Lecture 2

1. Lecture 2 Psychology & HCI – Visual System

2. Specific Objective • Explain the basic capabilities of the visual system as it relates to human computer interface

3. Content: • Vision: • Visual Acuity • Colour Perception • Dark Adaptation • Perception • Organisation and Pattern Recognition

4. Vision Human vision is a highly complex activity and a range of physical and perceptual limitations, yet it is the primary source of information for the average person. The visual system in humans allows individuals to assimilate information from the environment. Two stages in vision: • physical reception of stimulus • processing and interpretation of stimulus

5. The Eye - physical reception • mechanism for receiving light and transforming it into electrical energy • light reflects from objects • images are focused upside-down on retina • retina contains rods for low light vision and cones for colour vision • ganglion cells (brain!) detect pattern and movement

6. Interpreting the signal (cont’d) • Brightness • subjective reaction to levels of light • affected by luminance of object • measured by just noticeable difference • visual acuity increases with luminance as does flicker • Colour • made up of hue, intensity, saturation • cones sensitive to colour wavelengths • blue acuity is lowest • 8% males and 1% females colour blind

7. Interpreting the signal (cont) • The visual system compensates for: • movement • changes in luminance. • Context is used to resolve ambiguity • Optical illusions sometimes occur due to over compensation

8. Optical Illusions the Ponzo illusion the Muller Lyer illusion

9. Reading • Several stages: • visual pattern perceived • decoded using internal representation of language • interpreted using knowledge of syntax, semantics, pragmatics (phrases/sentences). • Reading involves saccades (jerky movements) and fixations • Perception occurs during fixations • Word shape is important to recognition • Negative contrast improves reading from computer screen

10. Visual Acuity • Visual acuity is defined as the clarity or sharpness of vision, which is the ability of the eye to see and distinguish fine details. Visual acuity is an important factor for a variety of everyday tasks, including reading text, recognizing symbols, and performing assembly work.

11. Typical Snellen chart used for visual acuity testing.

12. Colour Perception • Colour can only exist when three components are present: a viewer, an object, and light.  Although pure white light is perceived as colourless, it actually contains all colours in the visible spectrum.  When white light hits an object, it selectively blocks some colours and reflects others; only the reflected colours contribute to the viewer's perception of colour.

14. Dark Adaptation • Dark adaptation refers to how the eye recovers its sensitivity in the dark following exposure to bright lights.

15. Perception • In psychology and the cognitive sciences, perception is the process of attaining awareness or understanding of sensory information. • The word perception comes from the Latin words perception, percepio, meaning "receiving, collecting, action of taking possession, apprehension with the mind or senses.

16. Organisation and Pattern Recognition • Pattern recognition involves identification of faces, objects, words, melodies, etc. The visual system does more than just interpret forms, contours and colours. Pattern recognition refers to the process of recognizing a set of stimuli arranged in a certain pattern that is characteristic of that set of stimuli. Pattern recognition is an innate ability of animals.

17. Exercises Check this site out: http://www.echalk.co.uk/amusements/OpticalIllusions/illusions.htm

30. Good evening