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Gregory's constructivist theory of perception

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Richard Gregory (1970) argued that perception is constructed: the brain uses what it already knows to interpret incomplete sensory information. This is a top-down theory.

Key ideas

Sensory input: the information from our senses is often incomplete or unclear, so it is not enough on its own.
Prior knowledge: stored knowledge and past experience (schemas) about the world.
Inferences: we fill in the gaps by drawing conclusions from prior knowledge.
Perceptual hypothesis: our 'best guess' about what we are seeing. We test it against the evidence; if it fits, we accept it. Gregory estimated that much of the information reaching the eye is lost before it reaches the brain, so guessing is needed.

Strengths

It explains visual illusions: the Necker cube flips because the brain switches between two equally good hypotheses. Distortions such as the Muller-Lyer illusion happen when we apply depth knowledge wrongly.
It explains perceptual set: expectation, culture, motivation and emotion change perception, which shows prior knowledge matters.
Studies such as Carmichael et al (1932) show words (prior knowledge) change how a shape is remembered and drawn.

Weaknesses

If perception is guesswork, it should often be wrong, yet in everyday life we perceive accurately most of the time.
It does not explain how babies, with little prior knowledge, can perceive depth.
Much evidence comes from illusions and 2D pictures in the lab, which are artificial; real-world scenes are richer in information, as Gibson argued.
Worked example

Using Gregory's theory, explain why we see the Necker cube flip between two views.

  1. The drawing gives incomplete information about which face is at the front.
  2. The brain forms one perceptual hypothesis, then an equally good alternative, and switches between them.

Answer: Two equally likely perceptual hypotheses fit the same sensory input, so the brain alternates between them.

Key idea

Gregory's constructivist (top-down) theory says sensory input is incomplete, so we use prior knowledge to make inferences and form a perceptual hypothesis (a best guess). It explains illusions and perceptual set, but not why everyday perception is usually accurate.

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