Vision (Lecture 5) Harry R. Erwin, PhD COMM2E University of Sunderland

Vision(Lecture 5)

Harry R. Erwin, PhD

COMM2E

University of Sunderland

Resources

• Nicholls, Martin, Wallace, and Fuchs, 2001, From Neuron to Brain, 4rd edition, Sinauer. (Good for references, unless otherwise indicated, the primary reference for this lecture)

• Kandel, Schwartz, and Jessell, 2000, Principles of Neural Science, 4th edition, McGraw Hill. Covers the vestibular-ocular reflex.

• Dowling, 1992, Neurons and Networks, Belknap Harvard.

Outline

• Vision in the retina

• Vision in the cortex

• Visual sensorimotor integration

A Few Points from Kandel

• Vision is a creative process.• Visual information is processed in parallel by multiple

cortical areas– Motion, depth, form, and color are handled separately.– Two major pathways (dorsal or parietal for spatial/color and

ventral or temporal for object recognition)– One minor pathway (LGN—Superior Colliculus)

• Conversion between frames of reference is necessary. Cerebellum?

• Role of visual attention is probably important, particularly in proposing possible matches.

The Retina and Laternal Geniculate Nucleus (LGN)

• A portion of the CNS exposed to direct experimental observation.

• Multiple layers• Five main classes of neurons• Uses both electrical and chemical synapses. • Action potentials are used to communicate

down the optic nerve to the lateral geniculate nucleus (LGN) of the thalamus.

Retinal Anatomy

QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.

From <http://thalamus.wustl.edu/course/eyeret.html>

Structure of the Retina



Back

Front

Details

• The retina has seven layers, from inside to outside:– Optic nerve fibers– Ganglion cells– Inner plexiform layer– Horizontal, bipolar, and amacrine cells– Outer plexiform layer– Photoreceptors– Choroid

• To reach the photoreceptors, light must pass through five of these layers!

Photoreceptors


From <http://www.cis.rit.edu/people/faculty/montag/vandplite/pages/chap_9/ch9p1.html>

Receptor organization

• Outer segment traps the light using visual pigment. This is a modified cilium. Membrane potentials of about -40 mV due to the ‘dark current’.

• Inner segment contains the nucleus and organelles

Rods• Rods are black/white receptors. About 100,000,000 proteins

(rhodopsin) per rod, stored on several hundred disks. Rhodopsin is most sensitive to blue-green. Changes configuration in about 10-12 seconds. Regenerates over minutes.

• Cyclic GMP binding to cell membrane mediates the amplification of the signal. Produces constant output using ligand-activated channels that pass most cations. Light blocks cGMP binding.

• A single photon can be detected. The activation of seven rods by a photon can be consciously perceived.

• Rods become inactive during the day.

Cones

• The color receptors. The disklike infoldings holding the visual pigments are portions of the cell membrane. All three color pigments are closely related to rhodopsin. On X-chromosome, and red is the usual pigment involved in colorblindness.

• Mostly red (64%), some green (34%), and a few blue (2%), the three primary colors. Some girls can see four colors.

• Concentrated in the fovea.• Comparison of the outputs of different types of cones

produces color vision (Devalois and Devalois).




Back

Front

Retinal Processing

• Without light, receptors release glutamate (the “Dark Current”).• The receptors hyperpolarize in response to light at a rate that reflects the

rate at which photons are received. Cones are much less sensitive than rods. That stops glutamate release.

• Horizontal cells produce GABA and interact directly with the receptors in a feedback relationship. Also release GABA to bipolar cells and communicate with other horizontal cells by gap junctions.

• Bipolar cells receive input from the receptors (Glu) as long as there is no light.

• Amacrine cells synapse on bipolar and ganglion cells.• Ganglion cells receive inputs from bipolar and amacrine cells and

generate action potentials that travel to the LGN.

Receptive Field

• Key concept (Sherrington and later Hartline)

• Also applies in the cortex

• “The receptive field of a neuron is the area on the retina from which the activity of the neuron can be influenced by light”.

• There are neurons in the auditory cortex with visual receptive fields.




Back

Front

Bipolar Cells• Produce graded sustained changes in polarization based on the Glu

input. Some depolarize and some hyperpolarize based on their receptor types. Report small spots of darkness in light or light surrounded by darkness. Output via chemical synapses.

• Glutamate is inhibitory for on-center (H) bipolar cells (metabotropic Glu receptors), excitatory (normal) for off-center (D) bipolar cells.

• Rod bipolar cells listen to 15-45 rods. Detect large spots of light (D).• Midget bipolars listen to a single cone and are concentrated in the

fovea. Both H and D.• Other cone bipolars listen to 5-20 adjacent cones.

Horizontal Cells

• Release GABA continuously if not activated. Activation by receptors causes them to cease GABA release, preventing the cone and rod receptors from signaling light detection.

• Play a role in ‘center/surround’ detection.

• Best stimulus is illumination of a large area of the retina.

Amacrine (‘no-axon’) Cells

• Rod bipolars do not connect directly to ganglion cells, but rather indirectly via amacrine cells.

• In response to light, the rod bipolar depolarizes and releases Glu onto an amacrine cell.

• The amacrine cell generates an action potential, but has no axon. They output (+) via gap junctions (electrical synapses) to depolarizing cone bipolar cells and via Gly release (-) to ‘off’ ganglion cells, producing their ‘off’ responses.

• The depolarizing cone bipolars then trigger the ‘on’ ganglion cells.

• Hence the rod and cone systems trigger the same ganglion cells.

Ganglion Cells• Output of the retina: ‘on’ and ‘off’-center receptive cells• Two main categories of each (M & P)

– M cells project to the magnocellular division of the LGN, have large receptive field centers, low spatial resolution, are not color sensitive, and handle low contrast.

– P cells project to the parvocellular division of the LGN, have small receptive field centers, high spatial resolution, and are color sensitive. Require high contrast.

• The system elegantly deals with background light intensity variation.

• The eye adjusts automatically to a change in the light background, if necessary, switching between rods and cones.

Lateral Geniculate Nucleus (LGN)

• One on each side, receiving both P and M inputs from both eyes.

• Receptive fields are topographically mapped to the LGN.

• This is not just within layers but between layers.• Responses similar to ganglion cells. Contrast

mechanism is more finely tuned.• Some interneurons present. Function not understood.

May play a role in the gating of sensory afference.

Secondary Projections of the Retina

• Superior colliculus (SC)—controls saccades—where the eye moves between fixations. You do not see during this movement.

• Pretectum of the midbrain (near the SC)—handles pupillary reflexes

• ‘Blind-sight’ handled there as well.

Visual Sensorimotor Integration: Vestibular-Ocular Reflex (VOR)

• We perceive stable images on the retina much better than moving ones. During saccades, perception is shut down. If you move your head, your eyes correct via the vestibular-ocular reflex (VOR).

• The VOR is much faster than visual processing and even can anticipate image movement.

VOR Integration

• The VOR monitors the vestibular apparatus for head movement. Three types are detected using various types of hair cells:– Rotational movement—input from the semicircular canals in

the inner ear.

– Translational movement—input from the otolith organs in the inner ear. Takes into account the distance to the object being viewed.

– Head tilt in the vertical—input from the otolith organs.

The Vestibular System is Supplemented by the Optokinetic

System• Why:

– The vestibular apparatus habituates to rotational movement.

– The semicircular canals cannot detect slow movement well.

• The optokinetic system provides the central vestibular system with visual input.

Visual Cortex• 6-layered neocortex, moderately specialized, which appears

to consist of general-purpose computational elements.• The connections and functions have been mapped out, but…

– We don’t yet understand in any detail how neocortex performs its computational functions

– We know M and P cell inputs are kept separate in vision. This is an important structural constraint.

– Lesions in early stages of processing result in gaps in the visual field—’neglect’. The mind pretends they don’t exist.

• Cornelius Weber works in this area.

Neocortical Structure

• Two principle groups of neurons:– Stellate cells– Pyramidal cells

• Differ in axon lengths and cell body shapes• Stellate cells are local, and pyramidal cells are long-

range.• Smooth stellate cells are inhibitory.• Spiny stellate and pyramidal cells are excitatory.• Outputs are relative to a background level.

Six-Layer Cortex

• Three-layer cortex or paleocortex (olfaction) is simpler.• Varies from area to area. • Pulvinar (thalamic) input at layer 1.• Within-cortex input at layers 2, 3 and 5. Layers 2 and 3 output

within the cortex and project to 5.• Incoming geniculate fibers arrive mostly in layer 4 with some at

layer 6. Eye separation into ocular dominance columns here. Thereafter, binocular.

• Long-range cortical output from layers 2-4.• Deep structure output from layers 5 and 6. Layer 6 protects back to

the LGN, the claustrum and layer 4. Layer 5 goes to the superior colliculus.

Primary Visual Cortex

• Primary visual cortex, V1, striate cortex, or area 17. V2, which surrounds V1, is also visual.

• Most (80%) cells are already binocular.• Receptive fields result in simple and complex cells• Simple cells detect a number of patterns, but an important

one is a short bar based on: position, area, and angle• Edge detectors are also present, with length constraints.• Diffuse background illumination is ignored.

Complex Cells in V1

• Abundant in layers 2, 3 and 5• Specific field axis orientation of a dark/light

boundary• Diffuse illumination is ignored.• Accept freely positioned stimuli• Detects orientation without strict reference to

position.• Respond best to moving edges or slits.

Role of Left and Right Visual Cortices

• Each hemicortex handles half of the visual world, but using inputs from both eyes.

• Right/left connections exist between the hemicortices at the border.

• V1 communication is via the corpus callosum and involves cells in layer 3.

Two Visual Pathways

• Dorsal or parietal for spatial/color—“Over the top”. Parvocellular

• Ventral or temporal for object recognition —“Down the side”. Magnocellular

• There is evidence for a third pathway (“blindsight”) and perhaps of a fourth.

Motion, Depth, and Form

• Motion is a dorsal function– The middle temporal region solves the aperture problem

(partially hidden motion).

• Depth makes use of cues (LR) and binocular disparity (SR)– Combined in V1

• Object vision is a ventral function– V2 detects contours including illusions– V4 detects form– Complex forms (faces) in the inferior temporal cortex

Color Vision

• Captures properties of surfaces• Poor at capturing spatial detail

– Imagine a Dalmatian dog. Now count its spots.

• Color transformations are early in visual processing. P cells respond to:– Opposed signals from red and green-sensitive photoreceptors– Opposed signals from blue-sensitive photoreceptors and

some combination of red and green.

• There are multiple color pathways in the cortex.

Some Thoughts on Cortical Computation

• Sorting out how the cortex computes will be a key advance, probably worth a Nobel Prize or three.

• My questions:– Why is the neocortex so standardized in structure?– Why do pyramidal cells have such an interesting

compartmented structure?– How does the cortex learn? What does it learn?– What roles do inhibition and synchronization play?– How exactly is the neocortex wired during development and

why?

Conclusions

• I hope you find yourselves able to apply some of these questions.

Documents

Vision (Lecture 5) Harry R. Erwin, PhD COMM2E University of Sunderland