Sensation and Perception: Key Concepts and Applications

1. Sensation • Perception = gathering information from the environment • 2 stages: • Sensation = simple sensory experiences and translating physical energy from the environment into neural codes • Perception = organizing, identifying, and interpreting sensations to create mental representations of objects and events

2. Top-down and Bottom-up Processes in Perception • Bottom-up • “data driven” • “stimulus driven” • Example: different wavelengths of light produce different color sensations • Top-down • “conceptually driven” • “context driven” • Example: reading an exit sign more quickly because you know what it is likely to be

3. Topics in Sensation • Thresholds • Signal Detection Theory • Sensory Memory

4. Thresholds • Absolute threshold – the lowest intensity of a stimulus that can be detected • Subliminal stimuli • “Eat Popcorn” (McConnell et al 1958) • Backward messages • Self-help tapes (Greenwald et al 1991) • Priming • So do subliminal stimuli have real effects?

5. Thresholds: Theoretical and Actual Functions • Theoretical Threshold Functions (Step Threshold) • Actual Threshold Functions (Threshold is a continuous function) • Operational Definition of the Absolute Threshold • The stimulus intensity at which it is correctly identified 50% of the time • Is there anything else besides stimulus intensity that must be considered?

6. Signal Detection Theory • 2 Components of detection judgments • Discriminability • How much sensory evidence is there that a signal is present? • Controlled by stimulus • Bottom-up • Bias • How much evidence does the perceiver require before saying “yes”? • Controlled by a “pay-off matrix” (among other things) • Top-down

7. Separating Discriminability From Bias: 4 types of responses Stimulus Present Absent Response Yes No

8. Person A:Threshold with a Conservative Bias Loudness = 10 decibels Stimulus Present Absent Response Yes No

9. Person B:Threshold with a Liberal Bias Loudness = 10 decibels Stimulus Present Absent Response Yes No

10. Person C:Threshold with a Liberal Bias Loudness = 5 decibels Stimulus Present Absent Response Yes No

11. Class Data from Coglab*:Positive (Liberal) Bias Condition Stimulus Present Absent Response Yes No

12. Class Data from Coglab*:Negative (Conservative) Bias Condition Stimulus Present Absent Response Yes No

13. Discriminability and Bias • Signal and Noise distributions • X axis: Magnitude of internal sensory process (amount of neural excitation) • Y axis: frequency (probability density function) • Noise: effects of noise alone • Signal: effects of signal plus noise • Criterion • How much neural excitation does the perceiver require before saying “yes”? • Tutorial: http://psych.hanover.edu/Krantz/STD/

14. Signal Detection Measures of Discriminability and Bias • Discriminability • d’ (d prime) • distance between signal and noise distributions • 0 = no discriminability; higher values = more • Assumes that the signal and noise distributions are normal • Log(alpha) • another measure of discriminability that does not assume normality • 0 = no discriminability; 4.6 = perfect performance

15. Signal Detection Measures of Discriminability and Bias • Bias • Beta • 1 = neutral (when the criterion is through the point where the signal and noise distributions cross) • < 1 = liberal (“yes”) bias • > 1 = conservative (“no”) bias • C • 0 = neutral • < 0 = liberal • > 0 = conservative

16. Class Coglab Data Results1 (from a previous class. Your data are HERE) Hits False Alarms d prime Log-alpha C (bias)

17. Evidence for Signal Detection Theory • Swets, Tannar, & Birdsall, 1961 • 4 choice detection task (tones A, B, C, D) • After an incorrect answer, answered again • If the participants had no information about which tone it was when they were wrong, should be at chance on subsequent guesses • Results: after a missed guess, they were above chance on the next guess • Therefore, the threshold is a continuous function (as Signal Detection Theory predicts), not a step function.

18. The Problem of Bias • How can researchers avoid mistakenly attributing changes in response bias to perceptual effects (discrimination)? • Use Signal Detection Theory measures of discriminability and bias to distinguish the two • Induce a strong conservative bias • Use a forced-choice task

19. Importance of SDT • Stimulus detection is a function of not only how strong the signal is, but also the criterion that the perceiver chooses • Even basic sensory processes involve both bottom-up and top-down processes • Perception depends on contributions from both the stimulus and the perceiver • Sub-threshold stimuli can still provide information (and thus influence cognitive processes)

20. Sensory Storage • Sensory information is briefly stored at a low (relatively unprocessed) level • Icon – brief storage of visual information • Echo – brief storage of auditory information