Methods for studying the living brain Flashcards
Broca’s aphasia
A disorder of speech production, patients are still able to comprehend speech though
left hemisphere, posterior part of the inferior frontal gyrus
Apraxia of speech
speech is slow, laborious and non fluent
Agrammatism
loss of grammatical function words e.g. a, the, some, about but not content words e.g. boy, smart, university
Anomia
difficulty finding the appropriate word
Wernicke’s aphasia
a disorder of language comprehension
left hemisphere, posterior part of the superior temporal gyrus
Wernicke’s aphasia characteristics
- speech is fluent and unlaboured
- rise and fall of intonation is completely normal
- don’t realise that they have a speech deficit
- patient will use grammatical function words but may use few content words and often will insert nonsense of jargon
Limitations of clinical neuropsychology
- Patients are often difficult to test intensively
- Problem of replicability in single cases
- Assumes local lesions have local effects
- No control of lesion size or location in the brain- ‘experiments of nature’
Ablation studies in animals
the process of making small lesions in distinct parts of the brain of animals and to observe the effects of these lesions on behaviour
Delayed non-matching to sample
- the initial card shown to the monkey does not have the food reward
- normal monkey quickly learns that the reward is hidden under the card that was not rewarded in the initial trial
- if the monkey’s memory is impaired (ablation of the medial temporal lobes) it will select random cards. The longer the delay between learning the correct response and having to make a choice the more error
- therefore damaging the hippocampus impairs memory and learning
Limitations of ablation methods
- the experimental lesion may still effect distant brain structures
- there are many aspects of cognition that are difficult or impossible to test in other species e.g. language and complex reasoning
Extracellular microelectrode recording
recording the electrical activity (action potentials) from single neurons using a fine microelectrode located in the extracellular space.
Limitation: it is not possible to guarantee that the electrical signal being recorded is from one neuron, it could in fact come from several
To overcome this limitation intracellular recording is used where the microelectrode is placed inside the cell body of a neuron, this process is extremely technical
Neurons receptive field (the visual cortex)
single neurons respond to different visual properties, shape, colour, movement etc.
- most neurons in the visual cortex only respond to stimuli that appear in a limited region of space (the neurons receptive field)
- receptive fields of neurons in the primary visual cortex tend to be quite small whereas the receptive fields in higher association areas, quite large
- in the primary visual cortex, adjacent neurons tend to have overlapping receptive fields
- damage of the visual cortex can lead to blindness for the corresponding location of the visual field
Structural brain imaging techniques
provide a static snapshot of the anatomy of the brain
Functional brain imaging techniques
reveal which areas of the brain are metabolically active, either at rest or while a person is performing a particular cognitive task
Computerised tomographic (CT) scanning
allows us to see what is occurring in the brain structurally e.g brain shrinkage due to Alzheimer’s disease, region of damage caused by a stroke
X-ray detection in CT scanning
- X-ray beams are passed through the head
- a radiation detector opposite the beam picks up the amount of X-ray energy that emerges from the other side of the head
- computer reconstructs images
-tissues of different density absorb different amounts of x-ray energy
e.g. bone absorbs a large amount of x-ray energy = radio opaque
cerebrospinal fluid absorbs little x-ray energy because it is a liquid
On a typical CT scan, bone = white, CSF = black grey and white matter of the brain = different shades of grey
limitation: does not differentiate well between grey and white matter
poor spatial resolution (ability to discriminate fine structures)
Magnetic resonance imaging
- MRI exploits the magnetic properties of brain tissue (hydrogen)
- Enormous superconducting magnet inside the scanner that is cooled by liquid helium
- MRI is sensitive to the behaviour of protons in the nuclei of hydrogen atoms
- Hydrogen atom consists of nucleus with one proton and a single orbiting electron
- protons in the nucleus have a characteristic motion ‘spin’ creating a tiny magnetic field
- when the body is placed in the MRI protons align in a direction that is parallel with the magnetic field
- radio frequency pulse is passed through the head which perturbs the axis of spin of the protons. After pulse protons realign into the orientation of the static magnetic field
- synchronised relaxation (realignment) of protons produces energy that is picked up by detectors surrounding the head
computer reconstructs a 3D image, which reflects the distribution of protons and other magnetic agents in tissue
Characteristics of MRI
provides images with high spatial resolution
it can discriminate structures down to 1mm
excellent differentiation between grey and white matter (because the density of protons is much greater in grey matter
Scalp- recorded EEG
measures the weak electrical signals produced by the brain
- electrodes are placed at one or more points on the surface of the scalp
- the electrical signals detected by the electrodes are sent to an amplifier and are displayed on a computer monitor or printed onto paper
- the waveforms indicate the strength and rhythmicity of electrical activity in the brain
EEG has good temporal resolution (can discriminate very brief events in time), but poor spatial resolution.
Scalp- recorded event-related potentials (ERP)
- same EEG equipment
- measuring an event or interest e.g. the brains ability to process a brief sound
- can’t deduce anything from a single recording
- many trials averaged reveal the response to the signal
very good temporal resolution, but poor spatial resolution
Functional magnetic resonance imaging (fMRI)
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Transcranial magnetic stimulation (TMS)
- A TMS coil carrying an electrical current is held over the scalp
- focal magnetic pulse is generated activating a small region of the cortex (10-15mm)
- the activation acts like a ‘virtual legion’ temporarily disrupting the tissue for a few hundred milliseconds
- if a particular brain region is critically involved in a task, then TMS of that region should affect performance
