Neuro PQ 16-18 Visual

Human Visual Perception Range

The human visual perception range is typically from about 380 nm (violet) to 750 nm (red) in the electromagnetic spectrum.

What is Ultraviolet (UV) light?

Light with wavelengths shorter than visible light (10 nm to 400 nm). Not visible to humans but can cause skin damage.

What is Infrared (IR) light?

Light with wavelengths longer than visible light (700 nm to 1 mm). Not visible to humans; used in thermal imaging.

Structure Allowing Light to Enter Eyes

The cornea and pupil work together to allow light to enter the eye. The cornea focuses light, while the pupil adjusts the amount of light entering.

Basic Retinal Circuit Components

Photoreceptors (rods and cones)

Bipolar cells

Ganglion cells

Horizontal and amacrine cells (modulating signals)

Myopia

(Nearsightedness): Corrected with concave lenses (diverging).

Hyperopia

(Farsightedness): Corrected with convex lenses (converging).

Only Output from the Retina

The only output from the retina is the axons of the ganglion cells, which form the optic nerve.

Site of Phototransduction

Phototransduction occurs in the outer segments of photoreceptors (rods and cones) where light is converted into electrical signals.

Differences Between Rods and Cones

Rods: More sensitive to light, responsible for night vision, do not detect color, and have a high convergence ratio.

Cones: Less sensitive, responsible for color vision and high acuity, with three types (S, M, L) for different wavelengths.

S-cones

Sensitive to short wavelengths (blue).

L-cones

Sensitive to long wavelengths (red).

M-cones

Sensitive to medium wavelengths (green).

Hyperpolarization of Rod Photoreceptors

When light hits a rod, it activates rhodopsin, leading to a cascade that closes sodium channels, causing hyperpolarization (a decrease in the release of neurotransmitters).

Other Light-Sensitive Retinal Cells

Intrinsically photosensitive retinal ganglion cells (ipRGCs) are sensitive to light and play a role in regulating circadian rhythms.

On-Center Definition

An On-center cell is activated by light in its center and inhibited by light in its surround.

Off-Surround Definition

An Off-surround cell is inhibited by light in its center and activated by light in its surround.

Off-Center Definition

An Off-center cell is primarily activated by light in its surround and inhibited by light in its center.

On-Surround Definition

An On-surround cell is inhibited by light in its surround and activated by light in its center.

What are the characteristics of Parvocellular (Parvo) outputs?

Small cell bodies, high spatial resolution, color information, and sustained response.

What are the characteristics of Magnocellular (Magno) outputs?

Large cell bodies, low spatial resolution, motion detection, and transient response.

What are the characteristics of Non-M/P outputs?

Mixed features, involved in other functions like color processing.

Brain Areas Receiving Retinal Input Outside of LGN

Other brain areas include the superior colliculus (for eye movement), pretectal area (pupillary reflex), and the suprachiasmatic nucleus (circadian rhythms).

Cause of Tunnel Vision

Tunnel vision can result from peripheral vision loss due to conditions like glaucoma or retinitis pigmentosa, leading to a constricted visual field.

Retinotopic Map

A retinotopic map is an organized representation of visual information in the brain, where neighboring retinal cells project to neighboring brain areas, preserving spatial relationships.

Thalamocortical Projections from V1

Projections from the lateral geniculate nucleus (LGN) to V1 convey visual information for further processing in the visual cortex.

Intracortical Projections from V1

These projections within V1 facilitate communication between neurons, enhancing processing capabilities and integrating visual information.

Lateral Excitatory Projections from V1

Projections that spread laterally across the cortex to enhance edge detection and contrast in visual perception.

Corticothalamic Outputs from V1

Outputs from V1 back to the LGN, modulating the thalamic relay of visual information, influencing attention and perception.

Long-term Visual Deprivation and Cortical Reorganization

Long-term visual deprivation leads to a decrease in cortical stimulation, prompting the brain to reorganize connections to maximize functional efficiency, affecting visual processing.

Orientation and Direction-Selectivity

Orientation-selectivity: Neurons respond maximally to edges or bars of specific orientations.

Direction-selectivity: Neurons respond preferentially to motion in a particular direction.

Neuronal Activity Imaging Techniques

With Dyes: Calcium imaging uses fluorescent dyes to visualize calcium influx during neuronal activity.

With Dyes: Voltage-sensitive dyes monitor changes in membrane potential.

Without Dyes: Electrophysiology measures electrical activity using microelectrodes.

Cortical Module

A cortical module is a functional unit in the visual cortex, typically encompassing columns that process specific visual features like orientation and motion.

Two Major Visual Streams Outside of V1

Dorsal Stream: Projects to the parietal lobe, responsible for spatial awareness and motion processing ("where" pathway).

Ventral Stream: Projects to the temporal lobe, responsible for object recognition and form ("what" pathway).