The computerized data acquisition system
Most of the interesting physical characteristics of tissues and organisms can be measured electronically. The advantages of electronic measurement are that (1) it is precise, (2) you can record a lot of data easily, (3) you can automatically transfer the data to a computer for sophisticated analyses (and convenient display). This means that you have a lot more freedom to test different hypotheses than if you had to measure each variable by hand.
There are a number of components to our data acquisition system (Fig. 1). Transducers or electrodes collect data. Transducers produce a voltage in response to change in some physical variable. Electrodes record voltage differences that already exist. In either case, the recording is a voltage difference (which I will often refer to as a signal). This voltage is usually weak, so it must be amplified so the data is not lost in the omnipresent random voltage fluctuations of our environment (ie. electronic noise). The amplifier then passes this nice strong signal to the analog to digital converter; this is an electronic device that converts the electrical signal into something the computer can read (see appendix for more information, if you're interested). Once in the computer, specialized software can display and analyze the signal. Thus, the role of the computer is solely to tell the system what to do, then display the data as it comes in.
The first step in the process is the data collection. (1) Electrodes record and relay electrical signals created by tissues. They are useful for recording nerve and muscle activity. You almost always have two electrodes (and often a ground). This is because a voltage is a difference in electrical potential between two points; voltages are always measured relative to something. (2) Transducers, on the other hand, produce an electrical signal in response to changes in some other variable. For example, if you push or pull on a force transducer, it will produce a voltage difference that is proportional to the magnitude of the force. There are all kinds of clever ways of creating an electrical signal from specific physical variables. We can measure pressure, force, movement, temperature and light intensity in this way.
The type of amplifier you use depends on what
kind of electrical signal you record. There are two basic types -
AC and DC amps. Electrical activity of muscles and nerves is
high frequency so you need an AC amplifier. This records alternating
currents, ie. signals that change rapidly (faster than 0.1 sec).
On the other hand, electrical signals produced by transducers often change
slowly. Therefore you need a DC amplifier. DC stands for direct
current; the current generally doesn't change direction very rapidly
(one second is slow in the electronic world). DC amplifiers also have special
circuitry required for compatibility with the transducers.
AC amplifiers (A-M systems differential AC amplifier Model
1700)
The configuration of our AC amplifiers is straightforward.
The bold terms refer to specific controls (they are in bold so that you
can refer back to them easily, not so that you will memorize them).
1) As always, electrical devices usually only
work when the power is on (this is the single most common problem
people have when they ask me why their system doesn't work).
2) Mode should be set to "rec", for record
(you would only use "stim" if you were stimulating with the recording electrode,
which is an advanced technique).
3) "Input" is where the electrical
signal from the electrode comes into the amplifier, "output" is
where the amplified signal comes out. Output should be connected
to the analog to digital connector box. The amp has four channels,
meaning you can input the signals from four different transducers.
4) The "gain" switch determines the
amplification. Do you want to make the signal 100, 1000, or 10,000
times stronger (voltage-wise)? It depends on the strength of the
original signal.
5) The rest of the controls are filters.
You are surrounded by sources of electrical activity; these all affect
the magnitude of your recorded signal. It is often handy to filter
out this noise so that you only see the data you are trying to record.
The notch filter partially removes 60 hz noise. Virtually
all electrical fixtures in North America are wired to a 60 hz AC current,
so it is easy to pick up stray 60 hz signals. That's why we have
a special filter for it. The two top dials remove low and high frequency
noise. The low cut-off removes frequencies below the setting,
the high cut-off removes frequencies above the setting. Of
course your data is a mix of frequencies, so there is a tradeoff;
the narrower you make your cutoffs the more noise you remove, but you also
lose some of your data too.
6) Next to the power button, there is a socket
named "gnd". This is the instrument's ground, and all four
channels use this. It gives the amplifier a stable reference point.
DC amplifiers (World Precision Instruments - Transbridge TBM 4)
The configuration of our DC amplifiers is similar, although DC
signals need less filtering. This is also a four channel amplifier,
so there are four similar sets of controls.
1) The power button is on the back
of the amplifier, as are the outputs.
2) There is an input plug for the transducer
(make sure you input and output on the same channel).
3) Above the input are two indicator lights,
which are useful for determining what your baseline is with certain transducers.
4) Above the lights is a switch that determines
the bridge circuitry. If you are interested, I would be more
than happy to explain bridge circuitry to you. It is fundamental
to most transducers, and it is simple and elegant. If you would rather
not know, that's ok. What you need to know is that my homemade force
transducers work with the single-ended circuit, and the other transducers
use a differential (diff) circuit. If you set the switch to
ground
(gnd), then the amplifier will read the nice stable ground voltage.
This protects it from power surges when it is not recording.
5) The next dial up is the gain.
This determines how much you amplify your signal. (Again, you are
amplifying the voltage).
6) The top button is the position adjust.
This is how you set your baseline. You want your transducer to read
zero volts at rest, so turn the position adjust knob until it does so.
Otherwise your signal will likely be too high or low for the computer to
read.
The computer
The computer controls the acquisition, display
and analysis of data using a program called Superscope II. This software
allows me to create custom "instruments" for data collection. Each
lab handout will tell you what instrument you need to run. Each instrument
has a display modelled after a real electronic device. It will have
specific dials, buttons, and displays that you can manipulate to do everything
you need for the experiments. You will not have to learn any of the
menus unless you wish to do something more advanced.
1) Usually there will be a start button,
which will begin the recording.
2) There will be a display, which reads
like a chart recording of your signal. You can display a large number
of channels at once if necessary. Each channel will be labelled.
You can control the x and y axis scale to magnify your signal, or
zoom out. The controls for this are the arrow keys embedded at the
bottom of the vertical scroll bar, and the right of the horizontal scroll
bar. This is not amplification, it is analogous to looking at something
under varying degrees of magnification; the size of what you are
looking at doesn't change. The reason you can't just use this to
look at weak signals is that you magnify noise as well as your true signal;
amplifiers, on the other hand are designed to only amplify a specific signal,
and to eliminate noise. In addition to changing the axis scale, you
can scroll up and down to find your signal, or you can scroll horizontally
to look at your signal over a long time period.
3) The computer can stimulate tissues.
The output comes through two wires in the back of the analog to digital
connector box. For experiments involving stimulation, there will
be control bars and dials that let you control the magnitude and
duration of the stimulus.
4) There are often markers on the display.
These are vertical lines that mark points on your recording. Usually,
I will have a readout that will show what the magnitude of the voltage
is at the marked spot. This may give the absolute magnitude, or the
difference in voltage between the points marked by two different markers.
You can move the markers with the mouse, or with a special control dial,
depending on the program.
5) There may be other features referred to
in each handout.
Finding a signal
One final bit of advice that may be useful
deals with how to find your signal. Often you will record but not
see anything on the screen. This usually results from choosing the
wrong scale (See Fig. 2). You may need to decrease the magnification
(zoom out) on the screen. You may also need to use the position adjust
knob to bring your signal back to zero (if it is a DC amp). In general,
start at lower amplifications to find the signal, and zoom in, as you would
with a microscope. If you can see a nice signal, but it is very flat
even though you know something should be happening, you need to increase
the amplification (or magnify on the display); the signal is probably
too weak to register. If the signal is really jumpy, you may need
to turn down the amplification (or change axis scales).
Appendix 1. Analog to digital conversion.
Electrical signals are in the form of constantly
varying voltages. The logic boards of computers don't understand
constantly varying voltages, however. They understand digital signals;
the computer represents things with numbers (each number is represented
electronically as a series of on or off switches). So to record an
electronic signal you have to convert the analog (voltage) signal to a
series of numbers representing the magnitude of the voltage at each point
in time. This series of numbers is a digital signal. Hence,
we talk about analog to digital conversion when we record data with computers.
Recording signals digitally has enormous practical
relevance. Analog signals are prone to lots of noisy interference.
There are lots of electrical sources in the world, and these constantly
change the magnitude of the electrical potential (voltage) produced by
any transducer (such as a microphone). So that by the time the signal
has been amplified and played back (on a speaker for instance), all these
little distortions have given the sound a fuzzy quality not characteristic
of the original sound. By comparison, once something has been recorded
digitally, it is less prone to noise. A number is a number, and numbers
in a digital device or digital signal do not change on their own.
This has been a boon to the recording industry, as well as many other fields.