Visual Perception and Attention

1. Visual Perception and Attention Lecture 3

2. Why Attention? • Limited Capacities • We receive more sensory information than we can possibly process. • Example: You probably aren’t aware of the feeling of the socks on your feet because you have better things to attend to.

3. Limits of Attention • Selective Attention Errors • Focused Attention Errors • Divided Attention Errors

4. Limits of Attention • Selective Attention Errors Errors are caused by selecting the wrong information Example: paying attention to the radio rather than the road during poor weather conditions. • Focused Attention • Divided Attention

5. Limits of Attention • Selective Attention Errors Errors are caused by selecting the wrong information • Focused Attention Problems excluding irrelevant information Example: neighbor plays music too loudly while you are trying to study. • Divided Attention

6. Limits of Attention • Selective Attention Errors Errors are caused by selecting the wrong information • Focused Attention Problems excluding irrelevant information • Divided Attention Problems dividing attention between two sources of information at once. Example: Trying to pay attention to a cell phone conversation while also paying attention to the road.

7. Visual Limits • Visual Angle - measures the size of the image falling on the retina.

8. Visual Limits • If the object moves closer, the visual angle increases

9. Visual Limits • If the object moves closer, the visual angle increases

10. Structure of the Eye Lens Retina

11. Retina • 2 types of photocells Rods sensitive to shades of gray Cones maximally sensitive to red, green, or blue

12. Ganglion Cells

13. Spatial Frequency • used to describe the degree of spatial detail. High Frequency = fine detail Low Frequency = course detail

14. Spatial Frequency low frequencies only original

15. Spatial Frequency high frequencies only (ignore lack of color saturation in this example) original

16. Fovea • Fovea - located at the center of the retina

17. Fovea visual angle is roughly the size of your thumbnail at arms length

18. distribution of rods and cones • Cones are concentrated in the fovea. • Rods occur only in the periphery

19. Normal Image

20. Adjusted for retinal acuity

21. Low Light Conditions • Cones: relatively insensitive in low light conditions • Q: As the light level drops, what types of photoreceptors do we start to rely on more-and more? • A: rods!

22. Low Light Conditions • If we are seeing primarily with our rods, what does this do to our perception? • Black & White • Course detail/low frequency/low resolution • Little or no foveal vision

23. Simulated Low light low frequencies, no color original

24. Visual Systems

25. Cortical Magnification • A large chunk of the visual cortex is devoted to the fovea. • Fovea is 0.2% of the retina, but is represented by 25% of the visual cortex. • 1000x as much cortical tissue is devoted to processing information from the fovea compared to an equal sized patch from the periphery.

28. Why Cortical Magnification? • If the brain had the same amount of tissue as devoted to the fovea devoted to the rest of the visual field, then our visual cortex would have to be 100x bigger (i.e. we’d have huge heads). • Instead, we can make eye movements to move the fovea around.

29. Eyemovement terminology • Fixations • When the eyes are stopped on an object. • The eyes are not completely steady during a fixation, but show small, involuntary tremors. • Saccades • Smooth Pursuit • Dwell Times

30. Eyemovement terminology • Fixations • Saccades • The ballistic reorientation of the eyes from point of interest to another. • Saccades connect fixations together. • Visual processing is suppressed during saccades • Smooth Pursuit • Dwell Times

31. Eyemovement terminology • Fixations • Saccades • Smooth Pursuit • Occurs when the eyes track a moving object. • Not considered a saccade. • Dwell Times

32. Eye Movements • Fixations • Saccades • Smooth Pursuit • Dwell Times • How long the eyes dwell on an item of interest. • May consist of several fixations. • Time affected by difficulty of information extraction.

34. The amount of information we can usefully extract from the environment is not only determined by retinal acuity and cortical magnification, but is also determined by attention. Useful Field of View: is our attentional window on the world. • The UFOV is centered in the middle of our visual field. • It’s width is determined by the ease of image extraction and other factors such as the effects of aging.

35. Eye movements & UFOV • Saccades are used to connect attentional windows • What happens when items are difficult to discriminate? • UFOV shrinks -- fewer items examined in each fixation • Smaller UFOV leads to more eye movements

36. Eye movements & Attention • Covert attention can move around while the eyes are still. • Example: checking out someone out of the “corner of your eye” while pretending to read a newspaper.

37. Eye movements & Attention • Covert attention can move around while the eyes are still. • Overt attention: eye movements or any physical orienting of the body.

38. Eye Movements & Covert Attention • When the eyes are still, covert attention is free to move independently. • Programming an eye movement always leads to a shift of covert attention. • That is, covert attention gets to the intended target before the eyes begin to move.

39. If we ask you to move your eyes to a specific location (e.g. “left”), and… • flash a target right before your eyes move… • you will be more likely to identify the target when it appears where your eyes will land than when it appears somewhere else.

40. 0 msec 50 msec 100 msec 150 msec 200 msec “Left” T T

41. Attentional Guidance • Top-Down • Environmental Expectancies • Where • should I look on the ceiling for my car keys? • Attentional Set • What • is it smaller than a bread box?

42. Attentional Set • Singletons (Folk et al.) • items that are identifiable by a unique (compared to other visible items) feature • e.g. the only red car in the parking lot. • Search for feature singletons is extremely efficient.

43. Attentional Guidance • Bottom-up (attentional capture) • Onsets -- items that appear in a display. • Must include a luminance change • Color changes without a luminance change do not count as onsets. suddenly

44. I’m not an onset I am So am I

45. Attention Switching • Bottom-up (exogenous, involuntary) • Fast, automatic, unintentional • Top-down (endogenous, voluntary) • slow, intentional

46. voluntary arrow cue (75% valid) involuntary onset cue (25% valid -- chance)

48. “2 or 5”