Lecture 21 - Vision 3 - Ganglion Cells & Parallel Paths QA

What is luminance contrast?

Difference in the number of photons that fall upon the center versus the surround of a retinal bipolar cell's receptive field.

What do ganglion cells in the visual system primarily respond to?

Ganglion cells respond to contrast in luminance across their receptive fields.

What are ON-center ganglion cells characterized by?

They produce a burst of action potentials in response to a bright stimulus in the center of their receptive field.

What is the role of horizontal cells in the retina?

Horizontal cells contribute to the center-surround receptive fields that characterize bipolar cells.

What does hyperpolarization indicate in OFF-center ganglion cells?

A decrease in action potentials when light is introduced in the center of the receptive field.

What is the major difference between rods and cones in terms of light sensitivity?

Rods are extremely sensitive to light and can respond to a single photon, while cones require many photons to respond.

Describe the outer segments of rods.

Rods have long, consistently wide outer segments filled with photopigment-rich membranous discs.

What allows rods to have greater sensitivity to low light levels?

The stable bond between opsin and 11-cis-retinal in rods minimizes spontaneous detachment and dark noise.

How do cones differ in structure from rods?

Cones have fewer discs that taper off in width from the inner segment, making their outer segments variable in size.

What is the peak response wavelength for rods?

Rods have a peak probability of capturing photons at around 500 nm.

What are the three types of cones in humans?

S (Short-wavelength), M (Middle-wavelength), and L (Long-wavelength) cones.

What is the Law of Univariance?

A cone responds the same way to a captured photon regardless of the wavelength of light.

Explain the term 'scotopic conditions.'

Light conditions on a moonless night where rods are maximally active.

What role does recoverin play in rods and cones?

Recoverin inhibits GRKs when calcium concentration is high, affecting light sensitivity adaptations.

What characterizes a duplex retina?

Humans have both rod-rich areas for low light conditions and cone-rich foveas for high light conditions.

What is the significance of the fovea in visual acuity?

The fovea has a high density of cones, enabling fine spatial acuity under bright light.

Why do humans have difficulty perceiving fine details in dim light?

Dim light conditions deactivate the cone population responsible for high visual acuity.

What is chromatic contrast?

It occurs when the wavelengths optimal for the ON and OFF responses differ in midget ganglion cells.

What type of cells contribute to color vision?

Midget ganglion cells contribute through their responses driven by L (Red) and M (Green) cones.

How do parasol ganglion cells contribute to vision?

They provide a luminance channel without chromatic contrast due to inputs from both L and M cones.

What are AII amacrine cells' roles in visual processing?

They combine signals from rod bipolar cells and modulate the responses of bipolar cells to influence ganglion cell activity.

How do ganglion cells respond to stimuli in different light conditions?

They adapt to light levels, altering their spike rates based on brightness changes and luminance contrast.

What distinguishes the visual capabilities of Old World primates?

They possess three cone types allowing for a richer color discrimination compared to other mammals.

What is the significance of opsin gene expression among cones?

The specific opsin expressed defines the wavelength range each cone type is sensitive to.

What is termed the 'color confusion' in relation to genetic mutations?

It is the inability to distinguish between certain colors due to mutations affecting opsin genes.

What do anatomical differences in rod and cone distributions facilitate?

They allow efficient light capture in both bright and dim environments.

What are the colors associated with the L and M cones?

L cones are often associated with red, M cones with green, and S cones with blue.

What impact do unequal crossing over events during meiosis have?

They can lead to variations in cone opsin genes, potentially causing color vision deficiencies.

What is the role of chemical synapses in visual signal processing?

They can inhibit or excite downstream neurons affecting the interpretation of visual stimuli.

Define the term 'hyperpolarization' in photoreceptors.

A change in the membrane potential that decreases the likelihood of generating action potentials in response to light.

What is dark noise?

The spontaneous release of glutamate leading to non-visual signals in photoreceptors in the absence of light.

What happens in visual processing when luminance increases?

It leads to an increase in the number of photons captured by the photoreceptors.

Describe the visual processing differences between rods and cones in mesopic conditions.

Neither rods nor cones operate in their optimal ranges, causing decreased sensitivity and acuity.

How does the structure of the S cone opsin differ from those of the M and L cones?

The S cone opsin is designed to absorb at a lower wavelength (around 435 nm) compared to M and L cones.

What triggers action potentials in AII amacrine cells?

They generate spikes in response to the capture of light by rods via rod bipolar cells.

Define the function of midget bipolar cells.

They gather input from single cones and connect to midget ganglion cells, allowing fine spatial resolution.

What are the implications of opsin mutations in males specifically?

Due to having one X chromosome, mutations can lead to color vision deficiencies more commonly in males.

What is the relationship between luminance and chromatic contrast?

Luminance changes can affect the responses of different cone types, but color contrasts depend on relative changes in wavelength.

What adaptational change occurs in rods when exposed to light?

Rods gradually decrease their sensitivity as photon levels rise to prevent saturation.

What is a characteristic feature of midget ganglion cells?

They possess small receptive field centers and high visual acuity in the fovea.

Explain how color is perceived as a result of cone responses.

Perception of color arises from the differential responses of the three cone types to various wavelengths of light.

What role does the inner segment play in rods and cones?

It connects the outer segment to the cell's metabolic machinery, handling the signal transduction process.

Describe the effect of light adaptation on rod response.

Light adaptation leads to a smaller response per photon as photon capture increases.

How does the number of cones relate to color vision in mammals?

The number and type of cones dictate the range of visible colors and sensitivity to different light wavelengths.

What underlies the differences in color detection capabilities across species?

Variability in the number of cone types and opsin expression.

What is the purpose of electrical synapses in the retina?

They enable rapid signal transmission allowing faster communication between neurons.

How does the axon structure differ between parasol and midget ganglion cells?

Parasol ganglion cells have larger axons and receptive fields compared to the smaller, more specialized midget cells.

What do the S, M, and L cone designations represent?

They indicate short, middle, and long wavelength sensitivities, approximating blue, green, and red light.

Define the function of the locus control region in opsin gene expression?

It regulates the transcription of the cone opsin genes on the X chromosome.

How do horizontal cells affect both ON and OFF responses in bipolar cells?

They modify the neurotransmitter responses, affecting both types of bipolar cells in opposite ways.

What visually noticeable phenomena can result from color confusion in colorblind individuals?

Difficulty distinguishing between colors that are easily separable by normal trichromats.

What characteristic allows midget ganglion cells to support spatial vision?

Their small receptive fields enhance resolution and detail in central vision.

What is the impact of convergent wiring from multiple photoreceptors to ganglion cells?

It increases sensitivity at the cost of spatial resolution.

What structural adaptation helps categorize rods and cones functionally?

The differing shapes and sizes of their outer segments directly relate to their sensitivity and function.

How do cone types contribute to bright light environments?

Cones have a broader range of sensitivity allowing for adaptability in high luminance settings.

What is significance of studying the absorption spectra of photoreceptors?

It provides insight into how different photoreceptors detect and process light.

Define AII amacrine cell function in light detection.

They integrate signals from multiple rod bipolar cells and transmit visual information to ganglion cells.

What kind of external conditions lead to scotopic vision?

Very low light environments where rods become the primary functional photoreceptors.

What influences the dynamics of color perception in the visual system?

The interplay between the activity of multiple cone types and their responses to various wavelengths.

How do environmental changes affect ganglion cell response dynamics?

Changes in ambient light levels modulate the spike rates of ganglion cells to maintain visual acuity.

What happens when the luminance of a visual stimulus increases?

Cone responses become more pronounced, boosting color perception through increased photon capture.

Describe the structural differences between the foveal and peripheral retina.

The fovea has a high concentration of cones while the peripheral retina is rich in rods for low-light vision.

What type of synapses do AII amacrine cells form with ON and OFF bipolar cells?

They form sign-preserving electrical synapses with ON bipolar cells and sign-reversing chemical synapses with OFF bipolar cells.

How do ganglion cells transmit visual signals to the brain?

They send action potentials generated from photoreceptor input through their axons to various brain regions.

What adaptations allow the visual system to function well across varying light conditions?

The complementary roles of rods and cones facilitate a broad dynamic range in visual capacity.

How do changes in opsin gene structure relate to color blindness?

Mutations in opsin genes can lead to misinterpretation of color information due to altered absorption spectra.

What determines the unique response characteristics of ganglion cell types?

Differential input from specific cone types and photoreceptor wiring patterns.

What visual conditions primarily use cone pathways?

Bright light conditions exploit the cone pathways for detail and color perception.

Define the concept of 'chromatic purity' in ganglion cells.

When a ganglion cell's center is driven by a single type of cone, it is said to have chromatic purity.

How do differences in retinal circuitry affect color distinction?

Retinal circuits integrate inputs from different cones, allowing complex processing of colors.

What function do diffuse bipolar cells serve in visual processing?

They gather input from multiple cone types, facilitating additional color processing for ganglion cells.

What determines the pathways for color signals reaching the brain?

The types of ganglion cells and their connections to specific areas in the visual processing hierarchy.

How does the structure of the retina assist with visual acuity?

It includes specialized circuits that improve detail resolution, particularly in central vision.

What anatomical feature distinguishes midget ganglion cells from others?

They have very small receptive fields corresponding directly to single cones.

How do AII amacrine cells affect light detection thresholds?

By integrating signals from rod bipolar cells, they enhance the sensitivity of dark stimuli detection.

What is the relationship between opsins and color sensitivity?

Each opsin's structure is specifically adapted to maximize absorption at particular wavelengths.

Define the contribution of midget ganglion cells to spatial acuity.

They provide high spatial resolution due to their small receptive fields and dense packing in the fovea.

How do color vision anomalies arise in males more than females?

Males have a single X chromosome, making them more susceptible to mutations affecting color opsins.

What neural mechanisms lead to the modulation of visual signal intensity?

Signaling cascades are influenced by light levels, altering response rates in bipolar and ganglion cells.

Describe the impact of non-linear responses in photoreceptors under varying light levels.

They enable adaptation to a broad range of light intensities while maintaining sensitivity.

What role does calcium concentration play in phototransduction?

It affects the functioning of recoverin, which regulates GRK activity and, hence, photoreceptor sensitivity.

What part of the retina is adapted for detecting low light?

The peripheral retina, which is rod-dense, is specialized for vision in dim light conditions.

Explain the concept of overlapping receptive fields in ganglion cells.

It allows for integration of multiple inputs, enhancing brightness detection across varying light stimuli.

What visual challenges do animals face in detecting infrared light?

Animals lack photoreceptors for infrared light but may use thermoreceptors for temperature sensing.

Define the significance of opsin-gene evolution in Old World primates.

Gene duplication events allowed for the development of trichromatic color vision in these primates.

How do horizontal cells contribute to contrast in the visual output?

They modify synaptic inputs to bipolar cells, helping to create contrast between center and surround visual fields.

What physiological role do amacrine cells play in visual processing?

Amacrine cells modulate signal transmission and can integrate complex responses from multiple inputs.

Describe how neural adaptations facilitate color differentiation in bright environments.

Response dynamics of the cones adjust to increased luminance, providing a more distinct color perception.

What anatomical distinction contributes to the human capacity for color vision?

The presence of multiple cone types allows for the perception of a broader spectrum of colors.

What effect does light intensity have on the activation of specific ganglion cells?

Higher light intensity typically increases the spike rate of ON ganglion cells while inhibiting OFF ganglion cells.

How does visual information encode depth and spatial awareness?

The integration of signals from both eyes and the arrangement of receptive fields gives rise to depth perception.

What dynamics underlie the response variability in the presence of adaptive lighting conditions?

Continuous light adaptation allows vision to remain functional across different brightness contexts.

How do the physical properties of light govern perceptual processes in the retina?

The wavelength, intensity, and duration of light directly affect how visual signals are processed at the cellular level.

What characterizes the connection between the optic nerve and visual signals?

The optic nerve carries processed visual information from the ganglion cells to the brain for interpretation.

What consequences arise from mutations in opsin genes for individuals with color blindness?

These mutations disrupt normal color perception, leading to difficulties in distinguishing specific colors.

What role do cone types serve in human trichromatic vision?

Each type of cone contributes distinct information based on the wavelength it is most sensitive to, leading to color perception.

How does adapting to light influence perception of brightness in the visual system?

It allows the visual system to adjust and respond to varied lighting conditions, enhancing object recognition.

What factors create the potential for color confusion in an individual?

Mutations in opsin genes, differences in cone type function, or overlapping absorption spectra can lead to color confusion.

Explain the significance of the rods and cones distribution in the retina.

This distribution maximizes light capture ability under various lighting scenarios.

What neural strategies do ganglion cells employ to produce effective visual responses?

They utilize diverse synaptic connections to differentiate between light and dark stimuli through encoding.