The Retina - 2

Acuity

DEFINITION: ability to resolve or see fine details.

Visual Angle: DEFINITION: Angle formed by object on retina.

Types of Acuity: what is meant by acuity depends upon the stimulus used to measure it.

Detection: black bar on white field

Resolution: a grating

Recognition: e.g. Snellen, where

you read letters.

Acuity - 2

Measures of Acuity

20/20: can see at 20’ what a normal person can see at 20’.

This is normal, not perfect, vision.

20/200: can see at 20’ what a normal person can see at 200’.

Visual angle of the critical feature in a test, e.g. the width of the bars in a grating.

A typical population average is 1 arcmin (1/60 degree).

Acuity and Retinal Location:

Best at fovea. Falls off rapidly in periphery. Is tied to density of cones.

Eye Movements

We move our eyes because of the limited field with good acuity.

There are 6 eye muscles

4 rectus

2 oblique

Types of Eye Movements

Version: Both eyes move together

Vergence: Eye move in opposite directions

Eye Movements - 2

Version

Saccades, most common

places object on fovea.

can be > 400 deg/sec.

Takes ~ 200 msec to begin

Smooth Pursuit

track moving objects

relatively slow ~30 deg/sec.

Vergence:

Convergence (together) and divergence (apart)

Allows us to look at closer and farther objects.

relatively slow and also takes about 200 msec to begin


	DEFINITION: ability to resolve or see fine details.
	Visual Angle: DEFINITION: Angle formed by object on retina.
	Types of Acuity: what is meant by acuity depends upon the stimulus used to measure it.
		Detection: black bar on white field
		Resolution: a grating
		Recognition: e.g. Snellen, where
		you read letters.


Measures of Acuity
	20/20: can see at 20’ what a normal person can see at 20’.
		This is normal, not perfect, vision.
	20/200: can see at 20’ what a normal person can see at 200’.
	Visual angle of the critical feature in a test, e.g. the width of the bars in a grating.
		A typical population average is 1 arcmin (1/60 degree).
Acuity and Retinal Location:
	Best at fovea. Falls off rapidly in periphery. Is tied to density of cones.


	We move our eyes because of the limited field with good acuity.
	There are 6 eye muscles
		4 rectus
		2 oblique
	Types of Eye Movements
		Version: Both eyes move together
		Vergence: Eye move in opposite directions


Version
	Saccades, most common
		places object on fovea.
		can be > 400 deg/sec.
		Takes ~ 200 msec to begin
	Smooth Pursuit
		track moving objects
		relatively slow ~30 deg/sec.
Vergence:
	Convergence (together) and divergence (apart)
		Allows us to look at closer and farther objects.
		relatively slow and also takes about 200 msec to begin