Outline Sensation, Perception, Behavior Process of sensation Perceived vs. “real” world

1. Sensation and Perception Uta Wolfe St. Olaf College • Outline • Sensation, Perception, Behavior • Process of sensation • Perceived vs. “real” world • Properties of perceptual processes • - Adaptation, pattern coding and aftereffects • - Receptive Fields • - Lateral Inhibition/ Contrast enhancements • - Perceptual constancies • Top-Down Processing • Attention and Perception

2. Sensation, Perception, Behavior Sensation: - basic processes by which sense organs and nervous system respond to environmental stimuli - basic experiences resulting from these processes (eg: “yellowness”) Perception: - organizing and interpretation of sensory information ( eg: “The object is a banana”) Purpose of both: guidance of behavior most basic examples: phototaxis/ chemotaxis

3. External stimulus Sensation (Perception) Behavior

4. Process of sensation reception transduction coding Absorption of physical energy by specialized receptor transformation of one form of energy into another here: transformation of light, sound etc. into electrochemical change in receptor representation of stimulus aspects in temporal/ spatial pattern of neural firing Photon hits photoreceptor molecular change in pigment leads to channel closing and change in potential Action potential frequency signals intensity

5. Important point: Once transduced, light, sound, touch etc. are identical electrochemical signals. Question: Why do these identical signals lead to qualitatively different sensations? Why do we see light but hear sound? Answer: Law of specific nerve energies (J. Mueller, 1838): quality of sensation depends on which set of nerve fibers is stimulated. Demo: poke eye, hit back of head!

6. Perceived vs. “real” world Senses do not convey objective representation of environment: 1) Limitations of senses: - sensitive to only certain types of energy - sensitive to only small range of each energy spectrum - threshold (absolute or difference threshold)

7. Senses do not convey objective representation of environment: 2) Purposeful distortion of external stimulus: - extracts and emphasizes important stimulus aspects - leads to illusions - eg: adaptation: Locke’s observation on temperature lateral inhibition: Mach Bands

8. Properties of sensory systems: - Adaptation, pattern coding and aftereffects Adaptation: response of neuron decreases as a result of prolonged stimulation Neuronal response stimulus time Perceptual consequences: adaptation to pressure of clothes, continuous sounds, ambient temperature, dark/ brightness

9. Pattern coding: (aka population or across-fiber coding) Information about stimulus is conveyed by the pattern of firing across a group of neurons, rather than by one individual neuron Example: color R G Perception depends on relative responses of R and G R>G: Red G>R: Green R=G: White (Black)

10. Adaptation to green light: G’s response stimulus time What is subsequent perception of white light? R G Since G is adapted, R>G. Perception: Red

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13. Other aftereffects: Motion (Waterfall illusion) Spatial Frequency

15. Properties of sensory systems: - Receptive Field Receptive Field: Region of body/ world within which the activity of a neuron/ receptor can be influenced

16. Receptive field size and acuity/ two-point discrimination: Neurons 1, 2 Neuron 3 Neurons 1 and 2 can resolve the points as separate, neuron 3 cannot. Demos: two-point discrimination in touch and vision

18. Light Properties of sensory systems: - Lateral inhibition Lateral inhibition: mutual inhibition of adjacent neurons; enhances contrast Neurons Response Number of inhibitions 0 1 1 2 1 1 0

19. Perceptual consequences of lateral inhibition Hermann Grid Mach Bands

20. New Florida flag? Count and total black dots for Al Gore and white dots for George W. Bush. Recount to confirm.

21. Simultaneous contrast

22. Properties of perceptual processes: - Perceptual Constancies The challenge: To form a logical, stable representation of objects despite differences in viewing conditions, e.g. - distance size constancy - viewing angle shape constancy - illumination lightness constancy

23. “Excuse me for shouting- I thought you were farther away” Size constancy scaling depends on the ability to correctly infer relative distance from depth cues.

24. Misapplied size constancy: The tendency to misjudge the size of 2D stimuli when depth cues trigger size constancy scaling

25. A conscious percept of depth is not necessary for misapplied size constancy to occur. The Mueller-Lyer Illusion

26. Lightness constancy scaling depends on the ability to correctly infer illumination from contrast information. Demo Question: What color is the moon?

28. Illusions resulting from Lightness Constancy A B Bottom-Up explanation: Cell A receives less inhibition than Cell B. Top-Down explanation: A is in the shadow, yet it reflects as much light as B. Hence A must be lighter than B. Less inhibition; Signal: lighter More inhibition; Signal: darker

29. Interpretation: Shadow Strong Illusion Interpretation: Paint Weaker Illusion

31. Properties of perceptual processes: - Top-Down Processes Top-Down Processes: Use knowledge and expectations to form hypothesis about object/ scene - Gestalt Principles - Familiarity with objects - Familiarity with environment - Priming

32. Some Gestalt principles Similarity Figure-Ground Proximity Good continuation

33. Familiarity with objects: Hollow mask illusion

34. Familiarity with environment: Assumption: Light source is above scene

35. Priming: Expectation influences interpretation of figure