Visual Systems and Eye Anatomy Quiz

Visual System Overview

• The visual system is complex, involving light, the eye, and the brain.
• Humans rely on light waves for vision. Visible light has specific wavelengths corresponding to colors.
• Vision relies on light entering the eye through the pupil (controlled by the iris).
• The lens adjusts focus (accommodation).
• The amount of light hitting the retina is regulated by the iris.
• Pupil size compromises between sensitivity and acuity.
• Sensitivity is the ability to detect dim light, whereas acuity is the ability to see details.
• The fovea is the area of the retina with the highest acuity.

Structure of the Human Eye

• The eye's structure includes the cornea, iris, pupil, lens, ciliary muscle, sclera, retina, optic nerve, fovea, and blind spot.
• The cornea is the clear front surface of the eye.
• The iris controls the size of the pupil, which regulates the amount of light reaching the retina.
• The lens focuses light onto the retina.
• The ciliary muscle adjusts the shape of the lens.
• The sclera is the white outer layer of the eye.
• The retina converts light to electrical signals.
• The optic nerve carries these signals to the brain.
• The fovea is the center of the retina for sharp central vision.
• The blind spot is where the optic nerve exits the retina.

Pupil and Lens

• The pupil is the opening in the center of the iris.
• The iris controls the size of the pupil.
• The lens focuses incoming light onto the retina.
• Accommodation is the process where the lens changes shape to focus on near or distant objects.

Sensitivity and Acuity

• Sensitivity relates to the ability to see dim light, while acuity is the ability to see fine detail.
• Pupil size is a trade-off between sensitivity and acuity; a larger pupil improves sensitivity but decreases acuity.

Eye Placement and Binocular Disparity

• Predators typically have forward-facing eyes, providing binocular vision, which aids in depth perception.
• Prey typically have lateral eyes, offering a wider field of view for detecting predators from multiple angles.
• Binocular disparity refers to the slight difference in the images projected onto each retina, crucial for depth perception.

Structure of the Retina

• The retina comprises five layers, the receptor layer being farthest from the light.
• These layers include receptor layer (rods and cones), horizontal cells, bipolar cells, amacrine cells, and retinal ganglion cells.
• Incoming light must pass through layers before reaching the receptors.

The Fovea

• The fovea is the central part of the retina with the highest concentration of cones (photoreceptors).
• This region is crucial for high-acuity vision.

Blind Spot and Completion

• The blind spot is the area on the retina where the optic nerve exits, lacking photoreceptor cells.
• The brain compensates for this by filling in information using surrounding visual input.

Cones and Rods

• Cones are responsible for color vision and detail vision in bright light.
• Rods are responsible for vision in low light conditions. Cones have low sensitivity. Rods have high sensitivity
• The density of cones and rods across the retina varies; the fovea has a high cone density.

Cone and Rod Vision

• Photopic vision relies on cones to see in good lighting conditions.
• Scotopic vision utilizes rods for vision in dim light.

Convergence

• Convergence describes how multiple photoreceptors connect to a single ganglion cell.
• Rods typically exhibit high convergence, optimizing sensitivity for dim light
• Cones exhibit low convergence maximizing resolution in good light

Distribution of Cones and Rods over the Human Retina

• Cones and rods are distributed unevenly across the retina.
• The highest concentration of cones is located in the fovea, enabling sharp central vision. Rods have a higher density away from the fovea, adapting to low light conditions.

Eye Movement

• Fixations refer to steady eye positions for focusing on objects.
• Saccades are rapid eye movements that shift the point of focus.
• Stabilized retinal images disappear due to eye movements, and temporal integration combines inputs from these movements to produce clear vision.

Visual Transduction by Rods

• In darkness, rods are depolarized and release glutamate.
• When light strikes rods, rhodopsin bleaches.
• This bleaching hyperpolarizes rods and lowers glutamate release, which is a neural signal.
• Rods transmit signals via inhibition of other cells and the visual system.

Inhibitory Response of Rods to Light

• In the dark, sodium channels remain open, allowing continuous sodium influx and depolarization of the rod cells.
• In light, rhodopsin activation closes sodium channels, reducing the intracellular sodium flow and hyperpolarizing the rod.
• The reduced glutamate release in response to light activates neurons in the subsequent layers of retinal processing.

From Retina to Visual Cortex (Retina-Geniculate-Striate System)

• The primary visual cortex is the initial point of visual signal processing in the occipital lobe.
• The lateral geniculate nucleus functions as a relay station in the thalamus.
• Visual information from the retina travels through the geniculate nucleus to the primary visual cortex (V1).

Retinotopic Organization

• The visual cortex has a retinotopic organization, where different parts of the retina are mapped to different locations in the cortex.
• Stimulating specific areas of the visual cortex can evoke corresponding visual sensations in the blind.

Receptive Fields of a Sensory Cell

• Receptive fields are defined by areas of stimulation that elicit responses in visual neurons.
• Receptive fields vary in size and properties across different levels of the visual pathways (retina, lateral geniculate nucleus, and visual cortex).

Receptive Fields of the Retina and LGN

• Receptive fields in the retina and LGN tend to be of similar size and are typically circular.
• Receptive fields are smaller in the foveal area compared to peripheral areas

Receptive Fields of the Retina-Geniculate-Striate System

• Two main types exist:
  • On-center: Excitation in the center, inhibition in the periphery
  • Off-center: Excitation in the periphery, inhibition in the center.
• These cells respond to the contrast between light and dark, not solely the amount of light.

Receptive Fields of Primary Visual Cortex Neurons

• Simple cells in primary visual cortex respond best to edges or bars of light with particular orientations.
• Complex cells respond best to straight-line stimuli of specific orientations and are not as sensitive to exact location.

Changing Concept of Visual Receptive Fields

• Initial studies focused mainly on orientation and simple stimuli which are not comprehensive of how natural scenes are viewed.
• Visual receptive fields are intricate, responsive not only to simple stimuli but also to motion, illumination, orientation, and direction of movement.

Three Different Classes of Visual Cortex

• Distinct visual cortex areas process visual information.
• Primary visual cortex, often called V1, is located in the occipital lobe and receives input from the retina, acting as the very first part of visual processing in the cortex.
• Secondary visual cortex (prestriate cortex) surrounds V1 and receives input from V1. It's involved in more advanced aspects of visual processing.
• Visual association cortex (including the inferotemporal and posterior parietal areas) further processes visual information. These regions integrate input data from secondary visual cortex, as well as input from other sensory systems.

Functional Areas of Secondary and Association Visual Cortex

• Specific portions of the secondary and association visual cortices house visual analysis units.
• Modern techniques, such as PET and fMRI, aid in pinpointing functional specialization within these regions given their function in analyzing different types of visual stimuli.

Dorsal and Ventral Streams

• These are two distinct visual pathways in the brain.
• The dorsal stream ("where") processes spatial information, location, and movement.
• The ventral stream ("what") identifies objects and their characteristics.

Prosopagnosia

• Prosopagnosia is a face-recognition deficit due to brain damages.
• It's associated with damage to the fusiform face area, a brain region in the temporal lobe.

Akinetopsia

• Akinetopsia is an inability to perceive smooth motion due to damage to the medial temporal area (MT) of the visual cortex.
• This area in the brain processes information about object motion, and damage can impair the perception of smooth visual inputs/movements.