NSCI 175 Exam 2 Review: Central Visual System

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As you move from the retina --> LGN --> V1 --> dorsal and ventral streams, receptive fields become ____ (more/less) complex.

opsins

Cones contain ____ as their photopigments (3 types that detect short, medium, or long wavelengths of light!)

ocular dominance columns

Cortical columns (in V1) consisting of neurons that receive signals from the left eye only or the right eye only.

Photoreceptor to bipolar cells to ganglion cells

Direct pathway that visual information takes in the retina (horizontal and amacrine cells also involved)

6

Each LGN in the dorsal thalamus is arranged in ___ layers of cells, with layer 1 being the most ventral.

Only a SUBSET of layers receives input and sends output to different cortical/subcortical areas.

How is V1 different from the LGN?

central retina (only cones here!)

In photopic (daylight) conditions, where in the retina is our visual acuity higher?

dark current

In the darkness there is a steady flow of Na+ into the photoreceptor _____ that maintains the high RMP at -30 mV.

III

In which layer of the striate cortex is there mixing of input from the eyes?

magnocellular

Layer 1-2 of the LGN comprise the ____ LGN. These layers receive the retinal output from M-type ganglion cells.

parvocellular

Layers 3-6 of the LGN comprise the _____ LGN. These layers receive the retinal output from P-type ganglion cells.

ganglion, bipolar

Light passes through the ____ cells and ____ cells before reaching the photoreceptors (seems backwards!) Remember that light and information flow travel in opposite directions in the retina!

optic nerve, optic chiasm, optic tract

List the three structures of the retinofugal pathway in the order they receive visual information.

visual acuity

Measure of how much detail we see; sharper in the center of the visual field.

superior colliculus

Non-thalamic target of the optic tract in the midbrain; orients eyes in response to new stimuli (move fovea to objects of interest.)

pretectum

Non-thalamic target of the optic tract in the midbrain; pupil size and eye movements.

hypothalamus

Non-thalamic target of the optic tract; biological rhythms, sleep and wake.

pyramidal cells

Only ____ can send axons OUT of the striate cortex (only output.)

retinotectal projection

Projection from the retina to the superior colliculus (optic tectum) is called ____.

amacrine cells

Receive input from BIPOLAR cells and project to ganglion cells, bipolar cells, and other amacrine cells.

horizontal cells

Receive input from PHOTORECEPTORS and project to other photoreceptors and bipolar cells

on-center/off-surround

Referring to a concentric receptive field in which stimulation of the CENTER EXCITES the cell of interest while stimulation of the surround inhibits it.

off-center/on-surround

Referring to a concentric receptive field in which stimulation of the CENTER INHIBITS the cell of interest while stimulation of the surround excites it.

rhodopsin

Rods contain ____ as their photopigment.

scotopic, photopic, mesopic

Rods correspond to the ___ system and cones correspond to the ___ system and both correspond to the ___ system. Humans have a DUPLEX RETINA.

visual field

The entire region of space (measured in degrees of visual angle) that can be seen with both eyes looking straight ahead.

I and III

The koniocellular LGN projects to which layers of V1?

IVC (IVC alpha and IVC beta)

The largest number of LGN axons terminate in which layer of V1?

IVC alpha

The magnocellular LGN projects to which layer of V1?

IVC beta

The parvocellular LGN projects to which layer of V1?

on-center/off-surround and off-center/on-surround

The receptive fields of retinal ganglion cells are concentric and have two types: ____

M-type ganglion cells

This type of ganglion cell... * Constitutes 5% of the ganglion cell population * Has larger receptive field * Conducts APs more rapidly * Respond to color but are NOT color-specific. Not sensitive to differences in wavelength of light.

P-type ganglion cells

This type of ganglion cell... * Constitutes 90% of the ganglion cell population * Has sustained discharge as long as stimulus is on * Some color-opponent cells are sensitive to differences in wavelengths of light.

nasal retina of left eye and temporal retina of right eye

To see your LEFT visual hemifield, you use your...

nasal retina of right eye and temporal retina of left eye

To see your RIGHT visual hemifield, you use your...

True

True or false: Ganglion cells actually fire action potentials whether or not they are exposed to light, and light in the receptive field center or surround increases or decreases the firing rate.

True

True or false: In photopic conditions our visual acuity is much higher in our central retina because of the sole presence of cones there. This appearance is due to the lateral displacement of the cells above the photoreceptors, allowing light to strike the photoreceptors without passing through the other retinal cell layers.

False (overrepresented; there are many more neurons in the striate cortex that receive input from the central retina than the peripheral)

True or false: Our fovea is underrepresented in the striate cortex.

True (input is also segregated by retinal ganglion cell type input AND segregated by eye)

True or false: The anatomical organization of the LGN supports the idea that the retina gives rise to streams of information that are processed in PARALLEL.

True (the only major difference is in the type of OPSINS in the membranous disks of the cone outer segments)

True or false: The process of phototransduction in cones is virtually the SAME as in rods.

True

True or false: a discrete point of light can activate many cells in the retina (and even more cells in the target structure) due to the overlap of receptive fields.

True

True or false: as photoreceptor cells depolarize and hyperpolarize, the rate of NT release is proportional to the membrane potential.

False (rods have higher photopigment concentration)

True or false: cones have a higher photopigment concentration than rods.

True

True or false: input from the two eyes is kept SEPARATE in the LGN.

False (light is hyperpolarizing)

True or false: light is DEPOLARIZING.

True

True or false: most neurons in layers superficial to IVC are binocular and respond to light in either eye?

True (there is still segregation of inputs by eye in layer IV)

True or false: neurons connected to the left and right eyes are STILL distinct in layer IV of the striate cortex as they are in the LGN.

True

True or false: the only light sensitive cells in the retina are the rod and cone photoreceptors.

False (almost identical and are still very simple)

True or false: the receptive fields of LGN neurons are different from those of the ganglion cells that connect with them and are more complex.

False (most are!)

True or false: very few of the V1 neurons outside layer IVC are orientation selective.

False (viewed by the RIGHT hemisphere)

True or false: your LEFT visual field is viewed by the LEFT hemisphere.

fovea

Visual acuity is maximized at the ____ because only CONES are present; rods are absent as well as the other cellular layers are pushed out of the way.

LGN

Visual information travels through the ____ of the thalamus (the sensory relay nucleus in the brain.)

peripheral

We are more sensitive to low levels of light in our ____ retina. Our central vision is BLIND at scotopic light levels because rods are more numerous in the periphery.

outer segment, inner segment, synaptic terminal, cell body (same structural components)

What are similarities between cones and rods?

Hypothalamus, pretectum, superior colliculus

What are the non-thalamic targets of the optic tract?

Ganglion cells

What is the only source of OUTPUT from the retina?

Photoreceptor to bipolar cell to ganglion cell to optic nerve to optic chiasm to LGN to V1 (striate cortex, primary visual cortex)

What is the path that visual information takes to the cortex?

V1 (primary visual cortex)

What is the single major target of the LGN?

photoreceptor (converts EM radiation to neural signals)

What is the site of transduction in the visual pathway?

Light bleaches rhodopsin --> G-protein becomes active --> PDE activated which reduces cGMP levels --> reduced cGMP levels closes Na+ channels

What is the transduction pathway of light?

orientation selective

What type of receptive field in V1 responds best to an elongated bar of light moving across the receptive field, where orientation of the bar is critical?

direction sleective

What type of receptive field in V1 responds when a bar of light at the OPTIMAL ORIENTATION moves perpendicular to the orientation in one direction but not in the opposite direction?

Optic chiasm

Where do optic nerve fibers cross such that information about the left visual hemifield is directed to the right side of the brain and vice versa? (Where is the decussation in visual processing?)

2, 3, 5

Which layers of the LGN receive IPSILATERAL (from the same side) eye input?

Optic nerve

Which nerve is responsible for relaying information from the eye to the brain?

M-type

Which type of ganglion cell is more sensitive to LOW CONTRAST stimuli?

P-type and nonM-nonP type

Which types of ganglion cells encapsulate color opponent cells (sensitive to differences in wavelengths of light)?

amacrine

With the exception of _____ cells, ganglion cells are the only retinal neurons that fire action potentials. All other cells depolarize/hyperpolarize.

Rods (only cones are found in the fovea)

____ are absent in the fovea.

Retinotopy

organization of visual input; neighboring cells in the retina feed information to neighboring places in their target structures.