Introduction fMRI response uses the different signals; deoxyhemoglobin


Introduction

 

When studying the relationship
between the brain and behaviour, we can measure brain activity through
neuroimaging. A particular type of neuroimaging which is popular to researchers
is fMRI (functional magnetic resonance imaging). fMRI is better than other
techniques because it’s non- invasive (you are not exposed to radiation and you
do not have to ingest any substances). It also provides researchers with high
resolution images. fMRI determines brain activity from oxygen utilization
during MRI. This method of measurement uses the idea, from past research, that
blood flow within the brain and neural activity are linked. In fact, blood flow
increases in regions of the brain which are in use (Logothetis et al., 2001). We
use the BOLD (blood oxygenation level dependent) response when using fMRI which
looks at synaptic activity rather than neuronal activity and this has provided researchers
with a huge insight into cerebral activity. Ogawa et al. (1990) was one of the first to discover the use of BOLD
response in fMRI brain study. His research suggested that it can be used to
display mapping of blood oxygenation within the brain.

We Will Write a Custom Essay Specifically
For You For Only $13.90/page!


order now

 

What is the positive BOLD
response?

The brain needs oxygen in order to perform any kind of activity. When activity
occurs within a certain brain region, there is a greater flow of blood to that
region. This is because oxygen is needed to produce ATP (Adenosine Triphosphate)
to power structures on the cell membrane; ATP is a form of energy source from
glucose which is rechargeable. Oxygen is also supplied by a component of blood
called hemoglobin. A positive BOLD response enables us to measure the ratio of oxygenated
and deoxygenated hemoglobin within the blood. Either state of hemoglobin
produces different magnetic fields, oxygenated hemoglobin is diamagnetic and deoxygenated
hemoglobin is paramagnetic (Pauling 1936). Oxygenated and deoxygenated
hemoglobin also provides us with different signals in MR images (T2*) and a
positive BOLD fMRI response uses the different signals; deoxyhemoglobin changes
the MRI signal because as the concentration of deoxyhemoglobin decreases, the
fMRI signal increases. When certain regions of the brain are active, this
causes an increase in blood flow and therefore, an increase in oxyhemoglobin,
and a decrease in deoxyhemoglobin which results in the MRI signal increasing.

The certain course of a BOLD response is known as the hemodynamic
response (HRF). The hemodynamic response is heterogeneous as it varies between
brain regions, individuals, age, and different sensory, motor and cognitive
tasks. The HRF of a positive BOLD displays an initial dip which is said to be
because of the rapid deoxygenation of blood paired with the increased oxygen utilization
from increased synaptic activity. The HRF also shows a delayed peak (about 4-6
seconds) after activation. This is due to a large amount of blood flow and
oxygen thus changing the oxy/deoxy hemoglobin ratio. Once the peak has been
reached, the BOLD signal decays towards baseline level. The HRF of a positive
BOLD response is not like a neural response but more like a vascular response.

 

How does a positive BOLD
response relate to neural activity?

It has been understood from previous research that when you change the contrast
of a visual stimulus, the strength of the neural activity changes. Boynton et
al. (1996) was able to show that this was also reflected for BOLD fMRI. His
study consisted of a pulse visual stimulus which was in the form of a
flickering checkerboard pattern (contrast reversing with a flicker rate of 8
Hz) for a certain period of time “pulse duration”. After each stimulus cycle, a
uniform grey was shown upon the screen for 24 sec. The cycle was repeated six
times. Twenty-four pulse stimuli were viewed: the stimuli had one of four pulse
durations (3, 6, 12, and 24 sec) and one of four contrasts (0, 0.25, 0.5, and
1).

It was found that increasing the timing of the stimulus resulted in a
sustained hemodynamic response. If the stimulus was shown for a short duration,
the BOLD response was short, but if the duration was long, the BOLD response
increased. This suggests that the duration of the BOLD response is highly
related to the duration of neural activity. He also demonstrated that the
amplitude of the BOLD response varies in relation to the stimulus contrast. For
example, if the stimulus was of high contrast, it resulted in a high amplitude
BOLD response, and vice versa for a low contrast stimulus. It was concluded
that amplitude and timing of the BOLD response follows neural activity.

Research like this has enabled us to test the linearity of the BOLD
response and show that it is quite linear. However, there is evidence of non-
linearity if the stimulus is spaced closer than 5-6s apart (e.g. Wager et al.
2005).

 

Another example of research that has demonstrated how the BOLD response
relates to neural activity is that of Logothetis et al. (2001). The fMRI BOLD
response was monitored from implanted electrodes in cats and monkeys. It was
found that the BOLD response corresponds closely with the local field potential
which is the electrical field potential surrounding a group of cells. This
suggests that the BOLD response is actually more related to synaptic activity
than neural activity.

 

Evaluation of BOLD fMRI in neuromarketing

Neuroimaging methods are now being widely used for product marketing in
areas such as political, architectural, entertainment, early food product
design and price measurement. Neuromarketing can provide marketers with information
about consumer preferences that cannot be obtained through the standard
strategies used. “The most promising application of neuroimaging methods to
marketing may come before a product is even released — when it is just an idea
being developed”, (Ariely & Berns, 2010).

 

Berns & Moore (2012) were able to apply the use of fMRI to research
into neural activity in the music industry. Past research has found that
activity in certain regions of the brain (orbitofrontal cortex (OFC)) are
associated with reward and decision making such as future purchasing decisions
(Kennerley & Walton, 2011). Berns & Moore wanted to predict the future
popularity of certain music using fMRI. They conducted a lab experiment on a
small group of adolescents in which they measured the BOLD responses to songs
of unknown artists. To measure future popularity and success of the songs, sales
were calculated for three years after scanning. The researchers found that
neural responses were highly correlated with song success. Importantly, Berns
and Moore also asked participants how much they liked the songs, and although
subjective likeability had no influence on the future sales, activity within the
ventral striatum (associated with pleasure) was significantly correlated with number
of units sold. This research suggests that responses from fMRI can predict
purchase decisions of the population.

 

On the contrary, it has been noted by Poldrack (2006), that measuring an
increase in BOLD fMRI activity in a certain brain region, such as ventral
striatum or OFC, and subsequently saying that a ‘reward- related’ region has
become active, is an increasingly common finding in neuromarketing research. He
states that this deductive reasoning is called ‘reverse inference’ in which a particular
cognitive process has been inferred from a certain active brain region. He
suggests that these inferences are not “deductively valid” and argues that researchers
in neuromarketing should be careful to use reverse inference, especially if the
neural activity in a certain brain region is weak.

 

Some researchers in neuromarketing have shown a preference to use fMRI to
measure how price can influence consumer preferences. Karmarkar et al. (2015) measure
the fMRI response from showing the price of a product either before or after a
product was presented. The fMRI results found that the timing of when a price
is revealed can highly influence a person’s decisions to buy a product. When
the price was revealed before the product “price primacy”, the process of
evaluation on buying the product related to monetary worth such as “is it worth
it?”, whereas, viewing the product first resulted in evaluations related to the
products attractiveness such as “do I like this?”. These results suggest that when
advertising specific types of products (bargain- priced products), it would be better
for consumers to see the price early.

 

It was recognised by researchers such as Lee et al. (2007) that applying
BOLD fMRI to neuromarketing can present limitations in ecological validity. The
environment an individual is in when they receive stimuli in marketing research
can alter the results of product processing. For example, when in an fMRI lab
experiment, the individual is in a calm environment upon receiving stimuli, whereas,
if individual was in a real- life purchase environment, evaluating options with
other people, the activated brain regions may differ and consumer preferences
may also be different.

 

 

Conclusion

In conclusion, I believe that using BOLD fMRI can provide researchers
with insightful information into future predictions of marketing. However, as
it is an expensive procedure, I think it should only be used by companies who employ
certified neuroscientists with specialist training to help avoid problems such
as ‘reverse inference’.

x

Hi!
I'm William!

Would you like to get a custom essay? How about receiving a customized one?

Check it out