Psy 111 Basic concepts in Biopsychologyy p p y gy

Lecture 8: Vision & Audition

Objectives Describe nature and properties of light. Identify the route of light into the eye to the retina. D ib h i i d i i f h i Describe the organization and circuitry of the retina Explain transduction of light in the two types of photoreceptors

(rods and cones); how are they different? Describe the nature of color transduction Describe the nature of color transduction. Explain the inputs to bipolar cells and ganglion cells. Describe the output pathways from the retina. Describe the lateral geniculate nucleus Describe the lateral geniculate nucleus. Discuss the inputs and organization of primary visual cortex. Describe the secondary and association visual cortices; use face

cells example.p Identify the two streams of higher visual processing.

Principles of Sensory SystemsTransduction Mechanism – environmental energy -> biological energy

Receptive fields – each transducing cell or neuron is “receptive” to info from a specific subset of sensory information…often defined anatomicallyanatomically

Topographically Mapping/Arrangement – “shape” of information is maintained by anatomy and encoded at a “population” level.maintained by anatomy and encoded at a population level.

Parallel Pathways – “quality” of information is maintained

Relay Centers – series of projection neurons connect neural networksRelay Centers series of projection neurons connect neural networks

Hierarchical organization – convergence at relays to cortex

Cross Midline it just doesCross Midline – it just does…

Sensory systems

“getting info encoded in the primary sensory cortex”g g p y y

1 Principles of sensory systems1. Principles of sensory systems

2. Chemical sensory systemsy y

3. Vision: the photic sensory system

4. Hearing & Touch: the mechanical sensory systems

Electromagnetic Waves

“Light” is comprised of waves of photonsPhotons are emitted with different energies resulting in different wavelengths and different amounts resulting in differentwavelengths and different amounts resulting in different amplitudes of waves.

The visible spectrumShort Wavelength Long Wavelength

The visible spectrum is a small portion of the total spectrum of wavelength of light. g gKeep in mind we only perceive these as different colors of light; i.e. perception is a creative process of the brain!

Movement of Light

Absorption is how eye detects light energy; ie transfer of

T d tienergy = Transduction

Refraction is critical to pattern of light = shape detection

Structure of the Eye

But we really have two eyes…

Some perceptual processes (and many illusions) in vision result from having two independent sites of visual input:

•Convergence: turning inward to see objects up close

•Binocular disparity: difference in the position of the same image on 2 retinas enables you to construct a 3-D perception from two 2-D images on your retinas

Passage of Light into the Eye

Where light is TRANSDUCED into biological energy (i.e. detected).

Important for focus; uses refraction torefraction to appropriately bend light.

Where info isWhere info is conducted to rest of brain.

Organization of the Retina

Initial path of light

Primary visual

g

Processing

neuron (projection)

Processing

Light detection

Circuitry of the Retina

Ganglion Cells are primarGanglion Cells are primary visual neurons (which are distinct from the photoreceptor cells).

Input to ganglion cells isInput to ganglion cells is highly processed by horizontal, bipolar, and amacrine cellsamacrine cells.

Photoreceptors-detect light (transducer).

Light decreases Na+ conductances

Light causes photoreceptor cell to hyperpolarize.

Transduction of lightCell Membrane Cell Membrane

Photopigment = opsin + retinalPhotopigment = opsin + retinal-Retinal is sensitive to light i.e. light induces conformational change.-Opsin structurally similar to G-protein coupled receptorTransducin (G protein) is similar to G-Transducin (G-protein) is similar to Gi

-cGMP opens a Na+ channel which is closed in the presence of light (low cGMP)

Transduction in Photoreceptors

Photopigment is (roughly) equivalent to a G protein-coupled receptor.

Types of Photoreceptors

Cones have small outer segment (less photosensitive

Rods have large outer segment (lots of photosensitive p

protein) -> low sensitivity to light levels (need lots of

pprotein) -> high sensitivity to light levels (need just a levels (need lots of

light/photons)levels (need just a little light/photons)

Photopigments are contained in stacks in the outer segment

Responsiveness of Rods

Rods account for most of vision under low light (scotopic) conditions. They are much more sensitive to light (activated with less photons) than conesmuch more sensitive to light (activated with less photons) than cones.Broadly-tuned to light wavelengthOnly one type of rod (importance for color sensation/perception)….

Differential Responsiveness of Cones

Cones account for human vision under bright light conditions.Low sensitivity (needs more photons to activate).Relatively narrow-tuning (compared to rods)C h diff i i h diff i ll i d b ifiCones have different opsin proteins that are differentially activated by specific wavelengths of light; allows color vision when info is maintained in separate parallel pathways.

Regional Organization of Retina

Regional organization of:Distribution of photoreceptors hi h & l dcenter: high cones & low rods.periphery: high rods & low cones

Level of convergence in circuitry g ycenter: low convergence periphery: high convergence

Organization at the Fovea

Lateral displacement of non-photoreceptor cells allows high resolution image formation (decreases effect of light passingresolution image formation (decreases effect of light passing through other layers of cells).

Specialization of Fovea

• High density of photoreceptor cells• Low sensitivity photoreceptors (cones)• Differentially sensitive photoreceptor cells• Lateral displacement of retinal layers H d t ti f hi h it lit ti (i diff t Have detection of high acuity, qualitative (i.e. different

light wave lengths) information.

Receptive fields

Entire retina = receptive Area of input for a single cell =Entire retina = receptive surface contains 100,000,000’s+ cells.

Area of input for a single cell = “receptive field” [i.e. physical point where photos come in contact with the

ll i h i i fcell = input to the receptive portion of a photoreceptor cell]

Bipolar Cell Activation

Bipolar cells have input from both photoreceptors and horizontal cells in a “center-surround” fashion. Direct: photoreceptor to bipolar cell causes one response.Indirect: photoreceptor to horizontal cell to bipolar cell causes opposite responseresponse.This is lateral inhibition.Horizontal

cell

Bipolar Cell Activation

Bipolar cell response to photoreceptor transmitter can be depolarization or hyperpolarization

ll ll-OFF-cells versus ON-cells

Horizontal cells have opposite effect on Bipolar cell producing lateral inhibition.

Ganglion Cell Output

Ganglion cells – primary visual neuron (first cells to encode info with APs)•Innervated by Bipolar Cells and Amacrine cellsH “C t S d” (l t l i hibiti ) f ti ti•Have “Center-Surround” (lateral inhibition) of activation.

•Can be “OFF-center” = bipolar cell inhibits ganglion cell.

Ganglion Cell Output

ON-center OFF-center

Ganglion cells – primary visual neuron (encode info with APs)•Can be “ON” or “OFF” center but always have opposite center-surround.

Ganglion Cell Output

Retinal circuitry produces complex ganglion cell responses with maximal firing produce by light-dark borders whichwith maximal firing produce by light dark borders which makes vision highly contrast sensitive.

NB Opposite output pattern would be produced in a “ONNB. Opposite output pattern would be produced in a ON-center” ganglion cell.

Contrast Effects

Which is brighter? Center square on left or on right.Contrast effects are created because retinal output is

sensitive to relative stimulationsensitive to relative stimulation.

Perception is not perfect reflection of reality.

Types of Ganglion Cells

M CellsP Cells

What are the benefits and pitfalls of large receptive fields?

T d ff b t iti it dTrade-off between sensitivity and accuracy.-small receptive field high spatial acuity but only for small area; low sensitivity - need high level of stimulation over small area -large receptive field low spatial acuity but for a large area; but high sensitivity need low level of stimulation summed over large area

Types of Ganglion Cells

M t li l d l ti l f t d hi h l l fM-type ganglion also undergo relatively fast and high level of habituation; most sensitive to dynamic changes in light levels.

Color-opponent P Cell

Differential responses to color in P Ganglion cells are due toDifferential responses to color in P Ganglion cells are due to connections with bipolar and amacrine cells. (i.e. ganglion cells do not detect light but chemical messengers).

l ll f d (Note: M ganglion cells receive input from rods (not sensitive to wavelength of light)

Illusions created by color-opponent processingopponent processing

Color Information Coding

Color information is coded by the relative

i i factivation of three types of photopigments found in cones.

Optic nerves and Chiasm

Formed by axons of ganglion cells.

Non-thalamic targets of optic tract

Some axons from the optic nerve go out of main pathway and relay into the hypothalamus (important for sleep/wake cycle “entrainment”) and superior colliculus (orientation reflex).

Visual Pathways to the Cortex

Visual fields travel to contralateral cortexcortex.Note: fields are not defined by the eyes; cortex receives info from i il t l d t l t lipsilateral and contralateral eye.

Topographical mapping in visual paths

Thalamic relay

M i t f th i di id l l tMaintenance of the individual elements of visual stimulus requires distinct cellular encoding. g

Sensory quality & parallel pathwaysThe ability to capture individual elements of visual images in a precise f i i ffashion is dependent on the number of parallel pathways/distinct cellular relays. y-> information must be encoded by distinct cellular substrates. This is roughly equivalent to pixelThis is roughly equivalent to pixel density versus image quality. (although neural relays also involve convergence to allow more complex cellular information encoding)allow more complex cellular information encoding).

Lateral Geniculate Nucleus: Visual relay in thalamus

Color Vision

Again, we maintain important aspects of stimulus quality by parallel pathways.

Striate Cortex: Primary Visual cortex (V1)

Majority of LGN j yinput goes to layer IV of cortex

V1 Cells respond to specific visual ttpatterns.

Cell respond to a varietyCell respond to a variety of line orientation but maximal responding is i d d l b ifiinduced only by a specific orientation (i.e. pattern of input).p )

Column organization in Primary Visual Cortex.

Cells at same layers of cortex within differentcortex within different columns respond to different types of input; e g ma imal fi ing ithe.g. maximal firing with line of different orientation.

Column organization in Primary Visual Cortex.

Cells at different layers of cortex within a column respond t t f i t i l fi i ith li fto same type of input; e.g. maximal firing with line of same orientation.Column acts as a functional unit.

A word about cortex layers in general

• The cortex consists of 6 layers of ycells

• Each layer consists of different cell types

• Some layers receive information and process it

• Other layers send information following processing; i e cortexfollowing processing; i.e. cortex contains multiple-layer circuits.

• In most cortex, the cells responding to a certain type of information are yparranged in columns; comprises a functional unit

Two streams of visual processing

Dorsal stream is specialized for motion and believed to be involved in determining “where” stimuli are for the “control of behavior”control of behavior

Ventral stream is specialized for attributes and believed to be involved in determing “what” stimuli are for “conscious gperception”.

Cells respond to specific visual patterns.

Cell responds to movement of a stimulus in either direction butCell responds to movement of a stimulus in either direction but maximal responding is induced only by motion in one direction.

Higher cortical processing of vision• Information is then passed on to visual association cortexintegration with o a o s e passed o o v sua assoc a o co e eg a o w

other senses and generation of movement• As you move up the visual hierarchy, the receptive fields become larger and the

information processing is more complex and specializedp g p p• Information then passed on to Association cortex and subcortical

structuresemotion, memory etc

Neurons in the occipital lobeform columns that respond

Neurons in the

to basic shapes. e.g. line orientation

temporal lobe form columns that respond to categories ofcategories of shapes.

Higher cortical processing of vision• As you move up the visual hierarchy, the receptive fields become larger and s you ove up e v su e c y, e ecep ve e ds beco e ge d

the information processing is more complex and specialized • e.g. “Face cells” • may also be specific face cells e g “grandmother cell”• may also be specific face cells e.g. grandmother cell

Sensory systems

“getting info encoded in the primary sensory cortex”g g p y y

1 Principles of sensory systems1. Principles of sensory systems

2. Chemical sensory systemsy y

3. Vision: the photic sensory system

4. Hearing & (Touch): the mechanical sensory systems

Sound Waves & Air Compression

Sound waves = compressions in air produced by vibrations

Properties of Sound WavesProperties of Sound Waves

Human range is 20 –20K Hz

Real sounds are a mixture of waves with different amplitudes and frequencies which give complexity to hearing.

Structure of the Ear

Outer ear serves to focus sound waves and involved in localization of sound

Annimated summary of human ear function:Annimated summary of human ear function: http://www.sumanasinc.com/webcontent/anisamples/neurobiology/soundtransduction.html

The Middle Ear

Middle ear serves to transfer air compressions (of the outer ear) to fluid compressions (of the cochlea).This process also greater amplifies force of wave.

Functional Anatomy of the Cochlea

Cochlea is a pressure-conducting, cone-shaped tube from the Oval to Round windows

Wave formation and Resonance

Air pressure on the Tympanic membrane causes movement of the middle ear with the Stapes causing vibration of the Oval window res lting in fl id a es ithin the Cochlearesulting in fluid waves within the Cochlea.Waves resonate at specific point on the (flexible) Basilar membrane (i.e. specific anatomical site is associated with maximal displacement of membrane).Waves dissipate at Round Window.

Frequency Encoding

Point of resonance is determined by frequency of sound wave, providing a basis for anatomical encoding of pitch., p g g pi.e. more displacement/mechanical force at a point will produce more transduction…

The Inner Ear & Organ of Corti

Th f C ti li b t thThe organ of Corti lies between the (flexible) basilar membrane and the (rigid) tectorial membrane

Cochlea comprises 3 interconnected tubes (“scalae”) and the organ of Corti

The organ of Corti and cochlear waves

Waves move the basilar membrane but not the tectorial membrane resulting in conformational h i th t ili f h i llchanges in the stereocilia of hair cells.

Depolarization of hair cells

M t f b il b hMovement of basilar membrane changes conformation of stereocilia resulting in increased K+ conductance.

Endolymph has usually high K+.

Thus. increased K+ conductance depolarizes cell, opening vg-Ca++ channels, and increases transmitter releaseincreases transmitter release.

Intensity Encoding (Hair Cells)

Sound intensity/loudness (at same frequency) corresponds to height of wave. Higher waves spread out more

l b il balong basilar membrane.

Increased sound pressure (louder noise) produces higher graded receptor potentials in hair cells.

Frequency tuning of auditory neuronsNeurons respond maximally to “characteristic” frequencies.Higher characteristic frequencies correspond toHigher characteristic frequencies correspond to higher APs.

Population encoding is similar to in other senses…

Tonotopic mapping

Spatial location encodes (intermediate to high) frequencies

Central Auditory Pathways•Spiral ganglion cells synapse on complex brain stem circuitsth t d th l i l tthat precede thalamic relay to the cortex.•Brain stem circuits involved in auditory activated reflexes.

Summary of Auditory PathwaysSummary of Auditory Pathways1. Auditory nerve axons2. (Ipsilateral) cochlear nucleus3 S i li (bil t l3. Superior olives (bilateral

connections; sound localization)4. Inferior colliculi (involved in

i ti )orienting)5. Medial geniculate nucleus6. Primary auditory cortex

Auditory vs Visual Systems

thalamus

B i t f l ( th l i ) i fBrain stem neurons perform early (pre-thalamic) processing of auditory info as the retina does for visual info.

Auditory Cortexy2 - 3 areas of primary auditory cortex• Tonotopic organization (encodes frequency)• Tonotopic organization (encodes frequency)

– Functional columns (cells of a column respond to the same frequency)

About 7 areas of secondary auditory cortex• Secondary areas do not

respond well to pure tonesrespond well to pure tones and are poorly characterized

The vestibular system

The “labyrinth” contains the receptor cells for the vestibular system.

Vestibular system activation

Head movement activates the hair cells of the vestibular system.Transduction is similar to auditory system except “environmental” signal is produced by orientation of the body relative to gravity.