The Visual Pathway Sensory Systems What is a Sensory System? –Window to the physical energies of the external environment. –Gives rise to sensory perceptions

The Visual PathwayThe Visual Pathway

Sensory SystemsSensory Systems

What is a Sensory System?What is a Sensory System?

– Window to the physical energies of the Window to the physical energies of the external environment.external environment.

– Gives rise to sensory perceptions.Gives rise to sensory perceptions.


Several different modalitiesSeveral different modalities– VisionVision– AuditionAudition– SomatosensorySomatosensory– TasteTaste– SmellSmell

Laws of Specific Sense EnergiesLaws of Specific Sense Energies– Johannes Muller (1826) Johannes Muller (1826) – Labeled pathwaysLabeled pathways


Sensory ReceptorSensory ReceptorA specialized neuron that detects a particular A specialized neuron that detects a particular category of physical events.category of physical events.

– Sensory transductionSensory transductionThe process by which sensory stimuli are The process by which sensory stimuli are transduced into slow, graded receptor potentials.transduced into slow, graded receptor potentials.

– Receptor potentialReceptor potentialA slow, graded electrical potential produced by a A slow, graded electrical potential produced by a receptor cell in response to a physical stimulus.receptor cell in response to a physical stimulus.


Sensory ReceptorSensory Receptor– Some transduce and encode Some transduce and encode (e.g. somatosensory, olfaction)(e.g. somatosensory, olfaction)

– Some only transduce Some only transduce (e.g. vision, audition, taste)(e.g. vision, audition, taste)

– The region of the sensory surface that modulates the The region of the sensory surface that modulates the activity of sensory neurons is called the activity of sensory neurons is called the Receptive Receptive FieldField of the neuron.of the neuron.

The Visual SystemThe Visual System

Functions of the Visual System:Functions of the Visual System:– Answers two questionsAnswers two questions

What is it? (recognition)What is it? (recognition)

Where is it? (location)Where is it? (location)

– Transforms a distorted and upside-down 2-D Transforms a distorted and upside-down 2-D retinal image into the 3-D world we retinal image into the 3-D world we perceiveperceive..

The Visual SystemThe Visual System

Does the visual system create an exact Does the visual system create an exact copy of the external world?copy of the external world?– No!No!

The visual system creates a The visual system creates a perceptionperception of of reality.reality.– Chaplin MovieChaplin Movie

Visual IllusionsVisual Illusions





Visible LightVisible Light

Properties of LightProperties of Light– Wavelength – perception of colorWavelength – perception of color– Intensity(amplitude) – perception of brightnessIntensity(amplitude) – perception of brightness

Figure 6.2

The electromagnetic spectrum: colors and wavelengths visible to humans

Figure 6.4

The EyeThe Eye

LensLens – focuses – focuses light on the light on the retinaretina

Ciliary muscles Ciliary muscles alter the shape of alter the shape of the lens as needed the lens as needed

AccommodationAccommodation – – the process of the process of adjusting the lens adjusting the lens to bring images to bring images into focusinto focus

Figure 6.4

The BlindspotThe Blindspot

Eye Position and Binocular Eye Position and Binocular DisparityDisparity

ConvergenceConvergence – eyes must turn slightly inward when objects are closeeyes must turn slightly inward when objects are close

Binocular disparityBinocular disparity– difference between images on the two retinasdifference between images on the two retinas

Both are greater when objects are closeBoth are greater when objects are close– provides brain with 3-D image and distance provides brain with 3-D image and distance

informationinformation

The RetinaThe Retina

The retina is in a sense “inside-out”The retina is in a sense “inside-out”– Light passes through several cell layers Light passes through several cell layers

before reaching its receptorsbefore reaching its receptors

LIGHT-> retinal ganglion cells -> bipolar LIGHT-> retinal ganglion cells -> bipolar cells -> receptors cellscells -> receptors cells– Lateral communicationLateral communication

Horizontal cellsHorizontal cellsAmacrine cellsAmacrine cells

Figure 6.5

Duplexity theory of visionDuplexity theory of vision– cones and rod mediate different kinds of visioncones and rod mediate different kinds of vision

– Cones – photopic (daytime) visionCones – photopic (daytime) visionHigh-acuityHigh-acuityColor information in good lightingColor information in good lighting

– Rods – scotopic (nighttime) visionRods – scotopic (nighttime) visionHigh-sensitivity, low-acuity vision in dim lightHigh-sensitivity, low-acuity vision in dim lightLacks detail and color informationLacks detail and color information

Cone and Rod VisionCone and Rod Vision

Figure 6.8

Cone and Rod VisionCone and Rod Vision

More More convergence in convergence in rod system, rod system, increasing increasing sensitivity while sensitivity while decreasing decreasing acuityacuity

Only cones are Only cones are found at the found at the foveafovea

Distribution of rods and cones

Figure 6.9

SpectralSpectral SensitivitySensitivity

Lights of the same intensity but different Lights of the same intensity but different wavelengths may not all look as brightwavelengths may not all look as bright

A A spectral sensitivity curvespectral sensitivity curve shows the shows the relationship between wavelength and brightnessrelationship between wavelength and brightness

There are different spectral sensitivity curves for There are different spectral sensitivity curves for photopic (cone) vision and scotopic (rod) vision photopic (cone) vision and scotopic (rod) vision

Human photopic

and scotopic spectral

sensitivity curves

Figure 6.10

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The Visual Pathway Sensory Systems What is a Sensory System? –Window to the physical energies of the external environment. –Gives rise to sensory perceptions