LGN & Striate Cortex, Early Spatial Vision

Optic Nerve (II Cranial Nerve)

Bundle of axons of the retinal ganglion cells coming together at the optic disc, it exits the orbit via the optic canal

Optic Chiasm

Where optic nerves cross. ½ of the axons change direction here. Helps brain process visual information and create one image from both eyes.

Optic Tract

Bundle of nerve fibers in the brain that transmits visual information from the optic chiasm to the LGN of the thalamus.

Superior Colliculus

Located on the roof of the midbrain it receives direct input from the retina via retinal ganglion cells allowing it to process visual stimuli. Involved in reflexive eye movements and visual attention.

Optic Radiations

White matter tracts that connect the lateral geniculate nucleus to the primary visual cortex. Pathway allows the brain to interpret visual stimuli.

Afferent Neurons

Carry sensory information from sensory receptors to the central nervous system

Efferent Neurons

Transmit motor commands away from the central nervous system to muscles and glands initiating actions

Shortest Pathway from the eye to the brain

The retina through the optic nerve and optic chiasm, to the lateral geniculate nucleus and finally to the visual cortex

LGV (Lateral Gyrus of the Brainstem)

Responsible for processing visual information, receives input from the dorsal column-medial lemniscus and the spinothalamic tracts.

VPN (Ventral Posterior Nucleus)

Involved in relaying sensory information from the body to the cerebral cortex, receives input from the trigeminal nerve.

Dorsal column-medial lemniscus pathway

Major sensory pathway in the CNS responsible for transmitting fine touch, vibration, and proprioceptive information.

Spinothalamic Tract

Ascending pathway in the spinal cord responsible for conveying pain and temperature to the thalamus which then relays information to the somatosensory cortex for processing

Primary Visual Pathway

In each eye light coming from the right visual field falls on the left retina in both eyes which end up in the left side of the brain. (Right visual field = Left Cortex)

Lateral Geniculate Nucleus (LGN)

Structure in the thalamus that acts as a relay station for visual information, receiving signals from the retina and directing them to the primary visual cortex

Primary Visual Cortex (V1) (Straite Cortex)

Located in the occipital lobe at the back of the brain is the first cortical area to receive visual input from the LGN it is essential for the conscious perception of visual stimuli

Neurons in V1

Organized retinotopically, process basic visual features such as edges, orientation, and motion and respond to specific orientations of visual stimuli contributing to our ability to perceive shapes and patterns

Retinotopy

Adjacent neurons correspond to adjacent areas in the visual field. Connections are orderly.

Retinotopy - Horizontal

Staying in one layer and moving laterally, receptive fields are close together.

Retinotopy - Vertical

Moving through each layer radially, receptive fields on top of each other (essentially staying in one place on the retina)

Secondary Visual Cortex

Receives signals from the primary visual cortex and is responsible for processing motion, shape and position of objects. Neurons respond to more complex stimuli allowing for analysis of lines and whole objects.

Mapping of Input from the Visual Field

Divided by direction of visual world not by which eye. LGN consists of 6 layers (1-2 Magno and 3-6 Parvo)

P Cells (Parvo)

4 layers in LGN, respond to color and final details, connect small cell bodies, small receptive fields and is slow.

M Cells (Magno)

2 layers in LGN, respond to motion and contrast, connect large cell bodies, large receptive fields, and is fast.

LGN Cell Receptive Fields

Respond to simple patterns “donuts of light” As spot becomes bigger response is quicker, however response slows as stimulus reaches minus region, when stimulus completely covers receptive field response stops entirely.

Schiller, Logothetis & Charles Study

Monkeys injected with ibotenic acid (kills cell bodies) into LGN (Spot injected went down radially so RF ion top of each other)

After recovery:

Parvo: Loss of color, texture, fine pattern

Magno: Fail to see movement

Foveal Image

Image resolution across different regions is based on fixed points

Fovea

Area of the retina that provides the sharpest vision (contains a high concentration of cones) (More rods outside) Crucial for tasks requiring high visual acuity.

Straite Cortex

Is the primary visual cortex, axons from LGN come in through layer 4 then spread up and down.

Cortical Receptive Field (Circularly-Symmetric)

Unoriented, layer 4, similar RF to LGN cells, right place on retina/orientation doesn’t matter

Cortical Receptive Field (Simple Cells - Edge Detector)

Stimulus place and orientation in RF matter, responds to vertical edge (bright on the left and dark on the right)

Cortical Receptive Field (Simple Cells - Stripe Detector)

A cortical neuron whose receptive field has clearly defined excitatory and inhibitory regions, see lines and contours (vertical bar bright in the middle, dark on sides)

Orientation Tuning

The tendency of neurons in striate cortex to respond optimally to certain orientations and less to others.

Cortical Receptive Field (Complex)

Oriented, A cortical neuron whose receptive field does not have clearly defined excitatory and inhibitory regions, good at detecting motion.

Cortical Receptive Field (End-Stop Cell)

The process by which a cell in the cortex first increases its firing rate as the bar length increases to fill up its receptive field, and then decreases its receptive field, and then decreases its firing rate as the bar is lengthened further.

How Cortical Receptive Field Cells Work

Big sets of cells with receptive fields on the retina at each retinal location in the physical world have machinery that tell the brain what you are seeing

Orientation Columns

Organized regions of neurons that are excited by visual line stimuli of varying angles located in the striate cortex (V1). Each column responds to a particular orientation with the angle changing across adjacent columns

Ocular Dominance Columns

Stripes of neurons in the striate cortex (V1) that respond preferentially to input from one eye or the other. Organized in a striped pattern across the surface of the cortex with alternating bands dedicated to the left and right eyes, contributes to depth perception

Hubel and Wiesel

Revealed the arrangement of orientation and ocular dominance columns. Neurons in the striate cortex (V1) are tuned to line orientations which is crucial for constructing visual representations through edge detection.

Column

A vertical arrangement of neurons. Neurons within a single column tend to have similar receptive fields and similar orientation preferences.

Hypercolumn

A 1mm block of striate cortex containing two sets of columns, each covering every possible orientation (0-180 deg), with one set preferring input from the left eye and one set preferring input from the right eye.

Cytochrome Oxidase (CO) blobs

Cells that were stained with a particular substance that form blobs that respond well to color. The blob array suggests that there is an additional organizational layer on top of the orientation and ocular dominance arrays.

Adaptation

A reduction in response caused by prior to continuing stimulation.

Cortical Magnification

Many more V1 cells have RF in and near the fovea (spots on the retina that are magnified), similar to the distortions of the sensory homunculus in touch

Early Spatial Vision

Refers to black and white, no movement, basic vision “bottom up” (sensory information is interpreted directly from the environment, no prior knowledge)

Visual Acuity

The smallest spatial detail that can be resolved (detected) at 100% contrast

Point Spread Function

Refers to the shape of the blurred spot that results when a point source of light is imaged by a real optical imaging system. (used for improving the quality of images produced by imaging systems)

Two Point Acuity

Two objects in visual space that need to be separated enough to be able to tell them apart (1st step in acuity).

The “Rule of thumb”

Visual angle, how large an image is on the retina on the eye (2.10cm)

How to measure visual acuity (Optometrists)

Use distance (20/20) and optotypes

Optotypes

Letters or symbols shown on an eye chart to test the acuity of vision.

Herman Snellen

Invented method for designating visual acuity (letter chart)

Normal Vision Degree Size

Critical detail of 1/60 of a degree (1 arcmin) (stroke 1/60 deg, letter size 5/60 deg)

Snellen Chart

Most common way to measure acuity, 20/20 is normal vision

Numerator: you (20 feet from chart) Denominator: “normal vision”

Ex: 20:10 you see at 20ft what a normal vision person can see at 10ft

Legally Blind Status

20:200 you see at 20ft what a normal vision person can see at 200ft

How to measure visual acuity (vision scientists)

Use the smallest visual angle of a cycle of grating

20:20 is 30 cycles per 1 degree (best)

Grating

The ability to distinguish elements composed of alternating dark and lights stripes or squares (1 cycle = 1 dark, 1 light) (few cycled per degree (CPD) (low frequency))

When measuring visual acuity…

Always measure highest possible contrast (can change the difference just not overall average of contrast)

Phase

Position within receptive field

Contrast

One bar is one cycle per degree

Optics (PSF)

A measure of the system’s ability to resolve fine details, a narrower PSF indicates better resolution and image quality (in fovea) but a broader PSF (in the periphery) indicates worse performance - therefore sampling by photoreceptor mosaic is a limiting factor for acuity.

Tightly sampled

Enough cones across horizontal bars to tell difference between dark and bright bars

Spatial Frequency

The number of grating cycles (dark and bright bars) in a given unit of space

Cycles per degree

The number of grating cycles per degree of visual angle

Magnocellular Layer

Either of the bottom two neuron-containing layers of the LGN, the cells of which are physically larger than those in the top four layers

Parvocellular Layer

Any of the top four neuron-containing layers of the LGN, the cells of which are physically smaller than those in the bottom two layers.

Koniocellular Cell

A neuron located between the magnocellular and parvocellular layers of the LGN.

Contralateral

Reffering to the opposite side of the body (brain)

Ipsilateral

Referring to the same side of the body (brain)

Topographical Mapping

The orderly mapping of the world in the LGN and the visual cortex.

Visual Crowding

The harmful but not always obvious effect of clutter on peripheral object recognition.

Fourier Synthesis

Sum up sine waves to make a complex waveform (make square waves)

Fourier Analysis

Break a complex waveform into its sine wave components

Cycle

For a grating, a pair consisting of one dark bar and one bright bar.

Low Spatial Frequency (visible)

Cones fall directly in light or dark so can be determined (blurry)

High Spatial Frequency (Not visible)

Cones fall in both dark and light bars, so they are averaged (gray)

What grating pattern can you see best?

High contrast, Low spatial frequency

Retinal Ganglion Cells and Stripes

→ Respond well to spots and gratings (best response is medium frequency and high contrast)

→ Don’t care about orientation

Simple Cells and Gratings

Care About:

→ Frequency

→ Contrast (high)

→ Orientation

→ Phase

Tuned to a ranged of spatial frequencies

Complex Cells and Gratings

Care About:

→ Frequency

→ Contrast (high)

→ Orientation

Contrast Sensitivity Function (CSF)

A function describing how the sensitivity to contrast (reciprocal of contrast threshold) depends on the spatial frequency (size) of the stimulus.

→ Acuity Limit: 60 cpd

Contrast Threshold

The smallest amount of contrast required to detect a pattern.

CSF Hypothesis

CSF is the ‘envelope’ of a set of spatial frequency ‘channels’ tuned to different spatial frequencies with overlapping profiles

Selective Adaption (Pre-test)

Baseline (measure thresholds)

Selective Adaption (Adaption)

Stimulus is intense (high contrast)

→ Adapting to strong stimulus becoming less sensitive

Selective Adaption (Post-Test)

Study effect of adaption phase (measure thresholds)

Selective Adaption Response

Dip in the CSF curve shows selective adaption

→ Can reduce sensitivity temporarily

→ Can alter appearance of stimuli

Tilt Aftereffect

The perceptual illusion of tilt, produced by adaption to a pattern of a given orientation

Pre-Adaptive Sensitivity Responses (Left Tilt)

High-Medium-Low

Pre-Adaptive Sensitivity Responses (vertical)

Medium-High-Medium

Pre-Adaptive Sensitivity Responses (Right Tilt), dashes

Low-Medium-High

Post Adaptation Sensitivity Response (vertical), arrows

Low→Medium→High

Tilt Aftereffect Conclusion

Supports idea that the human visual system contains individual neurons selective for different orientations (can be adapted independently)

Selective Adaption Conclusion

Evidence that the human visual system contains neurons selective for spatial frequency.

Adaption Experiments Conclude

There is strong evidence that orientation and spatial frequency are coded by neurons in the human visual system

→ Cats, Monkeys, Humans: striate cortex

Spatial Frequency Aftereffect

The motion of a stationary stimulus is perceived to move in the opposite direction after prolonged exposure to a moving stimulus

Aliasing

A continuous signal is sampled at a frequency too low to accurately represent the original signal leading to distortion and loss of information