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 Sensation and Perception Uta Wolfe St. Olaf College

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

External stimulus Sensation (Perception) Behavior

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

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!

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)

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

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

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 RG Perception depends on relative responses of R and G R>G: Red G>R: Green R=G: White (Black)

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

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Other aftereffects: Motion (Waterfall illusion) Spatial Frequency

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

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

Properties of sensory systems: - Lateral inhibition Lateral inhibition: mutual inhibition of adjacent neurons; enhances contrast Light Neurons Response Number of inhibitions

Perceptual consequences of lateral inhibition Mach Bands Hermann Grid

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

Simultaneous contrast

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 angleshape constancy - illuminationlightness constancy

“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.

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

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

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

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. Illusions resulting from Lightness Constancy Less inhibition; Signal: lighter More inhibition; Signal: darker

Interpretation: Shadow Strong Illusion Interpretation: Paint Weaker Illusion

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

Some Gestalt principles Figure-Ground Good continuation Similarity Proximity

Familiarity with objects: Hollow mask illusion

Familiarity with environment: Assumption: Light source is above scene

Priming: Expectation influences interpretation of figure