Vision (3)

Question 1

What are the two main classes of primary visual cortex neurons identified by Hubel & Wiesel?

Answer 1

Simple cells and complex cells.

Question 2

What are the key features of simple striate cells?

Answer 2

Rectangular receptive fields with antagonistic “on” and “off” regions.

Respond best to bars or edges of a specific orientation.

Monocular (respond to only one eye).

Unresponsive to diffuse light.

Question 3

How do simple cells differ from lower layer IV neurons?

Answer 3

Borders between “on” and “off” regions in simple cells are straight lines rather than circular, and receptive fields are rectangular.

Question 4

What are the key features of complex striate cells?

Answer 4

Larger receptive fields than simple cells.

Cannot be divided into static “on” and “off” regions.

Respond to a preferred orientation anywhere in the field.

Many respond best to movement in a specific direction.

Many are binocular (respond to either eye).

Question 5

How do binocular complex cells respond to retinal disparity?

Answer 5

They fire most robustly when the preferred stimulus is presented to both eyes at slightly different positions, contributing to depth perception.

Question 6

What is ocular dominance in binocular cells?

Answer 6

Binocular cells respond more strongly to stimulation of one eye than the other.

Question 7

How is the primary visual cortex organized according to Hubel & Wiesel?

Answer 7

Organized into vertical columns: neurons in the same column respond to the same retinal area, the same eye (if monocular), and same orientation preference.

Columns are clustered for input from one retinal area; half for left eye, half for right eye.

Columns include neurons with different orientation preferences.

Question 8

How do neurons with simpler preferences contribute to complex preferences?

Answer 8

Neurons with simpler receptive fields converge onto neurons with more complex receptive fields, creating increasing complexity in the primary visual cortex.

Question 9

How has the understanding of retinal ganglion cell receptive fields changed since Hubel & Wiesel?

Answer 9

Retinal ganglion cells are now known to be highly diverse, with 20 types in primates and 40 in mice, responding to uniform illumination, orientation, motion, and direction of motion, not just on-center/off-center.

Question 10

How have views of lateral geniculate nucleus (LGN) receptive fields changed?

Answer 10

Some LGN cells respond to orientation, motion, and direction of motion, not just contrast, showing properties similar to retinal ganglion cells.

Question 11

What are contextual influences in visual processing?

Answer 11

Responses of visual neurons depend not only on stimuli inside their receptive field but also on stimuli outside it, including natural scenes, movement, and biologically relevant cues.

Question 12

How are receptive fields now viewed in terms of plasticity?

Answer 12

Receptive fields are plastic properties, continually fine-tuned by prior experience and current environmental signals, not static, hardwired features.

Question 13

What is the component (trichromatic) theory of color vision?

Answer 13

Proposed by Young and Helmholtz: There are three types of cones (short, medium, long wavelength sensitive). Color is encoded by the ratio of activity in the three cone types.

Question 14

What is the opponent-process theory of color vision?

Answer 14

Proposed by Hering: Color is encoded by opponent pairs: red/green, blue/yellow, black/white. Cells increase firing for one color and decrease firing for its complement. Explains phenomena like afterimages and why complementary colors cannot mix.

Question 15

How do the component and opponent-process theories coexist?

Answer 15

Cones operate on a trichromatic (component) basis. At higher levels (retina-geniculate-striate system), opponent processing occurs: cells respond oppositely to complementary colors.

Question 16

How do different species vary in color vision?

Answer 16

Most primates: trichromats (3 photopigments). Most mammals: dichromats (2 photopigments, poor red perception). Birds, reptiles, fish: 4 photopigments. Some insects: 5+ photopigments (dragonflies have 10).

Question 17

What was the significance of the Jacobs et al. (2007) study on mice?

Answer 17

Inserting a gene for a third photopigment converted mice from dichromats to trichromats, giving them the ability to perceive additional wavelengths.

Question 19

What is color constancy?

Answer 19

Color constancy is the tendency for an object to appear the same color under different lighting conditions, even though the wavelengths it reflects change.

Question 20

Why is color constancy important for survival?

Answer 20

It allows us to recognize and distinguish objects reliably in varying lighting, improving our ability to respond appropriately to objects in our environment.

Question 21

How did Edwin Land demonstrate color constancy?

Answer 21

Using Mondrians illuminated by three projectors (short, medium, and long wavelengths), Land showed that even when the reflected wavelengths were identical, participants still perceived the correct colors.

Question 22

What key condition is necessary for color constancy in Land’s experiments?

Answer 22

The object must be illuminated with light containing short, medium, and long wavelengths and viewed as part of a scene, not in isolation.

Question 23

What is Land’s retinex theory of color vision?

Answer 23

The retinex theory proposes that the perceived color of an object is determined by its reflectance (proportion of light reflected at different wavelengths) rather than the absolute wavelengths hitting the retina.

Question 24

How does the visual system achieve color constancy according to retinex theory?

Answer 24

The visual system compares the light reflected by adjacent surfaces across three wavelength bands (short, medium, long) to calculate the reflectance and perceive the object’s true color.

Question 25

How is the role of context in color perception similar to its role in edge perception?

Answer 25

Just as context enhances the perception of edges/contrast, the visual system uses context (surrounding surfaces) to determine color, supporting color constancy.