Brain Sciences 2 Flashcards
Give example single cell recording technique with visual neurons.
Consider cells in area MT (sometimes referred to as V5),Single-cell recordings reveal that neurons in this region do not show specificity regarding the color of the stimulus. But they are quite sensitive to movement and direction.
A rectangle was moved through the receptive field of this cell in various directions.
The polygon formed when the points are connected indicates that the cell was maximally responsive to stimuli that moved down and to the left; the cell
responded minimally when the stimulus moved in the opposite direction. (b) This graph shows speed tuning for an MT cell. In all conditions, the motion was in the optimal direction.
Must be within Receptive fields : Neurons only fire when the stimulus is in the right direction, has the right angle, and has the right speed.
How does the brain process touch?
The parietal lobe receives sensory information
about touch, pain, temperature sense, and limb proprioception (limb position) via receptor cells on the skin that convert it to neuronal impulses that are conducted to the spinal cord and then to the somatosensory relays of the thalamus. From the thalamus, inputs travel to the primary somatosensory cortex (S1) and The next stop is the secondary somatosensory cortex (S2), which is a unimodal association area that continues to process sensory information.
What is the single cell recording technique?
Insert a thin electrode through a surgical opening in the skull into the cortex or dee per brain structures. When the electrode is near a neuronal membrane, changes in electrical activity can be measured.
Place the electrode on the outside of the neuron. the tip will record the activity of a small set of neurons.
The primary goal is to determine which experimental manipulations produce a consistent change in the response rate of an isolated cell. For instance, does the cell increase its firing rate when the animal moves its arm?
How do we understand data from single-cell recording?
Because the activity of a single neuron is quite variable, it is important to record the activity from many trials in which a given stimulus is presented. The data are represented in what is called a raster plot, where each row represents a single trial and the action potentials are marked as ticks.
To give a sense of the average response of the
neuron over the course of a trial, the researcher sums the data and presents it as a bar graph called a peristimulus his togram, which allows scientists to visualize the rate and timing of neuronal spike discharges in relation to an external stimulus or event.
What is EEG (electroencelopraphy? And MEG? and is advantages/disadvantages?
PSPs of many neurons sum up to the signal that can
be detected at the scalp. EEG measures the electric fields (potential differences) created by the PSPs.
Thousands of neurons need to activate to create a dipole (diff. in potential) that can be sensed by EEG and MEG.
MEG measures magnetic fields that are created as a result of the electric fields (right hand rule). Uses superconductive quantum interference devices (SQUIDS) in a ‘dome’ over the head measures magnetic fields. This ‘dome’ is a dewar, filled with liquid helium at a very low temperature in order to maintain superconductivity.
HIGH TEMPORAL RESOLUTION but low spatial resolution.
How do we process and represent EEG/MEG data?
- Signal from sensor needs to be preprocessed to reduce noise.
- The signal called ERP (event related potential) happens in response to events such as stimulus or finger movement etvc.
- Brain has tendency to oscillate, i.e. be rhythmically active in canonical frequency bands.
- Time-frequency analysis: break down the signal into its different frequencies using a Fourier trasnformation which tells us how much power there is in each frequency band.
a. More power means event-related synchronizations.
What is neuropsychology and give examples of neuroimaging using lesions?
Neuropsychology allow to infer brain function from lesions (brain damage) to that area.
- A stroke occurs when the blood supply to part of your brain is interrupted or reduced, preventing brain
tissue from getting oxygen and nutrients.
By using neuroimaging (or neuropathology post mortem) to identify the overlap and dissociation between lesioned areas of the brain, one can infer something about the location or function in the normal brain.
- Broca’s aphasia is characterized by nonfluent speech.
- Wernicke’s aphasia may result in a complete absence of understanding language. Speech is,
by and large, fluent, but it may appear to not make sense to listeners, as the patients themselves cannot
understand what they are saying. - Severe frontal cortex lesion can cause dramatic change in personality : Phineas Gage and the
role of the prefrontal cortex.
How to use brain stimulation and lesions to the brain to figure out brain function?
Used to identify topographic maps of the motor and
somatosensory cortex.
* More cortex area for regions that require finer motor movement.
- Patient HM: due to severe epilepsy, he had his hippocampus removed but could not form new memories. Anterograde amnesia. Working and procedural memory worked just fine.
What is PET (Positron Emission Tomography)?
The participant receives an injection of radioactive tracer (an isotope).
Once arrived in the brain, the
isotope emits positrons that collide with electrons, releasing two gamma rays per collision.
These get detected by the PET scanner and the active brain areas (increased blood flow) can be localized.
A PET scanner is basically a gamma ray detector.
What are some other experimental manipulations of brain activity?
Transcranial Magnetic Stimulation :
* A strong electrical current runs through a tightly wrapped wire coil in an insulated sheath, creating
a magnetic field. When placed on the head, the field passes through the skull and causes neurons to fire.
* TMS can also disrupt normal neuronal activity and temporarily create a ‘virtual lesion’. E.g., rTPJ
stimulation alters moral judgment.
Transcranial Direct Current Stimulation (tDCS) :
* Apply a low direct current (mA) via electrodes on the head.
* No major safety concerns.
* Successes in the treatment of depression.
Deep Brain Stimulation (DBS) :
* Electrodes get surgically implanted in the brain and stimulate deep nuclei.
* Effective in movement disorder, also used in OCD, depression.
What is an MRI (magnetic resonance imaging) and how does it work?
- 3D ‘picture’ of your brain consisting of voxels.
- a very big magnet : A stronger magnet allows you to choose a smaller voxel size.
- MRI relies on the magnetic properties of water molecules to produce images. H consisting of one atom and one electron produce a magnitic field that make the atoms spin.
Steps:
1.Protons are spinning randomly outside the scanner.
2. Protons spin along the magnetic field vector of the scanner : when in the scanner, more atoms align
in the direction of the magnetic field B0 (low energy state) than in the opposite direction (high energy
state). The main magnet is responsible for the primary magnetic field.
- A radio frequency pulse changes the magnetization vector by a 90 degree angle. Gradient coils produce secondary magnetic fields, along the 3
dimensions for 3D imaging. RF coils, optimized for brain MRI, get low-energy protons to get into a high- energy state by disturbing
the proton alignment. - A radio frequency pulse is turned off. The net magnetization vector spirals back into its former state, along the longitudinal axis of the primary magnetic field of the scanner. This induces an electrical signal, picked up by the RF coils. The computer then performs an analog-to-digital conversion.
How do we process and analyze fMRI data?
fMRI gives us a time series of signal change due a hemodynamic response (changes in local blood flow that occur when active brain areas need more oxygen).
- HRF is a canonical representation of this hemodynamic response, of this indirect measure of neuronal activity.
- BOLD response signal must be pre-processed to account for individual differences, measurement error and eliminate noise.
- HRF used to transform information about the experiment (stimulus, onset of event etc), into a modelled signal using method called “convolution”.
- If modelled signal correlates well with the observed signal in a active brain area then one could conclude that this brain area is involved in econding the variable.
- For second-level analysis accross participants, you can a statistical parametric map SPM and find the t-score for every voxel in the brain and see if it’s significant.
Beware of false positives when doing this (some noise might pop up as significant). So need to correct for multiple comparison problem.
Solution: raise significance level. Divide p-threshold by amount of tests. Or use FWE which takes into account that nearby voxels are related.
What is fMRI and how does it work?
4D movie : recording activity over time. This activity is based on blood flow to brain areas that are more active.
Neuronal activity (postsynaptic potentials) consumes oxygen. To provide oxygen, there is a hemodynamic response, bringing more high-oxygen blood towards the brain area that needs it. This hemodynamic response has a canonical function. Note: it is slow!
Using a method called convolution, the shape of this time-resolved hemodynamic response function can
be described by a single parameter. In other words, one value describes the size of a detected BOLD
response.
HIGH SPATIAL RESOLUTION but low temporal resolution.
What is the difference between T1 and T2 weighted scans?
T2 weighted scans take into account local magnetic field inhomogeneities due to differences in oxygenated vs de-oxygenated hemoglobin. This is what allows researcher to track the BOLD (blood oxygenation level depedent) signal.
For T1 weighted scans:
- T1 relaxation, i.e. the time it takes for the proton to resume its spin along the longitudinal (z) axis, once the RF pulse is turned off. The T1 relaxation time varies per tissue (e.g., water, fat).
- Use these T1 times to differentiate : fat quickly realigns to the longitudinal axis of the scanner’s primary magnetic field, water realigns slower. Therefore, in a T1-weighed scan, fat appears bright, while water appears darker.
What analysis can we do with EEG/MEG and its advantages?
a. Sensor level analysis:
1. Knowing the shape of the electric/magnetic field around the dipole, we can infer its location because there are many sensors.
2. With an algorithm, we can try to find a solution to this inverse problem (ie. the signal could be coming from deep or superficial parts of the brain).
Changes in electric/magnetic field strength, or frequency power can be observed at the milisecond so high temporal resolution.
But Low spatial resolution because electric fields are smeared out by the skull and because of head movements in MEG.
