Perception and Perceptual Distortions

1. Perception and Perceptual Distortions Vaughan Bell vaughan@backspace.org

2. Outline • We are going to focus on Visual Perception • Approaches: • David Marr - Bottom-Up Computational Account • Richard Gregory – Top-Down Influence • JJ Gibson - An Ecological Approach • Sources of perceptual distortion and their relation to theories of visual perception

3. What needs explaining • The information we receive by our sense organs is relatively impoverished. • For example, the retina receives a grainy 2D image of the visual scene… • …that includes large gaps (blind spots)… • …and an uneven representation of colour (cones) and luminance (rods).

4. What needs explaining • This information is transformed into a rich visual experience. • Theories of visual perception attempt to explain how this happens. • And describe how perceptual distortions occur with the context of a model of normal psychological function.

5. Marr: Computational Approach • Marr wanted to understand mechanisms of vision rather than just behaviours associated with it. • Particularly, he wanted to link neurophysiology with psychology. • He took an information processing view of the mind… • …and aimed to describe perception in terms of computations on sense data… • …to extract high level visual experience.

6. Marr’s Stages of Visual Processing • Marr proposed there were distinct stages of processing in visual perception: • Raw Primal Sketch • Complete Primal Sketch • 2½D Sketch • Full 3D Representation

7. Marr: Early Primal Sketch This involves the extraction of information regarding edges and intensity changes.

8. Marr: Complete Primal Sketch • After the Raw Primal Sketch… • Marr argued we create a Complete Primal Sketch by grouping surfaces and common areas. • The Gestalt Psychologists of the early 19th Century demonstrated many different ways in which we can group objects.

9. Proximity Things that are close together seem part of the same group.

10. Similarity Things that are similar seem part of the same group.

11. Common Fate

12. Continuation We see this as two crossing lines, rather than two ‘V’ shapes.

13. Closure Images are seen to form closed rather than open patterns.

14. Marr: 2½D Sketch • After gaining information about groupings and surfaces the viewer needs some spatial information. • Marr called this stage the 2½D Sketch to emphasis that this stage didn’t give a full 3D representation. • Just an estimate of the spatial locations of objects and materials in relation to the viewer. • This is involved in some important processes…

15. Perceptual Constancy • We tend to experience objects as the same, despite the image they produce on the retina may vary greatly. • Shape constancy – objects seen from different angles do not appear different or to change shape • Size constancy – objects do not seem to change size when they move nearer or further away. • Colour constancy – differing illumination does not affect colour despite changes in the actual reflected light.

16. Figure / Ground Segregation

17. Figure / Ground Segregation • Symmetry is one of the factors known to be important:

18. 2½D Sketch: Depth Cues • We perceive much information from which we can infer depth: • Binocular disparity • Texture gradients • Occlusion • Convergence • Relative Size

19. Depth: Binocular Disparity Each eye receives a slight different image of the world from which 3D positions can be inferred.

20. Depth: Textural Gradient Surfaces appear to have a finer texture as they recede into the distance.

21. Depth: Convergence / Perspective Lines will appear to draw closer together as they go farther into the distance.

22. Depth: Occlusion Nearer objects appear to cover over more distant objects.

23. Marr: 3D Model Representation • The final stage of Marr’s theory. • A full 3D description of our spatial environment involving the identification of the structure of objects and materials in our visual scene. • It allows us to work out the 3D environment from a non-egocentric point-of-view.

24. Gregory: Top-Down Perception • Gregory believes that we use our experience of the world to shape how we perceive it. • In this way Gregory theories are Top-Down. • i.e. using high level concepts (knowledge of the world) to shape low level perceptions. • He demonstrated many of his points by using visual illusions.

25. Moving Dots Illusion

26. Gregory: Illusions • Gregory would argue that this is an example of our conceptual knowledge affecting how we perceive simple visual phenomenon. • The Necker Cube is another example.

27. Necker Cube

28. Gregory: Illusions • It would seem we can get two mutually exclusive percepts from a single visual input. • Gregory argues that Bottom-Up theories, such as those of Marr and Gibson, would not explain this. • Bottom-Up theories would seem to suggest that we should always perceive the same thing from the same input.

29. Rotating Mask

30. Rotating Mask • Gregory argues this sort of illusion happens because we are not used to seeing hollow faces. • Therefore our beliefs and expectations are applied to make best sense of the data. • Leading to the illusion.

31. JJ Gibson: Ecological Perception • Gibson sought to explain perception in terms of how it relates to properties of the world around us. • He saw movement and change within our visual field as providing crucial information to the viewer about the world and their place within it. • He also saw the perception of surfaces as important, rather than the interpretation of spots or patches of light.

32. JJ Gibson: Optic Flow • Gibson saw the perception of movement as much more than simply the effect of changes in the retinal image. • Gibson saw the perception of ‘optic flow’ as one of our main sources of visual information. • When we move our visual environment seems to flow past at different rates, closer things seem to move past faster than more distant things.

33. JJ Gibson: Optic Flow • This information can give us a great deal of information about our action within the world. • For example, the point of minimum optic flow within our field of vision is often where we are heading. • Optic flow can also give us information about the relative distances of objects and surfaces.

34. JJ Gibson: Surfaces • Gibson saw surfaces as one of the constants on which can base many perceptual judgements. • When we grow up we are constantly surrounded by surfaces which communicate different properties. • Differences in shading and texture of surfaces communicate a great deal about the shape and size of an object or area.

35. Gibsonian Optics Demonstration

36. Perceptual Distortions • Perceptual distortions are usually classified into: • Hallucinations – sensory perception without external stimulation of sense organ • Illusions – Misperceived or distorted perception of real physical stimuli. • There are many theories of how such distortions occur so the following is a taste of how some theories relate to theories of normal visual perception.

37. Bottom-Up: Form Constants • Mescaline is a phenethylamine hallucinogen and is found naturally in certain cacti. • Hallucinations associated with mescaline (et al) can be varied, although various commonalities have been observed. Peyote cactus • Named ‘form constants’ by Klüver (1966)

38. Mescaline Form Constants • These images appear in both eyes and move when the eyes move. • Suggesting they are generated early in the visual pathway.

39. Bressloff et al (2002) • Bressloff et al (2002) built a mathematical model of the primary visual cortex (V1). • This is largely involved with detecting lines, edges, contours etc. • Like Marr, Bressloff is taking a computational approach. • And modeling the equivalent to Marr’s ‘primary sketch’ stage.

40. Bressloff et al (2002) • Bressloff et al simulated destabilisation of V1 neurons in their resting state. • And produced results very similar to form constant images.

41. Integration • This suggests that form constant hallucinations can be explained in term of Marr’s theories. • Particularly as a disruption to the primary sketch stage of visual processing. • Which feeds up to the final experience of vision.

42. Top-Down: White Christmas • Mercklebach and van de Ven (2001) told participants they had hidden ‘White Christmas’ amid white noise. • Participants were asked to press a button if they heard any of the tune. • Actually, they was no song hidden in the noise. • 32% of the participants pressed the button at least once.

43. Top-Down: Random Patterns • Brugger et al (1993) ran a similar experiment with random dot patterns. • And found that some people will report pictures when none are there. • Particularly if they believe in ESP, or the patterns are presented to the left visual field.

44. Top-Down Distortions • This suggests prior expectations can influence what we perceive. • To the extent of experiencing meaningful information in a completely random background environment.

45. Ecological Approach • Gibson placed the environment at the centre of visual perception. • We know that perceptual distortions are not necessarily related to pathology and can be influenced by social and cultural factors. • Ohayon’s (2000) survey of over 13,000 people found that 40% had experienced hallucinations, and less than half were related to pathology.

46. Ecological Approach • Turner (1992) studied the Ndembu people of Zambia who regularly see spirit manifestations during rituals. • When talking part in a ritual, Turner reports seeing one herself. • Bartholomew (2001) reports on sightings of German bombers over Canada during times of national distress in World War I.

47. Ecological Approach • Romme and Escher (1993) report on many people who ‘hear voices’ but never become distressed or impaired, indeed voice hearing is considered normal in many cultures

48. Conclusions • Major themes in approaching visual perception include: • Bottom-Up – e.g. Marr • Top-down – e.g. Gregory • Ecological – e.g. Gibson • Perceptual distortions may occur due to influences on any of these processes.