Here are two description of the method:
“In the SSVEP technique, 64 electrodes are placed on the scalp,
covering all processing areas of the cortex. In this way, specific
areas of cortical activity can be identified, particularly against the
constant stimulus of the flashing light. When the data from the
electrodes is fed into a powerful computer, detailed maps of cortical
activity can be constructed. While being SSVEP-scanned, if a person is
asked to consciously do a particular mental task, he will
automatically activate specific brain areas related to the performance
of that task. In the area that is active, the SSVEP signal is reduced
and the degree of reduction is proportional to the degree of
activation of that area. This allowed us to draw an activation map of
the brain.”
“Previous studies (Silberstein et a!., 1995) have demonstrated that a
recently developed sophisticated type of EEG, Steady State Visual
Evoked Potential (SSVEP), was capable of generating maps of cortical
activity sensitive to cognitive processes. In SSVEP there are
transient reductions in amplitude that appear to index regional
increases in cortical activity associated with the performance of a
cognitive task. Because it is possible to estimate the amplitude of
the SSVEP using as little as 1 to 5 seconds of recorded activity, this
technique was chosen to investigate changes in brain activity
associated with attentional and decision making cognitive tasks.
In the SSVEP procedure a constant visual flicker was superimposed on
the viewing field generating the cortical activity recorded by the
SSVEP. This visual flicker generates a consistent 13 Hz sinusoidal
wave form in the EEG pattern. Silberstein et al. (1995) have shown
that cognitive tasks requiring attention and decision making cause a
reduction in the amplitude of the SSVEP in the prefrontal cortex,
suggesting increased activity, when paying attention and at the point
of decision making.”
(From articles about the Learning Enhancement Advanced Program (LEAP)
on the web site of the Complementary Medicine Association
http://www.compmed.com/leap.html and on a chiropractic web site
http://www.mybrainworks.com/studies/leap_cort.htm )
In a search of the scientific, I have found a number of published
papers about the measurement of steady state visual evoked potential
changes. These papers have been published in the peer reviewed
literature, and therefore can be assumed to have scientific
credibility. For example:
1. Looking at the response to odors:
Annals of the New York Academy of Science, 1998 Vol. 855: pp. 625-7
Steady state visual evoked potential (SSVEP) changes in response to
olfactory stimulation.
Patterson J, Owen CM, Silberstein RB, Simpson DG, Pipingas A, Nield G.
Brain Sciences Institute, Swinburne University of Technology,
Melbourne, Australia.
“The location, nature and characteristics of brain activity during
detection and identification of odors are of importance if brain
function techniques are to be of value to sensory systems…. The
results from a panel of 10 female subjects--who all identified the
butanol correctly--showed that butanol delivery resulted in sequences
of changes in SSVEP topography (amplitude and latencies) which
involved parietal, frontal and temporal regions. While consistent with
other studies (parietal changes), our results revealed more dynamic
temporal changes involving prefrontal and parietal regions at
different periods around odor delivery.”
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9929659&dopt=Abstract
2. Looking at differences in the changes in the brain during the
processing of visual input and in holding the information from that
input in short-term memory
International Journal of Psychophysiology 2001, Vol. 42: pp. 219-32
Steady state visually evoked potential (SSVEP) topography in a graded
working memory task.
Silberstein RB, Nunez PL, Pipingas A, Harris P, Danieli F.
Brain Sciences Institute, Swinburne University of Technology,
Melbourne, Australia.
“The steady state visually evoked potential (SSVEP) elicited by a
diffuse 13-Hz visual flicker was recorded from 64 scalp sites in 30
subjects performing a low and high demand version of an object working
memory task. During the perceptual component of the task, the SSVEP
amplitude was reduced at left and right parieto-occipital sites.
During the hold or memory component of the task, the SSVEP amplitude
exhibited a load-dependent increase at frontal and occipito-parietal
sites.”
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=2091717&dopt=Abstract
3. Looking at whether the need to pay attention causes any changes in
brain activity:
Brain Topographt 1990, Vol 3: pp337-47
Steady-state visually evoked potential topography associated with a
visual vigilance task.
Silberstein RB, Schier MA, Pipingas A, Ciorciari J, Wood SR, Simpson
DG.
Physics Department, Faculty of Applied Science, Swinburne Institute of
Technology, Australia.
“Fifteen right-handed males were required to view three times a series
of 180 geometrical shapes comprising a sequence of 60 squares, 60
circles and a further 60 squares... Trials 1 and 2 were identical
while trial 3 differed from the first two in that one of the circles
was modified. Subjects… prior to the third trial, were challenged to
identify the modified circle. A comparison of trials 2 and 3 indicated
that the appearance of the modified circle was associated with an
attenuation of the SSVEP in the occipito/parietal region. The same
comparison indicated a pronounced SSVEP attenuation in the
centro/parietal region during the interval that subjects were
anticipating the appearance of the modified circle.”
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=2091717&dopt=Abstract
4. Another study on attention
Clinical Neurophysiology 2000, Vol.111: pp. 1544-52
Concurrent recording of steady-state and transient event-related
potentials as indices of visual-spatial selective attention.
Muller MM, Hillyard S.
Department of Psychology, Cognitive Neuroscience and Neuropsychology,
Eleanor Rathbone Building, L69 7ZA, Liverpool, UK.
“The present study investigated the effects of spatial attention on
concurrently recorded visual event-related potentials (ERPs) and
steady-state visual evoked potentials (SSVEPs) to isoluminant color
changes embedded in rapidly flickering stimuli. METHODS: EEG was
recorded while subjects attended to flickering LEDs in either the
right or left visual hemifield and responded by a button press to
isoluminant color changes (targets)…. CONCLUSIONS: Results suggest
that the SSVEP and ERP reflect partially overlapping attentional
mechanisms that facilitate the discriminative processing of stimuli at
attended locations.”
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=10964063&dopt=Abstract
5. A study looking at whether it is possible to predict how well
television adverts are remembered
International Journal of Psychophysiology 2000, Vol 39: pp. 79-85
Frontal steady-state potential changes predict long-term recognition
memory performance.
Silberstein RB, Harris PG, Nield GA, Pipingas A.
Brain Sciences Institute, Swinburne University of Technology, P.O. Box
218, Hawthorn, Victoria 3122, Australia.
“Converging evidence from event-related potential and functional brain
imaging studies suggests that the brain activity at posterior regions
of the frontal cortex can predict the strength of long-term memory
traces… Thirty-five female subjects viewed an 18-min television
documentary program interspersed with 12 unfamiliar television
advertisements while brain electrical activity was recorded from four
pre-frontal, two posterior frontal and two occipital scalp sites.
After 7 days, the recognition memory was tested for images coinciding
with the 20 most prominent frontal SSVEP latency minima and maxima
during the viewing of ten contiguous advertisements (advertisements
2-11). We found that images coinciding with posterior frontal latency
minima were more likely to be recognized (58.7% recognition) than
images coinciding with SSVEP latency maxima (45.3% recognition).”
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11120349&dopt=Abstract
6.. Looking at whether it is possible to detect an emotional response
to pictures
Neuroimage 2002 Dec;17(4):1684-92 Related Articles, Links
Steady-State Visually Evoked Potential Topography during Processing of
Emotional Valence in Healthy Subjects.
Kemp AH, Gray MA, Eide P, Silberstein RB, Nathan PJ.
Brain Sciences Institute, Swinburne University of Technology, 400
Burwood Road, Hawthorn, VIC, 3122, Australia
“The International Affective Picture System (IAPS) is increasingly
used in brain imaging studies to examine emotional processes… The
aim of the present study was therefore to use steady-state probe
topography (SSPT) to examine the steady-state visually evoked
potentials (SSVEPs) associated with the processing of pleasant and
unpleasant images low in arousal content. Seventy-five IAPS images,
categorized as unpleasant, neutral, or pleasant, were presented to 16
healthy subjects while brain activity was recorded from 64 scalp
sites… Results demonstrate that both pleasant and unpleasant valence
is associated with transient, widespread, and bilateral frontal SSVEP
latency reductions. Unpleasant images were also associated with a
transient bilateral anterior frontal amplitude decrease… These key
findings support previous literature in terms of there being
substantial overlap in frontal neural circuitry when the brain
processes pleasant and unpleasant valence relative to neutral
valence.”
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=12498742&dopt=Abstract
7, There is some interest in using the technique for biofeedback
purposes, which has even been extended to the development of aids for
severely handicapped people:
AN EEG-BASED CURSOR CONTROL SYSTEM
Cheng Ming, Gao Shangkai, Senior Member, IEEE, Department of
Electrical Engineering
Tsinghua University, Beijing 100084, China
“ This paper describes an experimental brain-computer interface which
allows severely disabled persons to move the cursor on the screen. The
system is based on the detecting of steady-state visual evoked
potential (SSVEP). Four rectangular blocks around the cursor indicate
four directions
and flash at different frequencies on the screen. Once the subject
fixates on one of the blocks, the specific SSVEP is obtained. SSVEP
signal is processed online to determine which block the eyes are
focusing. As a feedback the cursor can be moved in corresponding
directions.”
http://www.resna.org/sigs/sig11/archive/EMBS.htm
Therefore, this technique does appear to have validity in recognising
and locating changes in brain activity that occur (a) in response to a
sensory stimulus, (b) to a stimulus which evokes an emotional
response, and (c) as a result of the processes of attention,
information processing and memory. Of these, (b) and (c) can be of
especial relevance to someone who is interested in the effect an
advertisement will have.
Among the limitations is the fact that different individuals can
manifest very different, apparently contradictory results, as reported
in one study:
“It is not clear whether or not all subjects employed the same
"strategy" in performing the selective vigilance task; it is possible
that individual differences in the approach to the task resulted in
differences in evoked potential findings. It may be that this
technique is sensitive to differences in individual "style," and that
such individual differences produce some of the observed disagreement
between results.” (From “Human Steady-State Visual and Auditory Evoked
Potential Components During a Selective Discrimination Task” by Thomas
A. Collura, Journal of Neurotherapy, Winter 1996
http://www.snr-jnt.org/JournalNT/JNT(1-3)1.html - this paper has a
very technical explanation of the theory underlying the method).
While I have not found any discussions on the Internet about the
limitations of using this technique to assess response to
advertisements, I would like to suggest a couple which sprung to my
mind.
Firstly, the technique has to be carried out in a highly artificial
environment. A person watching an advertisement, and at the same time
being confronted by a constantly flickering light, all the while with
64 electrodes attached to his or her scalp, is in a very different
situation to someone watching the same advertisement in the comfort of
home or cinema, no matter how much the researchers will have attempted
to provide comfortable seating etc. The artificiality of the
circumstances could possibly result in different responses from those
in normal surroundings.
Secondly, because this work has to be carried out in a laboratory or
test centre of some sort, it has to be done in volunteers who have
presented themselves for the job. To what extent is it possible to
ensure volunteers from all the various social, ethnic and other groups
in a population? It is likely that responses from these groups will
differ depending on the subject matter and presentation of the
advertisements. It would be necessary to ensure a sufficient sample
of people from all the groups targetted by the advertisements.
However, some groups will be less likely than others to volunteer, eg
very rich people, very busy people.
Hope this is what you were seeking. Please request further
clarification if you feel I have not covered the subject as required.
Search strategy:
On Google: 1. “Steady-State Probe Topography” 2. SSVEP limitations
3. “steady state visual evoked potentials” limitations
On Medline 1. SSVEP 2. .“steady state visual evoked potentials” and
use of the “related articles” function |
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