Perceiving and Recognizing Objects

4

Object Recognition

Objects in the brain• Extrastriate cortex: The region of cortex bordering

the primary visual cortex and containing multiple areas involved in visual processing

• After extrastriate cortex, processing of object information is split into a “what” pathway and a “where” pathway

“Where” pathway is concerned with the locations and shapes of objects but not their names or functions

“What” pathway is concerned with the names and functions of objects regardless of where they are

Figure 4.35 Visual cortical processing can be divided into two broad streams of processing

Object Recognition

Inferotemporal (IT) cortex: Part of the cerebral cortex in the lower portion of the temporal lobe, important for object recognition

• Part of the “what” pathway

When IT cortex is lesioned, it leads to agnosia

Object Recognition

Grandmother cells

• Could a single neuron be responsible for recognizing your grandmother?

Feed-forward process: A process that carries out a computation one neural step after another, without need for feedback from a later stage to an earlier stage

• Object recognition occurs so quickly that feed-forward processes must be occuring

SUMMARY

What is the role of middle vision?

organize visual input

Perceptual “committees”: many processes have to be in agreement

Gestalt rules: reflect regularities of physical world

Object recognition models:

templates: like photographs?

structural: relationship among parts important• Faces are prime example: viewpoint crucial

• Some brain areas are highly specialized

The Perception of Color

5

Basic Principles of Color Perception

Color is not a physical property but a psychophysical property

• Most of the light we see is reflected

• Typical light sources: Sun, light bulb, fire

• We see only part of the electromagnetic spectrum—between 400 and 700 nm

Figure 5.2 Lights of 450 and 625 nm each elicit the same response from this photoreceptor

Separate photoreceptor for each wavelength (color)?

Basic Principles of Color Perception

Problem of univariance: response from a single type of photoreceptor is ambiguous

same for different wavelengths

same for properly adjusted intensity

• Therefore, one type of photoreceptor cannot make color discriminations based on wavelength

Color perceived both in daylight and in darkness?

Photopic light:

bright enough for cone receptors

bright enough to “saturate” rod receptors

• Sunlight and bright indoor lighting are both photopic lighting conditions

Scotopic light:

bright enough for rod receptors

too dim for cone receptors

• Moonlight and extremely dim indoor lighting are both scotopic lighting conditions

• No color discrimination possible (color not physical!)

Figure 5.3 The moonlit world appears to be drained of color

Trichromacy: 3 types of cones

Cone photoreceptors: Three varieties:

• S-cones: Cones that are preferentially sensitive to short wavelengths (“blue” cones)

• M-cones: Cones that are preferentially sensitive to middle wavelengths (“green” cones)

• L-cones: Cones that are preferentially sensitive to long wavelengths (“red” cones)

Figure 5.4 The two wavelengths that produce the same response from one type of cone (M), produce different patterns of responses across the three types of cones (S, M, and L)

Trichromacy

Trichromacy: The theory that the color of any light is defined in our visual system by the relationships of three numbers, the outputs of three receptor types now known to be the three cones

• Also known as the Young–Helmholtz theory

Trichromacy: Issues and problems

Metamers: Different mixtures of wavelengths that look identical. More generally, any pair of stimuli that are perceived as identical in spite of physical differences

Physical stimulus Perception

A

P

B

Trichromacy

Additive color mixing: A mixture of lights

• If light A and light B are both reflected from a surface to the eye, in the perception of color the effects of those two lights add together

Figure 5.9 Georges Seurat’s painting La Parade (1887–1888) illustrates the effect of additive color mixture with paints

Mixing additively possible with paints too!

Trichromacy

Subtractive color mixing: A mixture of pigments

• If pigment A and B mix, some of the light shining on the surface will be subtracted by A and some by B. Only the remainder contributes to the perception of color

• Result of physical mixing of paints

Figure 5.7 In this example of subtractive color mixture, “white”—broadband—light is passed through two filters

Trichromacy

Color space: A three-dimensional space that describes all colors. There are several possible color spaces

• RGB color space: Defined by the outputs of long, medium, and short wavelength lights

• HSB color space: Defined by hue, saturation, and brightness

Hue: The chromatic (color) aspect of light Saturation: The chromatic strength of a hue Brightness: The distance from black in color

space

Figure 5.10 A color picker may offer several ways to specify a color in a three-dimensional color space

brig

htne

ss

saturation

Figure 5.11 The curvaceous triangle shown here represents all the colors that can be seen (at one brightness level) by the human visual system

green

red

Trichromacy

History of color vision

• Thomas Young (1773–1829) and Hermann von Helmholtz (1821–1894) independently discovered the trichromatic nature of color perception

This is why trichromatic theory is sometimes called the “Young–Helmholtz theory”

• James Maxwell (1831–1879) developed a color-matching technique that is still being used today

Figure 5.12 A modern version of Maxwell’s color-matching experiment

Task: match the reference light by mixing

Need at least three lights

Next week: opponent processes theory