We are examining the popular view that visual perception is a form of interpretation, specifically the interpretation of a stimulus. We should start by determining what, exactly, is a stimulus?
So as not to keep you waiting, the answer is roughly that a visual stimulus is visible light that is projected from the world to the retina. If that is unclear, or if you are interested in the biology, then keep reading.
In general, a stimulus is anything that can activate our sensory transducers, the cells that convert external signals into internal electrochemical signals that can be used by the rest of the nervous system. In the case of vision the transducers are in the eye, so let’s consider the general relationship between the world, the eye, and visual stimuli.
Our world is filled with objects that project light into our eyes. Let’s denote the set of such objects at a given time the scene. For example, the eye in Figure 1 is inspecting a scene that consists of two objects: a cube and a house. The eye seems to be looking directly at the cube, while the house is up to the left of the hovering orb.
Figure 1: An eye viewing a scene.
The light from the scene is projected to the retina by the eye's lens (Figure 2, top). The retina is a thin red sheet of tissue that coats the inside of the eye, biology’s grotesque movie screen.
Photoreceptors are probably the most important cells in the retina: they convert light into chemical signals that the rest of the nervous system can understand. However, the retina is much more than a sheet of photoreceptors. It is an extremely complicated neuronal processor in its own right. The retina contains multiple layers of neurons (Figure 2, bottom), and it is only the axons of the final layer of neurons that make their way through the optic nerve toward the brain.
Figure 2: Gross anatomy of the eye (top), and
cross-section of the retina (bottom).
The retina includes a special region, the fovea, which has an extremely high density of photoreceptors. When we look about the world, we typically direct our eye so the fovea aims at the most interesting parts of the scene, and this lets us take in more information from those regions. It’s like having a video camera with extremely high definition in the middle of the screen, but as you move away from the middle the picture gets quite fuzzy. Note that in Figure 1, the retina is painted onto the back of the eye in red, but the high-def fovea is represented by a small yellow region in the center of the retina.
Imagine gently peeling the retina from the inside of the eye and laying it flat on the page. Figure 3 is a graphical depiction of such a flattened retina, drawn so that the spacing of the grid lines represents the density of photoreceptors. The fovea is in the center of the graph, with finely-spaced grid lines that tell us we are in the high-def region. The density of photoreceptors quickly decreases as you move away from the fovea, as indicated by the more coarsely-spaced lines.
Figure 3: The retina from a functional
point of view.
You can directly experience this drop in resolution by trying to read this text while looking off to the side of the page. Even though you can still make out the coarse features of the page (e.g., there are words and pictures), it is extremely difficult to discriminate the finer-grained spatial properties such as individual letters and words.
There is one more notable feature of Figure 3. Namely, the top half of the retina is labeled ‘Bottom’ while the bottom half is labeled ‘Top.’ ‘Left’ and ‘Right’ also seem to be reversed. These labels are not mistakes, but highlight that an inverted image of the scene projects to the retina. For instance, an object located above the eye’s fixation point, such as the house in Figure 1, is projected to the bottom half of the retina.
I included the geometrical rationale for such image inversion in Figure 1 above. The light from the house travels from above, down to the eye, and the rays of light continue traveling down within the eye until they hit the lower half of the retina. For similar reasons, objects to the left of fixation project to the right-hand side of the retina.
All of this inversion business is illustrated in Figure 4, which shows the house/cube scene projected onto the retina. As we would expect, the fixated cube-face is projected squarely to the fovea. The image of the house, which arrives from the top-left part of the world, is projected to the lower-right quadrant of the retina. That is, the house projects to the quadrant labeled ‘Top Left,’ so-called because the image is projected from the top-left hand region of the world. Note that even the image of the house is inverted, so an upside-down house is projected onto the retina.
Figure 4: The scene from Figure 1 projected
onto the retinal graph from Figure 3. Note how
the projections are inverted.
Finally I think we have arrived at a respectable-enough understanding of visual stimuli. A visual stimulus consists of the images projected from the scene onto the retina. We should stipulate that only light that is able to activate the retina’s photoreceptors counts as a visual stimulus. Not all electromagnetic radiation (i.e., light) can activate photoreceptors. Photoreceptors are tiny light detectors that are sensitive to light within a certain narrow band of wavelengths, as shown in Figure 5. So, absorbing this wrinkle into our account of visual stimuli, and using the plural of ‘retina’ because we have two eyes, yields:
A visual stimulus is light, within the visible part of the electromagnetic spectrum, that is projected from the scene to the retinae.So, with that definition, I think we have a respectable characterization of visual stimuli.
There is one practical detail I should add before closing this discussion. Because vision is a distal sense that involves the perception of things far away from our bodies, in practice there is quite a bit of flexibility in what researchers count as a stimulus. They will sometimes identify the stimulus as the object itself out in the world (e.g., the Necker Cube drawing), the light emitted from the object, or the light from the object that hits the surface of the eye. In practice the things that researchers count as visual stimuli depend on the question being asked and the experimental setup. In the future I may call any of the above ‘stimuli’ when it seems appropriate, though typically I use the word to refer to the image projected to the retina.
Figure 5: The electromagnetic spectrum,
with the visible spectrum as a subset.
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