Cog. Neuro Exam 2

Structural brain imaging

maps of static brain structure based on different physical properties of different tissue types (skull, grey matter, white matter, etc.)

Functional brain imaging

maps of dynamic brain activity based on assumption that neural activity produces localized and transient physiological changes in the brain

Computerized tomography (CT)

measures using difference in X-ray absorption. 

Magnetic resonance imaging (MRI)

measure using difference in magnetic properties Essentially a big magnet 

Positron emission tomography (PET)

radioactive tracer used as an exogenous contrast agent to measure a change in blood flow

Functional magnetic resonance imaging (fMRI)

small differences in magnetic properties of hemoglobin serve as endogenous marker of blood flow 

Advantages of MRI

• Safe and noninvasive 

• Better tissue contrast 

• Much better spatial resolution and whole brain coverage 

• Widely accessible and inexpensive 

• Allows for both structural and functional imaging

How does fMRI work?

• H atoms have magnetic signals (initially no direction)

• When external magnetic field is applied, protons start to align with electromagnetic field of MRI

• Brief radio wave pulse orients H atoms to 90 degrees and produces measurable signal

• When pulse stops, protons relax back to orientation of magnetic field 

How does PET work?

• PET uses radioactive trace to act as exogenous contrast agent to measure change in blood flow 

What does PET measure?

PET measures change in blood flow to a region directly.

Differences between fMRI and PET

PET requires administration of a radioactive tracer and measures change in blood flow to a region directly 

FMRI is sensitive to concentration of oxygen in the blood and uses a naturally occurring signal in the bloodstream to do so 

What is the BOLD signal?

Blood-oxygen-level-dependent (BOLD) signal-measures relative difference between oxygenated blood and deoxygenated blood 

Decrease in signal= more deoxygenated blood 

What is hemodynamic response function (HRF)?

Initial Dip-Overcompensation-undershoot

Initial dip

Neurons consume oxygen and there is small rise in the amount of deoxyhemoglobin, which results in a reduction of the BOLD signal 

Overcompensation

in the response to increased oxygen, blood flow to region increases. Increase in blood flow is greater than the increased consumption (BOLD signal increases significantly)

This component is typically what is measured in fRMI and size of peak shows extend to which this region is active 

Undershoot

blood flow and oxygen consumption dip before returning to original levels 

May reflect relaxation of venous system, causing temporary increase in deoxyhemoglobin again 

What does it mean to say that fMRI has poor temporal resolution and strong spatial resolution? 

spatial resolution: determines quality of an image and describes how detailed an object can be represented by the image

temporal resolution: accuracy of scanner or how quickly the scanner can detect differences in brain activity

What is a ‘slice’

multiple voxels forming a 2D image

What is a voxel?

volume-based until; in imaging research the brain is divided into thousands of these

T1 weighted image

 image that demonstrates the differences in T1 times of tissues 

T1 time constant- how quickly protons return/relax to alignment with the magnetic field. 

T2 effect/contrast

describes how quickly protons decay away from 90 degree orientation

• Used for fMRI

• Driven mostly by local interactions and distortions 

• Optimal scanning parameters can be selected 

Cognitive subtraction

experimental logic/design in which activity for some baseline condition is subtracted from activity for an experimental condition 

Baseline task serves as the control condition 

Pure insertion

assumption that adding a cognitive component to a task/condition does not change the processing for other components 

Same for pure deletion (deleting does not change processing for other components)

Why is pure insertion a challenge for fMRI?

Challenge for fMRI because it measures difference between two different conditions because the brain is always active 

• Does not provide a baseline

Categorical/factorial

 factors in the experiment have categorical levels. 

Parametric designs

 dimension of factors varies continuously (/parametrically)

• Baseline/control is essentially “built in” 

• Emphasis on association variable/ brain activity (vs differences)

Block design

simple but powerful approach to find regions of the brain that are more active for Condition A>Condition B; presents stimuli in blocks

• We measure the BOLD signal for a single task condition 

• Each task condition only active by itself

• Same type of stimuli presented many times in a row 

Advantages and disadvantages of Block design

• Advantages

  • Greater precision with randomized block design than completely random design 

  • Simple statistical analysis 

  • No contamination 

• Disadvantages 

  • More assumptions required (no interactions between treatments and blocks and constant variance from block to block)

Event related design

Measure the BOLD signal for a single task condition 

Stimuli presented in a random order

Advantages/disadvantages of event-related design

• Advantages

  • Greater flexibility and randomization 

  • ER-fRMI allows experimenter to estimate the HRF from a single event type 

• Disadvantages

  • If stimuli presented too closely, response can carry over and contaminate response for next stimuli 

What does it mean for a brain region to be “active”?

• Activation and deactivation refer to the sign (positive or negative) of the difference in signal between two conditions 

  • Not to be confused with excitation or inhibition 

What are different ways that activity can support neural representations and transmit information?

• Activation: task A-taskB= (+)

• Deactivation: task A-taskB= (-)

• Tells us whether there is a difference in signal between conditions and the direction of that difference. 

What are some types of experimental questions that in general can only be answered using event-related design 

• How does memory change for remote vs recent events?

• How does emotion change for remote vs recent events?

Activation mapping analysis

 asks questions about which areas are more or less active 

Multi-voxel pattern analysis (MVPA)

fMRI analysis method in which distributed patterns of activity are linked to cognitive processes 

• Neural activity can be compared to previously known patterns of activity to determine the most probable category being viewed (can mind read with 96% accuracy)

Single dissociation-

when a patient with damage to region “X” is impaired on a certain task (task A), but not impaired for another task (task B)

• Broca’s patients impaired on speech production but spared for comprehension 

• Limitations and challenges in interpreting single dissociations

  • Cannot conclude that a region is only important for Task A and nothing else or that it is the only region important for task A

  • We can conclude that the region has a causal role in task A

  • Challenges: task resource artifacts and task demand artifacts

double dissociations

 two single dissociations with a complementary pattern 

• Region “X” is important for Task A but not Task B

• Region “Y” is important for Task B but not Task A

Task-resource artifacts

if two tasks share the same neural/cognitive resource but one task uses it more, then damage to this resource will affect the “harder” task more

Task-demand artifacts

a patient may perform worse on Task A vs Task B because they did not understand instructions for Task A (or chose bad strategy, etc.) 

Diaschisis

discrete, localized brain lesion disrupts functioning of spatially separate and structurally intact brain regions 

Cognitive neuropsychology

• concerned with WHAT the function impacts and how it relates to other areas of the brain 

• Concerned with what are the building blocks of cognition/cognitive processes 

Classical neuropsychology

• concerned with WHERE the brain function originates 

• Concerned with mapping functions to specific brain regions 

Single-case lesion study

data from different patient NOT combined data; multiple patients may participate but each patent’s data is examined individually 

Group lesion study

data from all patients combined in a lesion study 

Caramzza’s 3 assumptions that underlie the single-case research approach to studying brain lesion

1. Fractionation

2. Transparency

3. Universality

Fractionation

brain lesions can produce selective cognitive lesions 

Transparency

lesions can affect pre-existing cognitive modules, but do not create new ones 

Universality

all cognitive systems are basically the same 

Transparency assumption

 lesions affect one or more components within the pre-existing (or normal) cognitive system/architecture; do not result in the formation of a completely new cognitive system within the brain 

Safest conditions from transparency violations:

• Brian damage occurred in adult rather than child 

• Research occurs sooner rather than later after the brain damage 

• Patient’s cognitive profile has remained stable over time since the damage occurred

Voxel-based lesion-symptom mapping-

compares for each voxel, the social problem solving in patients with a legion at the voxel and a patient without a legion at that voxel

• form of group studies 

• Can reveal consensus about what type of brain damage tends to harm social problem solving

Transcranial magnetic stimulation (TMS)

a “coil generates a magnetic field that produces an electrical current in nearby neurons, generating action potentials 

Advantages of TMS over organic lessons:

• Non-invasive causal method 

• No reorganization/plasticity 

• Stimulation target is focal and experimenter-controlled 

• Multiple stimulation targets within the same participant 

• Online stimulation can determine timing of brain-behavior relationships and characterized ongoing neural processing 

• Potential to improve behavioral performance 

Advantages of organic lesions over TMS:

• Changes in behavior/cognition are more apparent 

• Subcortical lesions can be studied 

  • New research shows TMS can impact subcortical areas

Single pulse TMS

single pulse applied to a brain region at a certain time to impede that cognitive process

• TMS pulse used when WE KNOW exactly when the cognitive process being studied occurs 

Repetitive TMS (rTMS)

administering a “train” of pulses throughout a trial instead of just one, strategically timed single pulse 

• The more complex or prolonged cognitive ability being studied, the more likely you are to use this form of TMS

Transcranial direct current stimulation (tDCS)

• Causes a constant low current to the brain between 2 electrodes that were placed on scalp

• Where one electrode is placed on the region of interest and another electrode is placed on a control region 

In what ways to TMS (or tDCS) studies avoid violating assumption of transparency

The effects are temporary

Sensation

effects of a stimulus on the sensory organs

Perception

the elaboration and interpretation of the sensory stimulus by the brain

photoreceptors

cells in the retina that convert light into neural signals

rods

specialized for low level light intensity

cones

detect bright light, color, and higher details (more common near fovea)

 Ganglion cells are found along the inner margin of the retina, in the ganglion cell layer; have center-surround response properties 

On-center/off surround

Off center/on surround

Off center/on surround

• Light to center —> inhibits firing 

• Light to surround —> increases firing 

• Light to both —> baseline firing 

On-center/off surround

• Light to center —> increased firing 

• Light to surround —> inhibits firing 

• Light to both —> baseline firing

Major stops along the geniculostriate pathway 

retina-lateral geniculate nucleus (LGN) in thalamus- primary visual cortex in the occipital lobe (V1)

Primary cortex (V1) is also called…

striate cortex

This visual processing is lateralized

• which means separate processing for left vs. right visual hemifield begins in the retina and continues to right/left hemispheres of the brain 

  • Not left vs. right eye 

Simple cells

a simple cells responds to a particular orientation in a specific location of the visual field 

•  Derive their response by combining responses from several center-surround cells

Complex cells

derive their response by combining the response of several simple cells; respond to orientation (and sometimes motion)

**complex/hypercomplex cells respond to a larger portion of the visual field —> larger receptive fields

Hypercomplex cells

combine responses of complex cells to detect orientation/length/edges of shapes..

**complex/hypercomplex cells respond to a larger portion of the visual field —> larger receptive fields 

Receptive fields

different neurons/Brian areas respond to particular locations of the visual field 

• Spatial scope of receptive fields is very specific for ganglion cells and simple cells

• Receptive field size generally increases along the visual processing pathway 

Visual cortex shows…

retinotopic organization

retinotopic organization

• spatial arrangement of light on the retina is retained in primary visual cortex 

scotoma

small region of cortical blindness due to small, circumcised damage

hemianopia

cortical blindness for an entire left/right visual hemifield due to damage of right/left primary visual cortex 

Quadrantanopia

cortical blindness in one corner of right/left primary visual cortex  

Blindsight

 patients can accurately respond to some stimuli presented within the “blind” region of visual space 

• Example: other pathways to superior colliculus allow for rapid orienting to sudden stimuli

What are the major functions of the ventral “what” stream and the dorsal “where” stream for visual processing 

• Ventral: striate—> extrastriate —> (inferior) temporal lobe 

  • Processes stimulus features and identities; memory

• Dorsal: striate —> extrastriate —> parietal lobe

  • Processes motion and spatial attention; action 

Ventral

• striate—> extrastriate —> (inferior) temporal lobe 

  • Processes stimulus features and identities; memory

Dorsal

• striate —> extrastriate —> parietal lobe

  • Processes motion and spatial attention; action 

Color consistency

color of a surface/object is perceived as constant even when illuminated by different lighting conditions 

• Area V4 most directly supports it 

• Area V4 takes into account illumination and lighting conditions across large area of visual space (large receptive fields)

Achromatopsia

damage causes visual perception to appear in greyscale 

Hemiachromatopsia

 disorder of impaired color perception with relative preservation of form vision in one-half of the visual field 

Area V5/MT- primary (visual) motion center in the brain 

• Neurons respond to particular direction of motion 

• Relatively larger receptive fields combine input from smaller receptive fields to detect motion

Akinetopsia

damage causes failure to perceive visual motion

In object recognition, what is perceptual grouping?

is figure-ground segregation 

What are some “laws” of grouping that were first proposed by Gestalt psychologists

• Principles of perceptual grouping 

  • Law of proximity 

  • Law of similarity 

  • Low of good continuation 

  • Law of closure 

Object constancy-

understanding that the identity of an object remains the same independent of differences in viewing conditions (viewpoint, size, lighting, etc.)

structural description

representations of known objects in the memory cycle 

Apperceptive agnosia

deficit in object recognition due to impairment at the level of object perception 

associative agnosia

deficit in object recognition due to impairment at the level of knowledge/semantic memory

Integrative agnosia

deficit in object recognition due to impaired perceptual grouping of components into whole objects