proximal_distal.htmlTEXTUm6"595:Proximal and distal stimuli

Proximal stimulus vs distal stimulus

©Robert Sekuler 1999
In this course we'll adopt the convention that to qualify as a stimulus, an object or event
  • must be describable in physical terms, and
  • must register on a sensory receptor that's able to respond.
  • These two criteria do not completely settle all the details of how to define the stimulus. Consider the energy of a light bulb. There's no doubt that a light bulb can be a stimulus for human vision, but there are several different ways of describing this stimulus (or any other stimulus). One way describes the energy emitted by the bulb; another focuses on the portion of the bulb's energy that falls on the receptor surface of the human eye. After all, the argument goes, the energy actually striking the rec eptors is what does the real job of initiating sensory responses.

  • The first of these approaches defines what is called the distal stimulus. This is the energy emitted by or reflected from some object.
  • A second approach focuses on the energy falling on a receptor surface. This comprises what is known as the proximal stimulus .

  • The distinction between proximal stimulus and distal stimulus touches on something fundamental to sensory processes and perception. The proximal stimulus, not the distal stimulus, actually sets the receptors' responses in motion.

    To appreciate the distinction between proximal and distal stimuli, imagine that you are looking at a coin. No matter from what angle you view the coin, the coin's physical shape (the distal stimulus) is constant. The coin's shape does not change as you look at it from different angles. But think about the proximal stimulus, the pattern of energy actually falling on your eye's receptors. If you could take a picture of that energy pattern you'd discover that the shape of the proximal stimulus varies as y our angle of regard changes with respect to the coin. As a result, different sets of receptors in your eye are activated depending on your viewing angle. However, your perception of the coin remains constant.

    The situation has a hint of paradox: perception is invariant even though the proximal stimulus is not. The constancy of the perceptual response despite changes in the proximal stimulus is called perceptual constancy. And constancy is critical for any perceiver. Imagine the evolutionary advantage to any perceiver who achieves constancy and the evolutionary cost to any perceiver who cannot. Constancy allows us to recognize that some object is the same object we've seen or heard or smelled before despite all sorts of irrelevant changes in the proximal stimulus.

    Constancy, which cannot be explained strictly in terms of proximal stimulation, depends upon additional processing.
    Like any sensory phenomenon, constancy reflects the nervous system's processing of the proximal stimulus. As a shorthand device, we can call the nervous system's activity neural computations. A computation, whether carried out by an electronic device or by a biological system, is only as good as its data and its processing rules. The brain's perceptual computations may be accurate or they may be in error, depending on the quality of information supplied by the senses and on the brain's predisposition to process that information in certain ways.

    But if certain processing rules can lead to errors, why would the brain be predisposed to those rules? Such a predisposition seems at first to be self-defeating. For an explanation look to the information provided to our sensory systems.

    Information picked up by the senses is not just one random input after another; that information conforms well to particular, predictable regularities, sometimes described as invariants (James Gibson 1970). These regularities arise from the very nature of the physical world itself, the world in which our senses have evolved. If the brain's processing rules embodied these regularities, or constraints, that characterize the natural world, the brain's perceptual operation could be more efficient and rapid.

    Just as processing rules can be embodied within the microchips of an electronic device, rules that recognize the natural world's regularities could be embodied in the hardware of our brains.

  • For instance, in our world, objects tend to be compact. The various parts of any object tend to be near one another, not scattered at random all over the landscape.
  • In our world, the surface color or texture of most natural objects tends to change gradually rather than abruptly (Kersten, 1987).
  • In our world, light tends to come from above, rather than from below (Ramachandran, 1988).
  • Finally, in our world, objects tend to contrast with their backgrounds (because they are made of different materials); as a result, objects tend to be outlined visually by a border, a change in light intensity.
  • To take an auditory example, in our world, a surface's hardness determines how the surface reflects sound energy (Handel, 1989).

  • One of my least favorite sounds is the unmistakable clatter that signals a glass has fallen onto a hard surface and splintered into hundreds of pieces. Like many other, more pleasant sounds, the sound of a glass shattering is a kind of symbol. What we h ear as a result of sound energy reaching our ears tells us something about what's happened in the world. The sound represents or stands for something else, a symbolic function that is at the heart of a sensory response's usefulness.

    A sensory response can act as a symbol for something because it is reliably associated with that thing. Shattering glass produces a distinctive and informative sound. But try to imagine a world in which this were not true. Imagine a world in which a glass striking a hard surface gave rise to a sound that was totally random and unpredictable. Sometimes the falling, breaking glass would make a thud, other times a beep, or even a chirp or buzz. This random variation would be a real problem for the world's inhabitants. They would be unable to depend upon the sound as a reliable guide to the preceding event. Writers of fiction can create exoti c, erratic worlds. But in our world a particular consistency of cause and effect makes sensory responses reliable guides to objects or events. Time after time, a glass striking a hard surface tends to make pretty much the same sound (though differe nt glasses may make distinctly different sounds --good, thin crystal stemware versus heavy, durable juice glasses.) Time after time, a fresh lemon that has just been cut tends to elicit pretty much the same smell. But where do these invariants come from ? What factors make it possible to use sensory events as guides to the objects and events of the world?

    Invariant, dependable sensory responses arise from the consistent physical properties of the world and from the consistent responses of sensory systems.
    Invariant sensory responses, whose importance, you have just seen, depend upon two factors. The first is some invariant physical property that is associated with the object or event; the second is a sensory system that is capable of picking up and respo nding in a consistent way to the invariant physical property. Think about some of the basic, consistent physical properties that our senses depend upon and exploit:

  • White paper can be seen as such because white paper reflects a lot of the light that falls on, typically about 80%. Grey or black papers reflect less, about 20-30% and 8-10%, respectively. The proportion of incident light that is reflect ed is an invariant property of the paper.
  • When a glass strikes a really hard surface, like a marble floor or granite counter, the collision of glass and hard surface creates a characteristic brief, sharp disturbance of the surrounding air.
  • When a freshly cut lemon is exposed to air, a characteristic set of chemical molecules, called odorants, are released from the lemon into the air.
  • A spoonful of chocolate pudding releases other characteristic molecules, called tastants, onto the tongue and palette of the mouth. A blood-thirsty mosquito alighting on your arm bumps and bends a hair.