., in Broca’s area:
* where speech is produced, one neuron may be active during word perception,
* its neighbor may be active during word production.
* At the same time, specific stimuli or events may be associated with neuronal activity in a surprisingly large number of areas in both hemispheres.

what can also contribute to neuronal code?

inhibition of activation

what is encoded by sparse cortical activity?

well-learned behaviours

how are newly learn behaviours displayed?

accompanied by much more widespread cortical excitability

what are single-cell recordings limited by?

* recordings from relatively small num of neurons
* still very useful

what was the technique developed for recording elec activity over large regions of brain?

electroencephalograms (EEG) → placing leads from a voltmeter on skull, which records voltage fluctuations (“brain waves”)

how did tradition EEG arrangement look like?

* one electrode (active electrode) is attached or pasted on skull to detect elec activity underlying area.
* second electrode (indifferent electrode) attached elsewhere, where no changing elec activity exists (e.g., earlobe)
* The two electrodes detect the difference
in electrical potentials near the scalp electrode, thus revealing the underlying
brain activity.

what can elec fluctuations be displayed on once amplified?

polygraph

how were ele signals recorded in original polygraph?

by pens on long sheet of paper pulled by a motor

how are ele signals recorded in on polygraph today?

computers, smartphones store patterns and replay the elec signals on screen

what do EEG record?

* the sum of rhythmic graded potentials on many thousands of neurons

what are graded potentials?

small depolarizations and hyperpolarizations of membrane voltage

where does substantial portion of EEG signal come from?

the large pyramidal neurons of layers V and VI

how are cell rhythms produced?

1. in thalamus or brainstem act as pacemakers → driving graded potentials of cortical cells
2. interneurons withing the cortex, connected to many dozens of adjacent cells, also discharge rhythmically, driving those rhythms
3. cells also have intrinsic rhythms, and the connections between adjacent neurons can serve to synch those patterns
4. rhythm can fluctuate with heart rate or respirations ( processes that provide O2 and gluc to the cells)

what is the part of the neuron’s membrane that produces signal called?

signal’s generator

how are waves recorded from the skull conducted?

volume conducted through the brain and skull (like waves thru water)

what happens if electrode move farther away from source?

* wave amplitude from a given generator grows smaller
* so if multiple electrodes are placed on skull, amplitudes diff can be used to estimate the approx location of the generator that is producing a given set of waves

what is EEG value to study?

* states of consciousness
* waking, sleeping anesthesia
* diagnosing epilepsy and brain damage
* studying cog funcs
* including neural control of prosthetic devices

what do we see in EEG when person is aroused, excited or alert?

* low amplitude and high frequency
* known as beta rhythm

when do alpha waves emerge?

* when person is calm and resting quietly, esp with eye closed → created rhythmical, larger, slower brain waves
* amplitude wax n wanes at frequency of approx 11 cycles per seconds

where does largest alpha rhythm appear in humans?

from the region of the visual cortex at the back of the head

what happens if person is disturbed or opens their eyes?

alpha rhythms abruptly stops

what do voltmeter do?

transfer EEG waves into beeps so that the brain-wave rhythms can be heard

what else EEG indicate?

* changes that take place as a person moves from drowsiness to sleep and enters deep sleep


* EEG rhythms become slower in f and larger in amplitude
* 4- to 7 -cycle-per-second theta and finally 1 - to 3 -cycle-per-second delta waves are produced

what are EEG useful for in terms of anesthesia?

estimating the depth of anesthesia

what are EEG useful for in terms of brain damage?

evaluating the severity of head injury

what happens on EEG if brain ceases to func?

EEG trac becomes a flat line

what is epilepsy characterized by?

changed sensation, mood, or consciousness and/or by convulsions, referred to as seizures

what did EEG experiments find about epilepsy?

* the cause of seizures

what is partial seizure?

* a form of epilepsy
* abnormal elec discharges are restricted to only one or a few brain regions

what are partial seizures associated with?

* sensations or emotions (simple partial seizures)
* brief loss of consciousness (complex partial seizures)

what is generalized seizures?

* abnormal elec discharges occur in widespread regions of brain
* \

what are generalized seizures characterized by?

* abnormal body movements, include convulsions (tonic-clonic seizure)
* falling down without muscle tone (atonic seizure)
* loss of consciousness (absent seizure)

What can EEG provide info on about epilepsy?

cause & location

what does the duration of an epileptic attack correlate to?

the duration of abnormalities in EEG → large distinctive spikes, slowing of the elec signal waves or loss of the EEG signal

what does the location of abnormal rhythm induce?

* it is the focus in the brain region that first generates the abnormal activity

what method can be used to find location of epilepsy?

triangulation method

how can epilepsy spread across brain?

* waves recruits adjacent regions
* in pic, can tell RO has biggest spikes → suggest that location is here
* EEG waves compared via computer techniques

what do EEG for cog func produce?

ongoing “online” representation of the “working” brain

what is coherence theory?

* aims to relate the brain’s single-cell activity and EEG activity to the info processing required for behaviour,
* proposes that rather than brain activity consisting of a num of conscious “states”, a continuum-of-state exists, from high coherence to low coherence

when does high coherence occur in EEG?

* when it displays large, slow waves and the single-cell activity of neurons is highly correlated
* here, brain is idling

when does low coherence occur?

* when EEG displays low-voltage, high frequency beta pattern and single-cell activity is poorly correlated, with every neuron firing at diff times
* here brain is actively processing info

what is underlying theory of coherence theory?

1. measures, even small diff in EEG activity, can estimate diff brain regions’ info processing status


1. large num of EEG recording electrode placed across the skull can map the comparative involvment of diff brain regions in ongoing behaviour
2. reverse effect - that “brain info” in the form of EEG waves can become a tool to control external devices
3. much more info is contained in single-cell activity than EEG activity, so single-cell signals will be more effective in controlling BCIs than will EEG waves

what do EEG signals and coherence of underlying single-cell activity vary with?

* self-directed behaviours
* allows for individual to be able to change brain coherence by “thinking”
* e.g., to learn to control external device (computer) via mental activity
* using brain-computer-interfaces (BC) paralyzed person can learn to control computer cursor

what are event related potentials?

* brief changes in an EEG signal in response to a discrete sensory stimulus
* largely the excitatory and inhibitory graded potentials, the EPSPs and IPSPs that a sensory stimulus triggers on dendrites

why are EPSPs hard to detect ?

signal is mixed in with many other EEG signals from the brain

how to detect ERPs?

* repeatedly producing the stimulus and then avg the recorded responses
* avg cancels out irregular and unrelated elec activity, leaving only the graded potentials generated by the stimulus event
* an ERP produces a num of negative (N) and positive (P) waves in a period of a few hundred milliseconds after the stimulus

what are EEG waves depicted as going downward on the ERP graph called?

positive

what are EEG waves depicted as going upward on the ERP graph called?

negative

how are positive and negative waves numbered?

at the time they are produced

are all ERPs unique to a particular stimulus?

* no
* there are common ones to any auditory stimulus the brain perceives
* but the waves produced at longer latencies, 100-300ms after a stimulus is presented, likely are related to unique properties of the stimulus

what is another useful property of ERPs?

* can map the progress of response as it makes its way thru NS
* can assess both info processing in brain pathway and health of pathways
* neural response evoked by a sensory stimulus crosses many synapses between the sensory receptors and the cog processing regions in the cortex, where the stimulus info is further processed sequentially in a num of cortical regions
* at each neuron in such a pathway, a new ERP is generated

what do waves correspond to in brain mapping with ERP?

the successive activations of synaptic connections thru the auditory pathway from brainstem to cortex

where are ERP signals identified as I to VI from?

brainstem signal generators (neurons in the pathway)

where are ERP designated as N0 thru P1 from?

primary auditory cortex regions (A1)

where are ERP designated as N1 thru P3 from?

secondary + tertiary (association) regions of the cortex

what are dotted lines on ERP graph for brain mapping associated with?

indicate brain waves associated with thought processes in response to the signal (process of decoding the stimulus)

how to know what area is used most in ERP?

largest P3 corresponds to higher activation area

what else can ERPs reveal?

when and where in the brain actions are planned and executed

what are readiness potentials?

* ERPs
* produced in the motor cortex later than 300ms after stimulus presentation
* it signals both that motor cortex is preparing a movement and the part of motor area involved in executing the impending movement

what do MEG record?

records magnetic field (produced by many neurons) on the skull surface

what is the heart of a MEG probe?

* a sensing device containing the special superconducting coils needed to detect the brain’s very weak magnetic field
* SQUID (superconducting quantum interference device) is immersed in liquid helium to keep it at the low temp necessary for superconductivity
* one or more probes are moved across the surface of the skull, sending signals to the squid

what does each probe produce in MEG?

an “isocontour map” → chart with concentric circles (gradients) representing diff intensities in the neural magnetic field

what do isocontour maps allow investigators to do?

* calculate the location of the neurons generating the field in 3D
* convert MEG maps into a graph of elec events very similar to elec potentials recorded by EEG instruments

how do we induce an action potential in neurons?

placing an electrode near a neuron and passing an electrical current thru it

what can you do by stimulating neurons?

could learn abt the funcs of diff brain areas by electrically stimulating the tissue → region’s func can be inferred from any resulting behaviour

how can investigators construct a functional map of the brain?

by moving electrode and repeating brain stimulating procedure

what do results from earliest brain stimulation studies indicate?

* movements are elicited by stimulating the motor cortex
* sensations are elicited by stimulating the sensory cortex
* complex cog funcs such as speech are disrupted by stimulating association areas, such as the cortical speech areas

what do electrical stimulation of the brainstem produce in rats?

* many complex behaviours in rats
* including mating, aggression, nest building, and food carrying
* brainstem stimulation can be either positively rewarding (will seek out stimulation) or negatively (will avoid stimulation)

what were early brain investigations into elec stimulating the human brain directed at?

* investigating brain funcs
* controlling brain acitivity to remediate psychiatric diseases

what is DBS?

* deep brain stimulation
* has application in treating psychiatric, neurological, and psychological conditions

what are DBS stimulations used for?

* treating depression and OCD when other treatments have failed
* can treat epilepsy by coopting activity of neurons involved in abnormal activity and so preventing abnormal discharges

what is parkinsonism characterized by?

* tremors
* akinesia → absence or poverty of movement

how does DMB play a role for parkinson’s?

* when DBS electrode are implanted in the brain in a num of regions of the basal ganglia, including the globus pallidus or subthalamic nucleus, both tremors and akinesia are lessened
* treatment improves movement, cog, and mood

limitation of DBS?

* it is invasive → skull must be opened to introduced the electrode
* can damage brain or introduce infection
* so limited to when other treatment are not possible