Reading
through yet another study (Huebner and Gegenfurtner, 2012) using natural images
as stimuli, I started thinking about their contribution to our understanding of
visual processes. Using natural images has undeniably shown that current models
of visual processing are still incomplete. Indeed, most model based predictions
of neuronal responses to natural images are poor, including for cells in primate
primary visual cortex (V1), a cortical area considered by many to be relatively
“well-understood”.
Knowing
that a model is inaccurate or incomplete and gaining insight into the causes of
its failures are, however, two very different things. In my opinion, using
“natural” stimuli has hardly, if at all, contributed to the latter. I do
believe that the most important results concerning the functional properties of
visual neurons have been obtained, and will for a while continue to be, using
simple stimuli. Sinusoidal gratings, for example, allow us to use the powerful
tools of linear system theory , to systematically study failures of simple
linear models, and to uncover the important non-linearities in visual signal
processing.
Why is it
then that the number of studies using natural images has grown so much in
recent years (by my counting, only 2 papers used them in 1990, they were 62 in
2010, and there are already 62 half-way through 2012)? I think it became a
fashion due to the increasing ease with which these stimuli can be stored,
displayed and manipulated, and to the erroneous notion that using “natural” stimuli
would bring us closer an understanding of what neurons do outside of the lab,
in “nature”. Such a notion would make us
believe that Galileo Galilei should not have performed his
inclined plane experiments to study mechanics. Instead, he should have saved
the trouble by simply observing falling bodies in nature, thereby gaining
simultaneous insight into the laws of motion, inertia, acceleration, gravitation
and turbulent air resistance. Well, the
inclined plane experiments are now considered seminal in mechanics and a monument
of reductionist science, the fabled dropping of objects from the tower of Pisa
apparently never even occurred...
It may be
appealing to use complex stimuli to study the complexity of nature, it may even
become very fashionable, but the complexity may be so high that it masks the
underlying processes. Similar thoughts occur to me when I hear people
criticizing such old fashioned methods as extracellular single cell recordings.
It is of course much more fashionable to use optogenetics or two-photon calcium
imaging. The main question for me is whether we have learned everything we can
learn from the old methods using simple stimuli. New methods are great but it
doesn’t mean old ones should be abandoned…
- D. Kiper
