w1: history, methods Flashcards
Cognitive science
seeks to understand the information processing associated with functions like perception, memory, and decision making
Neuroscience
seeks to characterize the structure and function of the nervous system
Cognitive neuroscience
applies research methods from neuroscience to the functions and behaviors studied by cognitive science.
Cognitive neuroscientists use diverse research methods and experimental paradigms to develop models of mental function and behavior.
cognitive neuroscience vs cognitive science and neuroscience
seeks to create biologically grounded models of cognitive function.
Such models draw inspiration from prior work in cognitive science, while accommodating new developments and findings in neuroscience.
As a result, cognitive neuroscience models can inform and constrain prior cognitive science models, and point out new directions for neuroscience research.
neural correlate
…of cognition / of a function
mapping the brain regions that are activated during a psychological process
Using multiple methods provides two critical advantages:
convergence and complementarity
convergence
combining results from multiple experimental paradigms to illuminate a single theoretical concept
allows to measure mental phenomena that are not directly measurable through series of experiments with diverse stimuli and methods that are designed to all converge on a similar conclusion
example: consistent activation in lateral parietal cortex in different social cognition tasks
how did people come up with convergence?
Renaissance scholars noticed that scientific progress could be made not simply by recording facts about the world, but by developing new theories that combined a number of disparate insights into a common framework
primary approaches to meta-analysis
qualitative: identify a comprehensive set of studies on the same cognitive function and then looks for similarities among their results.
quantitative: combine results from multiple studies into a single statistical framework
semantic / ontological: combine studies according to similarity in their underlying concepts
meta-analysis makes use of which principle?
convergence
complementarity
each of the methods provides a different sort of information about brain function
Because brain function is expressed through many diverse physiological changes, cognitive neuroscientists use a welter of research methods that provide insight into different aspects of physiology: functional magnetic resonance imaging (fMRI), electroencephalography (EEG), positron emission tomography (PET), transcranial magnetic stimulation (TMS), single-neuron recording, neurological disorders, lesion studies, assessments of behavior, and others. None of these techniques provide a complete accounting of brain function
Perturbation techniques
alter brain function, and thus can be used to evaluate how specific brain regions or systems contribute causally to specific cognitive processes
TMS, drug administration, lesion studies
Neuroscience-based approaches can be divided into two broad categories:
studying changes in cognitive behavior when the brain has been perturbed in some way
measuring brain activity while cognitive tasks are being performed
homunculus
The layout of the representations in the somatosensory and motor cortices of the body parts on the contralateral side of the body by Wilder Penfield
Brain perturbation approach
Neuromonitoring approach
diaschisis
the effects of lesioning one part of the brain on the connected parts of the brain that causes misinterpretation of the lesion studies
If one area of the brain is lesioned, other areas of the brain innervated by the damaged area may, from the loss of input, also cease to function normally
DTI
diffusion tensor imaging
MRI-based technique to delineate the structural connections of the brain
quantifies the relative diffusivity of the water molecules in each voxel (imaging volume units) into directional components
axons are surrounded with water-repelling mielin and some fat, therefore water diffusivity is stronger along them than perpendicular to their membranes
the diffusivity characteristics of each voxel are represented by an ellipsoid whose dimensions reflect the relative diffusion rates along the different directions, with spheres representing isotropic diffusion (relatively equal in all directions) and narrow ellipsoids showing stronger diffusivity values along a particular direction (the longest axis of the ellipsoid). Using tractography approaches, white matter tracts can then be reconstructed from these data by finding the strongest continuous chain along the preferred diffusion directions across multiple voxels (shown here in red)
CT
computerized tomography
uses a movable X-ray tube that is rotated around the patient’s head
gathers intensity information gleaned from multiple angles through the imaging volume
These data are entered into a matrix, and the radiodensity at each point in the three-dimensional space of the head is calculated
then computed matrix generates “slices,” or tomograms (tomo means “cut” or “slice”), visualizing internal structures in various planes throughout the brain
today n largely superseded by MRI
MRI
magnetic resonance imaging
protons in hydrogen atoms of the brain become aligned with the very strong main magnetic field of the scanner. Perturbations with respect to this alignment provide a source signal that can be measured, analyzed, and used to construct an image.
protons in a strong magnetic field will efficiently absorb energy when the energy is delivered at a particular resonant frequency
excitation: MRI scanner emits energy in the form of radio waves at precisely the resonant frequency of protons
signal: the radio-wave energy is turned off, whereupon the protons begin to release the energy they absorbed. This energy is the signal measured by electromagnetic detectors around the head or other part of the body
electromagnetic coils in the scanner can cause the local magnetic field to differ in strength along specific directions. By varying these magnetic-field gradients in a systematic way along the x-, y-, and z-axes of the volume to be imaged, the MR signal is caused to vary correspondingly and systematically. variation in the signal emitted is decoded s to create an image that reflects the proton density and other tissue characteristics. e these characteristics are different for gray matter, white matter, the fluid in the ventricles, and other neural tissues
CT vs MRI
both noninvasive
CT faster and cheaper, can be used on patients with implanted metal devices
MRI has better spatial resolution
connectomics
analogy to genomics
reconstructing the connectivity of the entire human brain (or the brains of other species) at the level of neurons and synapses
entails computer-assisted image acquisition and analysis using high-speed methods, along with organizing the results into an extremely large database
so also all the synapses should be mapped
intracranial brain stimulation
direct electrical stimulation of a specific brain region
late nineteenth century
how does TMS work
if you apply a magnetic field, it induces an electrical current in the brain. Coils generate magnetic field.
will TMS activate or inhibit activity?
depends on the frequency
because if the frequency coincides with the frequency on which information normally travels through the brain then the area will be activated
rTMS
repetitive transcranial magnetic stimulation
applying a series of TMS pulses (e.g., one per second) over several minutes
the influence on a cognitive function of interest can then be examined by behavioral tests that can be administered during and after the TMS application (up till a couple of hours afterward)
drawbacks of TMS
affects a large area, limiting anatomical resolution
only up to 1.5 centimeters into the brain, so the cortex
tDCS
transcranial direct current stimulation
a constant, low-amplitude, electrical current is applied directly to the scalp
One of the electrodes, typically the smaller one, is placed over the area of interest to stimulate, and the other electrode is placed elsewhere to complete the circuit
anodal (positive) stimulation increases cortical excitability, cathodal decreases
Electrophysiological basis of EEG and ERP signals
Fluctuating voltages are generated when synaptic input to a cortical area results in a voltage gradient along the dendritic trees of large pyramidal neurons that are oriented more or less perpendicularly to the cortical surface. The electrode on the subject’s scalp picks up the associated voltages from currents that are volume-conducted through the skull and scalp tissues outside the neuronal dendrites.
LFPs
local field potentials
slower-frequency (compared to action potentials) dendritic field fluctuations
reflect variations in the polarization of the dendrites due to modulation by axonal input coming from other brain regions
frequency bands
their relative power is analyzed when analyzing EEG recording
delta (< 4 Hz), theta (4–8 Hz), alpha (8–12 Hz), beta (12–25 Hz), gamma (25–70 Hz), and, more recently, high gamma (70–150 Hz)
the inverse problem
a given distribution of electrical activity recorded at the scalp could have been produced by any one of a number of different sets of generators inside the head. Accordingly, such analyses, especially for more complex activity distributions and/or for studies without additional source-related information, must be viewed with caution and are an important limitation of the ERP approach.
These limitations can to some degree be circumvented by combining ERP methods with other techniques or approaches
PET
15O (oxygen-15, with a half-life of 2 minutes) incorporated into water molecules. Injected, goes to the areas that use it the most, so the most activated
As the unstable isotope decays, the extra proton breaks down into a neutron and an emitted positron. The emitted positron travels several millimeters, on average, until it collides with an electron. The collision of a positron with an electron destroys both particles, emitting two gamma rays that travel in opposite directions from the site of the collision. Gamma-ray detectors placed around the subject’s head are arranged to register a “hit” only when two detectors 180 degrees apart react simultaneously
The PET images of activity can then be superimposed onto magnetic resonance structural images from the same subject(s) to provide spatial information about specific brain areas involved in cognitive and other functions
Limitations: the short half-life of the reagents, the need for a nearby cyclotron to create the reagents, the use of radioactivity, the very poor temporal resolution, theoretical spatial resolution of several milimeters and practical even lower because of spatial smoothing and other steps in the analysis
fMRI
measures the blood oxygenation level–dependent (BOLD) signal. The vascular system supplies blood containing oxyhemoglobin to active regions of the brain. The influx of oxygenated blood to regions that become active reduces the local concentration of deoxyhemoglobin, which increases the BOLD signal; the difference in the signal provides a measure of local neuronal activity.
There are several ways to display functional MRI data on corresponding structural MRI scans, including overlaying onto an image slice; overlaying onto a three-dimensional image of the cortical surface extracted computationally from the MR structural image data; or overlaying onto the three-dimensional image of an extracted cortical surface that has been “inflated” by a computer algorithm
Experimental design: The fMRI response to a stimulus or cognitive event begins at about 1 to 2 seconds, peaks at about 5 to 6 seconds, and returns to baseline at about 12 to 15 seconds. it is fast enough to allow fMRI to be applied in an event-related manner
perturbation studies: associations and dissociations
type of analysis or experimental design
just the activation of the area is not enough to say that the area is involved in the task, but if the area is damaged and the performance drops - this is a causal relationship
DBS
deep brain stimulation
the implants to stimulate particular areas of the brain in living humans. For example stimulation of basal ganglia in patients with Parkinsons alleviates tremors immediately. Is also used agains major depression
The body starts to reject the implants at some point, so constant care is required, and also the implant doesn’t work forever, has to be replaced every couple of years.
optogenetic modulation
laser-activated channels in neurons, created by viruses (channels, not neurons)
can activate or inhibit neurons
kind of new and very exciting technique
experimental designs in PET and fMRI studies
repetition suppression
t maps
when we talk about fMRI, we usually present the data as statistical (t) maps. They show statistical contrast. You compute a t-score for each participant, which represents how significant is the response in one situation versus another situation
