An ecological understanding of perception derived from Gibson's early work is that of "perception-inaction", the notion that perception is a requisite property of animate action; that without perception action would be unguided and without action perception would serve no purpose. Animate actions require both perception and motion and perception and movement can be described as "two sides of the same coin, the coin is action". Gibson works from the assumption that singular entities, which he calls "invariants", already exist in the real world and that all that the perception process does is to home in upon them. A view known as social constructionism (held by such philosophers as Ernst von Glasersfeld) regards the continual adjustment of perception and action to the external input as precisely what constitutes the "entity", which is therefore far from being invariant.

Glasersfeld considers an "invariant" as a target to be homed in upon and a pragmatic necessity to allow an initial measure of understanding to be established prior to the updating that a statement aims to achieve. The invariant does not and need not represent an actuality and Glasersfeld describes it as extremely unlikely that what is desired or feared by an organism will never suffer change as time goes on. This social constructionist theory thus allows for a needful evolutionary adjustment.

A mathematical theory of perception-in-action has been devised and investigated in many forms of controlled movement and has been described in many different species of organism using the General Tau Theory. According to this theory, tau information, or time-to-goal information is the fundamental 'percept' in perception.

Perceptual Threshold

One aspect of the central nervous system's processing of sensory information is that of the "perceptual threshold", the level of stimulus intensity necessary for a conscious organism to be aware of a particular sensation. Stimuli bombard sensory receptors constantly, but the brain is able to filter out and "turn off" some stimuli. A human may experience a change in perceptual threshold when they "tune out" the radio whilst studying or when they "zone out" during a lecture. In both cases, the noise is adequate to stimulate sensory neurons in the ear, but neurons higher in the pathway dampen the perceived signal so that it does not reach the conscious brain.

Decreased perception of a stimulus is accomplished by inhibitor modulation, which diminishes a supra-threshold stimulus until it is below the perceptual threshold. Inhibitory modulation often occurs in the secondary and higher neurons of a sensory pathway. If the modulated stimulus suddenly becomes important, such as when a "zoned out" student is asked a question by a lecturer, the brain can consciously focus its attention and overcome the inhibitory modulation. At that point, the conscious brain seeks to retrieve and recall recent sound input from its subconscious so that the question can be answered.

Theories of Perception

• Interactive Activation and Competition

• Irving Biederman's Recognition by Components Theory

• Anne Treisman's Feature Integration Theory

The philosophy of perception concerns how mental processes and symbols depend on the world internal and external to the perceiver. Our perception of the external world begins with the senses, which lead us to generate empirical concepts representing the world around us, within a mental framework relating new concepts to preexisting ones. Perception leads to a person's view of the world, so its study may be important for better understanding communication, self, id, ego — even reality. While René Descartes concluded that the question "Do I exist?" can only be answered in the affirmative (cogito ergo sum), Freudian psychology suggests that self-perception is an illusion of the ego and cannot be trusted to decide what is in fact real. Such questions remain: Do our perceptions allow us to experience the world as it "really is?" Can we ever know another point of view in the way we know our own?

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