Cognitive Neuroscience boek
Chapter 1: Definitions, themes, and approaches
Cognition refers to the set of processes (cognitive functions) that allow humans and many
other animals to perceive external stimuli, to extract key information and hold it in memory,
and ultimately generate thoughts and actions that help reach desired goals.
Although the term phrenologist is nowadays applied dismissively to someone who describes
the brain in non-scientific terms, phrenology made an important contribution to modern
neuroscience: it introduced the idea that different parts of the brain contribute to different
sorts of information processing. This idea is known as localization of function.
Using multiple methods provides two critical advantages: convergence and
complementarity. Convergence describes the approach of combining results from multiple
experimental paradigms to illuminate a single theoretical concept.
The diversity of cognitive processes engaged by even a simple task may seem like a
fundamental limitation and it does make it difficult for cognitive neuroscientists to apply
results from a single experiment to real-world problems. But when the same result is
observed across a range of experimental tasks, then that commonality leads to a stronger
inference than could be drawn from any one experiment.
Cognitive neuroscience also benefits from the complementarity of its research methods,
each of which 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
The techniques vary in whether they are more sensitive to rapid changes in brain activity or
to slower changes, and whether they collect information from single neurons, small portions
of the brain, or the brain.
Chapter 2: The Methods of Cognitive Neuroscience
Neuroscience-based approaches can be divided into two broad categories: (1) studying
changes in cognitive behaviour when the brain has been perturbed in some way, and (2)
measuring brain activity while cognitive tasks are being performed.
Perturbations of the brain that impair or otherwise influence cognitive functions can come
about through a variety of mechanisms. Such perturbations often derive from brain damage
in patients due to stroke, trauma, or disease, but they can also be induced experimentally
using methods that employ pharma logical or electrical methods. Information from both
ways have greatly advanced our understanding of the neural underpinnings of cognitive
functions.
,Perturbations imposed by stoke, trauma, or disease
A major advantage of this approach is that if damage to a brain area or system
disrupts a cognitive function, it is likely that the damaged region is involved in some
critical way in the performance of that function.
A major limitation of clinical-pathological correlations in humans, however, is that
the brain damage is the result of many factors that are not under the control of the
experimenter.
Another way researchers have defined the relationship between brain damage and resulting
deficits in cognitive functions is by making restricted electrolytic or surgical lesions in
experimental animals. This approach allows the researcher to control the location and
extent of brain damage, limiting it to specific functional areas. The disadvantage is that the
training of animals carrying out cognitive tasks is more difficult than for humans and it can
raise ethical concerns.
Another way of perturbing cognitive function in the brain is via pharmacological
manipulation. Signalling between neurons involves the release of and response to
neurotransmitter molecules at synapses. Many drugs interfere with or augment these
processes and can thereby change cognitive functions. Drugs that bind to and activate
receptors in a manner like neurotransmitter are called agonists, whereas drugs that bind to
and block receptors are called antagonists.
Pharma logical manipulation can lead to interferences about the contribution of the
relevant neurotransmitter system to the cognitive processes they affect.
Perturbation by intracranial brain stimulation: In this approach, electrodes are placed onto
or into the brain of an experimental animal or a human patient during a neurosurgical
procedure. Electrodes can be placed transiently (just for the duration of the surgery) or
chronically. Chronically implanted electrodes allow researchers to assess the function of
individual neurons or groups of neurons as the animal carries out a cognitive task it has been
trained to perform.
A far less invasive approach that can be used to disrupt cognitive processing in normal
subjects is transcranial magnetic stimulation (TMS). In this technique, a strong but transient
and rapidly changing magnetic field is generated over a region of the scalp by passing an
intense, rapidly varying, electrical current through a set of coils. The rapidly changing
magnetic field in the coil induces a rapidly changing electrical field in the underlying brain
tissue, resulting in an extraneous flow of current that transiently interacts with local neural
processing. With strong stimulation, this interaction is typically disruptive, creating a
reversible brain lesion limited to the underlying area, in much the same manner that strong
direct electrical stimulation would. With weaker stimulation, this interaction can sometimes
facilitate activation of the underlying area.
, There are, however, several important drawbacks to TMS.
- TMS tends to affect a relatively large area.
- Transcranial stimulation can be delivered effectively to only relatively superficial
brain regions.
- The technique can result in concurrent stimulation of scalp and head muscles.
- Although TMS is in principle non-invasive, the stimulation entails some risk
The other main approach to understanding the relationship between cognitive functions and
the neural processes that give rise to them is to measure brain activity while a subject
performs specific cognitive tasks.
- Single-neuron electrical recording, which entails measuring the action potentials
produced by individual neurons. Such recording can record neural firing either
extracellularly (from the extracellular space adjacent to active neurons) or
intracellularly (from inside a single neuron).
EEG (Electroencephalography)
EEG recordings measure electrical brain waves that can be detected at the scalp. The
method makes use of a set of surface electrodes. The electrodes are brought into good
electrical contact with the skin by means of a conducting gel or salt paste and the pressure
from the elasticity of the cap.
Rather than reflecting the action potential firing typically measured in the single-
neuron recordings described in the previous section, the EEG signal derives from the
summed dendritic field potentials of groups of neurons that are varying together.
Ongoing EEG signals measured over time are widely used in clinical settings to assess various
aspects of brain function. These signals are typically analysed in terms of the power in
various frequency bands at each electrode location, the major band of interest being delta
(<4 HZ), theta (4-8 Hz), alpha (8-12 Hz), beta (12-25 Hz, gamma (25-70 Hz), and high gamma
(70-150 Hz)
Event-related potentials (ERP)
Although the ongoing EEG signal is useful for assessing the overall state of the brain, its
utility in investigating specific cognitive functions is relatively limited. The reason is that the
ongoing EEG record is not linked in time to any cognitive process or event. A more effective
way of relating scalp electrical activity to cognitive function is to implement a time-locked
averaging approach. The most common signals to extract from the ongoing EEG in this way
are event-related potentials (ERPs).
ERPs are small voltage fluctuations in an ongoing EEG triggered by sensory and
cognitive functions. However, because ERPs are generally smaller than the raw EEG signal in
which they are embedded, it is necessary to average multiple trials to extract ERP signals
from the background noise.
An ERP is revealed by extracting portions, or epochs, of the ongoing EEG time-locked to
repeated occurrences of a sensory, cognitive, or motor event and averaging the epochs
together. In this way, the background EEG signal that is unrelated to the event is averaged
out, revealing the average event-related response to the stimulus. Leaving only those
voltage changes specifically associated with processing of the event type of interest.
