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Chapter 5

Chapter 5. Spatial Vision and Form Perception. What Defines an Object?. Detection is the process of picking out an object from its surroundings. Discrimination is the process of distinguishing one object from another.

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Chapter 5

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  1. Chapter 5 Spatial Vision and Form Perception

  2. What Defines an Object? • Detection is the process of picking out an object from its surroundings. • Discrimination is the process of distinguishing one object from another. • Identification is the precise recognition of a particular object or a specific person.

  3. What are the components of visual form? • In the structuralist tradition, ideas and perceptions are created by combining components–simple sensations constitute the building blocks of perceived form. • How can we identify units of sensation with only minimal contributions from our prejudices and expectations?

  4. A Local Analysis: Differences between Neighbors • Each level of spatial information defines a scale. • The multichannel model: • Each subset of neurons conveys a characteristic kind of information defined by the spatial scale. • Gratings are special patterns used to test the multichannel model.

  5. Gratings as Tools for Exploring Form Perception

  6. Gratings as Tools for Exploring Form Perception • Spatial frequency (the number of light and dark regions imaged within a given distance on the retina) • Contrast (intensity difference between light and dark bars of the grating)

  7. Gratings as Tools for Exploring Form Perception • Orientation (axis of the grating’s bars) • Spatial phase (a grating’s position relative to some landmark)

  8. Using Gratings to Measure Performance • Transfer function specifies how contrast is transferred through the lens. • Cutoff frequency is point at which image contrast falls to zero.

  9. Using Gratings to Measure Performance • Fourier analysis: if some quantity changed in a complex manner over time, that complex function of time could be approximated by a series of simple sinusoidal functions.

  10. Contrast Sensitivity Function • The contrast sensitivity function (CSF) is the curve that describes the entire visual system’s sensitivity to contrast. • This curve defines the window of visibility.

  11. Contrast Sensitivity Function • The sensitivity of the visual system determines the threshold contrast needed to detect a given spatial frequency • CSF can be derived by measuring contrast thresholds for different spatial frequencies

  12. CSF at 3 Light Levels

  13. CSFs of Other Species • Why do cats chase “invisible” objects? • CSFs of nonhuman primates are similar to CSF of humans.

  14. Age and the CSF • Infants have a different window of visibility than adults do. • CSF of the infant improves steadily over first year of life. • Systematic changes in CSF begin reappearing after age 30.

  15. Structural Basis of the CSF • Selective adaptation effects suggest that different neural channels are used to detect different spatial frequencies. • Detection of any spatial target depends on responses in sets of neurons tuned to a certain size and orientation.

  16. Contrast Normalization • Visual responses are combined from all active neurons whose receptive fields fall within a local neighborhood. • This combined signal modifies the individual responses of those active neurons.

  17. Metamers • Metamers are two objects that are physically different but perceptually identical. • Metamers exist because of a blindness to certain stimuli characteristics.

  18. Form Defined by Texture Differences • Spatial discontinuities in texture, or texture contrasts, help visual system to detect and discriminate form. • Texture properties include regularity, granularity, and strong local orientation.

  19. Form Defined by Texture Differences • People are very good at detecting shapes that differ only in texture. • Rapid detection of a stimulus that differs from its immediate surroundings is called “visual pop-out.”

  20. Visual Pop-Out

  21. Global Context and Gestalt • Gestalt movement argued that objects appear as they do in virtue of the parts’ relations to one another. • Configural superiority effect: the whole is different from the sum of its individual parts.

  22. Global Context and Gestalt • Proximity • Similarity • Closure • Good continuation

  23. Global Structure and Symmetry • Symmetry: the tendency for parts of an object centered about a given axis to be highly similar in shape, texture, and color. • Most biological objects have the property of symmetry.

  24. How do we detect symmetry? • Detecting regions that differ from their immediate surroundings is an early step in detecting symmetry. • The visual system must also register similarities in the spatial arrangement of regions in the visual field.

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