introduction to course 9.04, vision and...

Vision and audition, Course 9.04Peter H. Schiller and Chris Brown, year 2006

Introduction to Course 9.04, Vision and Audition

Vision: Peter H. Schiller Audition: Chris Brown

Reading assignments

Research report

Exams

Introductions

What aspects of visual processing are we trying to understand?

Color vision

Movement pocessing

Depth perception

Object recognition

Generation of visually guided eye movements

Tools of the trade:

Psychophysics

Anatomy

Physiology

Pharmacology

Brain lesions

Imaging

The visual and oculomotor systemsPeter H. Schiller, Sep 11, 2006

Basic Wiring of the Visual System

The world seen by the two eyes

rabbit human

Seen by both eyes

Seen by left eye Seen by right eye

Seen by both eyes

Seen by left eye Seen by right eye

Figure by MIT OCW.

LGN

Cortex

Optic chiasm

Optic tectum

Fish and Amphibia

temporal nasal

opticchiasm

terminalnuclei

superiorcolliculi

NOT

LGNp

m

FRONTAL LOBE

PARIETAL LOBE

re i

ergh

h tp

hmis

eme

isft

h phele re

TEMPORAL LOBE

MT

V4

V3

V2V1

Primates left righthemifield hemifield

Horopter (Vieth-Muller circle)

fix

Basics of Retinal Connections

and Retinal Ganglion Cells

Ciliaryprocess

Retrolental space

Optic axis

Canal ofCloquet

Retina

ScleraChoroid

Lamina cribrosa

Horizontal section of the right human eye.

Nerve & sheath

Macula lutea

Fovea

Vitreous humor

Visual axis

Ora serrataCiliary epithelium

Zonule fibers

Ciliary muscleCanal of Schlemm

ConjunctivaIris

Cornea

Rectustendon

Lens

Anterior Chamber

Posterior chamberLimbal zone

Ciliary

body

Disc

Figure by MIT OCW.

Foveal cone density: 200,000/sqmm5 degrees out: 20,000/sqmm

10 degrees out: 10,000/sqmm

Fovea

Optic nerve

Human RetinaFoveal cone density: 200,000/sqmm

5 degrees out: 20,000/sqmm10 degrees out: 10,000/sqmm

Figure by MIT OCW.

Physiology of retinal ganglion cells

Recording Methods:

electrode in nerveelectrode in eye

fiber hook

optic nerve

Stimulation Methods:

direct light

on

ON cell

time

OFF cell

ON/OFF cell

reflected light

The receptive fields of three major classes of retinal ganglion cells

OFF

inhibition

ON

inhibition

ON/OFF

inhibition

ON

inhibition

Three populations of fibers

Rapid Slower Very slow

Conduction velocity in optic nerve fibers

Figure by MIT OCW.

MIDGET SYSTEM PARASOL SYSTEM

ON OFFON OFF

neuronal response profile

time

Photoreceptors

Distribution of rods and cones on the retina

Num

ber o

f rod

s or c

ones

in a

n ar

ea 0

.006

9 sq

mm

2400

2000

1600

1200

800

400

080 60 40 20 0 20 40 60 80

1.2

1.0

0.8

0.6

0.4

0.2

0

Rel

ativ

e vi

sual

acu

ity

Degrees from the foveaFovea

Distribution of rods and cones along a horizontal meridian. Parallel vertical lines represent the blindspot. Visual acuity for a high luminance as a function of retinal location is included for comparison.

Rods (number)Cones (number)Relative visualacuity in degreesfrom the fovea

Blin

d sp

ot

Figure by MIT OCW.

Some basic facts about the receptor array

1 degree = 200u on retina

Intercone distance in fovea = 2.4u (0.7 min)

200,000 cones per sq.mm. in fovea

20,000 cones per sq.mm. 5 degrees out

Thumbnail at arm's length = 1 degree

The 12 font letter "I" activates about 80 cones at 23 cm

Each rod has 1,000 disks, each with 10,000 molecules

Only 1 of 8 cones is blue. Red and green are equal.

:

Photoreceptor basics:

1. All photoreceptors hyperpolarize to light.

2. Depolarization of the photoreceptor releases glutamate.

3. Photon absorption by the photopigment results in isomerization of the chromophore from 11-cis to all-trans. This causes hyperpolarization thereby reducing neurotransmitter release.

4. Two classes of bipolars are the ON and the OFF. The synaptic junction of OFF bipolars is sign conserving; that of the ON bipolar is sign inverting.

5. The ON bipolar receptor is mGluR6. Its activation leads to closing of channels causing hyperpolarization.

Horizontal cells

Figure by MIT OCW.

A Horizontal cell with regularly arranged dendrites thatis connected to 15 cones in a cone density region of 20.

Amacrine cells

Figure by MIT OCW.

cholinergic cell

All cell

dopaminergic cell

All

Cholinergic

Dopaminergic

Electrical responses in the retina

Figure by MIT OCW.

all hyperpolarize to light

all hyperpolarize to light

some hyperpolarize and some depolarize

some give action potentials

all give action potentials

Ganglion Cells

Bipolar Cell

Receptor

Amacrine Cell

Horizontal Cell

Overview of retinal connections:

Photoeceptors all hyperpolarize to light. They produce only graded potentials. Glutamate is the neurotransmitter.

Horizontal cells all hyperpolarize tolight and produce only graded potentials.

Some bipolar cells depolarize (ON) and some hyperpolarize (OFF) to light. Bipolars produce only graded potentials.

Some amacrine cells produce actionpotentials. There are many classes including ON and OFF.

Ganglion cells produce action potentials.There are many classes including midgetand parasol that come either as ON or OFF.

Figure by MIT OCW.

R

GMGMG

FMB

IMB

A A

IDB RB

I

H

RC

R RC C C

R

Summary:1. In primates the right brain receives input from the left visual hemifield and the left brain from the right hemifield.

2. There are five major classes of retinal cells: photorecptors (rods and cones), horizontal cells, bipolar cells, amacrine cells, and retinal ganglion cells (RGC).

3. The receptive fields of RGCs have antagonistic center/surround organization.

4. There are several classes of RGCs, two of which are (a) the ON and OFF and (b) the Midget and Parasol.

5. All photoreceptors and horizontal cells hyperpolarize to light.

6. There are both hyperpolarizing and depolarizing bipolar cells.

7. Action potentials in the retina are generated only by amacrine and RGC cells.

8. The lateral geniculate nucleus of the thalamus is a laminated structure. What is segregated in the laminae varies with species.

9. The parvocellular layers receive input from the midget cells and the magnocellular layers from the parasol cells. Inputs from the left and right eyes are segregated in the laminae.

10. The receptive field properties of LGN cells are similar to those of the retinal ganglion cells.