PERCEPTION The feel of cotton wool (TOUCH) The smell of a cake cooking (SMELL)

1. PERCEPTION • The feel of cotton wool (TOUCH) • The smell of a cake cooking (SMELL) • The taste of chocolate (TASTE) • The sound of sad song (HEARING) • Knowing that we are sitting up (KINAESTHESIA) • And seeing a beautiful flower (VISION)

2. Visual Perception Visual Perception consists of two main processes: sensory processes and cognitive processes. Top down Bottom up Cognitive Processes: are where the brain processes, analyses, organises and interprets information so that it becomes meaningful. Sensory Processes: are involved with the detection, reception, conversion and transmission of raw data. (Adapted from Grivas, et al. 2004)

3. Blind Spot (and Fovea)

4. The visible light spectrum is just a tiny ‘slice’ of the total electromagnetic energy (radiation) spectrum. www.pl.euhou.net/

5. www.footprintsecurity.com.au/info_images/ligh...

6. www.visionweb.com

7. www.eyedesignbook.com/ch3/fig3-61retinarods

9. Thresholds: absolute and differential • How dim can a light be and still be visible? Is there a specific light intensity that represents the limit which a person is able or unable to perceive a visual stimulus? How much change in light intensity is required for our perceptual system to notice the difference? • When examining sensory processes, psychologists use the term threshold to refer to our ability to detect a stimulus or changes in a stimulus. There are two main types of thresholds for vision: absolute and differential. (Grivas, et al, 2004)

10. Absolute Threshold • The absolute threshold for vision refers to the minimum amount of light energy that is necessary in order for a visual stimulus to be perceived. (Grivas, et al, 2004). • Examples of what the absolute thresholds for our senses would be are as follows:

11. Vision: a candle flame seen from a distance of 50 kilometres on clear, dark night.

12. Hearing: the ticking of a watch under quiet conditions at a distance of six metres.

13. Touch: the wing of a bee falling on your cheek from a height of one centimetre.

14. Smell: one drop of perfume diffused throughout a three-room apartment.

15. Taste: one teaspoon of sugar dissolved in eight litres of water.

16. A Snellen Chart

18. THE NATURE OF PROCESSES IN VISUAL PERCEPTION • There are six key processes that occur in visual perception. These are: • The capture of visual information (reception): like taking a photo, the eye captures a visual image and focuses it on the photoreceptors in the retina. • Conversion of the information into a form that can be processed (transduction): photoreceptors in the retina convert light energy into neural impulses. • Sending the information from the eye to the brain (transmission): once converted, the information is transmitted from neuron to neuron to the visual cortex in the brain. • Identifying specific features of a visual stimulus (selection): involves differentiating between the various features that make up a visual stimulus such as colours, and orientation and angles of lines. • Arranging features of the visual stimulus into a form (organisation): in this process we reassemble the key elements of a visual stimulus in a meaningful way. We use ‘rules’ to do this. • Making sense of the visual information (interpretation): this involves cognitive processes and will vary between individuals. • (Grivas, et al, 2006)

19. The following slide provides and example of how we can interpret information differently depending on which features we focus on more.

21. ORGANISATION IN VISUAL PERCEPTION There are two broad categories of cognitive processes that we use when we assign meaning to incoming visual information. One category comprises the principles or ‘rules’ of visual perception that are believed to be innate or learned very early in childhood. The other category comprises psychological factors, which are predominantly learned. (Grivas, et al, 2006). The simultaneous use of these two cognitive processes is illustrated in our interpretation of the following picture…….

23. In order to perceive a Dalmation (dog) walking along and sniffing the ground from an apparent series of meaningless splotches your visual perception system would probably have first established that there are a number of black splotches against a white background. In doing so, you applied one principle of perceptual organisation (called figure-ground). Furthermore, to assist your interpretation of the various splotches, you probably used another perceptual principle (called closure). This involved completing the figure by mentally filling in the missing bits to help organise the splotches into a recognisable form). Another perceptual principle (called proximity) may have been used to support your interpretation of the dog’s legs and head. These forms were interpreted by organising them on the basis of how close the splotches were to each other, the assumption being that they were somehow related because they were close together. Still another factor is our individual experiences and psychological factors often unique to the individual. These can affect how readily we recognise different forms, for instance. Without past experience, which involves the use of information in memory, the interpretation of the dog would not be possible. (Adapted from Grivas, et al, 2006)

24. VISUAL PERCEPTION PRINCIPLES Visual perception principles can be classified into three broad categories: Gestalt principles, depth principles and perceptual constancies. Gestalt principles Gestalt principles of visual perception refer to the numerous ways in which we organise the elements in our visual field by grouping them into the perception of a whole, complete form, usually in the simplest possible way. For example, if part of what we are observing is hidden or obscured, then in order to make sense of what is being observed, we simply construct a whole or complete form by mentally filling in the parts that are not visible. The two following paintings by Giuseppe Archimboldo (1527-93) provide an example of how the ‘parts’ (fruit and vegetables) are organised into the ‘whole’ form of a person.

27. Do we process the parts or the whole first??? Do we act globally or locally?? Do you recognise the big letter before the smaller letters? H H HHH H H H H H H H HHHHHH HHH H H H H H H H H H HHH David Navon (1977) found that the local letter (the smaller one) had no impact on the speed of the recognition of the global letter (the bigger one), however, identification of the smaller, local letter was much slower when the global and local letters did not match. Navon concluded that we mentally process the whole before we analyse the parts. (Grivas, et al, 2006).

28. Types of gestalt principles 1. Figure-ground When we use figure-ground, we organise visual information by perceptually separating important aspects of the visual field into the ‘figure’, which stands out from the ‘ground’. You are doing this while reading this sentence. By making the object the centre of our focus it becomes the figure, while all other visual information becomes the ground, or background. This figure-ground organisation is achieved by the observer separating the figure from the ground using an existing (actual) or perceived (inferred) line. This line of separation between figure and ground is known as a contour. When this becomes difficult, whether intentionally or not, this creates camouflage which restricts our ability to mentally attach contours to figures to separate them from the background. (Grivas, et al, 2006).

29. These signs are designed to make the figure stand our clearly from the background. Artists have cleverly used figure-ground principles to create ambiguous contours.

33. Can you find 13 faces hidden in this artwork?

34. Types of gestalt principles 2. Closure Closure refers to our perceptual tendency to mentally ‘close up’, fill in or ignore gaps in a visual stimulus and to perceive objects as complete. This technique is common in advertising. With the Telstra logo, we mentally impose a missing vertical stroke to the letter ‘T’. And, with the IBM logo we fill in the gaps between each of the horizontal lines to mentally form solid letters and/or ignore the gaps that prevent solid letters. (Grivals, et al, 2006).

36. Types of gestalt principles 3. Similarity The tendency to perceive stimuli or parts of a visual stimulus that have similar features – such as size, shape, texture or colour – as belonging together in a unit, group or ‘whole’. Impressionist artists using a technique called ‘pointillism’ relies on our use of of the perceptual principle of similarity. (Grivas, et al, 2006)

39. The Ishihara tests are used to diagnose colour blindness. When tested, people are required to visually perceive the number in the stimulus figure. What two Gestalt principles would a person who did not suffer from colour blindness apply here?

40. Types of gestalt principles 4. Proximity Also called ‘nearness’, is the tendency to perceive parts of a visual stimulus that are positioned close together as belonging together in a group. This can be seen with letters of the alphabet. Several series of letters located in close proximity to each other will be perceptually groups and interpreted as words. For example: What’s that on the road ahead? What’s that on the road a head? The whole can be interpreted differently when there is a subtle difference in the proximity of just one of its parts. Also consider the spots on the dalmation from the earlier slide. (Grivas, et al, 2006)

41. Depth perception In addition to the principles that influence how we perceive objects, there is another group of principles that helps us to determine where objects are perceptually located. Depth cues are sources of information from the environment (external cues) or from within our body (internal cues) that help us to perceive how far away objects are (a perception of depth and/or distance). Furthermore, our retina actually holds two-dimensional image from the world around us, but we visually perceive the world in three-dimensional forms. Depth cues allow us to do this. Depth cues can be binocular (requiring the use of both eyes) or monocular (requiring the use of only one eye). (Grivas, et al, 2006)

42. Retinal Disparity Because our eyes are about 6 or 7 centimetres apart, each retina receives a slightly different visual image due to the different angles of view. Retinal Disparity refers to the very slight difference in the location of the visual images on the retinas which enables us to make judgements about the distance of an object from the viewer.

43. Convergence Convergence involves the brain detecting and interpreting depth or distance (up to 6 metres) from changes in tension in the eye muscles.

45. Monocular depth cues Require only the use of one eye to provide information to the brain about depth and distance, but they also operate with both eyes. Accommodation Involves the automatic focusing mechanism of the lens in the eye to adjust the shape of the lens in response to differing distances of view from the object. The lens bulges to focus on nearby objects and elongates to focus on distant objects. From Grivas, et al, 2006

46. Accommodation The brain monitors the movement of the ciliary muscles that control the shape of the lens. Information about how much the lens bulges or elongates is used by the brain to determine the depth and distance of the object in focus. Accommodation enables the continuous focusing of moving objects – even those travelling at great speed. From Grivas, et al, 2006

47. Pictorial cues Artists use these extensively to portray depth and distance on a two-dimensional surface. Pictorial cues include linear perspective, interposition, texture gradients, relative size, and height of the visual field. Linear perspective Is the apparent convergence of parallel lines as they recede into the distance.

48. Pictorial Cues linear perspective texture gradient

49. The Flight Into Egypt(1306) by Giotto. The Annunciation by Crivelli (1430-95)

50. Texture gradients Texture gradients refer to the gradual diminishing of detail that occurs in surfaces as they recede into the distance, compared with objects in the vidual field that are close and seen in fine detail. Thus, our judgement about depth and distance is influenced by the extent to which we can detect fine detail. (Grivas, et al, 2006)