Chapter Six: VisionAssorted Materials from Modules 1-3

Chapter Seven: AuditionModule 1

Visual Coding and Retinal Receptors

Reception- absorption of physical energy (electromagnetic waves) by receptors

Transduction-the conversion of physical energy to an electrochemical pattern in the neurons

Coding- one-to-one correspondence between some aspect of the physical stimulus and some aspect of the nervous system activity

Visual Coding and Retinal Receptors

From Neuronal Activity to Perception coding of visual information in the brain does not duplicate the

stimulus being viewed General Principles of Sensory Coding

Muller and the law of specific energies-any activity by a particular nerve always conveys the same kind of information to the brain

Now close your lid and poke your eye… do you see light (fosphenes)(sp?)

Qualifications of the Law of Specific Energies the rate of firing or pattern of firing may signal independent stimuli timing of action potentials may signal important information

indicating such things as movement the meaning of one neuron depends on what other neurons are

active at the same time

Visual Coding and Retinal Receptors-“Look into My Eyes!”

The Eye and Its Connections to the Brain Pupil-opening in the center of the eye that allows

light to pass through Lens-focuses the light on the retina Retina-back surface of the eye that contains the

photoreceptors The Fovea-point of central focus on the retina The Route Within the Retina

photoreceptors-rods and cones bipolar cells-receive input from rods and cones ganglion cells-receive input from bipolar cells optic nerve-made up of axons of ganglion cells blind spot-the point where the optic nerve leaves the eye

Figure 6.2  Cross section of the vertebrate eyeNote how an object in the visual field produces an inverted image on the retina.

Figure 6.4  Visual path within the eyeballThe receptors send their messages to bipolar and horizontal cells, which in turn send messages to the amacrine and ganglion cells. The axons of the ganglion cells loop together to exit the eye at the blind spot. They form the optic nerve, which continues to the brain.

Receptive Field- the part of the visual field to which any one neuron responds

Key Point: Fields have both excitatory and inhibitory regions•The receptive field of a receptor is simply the area of the visual field from which light strikes that receptor. •The receptive field for any other neurons in the visual system is determined by which receptors connect to the cell in question.

Figure 6.6  Two demonstrations of

the blind spot of the retina

Close your left eye and focus your right eye on

the o in the top part. Move the page toward you and away, noticing what happens to the x. At a distance of about 25 cm (10 inches), the

x disappears. Now repeat this procedure

with the bottom part. At that same distance what do you see?

Visual Receptors: Rods and Cones, Reception & Transduction continued

Rods-abundant in the periphery of the retina

-best for low light conditions

-see black/white and shades of gray

Conesabundant around fovea

best for bright light conditions

see color

Table 6.1 is very good

Transduction

Both Rods and Cones contain photopigments (chemicals that release energy when struck by light) 11-cis-retinal is transformed into all-trans-retinal in light

conditions

this results in hyperpolarization of the photoreceptor

the normal message from the photoreceptor is inhibitory

Light inhibits the inhibitory photoreceptors and results in depolarization of bipolar and ganglion cells

Theories Color VisionThe Trichromatic (Young-Helmholtz) Theory

KEY POINT: We perceive color through the relative rates of response by three kind of cones, each kind maximally sensitive to a different set of wavelengths, but receptors are not equally distributed across retina. (exercises)

Bowmaker & Dartnall (1980) projected a known amount of light directly through the outer segments of photoreceptors and measured how much light was absorbed by the photopigment molecules.

They found four classes of photopigments. The wavelength of maximum absorbance is indicated at the top of each curve. The 420 curve is for the short wavelength cones (blue), the 498 curve is for the rods, and the 534 and 564 curves are for the middle (green) and long wavelength (red) sensitive cones respectively.

More Theories of Color Vision

The Opponent-Process Theory we perceive color in terms of paired opposites

RED vs. GREEN; YELLOW vs. BLUE; BLACK vs. WHITE one color the result of excitation, the other the result of inhibition

of bipolar cells

Can explain negative color after-image effects The Retinex Theory

When information from various parts of the retina reaches the cortex, the cortex compares each of the inputs to determine the brightness and color perception for each area.

can explain color constancy only works when entire view has been tinted

Figure 6.12  Possible wiring for one bipolar cellShort-wavelength light (which we see as blue) excites the bipolar cell and (by way of the intermediate horizontal cell) also inhibits it. However, the excitation predominates, so blue light produces net excitation. Red, green, or yellow light inhibit this bipolar cell because they produce inhibition (through the horizontal cell) without any excitation. The strongest inhibition is from yellow light, which stimulates both the long- and medium-wavelength cones. Therefore we can describe this bipolar cell as excited by blue and inhibited by yellow. White light produces as much inhibition as excitation and therefore no net effect. (Actually, receptors excite by decreasing their usual inhibitory messages. Here we translate that double negative into excitation for simplicity.)

Neural Basis of Visual Perception An Overview of the Mammalian Visual System

Rods and Cones synapse to amacrine cells and bipolar cells

Bipolar cells synapse to horizontal cells and ganglion cells

Axons of the ganglion cells leave the back of the eye

The inside half of the axons of each eye cross over in the optic chiasm

Pass through the lateral geniculate nucleus

Transferred to visual areas of cerebral cortex

Neural Basis of Visual Perception

Concurrent Pathways in the Visual System In the Retina and Lateral Geniculate

Two categories of Ganglion cells– Parvocellular-smaller cell bodies and small receptive

fields, located near fovea; detect visual details, color– Magnocellular-larger cell bodies and receptive fields,

distributed fairly evenly throughout retina; respond to moving stimuli and patterns

In the Cerebral Cortex V1-Primary Visual Cortex-responsible for first stage visual

processing V2-Secondary Visual Cortex-conducts a second stage of

visual processing and transmits the information to additional areas

Ventral stream-visual paths in the temporal cortex Dorsal stream-visual path in the parietal cortex

Figure 6.20  Three visual pathways in the cerebral cortex(a) A pathway originating mainly from magnocellular neurons. (b) A mixed magnocellular/parvocellular pathway. (c) A mainly parvocellular pathway. Neurons are heavily connected with other neurons in their own pathway but only sparsely connected with neurons of other pathways. Area V1 gets its primary input from the lateral geniculate nucleus of the thalamus; the other areas get some input from the thalamus but most from cortical areas. (Sources: Based on DeYoe, Felleman, Van Essen, & McClendon, 1994; Ts’o & Roe, 1995; Van Essen & DeYoe, 1995)

Neural Basis of Visual Perception- The Cerebral Cortex: The Shape Pathway

Hubel and Wiesel’s Cell Types in the Primary Visual CortexSimple Cells

has fixed excitatory and inhibitory zones in its receptive field

Complex Cellsreceptive fields cannot be mapped into fixed

excitatory and inhibitory zonesRespond to a pattern of light in a particular

orientation

Figure: The receptive field of a complex cell in the visual cortex: -It is like a simple cell in that its response depends on a bar of light’s angle of orientation. -It is unlike a simple cell in that its response is the same for a bar in any position within the receptive field.

Neural Basis of Visual Perception- The Columnar Organization of the Visual Cortex

Columns are grouped together by function– Ex: cell within a given column respond best to lines of

a single orientation Are Visual Cortex Cells Feature Detectors?

Feature Detectors-neurons whose responses indicate the presence of a particular feature

Shape Analysis Beyond Areas V1 and V2 Inferior Temporal Cortex (V3)-detailed information about

stimulus shape (V4)-Color Constancy; Visual Attention (V5)-Speed and Direction of Movement

Neural Basis of Perceptual Disorder

Disorders of Object Recognition Visual Agnosia-Inability to Recognize Objects Prosopagnosia-Inability to recognize faces

Color Vision Deficiencies Complete and Partial Color Blindness-inability to perceive

color differences Generally results from people lacking different subsets of

cones genetic contributions- same photopigment made on

medium and longwave wavelength receptors

Neural Basis of Visual Perception-The Cerebral Cortex

The Cerebral Cortex: The Color Pathway Parvocellular to V1 (blobs) to V2, V4, and Posterior Inferior

Temporal Cortex The Cerebral Cortex: The Motion and Depth Pathways

Structures Important for Motion Perception Middle-temporal cortex-V5-speed and direction of

movement Motion Blindness-Inability to detect objects are moving

Experience and Visual Development

Early Lack of Stimulation of One Eye-blindness occurs in that one eye

Early Lack of Stimulation of Both Eyes-if this occurs over a long period of time, loss of sharp receptive fields is noted

Restoration of Response and Early Deprivation of Vision-deprive stimulation of the previously active eye and new connections will be made with the inactive eye

Uncorrelated Stimulation in Both Eyes-each cortical neuron becomes responsive to the axons from just one eye and not the other

Experience and Visual Development

Early Exposure to a Limited Array of Patterns—most of the neurons in the cortex become responsive only to the patterns that the subject has been exposed to

Lack of Seeing Objects in Motion-become permanently disable at perceiving motion

Effects of Blindness on the Cortex-parts of the visual cortex become more responsive to auditory and tactile stimulation

Chapter SevenThe Nonvisual Sensory Systems- Auditory SystemModule One

Audition Sound and the Ear

Physical and Psychological Dimensions of Sound Amplitude=intensity of

wave=loudness frequency=number of

waves/second=pitch

Figure 7.1  Four sound wavesThe time between the peaks determines the frequency of the sound, which we experience as pitch. Here the top line represents five sound waves in 0.1 second, or 50 Hz—a very low-frequency sound that we experience as a very low pitch. The other three lines represent 100 Hz. The vertical extent of each line represents its amplitude or intensity, which we experience as loudness.

Anatomy of the Ear Structures of the Ear

Pinna-cartilage attached to the side of the head

Tympanic Membrane-eardrum middle ear

bones-hammer/anvil/stirrup oval window-membrane leading to

inner ear cochlea-three fluid-filled tunnels

scala vestibuli scala media scala tympani

basilar membrane-flexible membrane

tectorial membrane-rigid membrane

hair cells-auditory receptorsFigure 7.2  Structures of the earWhen sound waves strike the tympanic membrane in (a), they cause it to vibrate three tiny bones—the hammer, anvil, and stirrup—that convert the sound waves into stronger vibrations in the fluid-filled cochlea (b). Those vibrations displace the hair cells along the basilar membrane in the cochlea. (c) A cross section through the cochlea. The array of hair cells in the cochlea is known as the organ of Corti. (d) A closeup of the hair cells.

Pitch Perception Theories of Pitch Perception

Frequency theory- the basilar membrane vibrates in synchrony with a sound,

causing auditory nerve axons to produce action potentials at the same frequency

Place theory- the basilar membrane resembles the strings of a piano in that

each area along the membrane is tuned to a specific frequency and vibrates whenever that frequency is present

Volley principle- the auditory nerve as a whole can have volleys of impulses up

to about 5,000 per second, even though no individual axon can approach that frequency by itself

Figure 7.4  The basilar membrane of the human cochleaHigh-frequency sounds produce their maximum displacement near the base. Low-frequency sounds produce their maximum displacement near the apex.

Figure 7.5  Traveling waves in the basilar membrane set up by different frequencies of soundNote that the peak displacement is closer to the base of the cochlea for high frequencies and is toward the apex for lower frequencies. In reality the peak of each wave is much narrower than shown here.

Pitch Perception in the Cerebral Cortex

Primary auditory cortex Each cell responds best to one tone Cells preferring a given tone cluster together

Secondary auditory cortex Each cell responds to a complex combination of sounds

Figure 7.6  Route of auditory impulses from the receptors in the ear to the auditory cortex

The cochlear nucleus receives input from the ipsilateral ear only (the one on the same side of the head). All later stages have input originating from both ears.

Hearing Loss

Conductive Deafness bones of the middle ear fail

caused by tumors, infection, disease

usually corrected by surgery or hearing aids

Nerve Deafness damage to cochlea, hair cells or auditory nerve

usually treated with hearing aids

caused by genetics, disease, ototoxic drugs, etc.

Localization of Sound

Sound Shadow-loudest in nearest ear

Time of arrival-arrives at one ear soonest

Phase difference-sounds arrive out of phase dependent on frequency

Figure 7.10  Phase differences between the ears as a cue for sound localizationNote that a low-frequency tone (a) arrives at the ears slightly out of phase. The ear for which the receptors fire first (here the person’s left ear) is interpreted as being closer to the sound. If the difference in phase between the ears is small, then the sound source is close to the center of the body. However, with a high-frequency sound (b) the phase differences become ambiguous. The person cannot tell which sound wave in the left ear corresponds to which sound wave in the right ear.