Lab Experiment 1: Reaction time and experimental design
Objectives:
1. Understand experimental design; dependent and independent
variables
2. Become familiar with the BioPac hardware and software
3. Determine if caffeine increases reaction time
4. Understand how statistics are used to evaluate data
Introduction
Many of the experiments we will do in the lab this semester will focus on
recording and measuring specific physiological processes. The BioPac system is
specifically designed for this purpose, and consists of an acquisition unit (the hardware)
and software that record and analyze physiological information, such as heart activity,
muscle contraction, neuronal activity, and respiratory function. Your body is, essentially,
a vessel of electrical activity, and the BioPac unit reads these electrical signals and filters
out any “noise”. The software then converts these signals into waveforms that can be
displayed on the computer for easy visualization. Since the BioPac unit takes roughly a
thousand measurements every second, a waveform is simply a line that connects all these
many data points.
In this experiment, we will measure reaction time to an auditory stimulus. This
experiment is designed for two purposes. First, it will be a very straightforward use of
the BioPac system, and thus will allow you to become familiar with its use (hardware and
software). Second, even though it is a relatively simple experiment, it is representative of
a powerful way in which physiological systems can be manipulated to ascertain the effect
of some factor on the system, such as testing the efficacy of a new drug on a group of
people. In this example, the drug being tested is called the independent variable, since it
is the variable being controlled by the investigator (dose, timing of administration, etc).
The effect being measured is called the dependent variable; in this case, the specific
effect of the drug. For example, if this were a drug designed at lowering blood pressure,
then blood pressure levels would be the dependent variable.
In today’s experiment, we will be determining the effect of caffeine on reaction
time to an auditory stimulus (what are the dependent and independent variables in this
experiment?). Caffeine is a central nervous system stimulant and is used globally for
improving alertness. Caffeine actually works as an anti-depressent, since it functions to
block the effect of chemical signals in the brain that play a role in the sleep-wakefulness
cycle (specifically, those that induce a state of rest in the brain). The BioPac system will
be used to record and calculate the time interval between the delivery of an auditory
stimulus (a short clicking sound) and the subject’s pressing of a finger switch. When the
experiment is performed before and after the administration of caffeine, the data can be
used to determine the effect of caffeine on reaction time. However, experiments like this
must be carefully controlled to ensure that, should some difference be observed, it
actually is the caffeine causing the difference. Thus, a negative control, or placebo, is
administered to a group of subjects as well. In our case, half the subjects will drink
sugar-free juice and the other half will drink sugar free juice containing 100mg of NoDoz
(about the same amount in one cup of coffee). The experiment will be conducted in a
double-blind format, where both the subjects and those administering the drink do not
know to which group they belong. After the data are recorded, the instructor will reveal
which subjects were in which group, and the data will be analyzed using a statistical
program to determine whether there is a significant difference in reaction time after
caffeine consumption.
Procedure:
1. At your workstation, click on the “BioPac Student Lab” alias on the desktop.
Choose lesson “L11 – React-1”. The computer will prompt you to enter a file
name. The first subject should enter his/her first and last name here with the word
“before”. After the first subject has finished the experiment, the second subject
will be able to create a file in the same way and then do the experiment.
2. The experiment window will come up, and you will see two panes: the top one is
where the hand switch activity will be recorded, and the bottom one contains
instructions and will be used to record the data. Follow the instructions for
calibrating the machine.
3. Once you finish the calibration, the program will ask you to continue to the data
recording phase. This will happen in four “segments” of 10 stimuli per segment,
and the program will stop between each segment. The first two segments will
present stimuli at random intervals (between one and 10 seconds), while the
second two segments will present stimuli at fixed intervals (every 4 seconds). As
soon as you click “Record”, the first segment of 10 auditory clicks will begin.
The other three segments will start each time “Resume” is clicked. At the end of
the fourth segment, click “Done”. During each segment, the subject should have
his/her eyes closed and use the finger switch in the dominant hand.
4.
Once the first subject has done all four segments and clicks “Done”, choose
“Yes” to save the data. The program will then ask you what you want to do next;
choose “Record from another subject”. Repeat steps 1 to 4 with the second
subject.
5. After the second subject has finished and has saved the data, choose “analyze
current data file”. STOP at this point and each subject should drink a cup of juice
Each cup is labeled with a number – RECORD THE CUP NUMBER in your lab
notebook. Each subject must then wait at least 30 minutes before doing the
experiment again. RECORD THE TIME in your lab notebook when you drank
the juice.
6. All the raw data from each of the four segments (for subject #2) will be displayed
in the upper pane, and the time intervals are recorded in the lower pane. To
understand how the time interval is calculated, click on the magnifying glass icon
(bottom right corner of upper pane) and then highlight the first waveform. This
will expand the X-axis (seconds). You will see an inverted triangle in the grey
area directly above the recording window, indicating when the auditory stimulus
was delivered. Now, click on the waveform cursor icon (middle button in the
lower right corner of the upper pane) and then highlight the area from the triangle
to about halfway up the hand swich trace. The value will be displayed in the
“DeltaT” box above the upper panel. This value should closely match the value
recorded in the lower panel for the corresponding data point.
7. Transfer the data to the provided Excel spreadsheet (found on the desktop) in the
appropriate cells in the sheet labeled “raw data”(make sure the data are placed
under “subject #2 before” category)
8. Once subject 2 has recorded the data into the Excel spreadsheet, subject 1 should
do the same. To do this, from the “lessons” menu at the top, choose “review
saved data”. You’ll get the Mac finder display, with “BioPac Student Lab”
highlighted. Choose “Data Files” in the window on the right, and then your
filename, and then the actual data file (it will have the suffix “-L11”). Open the
file, and repeat steps 6 and 7.
9. After at least 30 minutes has passed, each subject should repeat the entire
experiment. From the “Lessons” menu, choose “L11-React 1”. You will be
prompted to enter a file name. Subject 1 should enter his her full name and then
the word “after”. Repeat steps 2-9, so each subject has done the experiment again
after consuming the juice.
10. When all the data have been recorded and transferred to the Excel sheet, you will
perform a statistical analysis to determine if caffeine had any significant effect on
reaction time. Since sample size plays a critical role in determining the
significance of the data, we will use an average of all the data recorded from all
subjects to do this. The instructor will inform each subject which type of juice
was consumed. We will evaluate the data separately for the pseudo-random and
the fixed-interval experiments, according to the following:
a. For both experiments, you will need to choose the average for one of the
two segments (average from 10 data points). On your Excel sheet, transfer
the average to the appropriate columns in the sheet labeled “paired t-test”,
depending on whether you consumed the caffeine or placebo. Since each
student will be either in the caffeine or placebo group, after the entire lab
(both sections, all students) has done the experiment, we will have at least
13 students in each group.
b. A paired P-value (which is the actual measurement of statistical
significance for a “before” and “after” treatment) will be calculated based
on the data. Remember that a P-value of < 0.05 indicates statistical
significance.
c. Below this section, you will see a heading that says “perform unpaired ttest”. This section is comparing the data obtained in the caffeine group
with that of the data obtained from the placebo group, by first finding the
difference in average reaction time for each subject, and then using those
differences to calculate a P-value. If caffeine has a specific effect on
reaction time, you would expect to see a P-value of < 0.05 for this
calculation.
d. Another variable that could influence the significance of your data is the
possibility that you simply “learned” to decrease your reaction time as the
experiment progressed; i.e., you acclimated to the experimental
parameters and were able to decrease reaction time accordingly. To
evaluate this, we will determine if there is any significant difference
between the reaction time of the first and tenth auditory click for both
experiments for all subjects. Under the “learning” tab, transfer the data
from your “before” experiment as directed to the columns under ”evaluate
learning”. What type of P-value would you expect to see if learning had
no effect on the interpretation of your caffeine/placebo experiment?
11. You should print the excel sheets and cut and tape the data into your lab notebook
(you may want to shrink them down first).
12. Draw appropriate conclusions from your data:
a. Does consumption of caffeine significantly affect reaction time? What
statistical results do you base your conclusion on? If it does not, offer
some suggestions why this may be the case.
b. Does “learning” affect reaction time? If so, how should this be considered
when evaluating caffeine’s role in altering reaction time?
c. What other parameters (i.e., variables) can you think of that could affect
the outcome of the experiment? Try to answer the question this way: if
your job as the head of the clinical research group for a drug or vaccine
company was to design a trial that would evaluate a drug/vaccine’s
efficacy, what independent variables would you be considering as you
designed the trial (there are two main ones)?