Below you'll find a series of quotations that highlight the topics we have been discussing in the last nine posts. I chose them for their exceptional eloquence, clarity, and influence. They are not in chronological order, but are roughly in ascending order of specificity of the claims. This will be the final post in this narrative arc.
[V]ision is the process of discovering from images what is present in the world, and where it is.
-David Marr (1982)
We may define visual perception as attributing objects to images.
-Richard Gregory (2009)
Visual perception involves coordination between sensory sampling of the world and active interpretation of the sensory data. Human perception of objects and scenes is normally stable and robust, but it falters when one is presented with patterns that are inherently ambiguous or contradictory. Under such conditions, vision lapses into a chain of continually alternating percepts, whereby a viable visual interpretation dominates for a few seconds and is then replaced by a rival interpretation. This multistable vision, or ‘multistability’, is thought to result from destabilization of fundamental visual mechanisms, and has offered valuable insights into how sensory patterns are actively organized and interpreted in the brain…
-Nikos Logothetis (2002)
[The Necker cube] has an interesting property. Look at it fairly steadily for a while, and the cube will invert, as if it were being viewed from another angle. After a time the percept switches back to the original one, and so on. In this case there are two equally plausible 3D interpretations of the image, and the brain is uncertain which it prefers. Notice that it only chooses one at a time, not some odd mixture of both of them…
The reason you normally see without ambiguity is that the brain combines the information provided by the many distinct features of the visual scene (aspects of shape, color, movement, etc) and settles on the most plausible interpretation of all these various visual clues taken together…[W]hat the brain has to build up is a many-leveled interpretation of the visual scene, usually in terms of objects and events and their meaning to us.
-Francis Crick (1995)
We don’t directly experience what happens on our retinas, in our ears, on the surface of our skin. What we actually experience is a product of many processes of interpretation—editorial processes, in effect. They take in relatively raw and one-sided representations, and yield collated, revised, enhanced representations, and they take place in the stream of activity occurring in various parts of the brain. This much is recognized by virtually all theories of perception…
-Dan Dennett (1991)
The mental activities that lead us to infer that in front of us at a certain place there is a certain object of a certain character, are generally not conscious activities, but unconscious ones. In their result they are equivalent to a conclusion, to the extent that the observed action on our senses enables us to form an idea as to the possible cause of this action; although, as a matter of fact, it is invariably simply the nervous stimulations that are perceived directly, that is, the actions, but never the external objects themselves. But what seems to differentiate them from a conclusion, in the ordinary sense of that word, is that a conclusion is an act of conscious thought… Still it may be permissible to speak of the mental acts of ordinary perception as unconscious conclusions, thereby making a distinction of some sort between them and the common so-called conscious conclusions.
-Hermann von Helmholtz (1866)
Perception consists of interpreting two-dimensional retinal images of a three-dimensional world. The process of projecting a three-dimensional scene onto a two-dimensional retina necessarily discards information about the three-dimensional structure of the scene. This makes it impossible, in principle, to deduce all of the three-dimensional structure of a scene…However, even though such problems cannot be solved by deduction, acceptable solutions can be found using statistical inference.
-JV Stone (2009)
Our visual experience evidently is the product of highly sophisticated and deeply entrenched inferential principles that operate at a level of our visual system that is quite inaccessible to conscious introspection or voluntary control. We do not first experience a two-dimensional image and then consciously calculate or infer the external three-dimensional scene that is most likely, given that image. The first thing we experience is the three-dimensional world—as our visual system has already inferred it for us on the basis of the two-dimensional input. Hermann von Helmholtz, the great nineteenth century scientist who more than any other single individual laid the foundations for our present understanding of visual and auditory perception, expressed this by characterizing perception as “unconscious inference.”
-Roger Shepard (1991)
[T]he brains’ representations are hypotheses, predictive like the hypotheses of science. Like science, perception bets from available evidence on what is likely to be true…For perception, there is always guessing and going beyond available evidence. On this view, the closest we ever come to the object world is by somewhat uncertain hypotheses, selected from present evidence and enriched by knowledge from the past. Some of this knowledge is inherited—learned by the statistical processes of natural selection and stored by the genetic code. The rest is brain-learning from individual experience, especially important for humans.
-Richard Gregory (2009)
It is the business of the brain to represent the outside world. Perceiving is not just sensing but rather an effect of sensory input on the representational system. An ambiguous figure provides the viewer with an input for which there are two or more possible representations that are quite different and about equally good, by whatever criteria the perceptual system employs. When alternative representations or descriptions of the input are equally good, the perceptual system will sometimes adopt one and sometimes another. In other words, the perception is multistable.
-Fred Attneave (1971)
We have suggested that the biological usefulness of visual consciousness in humans is to produce the best current interpretation of the visual scene in the light of past experience, either of ourselves or of our ancestors (embodied in our genes), and to make this interpretation directly available, for a sufficient time, to the parts of the brain that contemplate and plan voluntary motor output, of one sort or another, including speech.
-Francis Crick and Christof Koch (1998)
Attneave, F (1971) Multistability in perception. Sci Am. 6: 63-71.
Crick, Francis (1995) The Astonishing Hypothesis: The Scientific Search for the Soul. Scribner.
Crick, F, and Koch, K (1998) Cerebral Cortex, 8:97-107.
Dennett, D (1991) Consciousness Explained. Back Bay Books.
Gregory, RL (2009) Seeing Through Illusions. Oxford University Press.
Helmholtz, H. von 1866 Concerning the perceptions in general. In Treatise on physiological optics, vol. III, 3rd edn (translated by J. P. C. Southall 1925 Opt. Soc. Am. Section 26, reprinted New York: Dover, 1962).
Leopold, Wilke, Maier, and Logothetis (2002) Stable perception of visually ambiguous patterns. Nature Neuroscience 5: 605-609.
Marr, D (1982) Vision. WH Freeman, NY.
Shepard, RN (1991) Mind Sights, W.H.Freeman & Co Ltd.
Stone, JV, Kerrigan, IS, and Porrill, J (2009) Where is the light? Bayesian perceptual priors for lighting direction. Proc R Soc B 276: 1797-1804.
Table of Contents of posts on consciousness.