While elaborating on the parallels between perception and language interpretation, we have unpacked many features of the nature of visual perception that should hold up even if we end up finding the view of perception-as-interpretation wanting. In this post I’ll briefly integrate the data and theory from the past seven posts into a more tidy and (hopefully) coherent story.
As we discussed in some detail in post ten, the contents of experience have properties that are, on the surface, quite different from the properties of the underlying neural machinery doing the experiencing. I can see an iridescent jewel two feet in front of me (that’s the content), but the vehicle doing the experiencing is neither iridescent nor two feet in front of me.
We can be intimately familiar with the contents of our experience while remaining in complete ignorance of facts about nervous systems. I hope I don’t offend my fellow neuroscientists when I claim that our species’ great artists, playwrights, musicians, and novelists have revealed more about the contents of experience than any neuroscientist. Yet most of these artists worked without knowing the most basic facts of neuroscience. The vehicles of experience are effectively invisible to us, while the contents of experience are as familiar as breathing. Anyone that has savored an authentic lobster roll from a rundown shack on the coast of Maine knows what it is like to revel in the contents of experience (and those who have not have yet to fully live).
In sum, the contents of our experience seem to be a neurally-constructed portrait of what is happening beyond the brain. The brain faces some rather severe obstacles if its goal is to make this portrait accurate. For one, a great deal of information is lost in the projection from the scene to the retina (a projection we discussed in some detail in post nine).
Consider the case in which a projection onto the retina is square-shaped. What can we say about the object that generated that projection? Assuming there are no distance cues present, the same square shape on the retina could be produced by a tiny square that is extremely close to the eye, a medium-sized square a moderate distance away, or a colossal square that is extremely far away. It could even be generated by non-square shapes transmitted through a distorting funhouse-type medium.
Purves and Lotto state the point nicely:
[T]he retinal output in response to a given stimulus can signify any of an infinite combination of illuminants, reflectances, transmittances, sizes, distances, and orientations in the real world. It is thus impossible to derive by a process of logic the combination of these factors that actually generated the stimulus[.]In other words, given only retinal movies as data, the brain cannot determine with perfect accuracy the scene in the world that generated said movies. Given the often striking ambiguity of the source of a retinal projection, it is remarkable that our visual system usually locks in on a single perceptual response to a given stimulus. Even during bistable perception we typically experience one object at a time, not a superposition of two objects.
How does the brain settle on a unique percept when provided with an inherently ambiguous retinal projection? It seems the brain uses context (post eleven) as well as background assumptions and knowledge (post twelve) to help narrow down the range of reasonable interpretations. Bistable percepts merely serve to highlight those rare instances when these contributions from the brain are not sufficient to settle on a single interpretation for an extended period of time.
In general, while we know that the retinal movies strongly influence the brain’s construction of experience, our experience is obviously not a mere report or transcription of what is happening in the retinae. If it were, ambiguous stimuli wouldn’t spontaneously reorganize in such drastic ways such as we observe in the Spinning Girl and Rotating Necker Cube (post seven), the angles in Purves’ Plumbing would look the same, the tabletop dimensions in Turning the Tables would look identical (post twelve), the yellow and blue squares in Purves’ Cubes would look grey, Shepard's subterranean monsters would look identical in size (post eleven), etc..
Hopefully the previous seven posts have made it clear why psychologists often say that the brain constructs interpretations of stimuli in a context-sensitive way, based on background knowledge and assumptions, in the light of sometimes intense ambiguity of the actual source of the stimulus. If we were forced to choose between the false dichotomy of saying that experience is an interpretation of what is happening in the retinae versus a transcription of what is happening on the retinae, I think the choice is clear.
Richard Gregory (1966) summed up the view quite well when he said that “Perception is not determined simply by the stimulus patterns; rather it is a dynamic searching for the best interpretation of the available data.” Our visual experience is clearly the result of neuronal events downstream from the stimulus, a construction of an experience whose contents mostly include worldly events beyond the eyes. It is such worldly events that we must engage with, after all, and such engagement with the world determines whether we eat, reproduce, flee, or die.
In the next couple of posts we’ll persue the idea that perception is interpretation down more specific paths, looking at a prominent view that the mechanism of interpretation is a kind of unconscious inference, and finally we’ll end up heading into the brain, looking at the neuronal basis of these internal “portraits” of the world.
Gregory, RL (1966). Eve and Brain. London: Weidenfeld and Nicolson.
Purves, DP, and Lotto, RB (2003) Why we see what we do: An empirical theory of vision Sinauer Associates.
Table of Contents of posts on consciousness.