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HUL 211 Methods of studying objects - Biological measures

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					Methods of studying objects
   Biological measures



       Snehlata Jaswal


        HUL 211 OBJECT PERCEPTION AND MEMORY
              Basic question
How does the brain analyze, interpret and represent
the external reality?
       Electroencephalography (EEG)
Electroencephalography (EEG) is the recording of electrical activity
along the scalp. EEG measures voltage fluctuations resulting from
ionic current flows within the neurons of the brain.
Despite limited spatial resolution, EEG continues to be a valuable
tool for research and diagnosis, especially when millisecond-range
temporal resolution (not possible with CT or MRI) is required.

Derivatives of the EEG technique:
• Evoked potentials (EP), which involves averaging the EEG activity
  time-locked to the presentation of a stimulus of some sort
  (visual, haptic, or auditory).
• Event-related potentials (ERPs) refer to averaged EEG responses
  that are time-locked to more complex stimuli.
Electroencephalography (EEG)
Brain waves
              Delta

              Theta

              Alpha

              Mu

              Beta

              Gamma
                        Brain waves
Delta
• Delta is the frequency range up to 4 Hz.
• It tends to be the highest in amplitude and the slowest waves.
• It is seen normally in adults in slow wave sleep.
• It is also seen normally in babies.
• It is usually most prominent frontally in adults (e.g. FIRDA - Frontal
  Intermittent Rhythmic Delta) and posteriorly in children (e.g. OIRDA -
  Occipital Intermittent Rhythmic Delta).

Theta
• Theta is the frequency range from 4 Hz to 7 Hz.
• Theta is seen normally in young children. It may be seen in
  drowsiness or arousal in older children and adults; it can also be
  seen in meditation.
• Excess theta for age represents abnormal activity.
• On the contrary this range has also been associated with reports of
  relaxed, meditative, and creative states.
                           Brain waves
Alpha
• Alpha is the frequency range from 8 Hz to 12 Hz.
• Berger named the first rhythmic EEG activity he saw as the "alpha wave".
   This was the "posterior basic rhythm" (also called the "posterior dominant
   rhythm" or the "posterior alpha rhythm"), seen in the posterior regions of the
   head on both sides, higher in amplitude on the dominant side.
• It emerges with closing of the eyes and with relaxation, and reduces with eye
   opening or mental exertion.
• The posterior basic rhythm is actually slower than 8 Hz in young children
   (therefore technically in the theta range).
• Alpha can be abnormal; for example, an EEG that has diffuse alpha occurring
   in coma and is not responsive to external stimuli is referred to as "alpha
   coma".

Mu
• Mu ranges 8–13 Hz., and partly overlaps with other frequencies.
• It reflects the synchronous firing of motor neurons in rest state.
• Mu suppression is thought to reflect motor mirror neuron systems, because
  when an action is observed, the pattern extinguishes, possibly because of the
  normal neuronal system and the mirror neuron system "go out of sync", and
  interfere with each other.
                           Brain waves
Beta
• Beta is the frequency range from 12 Hz to about 30 Hz. It is seen usually
  on both sides in symmetrical distribution and is most evident frontally.
• Beta activity is closely linked to motor behavior and is generally attenuated
  during active movements.[
• Low amplitude beta with multiple and varying frequencies is often
  associated with active, busy or anxious thinking and active concentration.
• Rhythmic beta with a dominant set of frequencies is associated with
  various pathologies and drug effects.
• It may be absent or reduced in areas of cortical damage.
• It is the dominant rhythm in patients who are alert or anxious or who have
  their eyes open.

Gamma
• Gamma is the frequency range approximately 30–100 Hz.
• Gamma rhythms are thought to represent binding of different populations
  of neurons together into a network for the purpose of carrying out a certain
  cognitive or motor function.
              Magnetoencephalography
Magnetoencephalography (MEG) is a technique for mapping brain activity
by recording magnetic fields produced by electrical currents occurring
naturally in the brain, using arrays of SQUIDs (superconducting quantum
interference devices).

The MEG (and EEG) signals derive from the net effect of ionic currents
flowing in the dendrites of neurons during synaptic transmission. Any
electrical current produces an orthogonally oriented magnetic field. It is
this field which is measured.

To generate a signal that is detectable, approximately 50,000 active
neurons are needed. Since current dipoles must have similar orientations
to generate magnetic fields that reinforce each other, it is often the layer
of pyramidal cells, which are situated perpendicular to the cortical surface,
that give rise to measurable magnetic fields. Deep brain processes cannot
be measured in this way. Thus MEG is limited to recording activity that
occurs near the surface of the brain.
               Single cell recordings
Single-unit recordings provide a method to measure the
electrophysiological responses of a single neuron using
a microelectrode system. When a neuron generates an action
potential, the signal propagates down the neuron as a current
which flows in and out of the cell through excitable membrane
regions in the soma and axon. A microelectrode is inserted into
the brain where it can record the rate of change in voltage with
respect to time. These microelectrodes must be fine-tipped,
high impedance conductors; they are primarily glass
micropipettes or metal microelectrodes made of platinum or
tungsten. Microelectrodes can be carefully placed within or
close to the cell membrane, allowing the ability to
record intracellularly or extracellularly.

Single unit recordings are widely used for cortical mappping.
Computerized Axial Tomography (CT Scans)
In a conventional x-ray exam, a small amount of radiation is aimed
at and passes through the body, recording an image on
photographic film or a special image recording plate. Bones appear
white on the x-ray; soft tissue, such as organs like the heart or liver,
shows up in shades of gray and air appears black.

With CT scanning, numerous x-ray beams and a set of electronic x-
ray detectors rotate around you, measuring the amount of
radiation being absorbed. As the examination table is moving
through the scanner, the x-ray beam follows a spiral path. A special
computer program processes this large volume of data to create
two-dimensional cross-sectional images, which are then displayed
on a monitor.
Computerized Axial Tomography (CT Scans)
• Computed tomography of the brain from bottom to top
• Shows only structural problems or changes
       Magnetic Resonance Imaging
• MRI uses a powerful magnetic field, radio frequency pulses and a
  computer to produce detailed pictures of organs, soft tissues, bone
  and virtually all other internal body structures.
• MRI can show tissue damage or disease, such as infection,
  inflammation, or a tumor that may not be assessed adequately
  with other imaging methods such as x-ray, CT, or ultrasound.
• MRI does not use ionizing radiation (x-rays).
• Currently, MRI is the most sensitive imaging test of the brain
How it works:
The individual is placed in a magnetic field. The magnetic field is
produced by passing an electric current through wire coils in most
MRI units. Other coils, located in the machine and in some cases,
placed around the part of the body being imaged, send and receive
radio waves, producing signals that are detected by the coils. A
computer then processes the signals and generates a series of images
each of which shows a thin slice of the body.
A major problem with the MRI machine is that it is very noisy.
Magnetic Resonance Imaging
Magnetic Resonance Imaging
                    Functional MRI
MRI provides only a static structural view of brain matter.

In contrast, fMRI studies the changes in the brain while a task is being
performed, and brain neurons are active.

When neurons become active, blood flow to those brain regions
increases, and oxygen-rich (oxygenated) blood displaces oxygen-
depleted (deoxygenated) blood. Oxygen is carried by
the hemoglobin molecule in red blood cells. Deoxygenated
hemoglobin (dHb) is more magnetic than oxygenated hemoglobin
(Hb), which is virtually nonmagnetic. Since the nonmagnetic blood
interferes less with the magnetic MR signal, when present,
oxygenated blood leads to an improved MR signal. This improvement
shows which neurons are active at a time.
Functional MRI
      D'Esposito and Ranganath (2000)
      1: Subject is asked to remember a face.
      Areas at the rear of the brain that
      process visual information are active
      during this task, as is an area in the
      frontal lobe.
      2: Subject is asked to "think about this
      face." The hippocampus is activated -
      specifically during the time when we
      are remembering new information.
      3 and 4: Subject was asked to compare
      another face to the remembered face.
      Some of the same visual areas are
      activated as during the initial memory
      task, but other areas, such as part of
      the frontal lobe, are involved in making
      a decision.
Structural vs. Functional MRI
       Positron Emission Tomography
PET is a nuclear medicine technique that produces a three-dimensional
image or picture of functional processes in the body. The system detects
pairs of gamma rays emitted indirectly by a positron-
emitting radionuclide (tracer) which is introduced into the body on a
biologically active molecule, usually using an injection.

Molecules accumulating at a specific target site are labeled with a
positron emitting radionuclide. The emitted positron traverses a short
distance until it collides with an electron, annihilating into two gamma
rays of 511 KeV. The coincident detection of the two gamma rays that
are emitted ˜180 degrees apart from each other can be recorded and
used to reconstruct an image showing the location and amount of
positron radio-nuclides in the body of a living subject.
Positron Emission Tomography
       Positron Emission Tomography
Unlike FMRI, PET Scanners are not noisy.

They also offer better resolution.

But they do require injection of radioactive material. Hence they are not
popular.

PET scans are increasingly read alongside CT or magnetic resonance
imaging (MRI) scans, with the combination (called "co-registration")
giving both anatomic and metabolic information (i.e., what the structure
is, and what it is doing biochemically).

Unavoidable for some kinds of studies.
Positron Emission Tomography
       Eye movement recordings
One of the strongest ‘behavioural’ measures in
cognitive science

We can assess:
• Duration of fixation
• Distance of saccade
• Direction of saccade



                HUL 211 OBJECT PERCEPTION AND MEMORY
HUL 211 OBJECT PERCEPTION AND MEMORY
Eye movement recordings




      HUL 211 OBJECT PERCEPTION AND MEMORY
Eye movement recordings




      HUL 211 OBJECT PERCEPTION AND MEMORY
Eye movement recordings




Distance and direction must be recorded
         HUL 211 OBJECT PERCEPTION AND MEMORY
EMR Methods - Moving window




        HUL 211 OBJECT PERCEPTION AND MEMORY
EMR Methods - Moving scotoma




        HUL 211 OBJECT PERCEPTION AND MEMORY
EMR – Some results – Person perception




            HUL 211 OBJECT PERCEPTION AND MEMORY
EMR – Some results - Reading




        HUL 211 OBJECT PERCEPTION AND MEMORY
EMR – Some results – Scene perception




            HUL 211 OBJECT PERCEPTION AND MEMORY
EMR – Some results - Face perception




            HUL 211 OBJECT PERCEPTION AND MEMORY
         Thank you




HUL 211 OBJECT PERCEPTION AND MEMORY

				
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Description: Object Perception and Memory Lecture Series