Categories of Perception

We can categorize perception as internal or external.

Internal perception (proprioception) tells us what's going on in our bodies. We can sense where our limbs are, whether we're sitting or standing; we can also sense whether we are hungry, or tired and so forth.

External or Sensory perception (exteroception), tells us about the world outside our bodies. Using our senses of sight, hearing, touch, smell and taste, we discover colors, sounds, textures, etc. of the world at large. There is a growing body of knowledge of the mechanics of sensory processes in cognitive psychology.

The philosophy of perception is mainly concerned with exteroception. When philosophers use the word perception they usually mean exteroception and the word is used in that sense everywhere.

Scientific Accounts of Perception

The science of perception is concerned with how events are observed and interpreted. An event may be the occurrence of an object at some distance from an observer. According to the scientific account this object will reflect light from the sun in all directions. Some of this reflected light from a particular, unique point on the object will fall all over the corneas of the eyes and the combined cornea/lens system of the eyes will divert the light to two points, one on each retina. The pattern of points of light on each retina forms an image. This process also occurs in the case of silouettes where the pattern of absence of points of light forms an image. The overall effect is to encode position data on a stream of photons and to transfer this encoding onto a pattern on the retinas. The patterns on the retinas are the only optical images found in perception, prior to the retinas, light is arranged as a fog of photons going in all directions.

The images on the two retinas are slightly different and the disparity between the electrical outputs from these is resolved either at the level of the lateral geniculate nucleus or in a part of the visual cortex called 'V1'. The resolved data is further processed in the visual cortex where some areas have more specialised functions, for instance area V5 is involved in the modelling of motion and V4 in adding colour. The resulting single image that subjects report as their experience is called a 'percept'. Studies involving rapidly changing scenes show that the percept derives from numerous processes that each involve time delays.

Recent fMRI studies show that dreams, imaginings and perceptions of similar things such as faces are accompanied by activity in many of the same areas of brain. It seems that imagery that originates from the senses and internally generated imagery may have a shared ontology at higher levels of cortical processing.

If an object is also a source of sound this is transmitted as pressure waves that are sensed by the cochlear in the ear. If the observer is blindfolded it is difficult to locate the exact source of sound waves, if the blindfold is removed the sound can usually be located at the source. The data from the eyes and the ears is combined to form a 'bound' percept. The problem of how the bound percept is produced is known as the binding problem and is the subject of considerable study. The binding problem is also a question of how different aspects of a single sense (say, color and contour in vision) are bound to the same object when they are processed by spatially different areas of the brain.

Philosophical Accounts of Perception

Historically, the most important philosophical problems posed by perception concerned the epistemology of perception-the question of how we can gain knowledge via perception. However, the problems raised by perception also touch on other fields of philosophy-the nature of qualia is an important topic in the philosophy of mind. Moreover, any fully explicit account of perception requires a commitment to one of a variety of ontological (metaphysical) viewpoints on a spectrum of direct realism, indirect realism and idealism.

The most common belief about perception, probably universal in childhood, is naive realism, in which people believe that what they perceive are things in themselves. Many people who have not studied biology carry this belief into adult life. In this form, naive realism is not strictly a theory but rather an axiom on which all thought and use of language is based. In a sense it is transparently true. If I see a chair it is a chair that I see. When biologists say that this is mistaken, there has been a subtle change in the meaning of the word see (or perceive) that is necessary for a scientific account of how the brain works but unfortunately is not made clear by new terminology. Cross purpose arguments can result. That childhood naive realism is indeed a belief that amounts to an implicit theory is shown by the common humbling experience of arguing its validity as a first year biology student only to be forced to admit that one's position is self-contradictory even without the results of experiments on perception that demonstrate its absurdity. Within the biological study of perception naive realism is unusable. However, outside biology modified forms of naive realism are defended. Thomas Reid in the eighteenth century realised that sensation was composed of a set of data transfers but declared that these were in some way transparent so that there is a direct connection between perception and the world. This idea is called direct realism and has become popular in recent years with the rise of postmodernism. Direct realism does not clearly specify the nature of the bit of the world that is an object in perception, especially in cases where the object is something like a silhouette.

The succession of data transfers that are involved in perception suggests that somewhere in the brain there is a final set of events, in which sense data are somehow available to a perceiving subject, that is the substrate of the percept. Perception would then be some form of brain activity and somehow some part of the brain would be able to perceive signals provided by some other (or the same) part of the brain. This concept is known as indirect realism or representative realism. In indirect realism it is held that we can only be aware of external objects by being aware of representations of objects. This idea was held by John Locke and Nicolas Malebranche. The common argument against indirect realism is that it implies a homunculus with an infinite regress (a perceiver within a perceiver within a perceiver...). However, as long as each stage of sensory processing achieves a different task a finite regress is perfectly possible. The above argument against indirect realism has also been challenged on the grounds that it assumes that perception is entirely due to data transfer and classical information processing. It is suggested that the argument can be avoided by proposing that the percept is a phenomenon that does not depend wholly upon the transfer and rearrangement of data. The real problem here probably relates not so much to issues of infinite regress as to basic ontological issues of the sort raised by Leibniz Locke, Hume, Whitehead and others, which fall beyond the scope of this account.

Direct realism and indirect realism are known as 'realist' theories of perception because they hold that there is a world external to the mind. Direct realism holds that the representation of an object is located next to, or is even part of, the actual physical object whereas indirect realism holds that the representation of an object is brain activity. Direct realism proposes some as yet unknown direct connection between external representations and the mind whilst indirect realism requires the resolution of ontological issues relating to fundamental physics which remain outstanding, particularly in relation to the binding problem. Indirect realism provides an account of issues such as:

Quality themes, imaginings, hallucinations, illusions, the resolution of binocular rivalry, the resolution of multistable perception, the modelling of motion that allow us to watch T V, the sensations that result from direct brain stimulation, the update of the mental image by saccades of the eyes and the referral of events backwards in time whereas direct realism has to argue either that these experiences do not occur or avoids the problem by defining perception as only those experiences that are consistent with direct realism.

Apart from the realist theories of perception there are also anti-realist theories. There are two varieties of anti-realism: Idealism and Skepticism. Idealism holds that reality is limited to mental qualities, while skepticism challenges our ability to gain knowledge of any reality external to our mind. One of the most influential proponents of idealism was George Berkeley who maintained that everything was mind or dependent upon mind. Berkeley's idealism has two main strands, phenomenalism in which physical events are viewed as a special kind of mental event and subjective idealism. David Hume is probably the most influential proponent of skepticism.

A third theory of perception attempts to find a middle path between realist and anti-realist theories. Called enactivism, the theory posits that reality arises as a result of the dynamic interplay between an organism's sensorimotor capabilities and its environment. Instead of seeing perception as a passive process determined entirely by the features of an independently existing world, enactivism suggests that organism and environment are structurally coupled and codetermining. The theory was first formalized by Francisco Varela, Evan Thompson and Eleanor Rosch in "The Embodied Mind".

Cognitive Processing and Epiphenomenalism

Perception is sometimes referred to as a cognitive process in which information processing is used to transfer information from the world into the brain and mind where it is further processed and related to other information. Some philosophers and psychologists propose that this processing gives rise to particular mental states (cognitivism) whilst others envisage a direct path back into the external world in the form of action (radical behaviourism).

Many eminent behaviourists such as John B. Watson and B.F. Skinner have proposed that perception acts largely as a process between a stimulus and a response but despite this have noted that Ryle's "ghost in the machine" of the brain still seems to exist. As Skinner (1953) wrote:

"The objection to inner states is not that they do not exist, but that they are not relevant in a functional analysis".

This view, in which experience is thought to be an incidental by-product of information processing, is known as epiphenomenalism.

Perceptual Space

Another aspect of perception that is common to both realists and anti-realists is the idea of mental or perceptual space. David Hume considers this at some length and concludes that things appear extended because they have the attributes of colour and solidity. A popular modern philosophical view is that the brain cannot contain images so our sense of space must be due to the actual space occupied by physical things. However, as René Descartes noticed, perceptual space has a projective geometry, things within it appear as if they are viewed from a point and are not simply objects arranged in 3D. Mathematicians now know of many types of projective geometry such as complex Minkowski space that might describe the layout of things in perception (see Peters (2000)). It is also known that many parts of the brain contain patterns of electrical activity that correspond closely to the layout of the retinal image (this is known as retinotopy). There are indeed images in the brain but how or whether these become conscious experience is a mystery (see McGinn (1995)).

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