Psychology - Perception

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Perception refers to the ways in which information is received through our sense organs. Perception is a cognitive process involving the recognition and interpretation of stimuli once they have registered.

Our perception of the world appears to be immediate and we have no conscious awareness of the brain activity underlying this ability. However, in spite of this it involves complex brain processes.


These are visually perceived images that are different from objective reality. The information received by the eyes is processed and gives a perception that is not in line with that of the source.

Physiological illusions. 

  • These are presumed to be the effects on the eyes or brain of excessive stimulation of a specific type, e.g. brightness, colour or movement.
  • Example = Hermann grid illusion, where grey sports appear at the intersections as a result of the dark surround.

Cognitive illusions. 

  • These are thought to arise from our own assumptions on the world, leading us to make unconscious conclusions. 

- Ambiguous illusions = these involve a perceptual switch between alternative explanations.

- Distorting illusions = these are characterised by distortions of size or length. Example = Muller-Lyer illusion/cafe wall illusion.

- Paradox illusions = these are generated from objects that are impossible. Example = Penrose triangle (which is dependent on a cognitive misunderstanding that adjacent edges must join)/impossible staircase.

Example = Kanizsa triangle. The brain has a need to see familiar simple objects and has a tendency to create a whole image from individual elements. Our brain makes sense of shapes and symbols, putting them together like a jigsaw puzzle, formulating that which isn't there to that which is believable. This means that, a floating white triangle, which does not exist, is seen.

Example = the Ponzo illusion. This uses cues of depth perception to fool the eyes. The converging parallel lines tell the brain that the image higher in the visual field is further away and therefore perceived to be longer, although the two yellow lines are of equal length.

Example = colour and brightness constancies. The contrast of an object will appear darker against a black field which reflects less light compared to a white field, even though the object itself has not changed in colour. The horizontal bar is the same shade of grey throughout.

Example = the spinning dancer. Some observers initially see the figure as spinning clockwise and some counter clockwise. Additionally, some may see the figure suddenly spin in the opposite direction. The illusion derives from an ambiguity from the lack of visual cues for depth.

Example = the Poggendorff illusion. This is an optical illusion that involves the brain's perception of diagonal lines and horizontal/vertical edges. In the picture, a straight back and red line is obscured by a grey rectangle. The blue line, rather than the red line, appears to be a continuation of the black one, which is clearly shown not to be the case on the second picture. To this day, it is not known why this illusion…


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