Brain Imaging Techniques Test Questions – Flashcards

properties of the world that are manifested in cognitive systems (mental representation) and neural systems (neural representation).

AKA single unit recordings. measures the responsiveness of a neuron to a given stimulus (in terms of action potentials per second). micro electrodes (invasive, not on humans often). plug direct into neuron and get activity from single neuron- can either be implanted right into axon intracellular or outside membrane of the neuron extracellular. rate coding and temporal coding. e.g. looking at bar of light and measuring one neuron's activity and you get spikes which are each action potentials and usually measure them in one second.

when you use single cell recordings and find that one neuron is very active during one task and then quiet on another. you make an inference. if there's a lot of activity, you infer that the neuron has to do with that processing. so rate coding is what's the rate of action potentials from that neuron? if it's a high rate, then it has a lot to do with what the subject is doing. some neurons do some jobs and others do other jobs.

the time taken between the onset of a stimulus/event and the production of a behavioral response (button press). RT is usually measured with a computerized test in ms. ERP is also measured in ms so it's also important in mental chronometry. so to get mental chronometry, you can use reaction time and ERP.

AKA multi unit recordings. the electrical activity of many individually recorded neurons.

records electric signals generated by the brain through electrodes placed on different points on the scalp. noninvasive and involve recording (no stimulation). for there to be an electrical signal at scalp, first, a whole pop. of neurons must be active in synchrony to generate a large enough electric field. second, this pop. must be aligned in a parallel orientation so they summate rather than cancel out. to gain EEG measure, need to compare the voltage between 2 or more different sites. a reference site that is likely to be uninfluenced by variable under investigation is chose. one common reference point is the mastoid bone.

when doing an EEG, usually pick a ref. point or you can reference to the average of all the electrodes. electrodes are arranged at various locations on the scalp and described in reference to the 10 20 system of Jasper. electrodes are labeled according to their location (F=frontal, P=parietal,O=occipital, T=temporal, C=central) and the hemisphere involved (odd numbers=left, even numbers=right, z=midline). for example, O2 is over the right occipital lobe and the Fz is over the midline of the frontal lobe.

the electrophysiological changes elicited by particular stimuli and cognitive tasks that come from an EEG. ERPs become of use for the timing and amplitude of the peaks. an ERP waveform consists of a series of peaks and troughs that vary continuously over time. ERPs are not invasive, good temporal resolution but poor info about where the neural activity originates from and is based on the electrical activity of neurons.

some of the activity from EEG waveform (ERPs) may relate to the current task (reading) but most of it will relate to spontaneous activity of other neurons that don't contribute to the task. therefore, the STNR in a single trial of EEG is very low and poor (the signal being the electrical response to the event and the noise being the background level of electrical activity). to increase and better the ratio, we average the EEG signal over many presentations of the stimulus (100 trials). results are represented graphically by plotting time (ms) and electrode potential (microvolts) on a graph. graph will show series of positive and negative peaks with asymptote at 0 uV. the positive and negative peaks are "P" and "N" and their corresponding number. thus, P1, P2, P3 refer to the first, second and third positive peaks. they can also be labeled P300 or N400 to refer to positive peak at 300 ms or negative peak at 400 ms.

ERP latency- in milliseconds ERP amplitude- in microvolts

related to properties of the stimulus

related to properties of the task

difficulty of locating the sources of electric activity from measures taken at the scalp (in ERP research). in ERP, the electrical potential at scalp is known but the number, location and magnitude of the electrical sources are unknown. the most common way to solve this problem is dipole modeling. meaning you make assumptions about how many regions of brain are involved with generating pattern of scalp potentials. therefore, you want to use other forms of measure that have better spatial resolutions like fMRI or PET.

measures of the spatial config. of different types of tissue in the brain (CT and MRI). based on the fact that different types of tissue (skull, gray, white, cerebro. fluid) have different physical properties. these different properties can be used to construct detailed static maps of the physical structure of the brain.

based on assumption that neural activity produces local physiological changes in that region of the brain. can be used to create dynamic maps of the moment to moment activity of brain when engaged in task. most common methods are PET and fMRI which are based on a hemodynamic measure.

when the metabolic activity of neurons increases, the blood supply increases to that area. techniques like PET measure the change in blood flow to a region directly whereas fMRI is sensitive to the concentration of oxygen in the blood.

CT scans are constructed according to the amount of x ray absorption in different types of tissue. the amount of absorption is related to tissue density- bone absorbs the most (so skull appear white), cerebro. fluid absorbs the least (so ventricles appear black) and brain matter is intermediate (and appears gray). CT scans expose person to small amount of radiation. used to diagnose tumors or find hemorrhaging. CT can't distinguish between gray and white matter like an MRI.

advantages over CT- no radiation so it's safe, better spatial resolution which allows gyri to be seen, better discrimination between white and gray, can be adapted for use in changes in bood (fMRI). MRI is used to create images of soft tissue which xrays pass through. most human tissue is water based and the amount varies in each type of tissue so they will respond and behave differently which allows for a 3d image to be created of these tissues.

magnetic field is applied across body part being scanned. protons in water molecules in body have weak magnetic fields. when MRI strong external field is applied, a small fraction of hydrogen nuclei in water will align themselves with the field. strength of the magnetic field is measure in Tesla (T). when the protons are aligned, a brief radio frequency pulse is applied that knocks the orientation of protons by 90 degrees to their original orientation. as protons spin in a new state, they produce a detectable change in magnetic field and this is what forms the basis of the MR signal.

a technique for segregating and measuring differences in white and gray matter concentration. VBM and DTI are two important methods in getting info about how individual differences in brain structure are linked to individual differences in cognition. VBM capitalizes on the ability of MRI to the differences between white and gray. VBM does this by dividing the brain into tens of thousands of little regions (called voxels) and then estimating the concentration of white/gray in each voxel. with this info, it is then possible to compare across individuals such as- which brain regions are larger or smaller, in people with good social skills versus those with bad skills?

uses MRI to measure white matter connectivity between brain regions. DTI can measure white matter connectivity between regions because water molecules trapped in axons tend to diffuse in some directions but not others. when many axons that are experiencing this are arranged together, it is possible to quantify this effect this MRI.

uses radioactive tracer INJECTED into the bloodstream. the greater blood flow to a region, the greater the signal emitted by the tracer into that region. temporal and spatial resolution is worse than fMRI. PET takes 30 seconds for tracer to enter brain and 30 seconds for radiation to peak. this is critical time window for obtaining changes in blood flow related to cognitive activity. therefore, temp. resolution for PET is like 30 seconds. given that most cognitive events take place in a second, this is not good.

unlike PET, no need for radiation and takes much less time than PET. can be done in under an hour. component of MR signal that is used is sensitive to the amount of deoxyhemoglobin in the blood. when neurons consume oxygen, they convert oxyhemoglobin into deoxyhemoglobin which is strong and can make distortions in the magnetic field. this distortion can be measured to give indication of the concentration of deoxyhemoglobin in the blood. this has been termed BOLD. the temporal resolution is several seconds which is still slow but faster than PET. also fMRI spatial resolution is better than PET.

blood oxygen level dependent contrast. the signal measured in fMRI that relates to the concentration of deoxyhemoglobin in the blood.

the way that BOLD signal evolves over time in response to an increase in neural activity is the HRF. HRF has three phases 1) initial dip- as neurons consume oxy, small rise in amount of deoxy which reduces BOLD signal, 2) overcompensation- in response to increase consumption of oxy, the blood flow to the region increases and the blood flow increase is greater than the increased consumption which means that the BOLD signal increases significantly and this is measured in fMRI, 3) undershoot- the blood flow and oxy consumption dip before returning to their original levels.

assumes that different cognitive components can be added or deleted without changing other aspects of the task. a type of experimental design in functional imaging in which activity in a control task is subtracted from activity in an experimental task.

in parametric design, the variable of interest is treated as a continuous dimension rather than a categorical distinction. one is measuring associations between brain activity and changes in the variable of interest rather than measuring differences in brain activity between two or more conditions.

in addition to choosing your experimental design, you also have to choose how different stimuli will be ordered. there is block design and event related design. block design is stimuli that belong together in one condition could be grouped together. choice of design is determined by the task and whether data comes from PET or fMRI. for PET, block is required. in fMRI you can do event related or block. the advantage of block is that the method has more power because it is more able to detect significant but small effects.

stimuli from two or more conditions are presented randomly now you have to choose to order these stimuli with block or event related. in event related, the different intermingled conditions are separated for analysis. in fMRI you can do event related or block. the advantage of event related is that they enable a much wider range of experimental designs and are more closely related to the typical design structure of most cognitive psych experiments.

redistributing brain activity from neighboring voxels to enhance the signal to noise ratio. after mapping each brain onto a standard brain map, the process of smoothing occurs. this spreads some of the raw activation level of a given voxel to a neighbor voxel. the closer the neighbor is, the more activation it gets. consider a voxel prior to smoothing that is inactive but because it has so many active neighbors, the voxel gets switched on by the smoothing process. in contrast, if a voxel is initially active but neighbors are inactive, it gets turned off by smoothing. so smoothing enhances the signal to noise ratio. in this sense, one would assume that the signal corresponds to the larger cluster of activity and the noise is the isolated voxel.

a volume based unit. in imaging the brain, the brain is divided into thousands of these.

false positive. when you detect a significant result but there really isn't one.

both have to do with the nature of the mechanism by which neurons communicate. inhibition= reduction of the activity of a brain region triggered by activity in another region/process. excitation= an increase of the activity of a brain region (or cognitive process), triggered by activity in another region/process.

refers to the positive and negative signs of the difference in signal between two conditions. activation= an increase in physiological processing in one condition relative to some other conditions. deactivation= a decrease in physiological processing in one condition related to some other condition.

in functional imaging, some of the regions that appear "active" may indeed be used during performance of the task but may not be critical to the task. as such, functional imaging gives up a better idea of which regions may be sufficient for performing a task but not always which regions are crucial for performing a task.