Cognitive Psych: 9/14/26 Lecture

Sensation

he process by which our sensory receptors and nervous system receive and represent stimulus energies from our environment. It involves the detection of physical stimuli (e.g., light waves, sound waves) by the sensory organs (e.g., eyes, ears) and converting these signals into neural signals.

Perception

Conscious experience that results from stimulation of the senses

• recognizing, organizing, and interpreting these signals; the resulting experience (doing something with the info from sensation)

Problems our visual system addresses:

The input (light waves) gives limited information

Crucial summary of input-find edges

• An edge is a sudden discontinuity in intensity

• Matters a lot bc edges and line star super important

Why would our visual system care about edges

• Edges frequently correspond to the boundaries of objects

• Edges are invariant to lighting conditions

Problems our visual system addresses: The stimulus on the receptors is ambiguous

• Edges could correspond to many different objects; inverse projection problem

Problems our visual system addresses: objects can be hidden or blurred

• Occlusion: think of it as hiding something

Problems our visual system addresses: the same object has many different views

• Although we can recognize most objects from many different viewpoints (viewpoint invariance) we are quicker to recognize objects in familiar orientations.

The complexity of perception

The problems show us that perception is actually quite complex, and MUST rely on more than just computation of light waves

Bottom-up processing:

Processing that starts with information received by the receptors. This type of processing is also called data-based processing.

Top-down processing

Processing that involves a person’s knowledge or expectations. This type of processing has also been called knowledge-based processing.

Which of the following would be an example of top-

down information?

a. Retinal signals to the brain

b. Memory establishing a category

c. Distinguishing the orientation of light

b. Memory establishing a category

Role of top down processing

changes how we interpret an image

Theory of object recognition:

regularities leading to assumptions

• Many of the qualities of objects that we would like to know about trade off with other qualities

• Our visual system makes assumptions (based off regularities) to limit the possibilities

Shape/orientation trade off

Shape is different depending on the orientation… the way it is turned

Shape/orientation: Perkin’s law

A perceptual strategy that assumes conjunctions of lines corresponds to different shape and orientations of objects.

 Shape/orientation: oblique effect

Vertical and horizontal orientations can be perceived more quickly than other (slanted) orientations.

color constancy:

tendency of objects to appear the same color even with changing illumination.

Local contrast

Assume that the brightest thing around is white; the darkest thing around is black.

Size/distance tradeoff: Familiar size

• Familiar size is actually not that powerful of a cue

Size/distance tradeoff: convergence

• Convergence: not very effective

• Most useful for very short distances

Size/distance tradeoff: texture gradient

• Objects that are closer to us we appear to be closer because they have more detail

Size/distance tradeoff: linear perspective

Lines appear to converge at the horizon

size/distance tradeoff: height

Assume that higher objects are farther away

Size/Distance Tradeoff: Atmospheric Perspective

Most useful for great distances

Size/distance tradeoff: stereopsis and retinal disparity

• Each eye gets a slightly different view

• Retinal disparity: the farther away the object, less difference between what the two eyes see

Why do we make these particular assumptions? (size/distance trade off)

They can even be misleading

Regularities: The preceding are all physical regularities