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SENSORY SYSTEMS:

THE VISUAL SYSTEM

MODULE 2: NEUROSCIENCE

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The Auditory System

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The Tactile System

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The Visual System

SECTION OVERVIEW

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WHAT WE KNOW...

The auditory system works according to a tonotopic map, which means that the various receptive areas are arranged by frequency.
We will see a similar concept at play in studying the visual system.

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What is the visual cortex?

How is vision processed?

How can vision be restored?

KEY QUESTIONS:

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THE VISUAL CORTEX

Location: Occipital Lobe

Subdivisions: V1-V5, other higher processing areas

Organization: Retinotopic map

Inputs: contralateral

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The Eye and the Retina

The cornea and lens focus light onto the retina
Pupil acts as an aperture by constricting and relaxing to control light levels
Retina is the tissue and the retinal pigment epithelium (RPE) contains highly pigmented cells
Retina contains a pit called the fovea (specialized to contain the highest acuity vision due to less cell layers)

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Rods and cones are intermixed in the retina

At each little position in the retina, there are photoreceptors that are detecting dim light (rods) or bright light for the detection of color (cones)
Rods and cones are not uniformly distributed

This makes regions of the retina specialized for different types of vision

Fovea only has cones
Rods located in periphery of retina

peripheral vision is more light sensitive, enabling you to see dimmer objects in your peripheral vision

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PHOTORECEPTORS�Cell types: photoreceptor cells (rods and cones); specialized phototransduction that transforms stimulus (photons) into neural signals which detect brightness, contrast

Rods

Cell Structure: free floating disks within membrane

Function: detect objects under dim light

very light sensitive, large, concentrated in peripheral retina

Cones

Cell Structure: disks continuous with membrane

Function: detect objects under normal light

less sensitive, smaller, concentrated in the fovea, responsible for color vision

Types: Longwave, mediumwave, shortwave

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Vision processing begins in the retina, which is made up of two types of photoreceptors. The photoreceptors can be distinguished by their shape as well as the type of light they respond to. Rods are long, cylinder shaped cells that appear to have free-floating stacks of disks within the membrane. Rods are sensitive to low levels of light, and are therefore responsible for vision under dim lighting. Cones are responsible for vision in regular light, and the disks are continuous with the outer membrane. Cones can be further categorized by the wavelength of light that they respond to: long wave, medium wave, and short wave. It is the integration of these wavelength specific signals that allows for color vision, as different colors correspond with different wavelengths. Upon reception of a photon, the membrane of the photoreceptors is actually hyperpolarized, which is different than other cells, and through a series of biochemical events that signal is transformed into vision. We will be looking more at the broad circuitry and less into the specific biochemistry.

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Phototransduction

Phototransduction: converts light to hyperpolarization
In the dark, rods are depolarized (-40 mV) and steadily release glutamate
Rods do not spike – release is continuous and proportional to Vm (membrane potential)
Light hyperpolarizes the membrane and decreases glutamate release

This is an analog/graded response – the brighter the light, the stronger the hyperpolarization

Photons are absorbed in the outer segment

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VISUAL PROCESSING

Photoreceptors

Optic nerve

Thalamus

From the photoreceptors, the signal moves through the retinal ganglion cells that make up the optic nerve. From there, the signal travels to the thalamus, where it leaves on one of three pathways. The M cells take information about movement, the P cells take information about spatial resolution, and K cells take information about color. After processing, the signal is sent to the temporal lobe or the parietal lobe through the ventral or dorsal streams, respectively.

Visual Cortex

Temporal Lobe/Parietal Lobe

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From the photoreceptors, through the retinal ganglion cells, whose axons make up the optic nerve. There are two types of ganglion cells: ON cells and OFF cells. ON cells decrease firing when an object is dark, and off cells increase when an object is in the dark. This specificity allows for integration of information before the signal reaches its next destination: the lateral geniculate of the thalamus. Remember that the thalamus is the hub of information for the brain, and has projections to all major areas of the cortex. From the lateral geniculate, the signal is sent in one of three pathways to the primary visual cortex. M cells take information about movement, P cells take information about spatial resolution, and K cells taking information about color. Once the signal has been processed in the visual cortex, it is sent in either the ventral or dorsal stream. The ventral stream projects to the temporal lobe and is thought to be associated with recognition of objects. The dorsal stream projects to the parietal lobe and is associated with visuomotor processing.

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RESTORING VISION

artificial retina approved by FDA in 2013
replaces damaged photoreceptors
microelectrode array implanted on the back of the retina

connected to a camera/receiver
encoded info passes through the electrode array to the optic nerve

issues: overheating

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Summary/What’s Next

Summary:

Vision processing starts with the photoreceptors in the retina, and follow retinotopic pathways through the optic nerve to the thalamus and then the visual cortex. From the visual cortex, information is sent to the temporal and parietal lobes. Damaged photoreceptors can be replaced by an artificial retina.

What’s next:

Tactile Sensation

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THANKS FOR LISTENING!

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