Chapter 5: Spatial Vision & Form Perception

• Detection

• Discrimination

• Identification

• Objects & Forms– Light can vary by intensity, color, pattern.

Analysis of Form

• Titchener & Analytic Introspection

• Campbell & Robson’s Analysis by Scale (a.k.a. the multi-channel model)

Gratings (Gabors)

Grating (Gabors)

• Spatial Frequency

• Contrast

• Orientation

• Spatial Phase

Orientation

Spatial Phase

Spatial Phase

An analogy between lens and vision

• The Transfer function (the lens)

• The context sensitivity function (vision)

Gratings and the Transfer Function

The transfer function vs. the contrast sensitivity function

• In both cases, we can have gratings as targets.

• In the eye, we don’t want the retinal image, but some perceptual fact. That is, we want both to analyze a perceptual function.

• So, we go for contrast thresholds

Contrast thresholds

• At some levels of contrast, humans can see a grating.

• At other levels of contrast, humans cannot see a grating.

• We want to plot the line that separates the two.

TF & CSF

• Both have a high end cut-off.

• CSF has a low-end cut-off; TF does not.

• CSF: There is a maximal spatial frequency for contrast sensitivity.

CSF and night vision

• High spatial frequency fade first with decreasing light.

• So, at night, people can see big things, but not small things.

How does this fit in with the “convergence trade-off?”

How does this fit in with the “convergence trade-off?”

• What is the convergence trade off?

– The lower the convergence, the greater the resolution.

– The higher the convergence, the greater the light sensitivty.

We can understand why fine details disappear first. The ability to see fine details is bought at the expense of light sensitivity.

The CSF is species specific

• Different species have different CSFs.

• CSFs have been measured in over a dozen different species.

Evidence for spatial frequency channels

• Selective adaptation

Evidence for spatial frequency channels

• Selective adaptation

• People respond more quickly to low frequency information that to high frequency information.

• In short presentations, only low-frequency information is perceived; high-frequency information is “blotchy.”

Metamers

• Metamers are objects that are physically different, but are perceptually indistinguishable from each other.

Whether or not two objects are metamers depends on viewing conditions.

(See Figure 5.28 on p. 20).

For example,

Lighting

Distance

• Form Defined by Contrast

• Form Defined by Texture

Gestalt Principles

• Proximity

• Similarity

• Closure

• (Good) continuation

Gestalt principles interact

• In the window, proximity indicates grouping of the face with the bars. Texture and luminance (similarity) indicate non-grouping.

• Proximity promotes perceptual grouping.

Why do the Gestalt Principles work?

• They embody cues that help identification in the natural world. So, they are fostered by natural selection.

Reinforcement between hypercolumns

• Bill Geisler, et al., at University of Texas studied many natural scenes.

• In short, similarity of orientation is proportional to separation.

In other words:

• Adjacent segments of any single natural contour tend to have very similar orientations.

• Distant segments of any single natural contour tend to have dissimilar orientations.

Perception of Symmetry (Sometimes Classified as a Gestalt

Principle)• Faces are approximately bilaterally

symmetric.

• Human body is approximately bilaterally symmetric.

• Symmetry is widely thought to contribute to attractiveness.

Rentsler, et al.

• People prefer more symmetry.

• People prefer greater simplicity.

• But, simplicity is a function of experience, where symmetry is not.