Applications - Quantitative Neurophysiology - page 105

CHAPTER 10
Applications
10.1 SCIENCE AND SCIENCE FICTION
A compact definition of science fiction is science that hasn't happened yet . Using this definition we can
ask when some science fiction of the brain will become science. A quick overview of some of the most
popular science fiction topics and movies will show that there are (and have been) some incredible leaps
of imagination in what people and machines may be able to do in the future. Although at the time these
ideas were far fetched, the pace of breakthroughs is such that some of these wild ideas may not be far
from realization.
The purpose of this text is to provide a basic quantitative background from which to jump off to
more advance areas of study. Although we cannot go into the details of each of these very interesting
research areas, this chapter is an overview of some of the most promising new directions in neuroscience
and neuroengineering. As you read each section, try to consider the technology from two perspectives.
First, how will the quantitative background of the previous nine chapters inform and supplement these
new lines of research? Second, given these new technologies, what ideas do you have for future experiments
or devices?
10.2 NEURAL IMAGING
The electrical function was uncovered using primarily patch clamp electrodes and extracellular electrodes.
These two techniques are very efficient and can detect small changes over a short period of time. Over
the past four decades, however, three new imaging techniques have enabled a fresh and complimentary
view of the function of the brain.
10.2.1 Optical Recordings
In the late 1960s and early 1970s, the work of the three research groups of Tasaki et al. found that small
molecules could be embedded in the cell membrane (Fig. 10.1) that would absorb light at one wavelength
(color) and emit light at a different wavelength (left panel of Fig. 10.2). Furthermore, for some molecules,
called fluorescent dyes , the intensity of the emitted light was proportional to V m within a range that spanned
the normal action potential (right panel of Fig. 10.2). Some subset of these molecules could also respond
to changes less than 1 msec , short enough to resolve the action potential upstroke.
A typical imaging system is shown in Fig. 10.3 and consists of two light pathways. In the first
pathway (solid line), a laser is used to excite the tissue at a high frequency through a dichromatic mirror.
The function of the dichromatic mirror is that it will pass light of a high frequency but reflect light of
lower frequencies.The cell then absorbs high frequency photons and emits low-frequency photons. In the
second pathway (dotted line), the low-frequency light emitted from the cell is reflected off of the mirror,
through a microscope and then to a collection device (usually a CCD camera or photodiode array).
The optical imaging method provides at least three advantages over the use of extracellular elec-
trodes. First, as demonstrated in Ch. 8, a single φ e recording is a weighted average of all surrounding
electrical activity. On the other hand, optical mapping enables V m to be recorded from many locations
simultaneously. Second, when coupled with a high power microscope, the field of view can be adjusted