Colour Perception L4

What type of cells collect information from photoreceptors in the retina?

Retinal ganglion cells (RGCs).

What mechanism do retinal ganglion cells operate with?

A centre-surround antagonistic mechanism.

What are the subcategories of retinal ganglion cells?

M cells, P cells, and K cells.

How many layers does the LGN have?

The LGN has 6 layers.

How does the size of receptive fields vary in the LGN?

LGN RF sizes vary, with the smallest devoted to the fovea and the largest in M layers.

How many cells does the primary visual cortex (V1) have per hemisphere?

About 100 million cells.

What happens to the retinotopic map in the visual cortex compared to the LGN?

The retinotopic map is maintained, but the distribution of cells is distorted, with more cells devoted to the central visual field.

What is the primary pathway for visual information from the eye to the brain?

The optic nerve to the optic chiasm, then to the optic tract, and finally to the LGN.

What is the significance of the superior colliculus in visual processing?

It is involved in eye movements and receives some input from retinal ganglion cells.

What is the function of the hypothalamus in relation to visual input?

It helps regulate circadian rhythms based on light information.

What is the difference in response speed between M and P cells?

M cells respond quickly, while P cells respond more slowly.

What is the role of simple cells in the visual cortex?

Simple cells respond optimally to oriented stimuli at specific positions within their receptive fields and are phase sensitive.

What distinguishes complex cells from simple cells?

Complex cells respond optimally to oriented stimuli regardless of their position within the receptive field and are phase insensitive.

What is a hypercolumn in the visual cortex?

A collection of adjacent columns that analyze multiple attributes of an image, such as orientation, size, color, and direction of motion.

What is the primary role of V1 in visual processing?

V1 processes visual information, with a majority dedicated to central vision due to cortical magnification, and organizes cells into columns based on orientation and ocular dominance.

Why is color vision important?

Color vision aids in discrimination and detection, influencing tasks such as food selection, traffic navigation, scene segmentation, and communication.

What is color opponency?

A mechanism in color vision where certain colors are perceived in opposition to others, aiding in color discrimination.

How does chromatic processing occur in the visual pathway?

Chromatic processing involves the interaction of different types of color-sensitive cells at various stages, including the retina, LGN, and cortex.

What are some common defects of color vision?

Defects can include color blindness, where individuals have difficulty distinguishing between certain colors due to genetic or environmental factors.

What is the significance of visual memory in color perception?

Visual memory allows individuals to recall and recognize colors, which can signal emotions and aid in communication.

What is the relationship between orientation selectivity and visual perception?

Orientation selectivity enhances the ability to detect edges and shapes, which are critical for visual recognition and interpretation.

What is the role of ocular dominance columns in the visual cortex?

Ocular dominance columns are organized groups of neurons that respond preferentially to input from one eye, contributing to depth perception.

What happens to cortical cell sensitivity after adaptation?

Following adaptation, the sensitivity of active cells is reduced, altering their response to new stimuli and affecting perception.

What is the function of pinwheel maps in the visual cortex?

Pinwheel maps illustrate the orientation preference of cortical cells, providing insight into how visual information is processed.

How does the visual cortex process motion?

The visual cortex contains cells that are specifically tuned to detect motion direction and speed, allowing for dynamic visual perception.

What is the significance of receptive fields in cortical cells?

Receptive fields determine how cortical cells respond to visual stimuli, influencing their selectivity for orientation, size, and motion.

How does color help us recognize objects?

Color recognition is influenced by the usual color of objects; congruent conditions enhance labeling accuracy compared to incongruent conditions.

What determines the color of an object?

The color is mainly determined by the wavelengths of light reflected by the object; for example, an object appears red when it reflects red wavelengths more than others.

What are achromatic colors?

Achromatic colors, such as white and black, occur when all wavelengths are reflected equally.

Define hue, brightness, and saturation in color terms.

Hue (H) distinguishes colors like red from blue; brightness (V) is the perceived intensity of light; saturation (S) characterizes a color as pale or vibrant.

What is the nature of color perception?

Color is a psychological phenomenon, entirely subjective, dependent on the neural apparatus and the correct photoreceptors.

What is a metamer in color perception?

A metamer is a sensory stimulus that appears identical to another stimulus but is physically different, indicating that the visual system produces identical neural responses to different stimuli.

How do we see more colors with cone cells?

Humans have three types of cone cells (S, M, L) with different peak absorption wavelengths, allowing for the perception of a wide range of colors.

What is the principle of univariance?

The principle of univariance states that a single photopigment cannot distinguish between different wavelengths and intensities, as the same neural response can occur for different light combinations.

How do different animals perceive color?

Humans are trichromats with three cone types, while some mammals are dichromats with two pigments, and some birds are pentachromats with up to five pigments.

What is retinal topography in relation to cone cells?

Retinal topography refers to the layout of cone cells in the retina, which varies among individuals, affecting the distribution of S, M, and L cones.

How do parvocellular RGCs contribute to color perception?

Parvocellular RGCs have chromatically opponent receptive fields, where the center may be excited by one color while the surround is inhibited by its opponent.

What is the role of the LGN in color processing?

The LGN layers 1 & 2 receive input from M RGCs, contributing to the achromatic luminance channel.

What is the significance of cone distribution in color perception?

The distribution of cone types does not impact the ability to perceive colors, as individuals can have varying ratios of L and M cones.

What happens when you fixate on a color for an extended period?

Fixating on a color for about 30 seconds can lead to an afterimage of its opponent color when looking at a white surface.

What is the relationship between color experience and physical light properties?

The subjective experience of color is connected to the physical properties of incident light waves, with different wavelengths potentially resulting in identical color experiences.

What is the function of cone cells in color vision?

Cone cells are responsible for color vision, with S cones sensitive to blue, M cones to green, and L cones to red, enabling the perception of a wide color spectrum.

How does the visual system encode color?

The visual system encodes color based on the combination of signals from different cone classes, each with unique spectral sensitivities.

What is the significance of the peak absorption wavelengths of cone cells?

S cones peak at 420nm (blue), M cones at 530nm (green), and L cones at 565nm (red), which are crucial for color differentiation.

What does it mean for a color to be purely psychological?

It means that color perception is subjective and relies on the neural responses to different wavelengths rather than inherent properties of the objects themselves.

What layers of the LGN receive input from M RGCs?

Layers 1 & 2, which are responsible for the achromatic luminance channel.

Which layers of the LGN receive input from P RGCs?

Layers 3-6, which are responsible for the two chromatic channels, known as cardinals.

What is the preferred stimulus for nearly all LGN cells?

Stimuli modulated along the cardinal directions of colour space: red-green and blue-yellow.

How do cortical cells differ from LGN cells in terms of colour tuning?

Cortical cells prefer a wider range of hues, not just the cardinal directions.

What is a double opponent receptive field (RF)?

A type of cortical cell RF that is excited by one colour and inhibited by its opponent colour.

What is colour constancy?

The ability to perceive a fixed colour for an object despite changes in illumination conditions.

What phenomenon explains why squares appear the same under different lighting?

Chromatic induction, where perceived colour changes due to background lighting.

What percentage of trials did participants correctly identify paper colours under varying lighting conditions?

55%, indicating above chance level performance.

What is memory colour in relation to colour constancy?

The influence of prior knowledge of object-colour associations on colour perception.

How does chromatic adaptation contribute to colour constancy?

It decreases visual sensitivity to a specific illuminant over time, aiding in consistent colour perception.

What is the role of surroundings in achieving colour constancy?

The visual system uses information about lighting effects on objects to adjust perceived colours.

What is congenital colour vision deficiency (CVD)?

A hereditary condition often linked to an X-linked recessive gene affecting colour perception.

What is cerebral achromatopsia?

An acquired colour vision deficiency due to damage in the V4 area of the brain.The individual's eyes can detect light and send signals correctly, but the brain cannot interpret these signals as colour so they see in greys black and white

What characterizes a monochromat?

A rare form of CVD with no functioning cones, resulting in black and white vision.

What is the difference between dichromats and trichromats?

Dichromats miss one pigment, while trichromats have all three types of cone pigments.

What difficulties do individuals with missing M cone pigment experience?

They struggle to distinguish between green and red, perceiving greens as neutral or brownish.

What challenges do individuals with missing L cone pigment face?

They have difficulty distinguishing red shades and may perceive red as darker or black.

What issues arise for individuals with missing S cone pigment?

They struggle to differentiate blue and yellow, confusing blue with green and yellow with light pink or grey.

What is an anomalous trichromat?

An individual with all three cone types but altered sensitivity in one, leading to impaired colour discrimination.

How is colour vision deficiency detected?

Using Ishihara colour plates, where visible numbers indicate the type of deficiency.

What is the relationship between colour perception and visual judgments?

Colour perception aids in making visual judgments, influenced by the interaction of wavelength and visual machinery.

What is the significance of chromatic opponency in LGN cells?

It allows LGN cells to be tuned along cardinal hue axes, enhancing colour discrimination.

What is the impact of visual experience on colour perception?

Familiar objects are judged as richer in colour compared to unfamiliar objects reflecting the same wavelength.

What are the sources referenced for the lecture on colour perception?

Goldstein's 'Sensation and Perception', Bear et al.'s 'Neuroscience: Exploring the Brain', and Gegenfurtner's review.