Using neuroimaging and pschophysics, I want to find out more about perception and cognition as a
function of neural activity. In particular I am interested in
attention, consciousness and their relation to visual perception. In
order to investigate this relation we use various brain imaging
techniques, such as:
Cognitive neuroscience is the study of the neural basis (biology) of cognition (perception,
memory, knowledge). It is multidisciplinary, encompassing the domains
of psychology, computer and information sciences, philosophy, biology
and neurosciences. The goal is to disentangle the neurophysiological
substrate of cognitive functions. It is based on the commonly accepted assumption
that specific brain processes, regions or pathways are responsible
for mediating cognitive function, which is why
brain imaging techniques play such an important role.
EEG, or Electroencephalogram, is a technique invented by Hans Berger
in 1924. It is a graphic record of the electrical potentials generated
by nerve cells in the cerebral cortex. Electrodes are placed on
the scalp where good mechanical and electrical contact is made.
These electrodes are carefully placed along the head at regular
intervals and sense the sum of electrical changes in the nerve cells
(postsynaptic potentials). In digital EEG, the electric signals
obtained by electrodes placed on the head are run through an amplifier
and converted by a digital signal processor connected to a computer.
Our EEG equipment contains small pre-amplifiers on each electrode,
which greatly enhances the signal and makes skin scrubbing to enhance
Due to its high temporal resolution, EEG provides unique and important
timing information about brain processing. Its spatial resolution
is rather bad, especially if one takes into account the microscopic
scale on which things take place in the brain. This is why we use
EEG mainly to find out WHEN things happen in the brain, as opposed
to WHERE they happen.
We work with a Biosemi ActiveOne active EEG system. The results are analyzed using a combination of EEGLAB, FieldTrip, and a custom Matlab toolbox called ADAM (Amsterdam Decoding and Modeling toolbox).
MEG (magnetoencephalography) measures the tiny magnetic fields
(fT range) created by active areas in the brain with highly sensitive
measurement devices called SQUIDs (superconducting quantum interference
devices). MEG has the same temporal resolution as EEG but signals
are less affected by the conductivity profile of the brain, skull
and scalp. Thus MEG is superior to EEG. The spatial resolution is
less than for MRI.
Functional Magnetic Resonance Imaging uses the technique of nuclear
magnetic resonance. This technique allows you to detect slight changes
in the magnetic properties of the substance under investigation.
In the case of brain activity one exploits the fact that a neuron
becoming active results in an increased oxygen level in the blood
vessels around it. The oxygen in the blood is carried by haemoglobin,
whose magnetic properties change when the oxygen level is rising.
This change is detected by MRI and thus indicates which area of
the brain is relatively 'active' during measurement. In order to
detect such changes an enormously strong magnetic field is required.
Even the weakest MRI equipment is over 1 Tesla, which is 20,000
times stronger than the earth's magnetic field. The 'f' in fMRI
stands for 'functional' and represents the fact that MRI is used
to image brain function.
I usually scan on a 3T scanner, and use various sofware package to process my data: FSL, FreeSurfer, Matlab, Brainvoyager and whatever else is needed to get the job done.
Eye tracking equipment allows one to track where a subject is looking. This way it is possible to check whether subjects are following task instructions, but the eye tracker can also serve as dependent variable, to measure which items are selected by a subject when making eye movements. It is possible to track eye-movents in the scanner as well as during fMRI.
In trying to find out how things work, one usually tries to establish
a link of cause and effect. In psychology it has been notoriously
difficult to establish such links. Partially, this is due to the
fact that there are so many variables to consider in human behaviour. Another difficulty
is that it is often hard to determine the
nature of a relation. Is a mother irritable because her child is
crying, or is the child crying because its mother is irritable?
Or are both caused by the noisy neighbours (in which case the crying and irritability are correlational, but do not affect each other)? To find out what is what, researchers often try to isolate one variable (the noisy neighbours for instance),
and see if - all other things kept equal - its presence or absence causes
the crying behaviour of the baby and/or the irritability of the
mother. But simply manipulating the noise caused by the neighbors may not be enough. For example, the crying may increase heavily when the noise increases, but this does not mean that the noise causes the crying. The noise may simply be a modulating force, but not a causal one. Such modulatory relations are actually very common. Attention for example may greatly enhance processing of some visual input over other input, but many researchers still make the mistake to conclude that attention causes the processing of visual input.
In cognitive neuroscience we not only want to find out what external
factors influence behaviour, but we also try to establish what brain functions are causal in bringing about mental phenomena (in our case conscious experience).
As we are trying to figure out the relationship between neural processes and mental phenomena, one of the biggest challenges
is establishing that brain process X causes mental phenomenon Y. This can most convincingly be done by taking route A, in which process X is abolished to see if Y subsides, for example by using paradigms in which mental phenomenon Y is abolished through an experimental manipulation to see what neural processes have subsided.
Great care should
be taken to confirm that no other processes
critically depend on X. For example, taking out the eyes will abolish conscious perception, but it would be a mistake to conclude that the eyes cause conscious perception (even though they may be necessary, they are most likely not sufficient). In such cases, other processes that depend on X might be causal in bringing about Y. Conversely, we should be aware of situations in which brain process X remains active even though mental phenomenon Y is presumably absent. In such cases one can either conclude that process X does not cause mental phenomenon Y, or one can conclude that mental phenomenon Y is still occurring, but that the subject is not able to gauge their internal state properly. The observation that neural processes involved in generating conscious experience continue to operate even when subjects carry out attentionally demanding tasks (not allowing them to access these representations) has sparked the controversial idea that the neural correlate of conscious experience (phenomenal consciousness) does not depend on a subject's ability to consciously report the contents of experience (access consciousness).
In order to distinguish between such complex hypotheses regarding the relationship between body and mind, we have
to take meticulous care that all things are kept equal between experimental
conditions, except those things we are interested in. We need
to have research paradigms that allow us to uniquely manipulate the presence
and absence of awareness or attention. Backward masking
and dichoptic fusion are examples of paradigms that allow us to manipulate conscious experience, whilw inattentional blindness and the attentional blink allow us to manipulate attention and conscious access. Some manipuations, such as binocular rivalry (but also masking) potentially interfere at multiple levels of the visual processing hierarchy, and the implications of such manipulations should be evaluated on a case-by-case basis.
Backward masking is a widely used technique in vision research.
Presentation of a short stimulus is followed by a mask after some
variable interval. At short intervals, the masking stimulus interferes
with recognition of the target stimulus. Even though the target
stimulus is not seen, it may to some extent still be processed.
In our research for example, we try to establish that information
about the location of a stimulus may still be processed in the absence
of awareness. There are different types of masking. In structural
masking the mask is structurally similar or equal to the target
stimulus. In meta-contrast masking the inner border of the mask
follows the outline of the stimulus, thereby forming an inverse
contrast with it. The masking paradigm allows us to compare information
processing during conscious vision with information processing in the absence
of conscious experience. This dissociation may give us important insights regarding
the neural basis of visual consciousness.