Chapter SixChapter Six

Vision

Sight begins with Light• The advantages of electromagnetic radiation

(Light) as a stimulus are…– Electromagnetic energy is abundant, travels VERY quickly and

in fairly straight lines

• The Electromagnetic Spectrum– Range of energy visible to humans falls between 400 and 700

nanometers. Wavelength determines the color we see.

• Absorption, reflection and refraction– Absorption and reflection influence the colors we see– Air and water refract, or change the direction, of traveling

waves of light– Wave height or amplitude determines how bright light

appears to us.

Figure 6.1 The Visual World of Dog and Human

Figure 6.2 Dimensions of Electromagnetic Radiation

Figure 6.3 The Electromagnetic Spectrum

Figure 6.4 Light Interacts with the Environment

The Structures & Functions of the Visual System

• Protection for the Eyes– Located in bony orbit of the skull, cushioned by fat

– Eyelids and blinking

– Tears

• Eyes in front of our heads offers us a limited range of view, but allows for depth perception

• The Anatomy of the Eye

• The Sclera is the white tough outer coating that helps our fluid filled eyes keep their shape

• The Cornea – curved and transparent (no blood supply) first level of focus for incoming light

More Eye Anatomy!

• Anterior Chamber – provides nutrients to the Cornea – fluid is aqueous humor

• The Pupil – an opening in the Iris – manages incoming light levels. Color from melanin.

• The Lens – focus point #2 – also no blood supply so transparent. Allows for accommodation.

• The Vitreous Chamber – The major interior chamber. Has a blood supply, but vessels are stationary & are therefore ignored.

• The Retina – where transduction takes place

Figure 6.6 The Structure of the Eye

More stuff about the Retina• The Optic Disc is where the optic nerve leaves

• The Macula is a central area where there is a higher concentration of photoreceptors

• Within the macula is the Fovea (small pit) with a VERY high concentration of Cones

• Cones are the photoreceptors designed to pick up fine detail, and also color information

• Rods are scattered around the remainder of the retina, and they do B&W and work in dim light

• An epithelium layer picks up stray random light

Figure 6.7 Landmarks of the Retina

More about the retina & photoreceptors

• The Layered Organization of the Retina• Ganglion cell layer: axons here make up the optic nerve

• Next are the Amacrine cells, & then Bipolar Cells

• Finally we get to Horizontal Cells & Photoreceptors

• Rods (120 million) provide scotopic vision (seeing in dim light). Photopigment is Rhodopsin. Scattered around the periphery of the retina.

• Cones (6 million) provide photopic vision (seeing in bright light). Color and detail processing here! Most concentrated in the fovea.

• Transduction by photoreceptors – process of transmutting physical stimulus into electrical signals

Figure 6.9 The Structure of the Retina

Figure 6.10 Rods and Cones

Table 6.1 Scotopic and Photopic Vision

How to Rods & Cones Transduce Light?

• Differences between Rods and Cones• Photopigments each have different peak sensitivities

• Three types of cones are:

• Blue/short wavelength

• Green/middle wavelength

• Red/long wavelength

• Rhodopsin in rods is most sensitive to bluish-green wavelengths

• Rods and cones need different amounts of light to respond. They release the neurotransmitter Glutamate

• They produce graded (not action) potentials

Figure 6.12 The Responses of Rods and Cones to Different Wavelengths

Photoreceptors pass along their info how?

• Processing by Retinal Interneurons• Horizontal Cells – form connections with

photoreceptors (pick up the graded potentials) and also bipolar cells

• Bipolar Cells• Receptive fields allow for the “compression” of lots of ‘data’

• Antagonistic center-surround organization

• Lateral inhibition helps the system figure out where boundaries are

• Amacrine Cells – form connections between bipolar, ganglion, and other amacrine cells

• Ganglion Cells – receive input from bipolar, amacrine cells & replicate the info passed to them by bipolar cells

• Three types of Ganglion Cells – M, P, and K cells

Figure 6.13 A Retinal Bipolar Receptive Field

Table 6.2 The Three Types of Ganglion Cells

Data’s been transduced. Now what?

• Optic Nerve Connections• Ganglion cell axons exit the eye through the optic disk,

forming an optic nerve leaving each eye

• Left & Right visual field info crosses & is reconstituted at the Optic Chiasm. Most (80%) data heads off to the

• Lateral Geniculate Nucleus of the Thalamus which has distinct stacked layers. From there it heads to the

• Primary Visual Cortex (Striate) in the Occipital Lobe.

• A little data also goes to the The Superior Colliculus • In humans used to guide movements of the eyes and head toward

newly detected objects

Figure 6.16 The Pathways from the Eye to Cortex

Figure 6.17 The Lateral Geniculate Nucleus (LGN)

Are we actually seeing anything yet?

• The Striate Cortex – located in the occipital lobe begins the process of perception

• 6 Layers of neurons analyze the incoming stuff

• Simple Cells respond to lines, bars & orientation

• Complex cells respond to motion and direction

• Cortical Columns include hyper columns, occular columns and orientation columns

• Let’s not forget Oxidase Blobs (for color), and lastly

• Cortical Modules that put it all together for us

Figure 6.18 The Striate Cortex of the Occipital Lobe

Figure 6.20 Cortical Modules

The Primary Visual cortex has help!

• Visual Analysis Beyond the Striate Cortex• At least a dozen other areas of human cerebral

cortex participate in visual processing

• The Dorsal Stream (A.K.A. the “Where” Stream)

• Essential for analyzing movement, object locations, and for ‘eye hand coordination’, like grasping

• The Ventral Stream (A.K.A. the “What” Stream)

• Responds to shapes and colors. Helps us recognize objects. This stream also includes the

• Fusiform Face Area (specializes in recognizing faces)

Figure 6.21 The Ventral “What” Stream and the Dorsal “Where” Stream

Figure 6.22 The Fusiform Face Area

Visual Perception Theories

• Hierarchies– Simple cells input to increasingly complex cells

– Feature detectors

• Spatial Frequencies– Gratings: simplest patterns of lines

– Contrast Sensitivity function

• How do we perceive depth?– Monocular cues – convergence, texture gradients, position

– Binocular depth cues – retinal disparity

• How do we perceive color?– Trichromatic theory works at the receptor level

– Opponent Process theory works at the ganglion level

Figure 6.23 Problems for the Hierarchical Model of Vision

Figure 6.24 Spatial Frequencies

Visual Perceptual Processes Continued

• Color Blindness?– Dichromacy (R or G photopigment issues) – much

more common in males; issue on the X chromosome

– Monochromacy (True color blindness)

– Tetrachromats have an extra pigment (PINK!)

• Color Constancy

• Color Contrast

• Primary colors of LIGHT are Red, Green, Blue, and together they mix to white

Figure 6.27 Mixing Lights

Figure 6.29 Opponency

Figure 6.30 Looking Through the Eyes of a Dichromat

Figure 6.31 Color Contrast

Visual System Development & Issues

• Newborns and very young kids have limited ability to see detail at a distance

• Presbyopia – “old sight”: reduced ability of the lens to provide accommodation

• Cataracts – a clouding of the lens

• Myopia – elongated eye shape – nearsighted

• Hyperopia – shortened eye – farsighted

• Astigmatism – unevenly shaped cornea

• Blindness – eye or optic nerve damage, or striate cortex damage (scotoma)

Figure 6.32 The Development of Contrast Sensitivity

Figure 6.34 Eyeball Shape Influences the Quality of Vision