The Visual System

LightEnters the eye as electromagnetic radiationTravels in a wave that vary in amplitude (height) and wavelength (between peaks)Amplitude= brightnessWavelength= color (hue)

Humans see a mixture of several wavelengthsVaries puritySmall portion of the spectrum

Parts of the eye

Parts of the EyeCornea (1) transparent covering/protectionPupil and Iris (2) bright light/contracts, dim light/relaxLens (3) focuses light on the retina, accommodationRetina (4) neural tissue that absorbs lightOptic disk & Blind Spot (5)Optic Nerve (6)Fovea (7)Blind Spot Activity!!

The Retina Millions of receptor cells10 % of lightRods- 100 to 125 millionCones- 5-6 millionExpt- What Color is it??

Light and Dark Adaptation

From the Eye to the Brain

Processing in the Visual CortexFeature Detectors- neurons respond to specific features of a visual stimulus (lines, edges, movement)Simple CellsComplex cellsHubel and Wiesels research

Processing in the Visual CortexVentral (what) pathwayTemporal lobeobject identificationDorsal (where) pathwayMotionParietal lobe

Color Theory Psychological interpretation- Blends of three aspects or properties of lightThree theories regarding color

Trichromatic Theory- Young and Von Helmholtz

The eye does the mixing by varying the ratio of neural activity among these 3 typesColor blindnessDichromatsMonochromats

Opponent Process Theory- HeringColor perception depends on receptors that make anatagonistic responses to 3 pairs (red v. green) (yellow v. blue) (white v. black)When excited they respond to one color and when inhibited they respond to the other

Opponent Process Theory- HeringComplementary colors (p.101)

Afterimage

Reconciling TheoryBoth theories correctPerception of color is processed in stagesBeginning stages- trichromaticLater stages- opponent processing

Form PerceptionPerceptual Set - The influence of prior assumptions and expectations on perceptual interpretations

Perceptual SetWhat do you see in the center picture: a male saxophonist or a womans face? Glancing first at one of the two unambiguous versions of the picture is likely to influence your interpretation.

Feature AnalysisBottom Up Processing & Top Down Processing

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Gestalt Principles (p105)Figure GroundProximityClosureSimilaritySimplicityContinuity

Phi PhenomenonThe illusion of motion when fixed lights are turned on and off in a sequenceStare at the X in the middle and notice what happens. Is the Green Dot Moving?

http://www.weeville.com/eyetest.htm

Relationship Between Perceived Size and Perceived DepthTo perceive the size of objects accurately we must also perceive their distance accuratelyThus, many visual illusions occur simply because a particular image lacks sufficient depth cuesThis figure shows that image size depends upon both object size and distance

Depth PerceptionBinocular depth cuesRetinal disparity- objects 25ft project images to different locations on the right & left retina

Monocular Depth CuesLinear Perspective

Motion Parallax

Monocular Depth CuesTexture gradient

Interposition

Monocular Depth CuesRelative SizeHeight in a plane

Size ConstancyPeople are the same size even though their image sizes differThe depth cues such as linear perspective and relative size help the visual system judge the size accurately

Size Distance Relationship

Shape ConstancyThe understanding that an objects shape remains the same even though the angle of view makes the shape appear changed

Shape ConstancyIt is hard to tell if the figure on the upper right is a trapezoid or a square slanted backward.

If we add texture, the texture gradient helps us see that it is actually a square

Brightness ContrastPerceived lightness stays roughly constant as long as the context or surroundings stay the same.When the context changes you may perceive the color as changing.Although the interior squares are in fact identical, we perceive the one as lighter or darker because of the contrast with its surroundings. Want to see more of these perceptions? Goto http://www.skidmore.edu/~hfoley/Perc4.htm#lightcon

Sensory Deprivation & Restored Vision Early visual experience can have a profound effect on perception. Do the kittens ever fully regain normal sensitivity to horizontal or vertical lines? NO.

From the time their eyes first opened, and until the age of 5 months, these kittens were removed from darkness each day to spend 5 hours alone in a black-and-white striped cylinder with a clear glass floor. A stiff collar prevented the kittens from seeing anything else, even their own bodies. Afterward, these kittens had difficulty perceiving horizontal forms, compared with other kittens exposed only to horizontal forms.

Sensory Deprivation & Restored VisionThese experiments show that lacking stimulation, the cortical cells had not developed normal connections making them functionally blind to shape.A sensory restriction does no permanent damage if it occurs later in life. This suggests that visual experiences during infancy are a critical period for normal sensory and perceptual development. Experience guides the organization of the brains neural connections.If deafness or blindness is corrected as an infant, it awakens the pertinent brain area. Nurture sculpts what nature has endowed.

ContextThe setting or environment in which we interpret sensory stimuliCulture can also influence how we perceive information.People actively construct their perceptions by drawing on their prior learning and cultural experiences. People living in urban and industrialized environments where there are more right angles and straight lines will be more susceptible to the Muller-Lyer Illusion than people in non-carpentered natural environments.

Context & CultureWhat is above the womans head? In one study, nearly all the East Africans who were questioned said the woman was balancing a metal box or can on her head and that the family was sitting under a tree. Westerners, for whom corners and boxlike architecture are more common, were more likely to perceive the family as being indoors, with the woman sitting under a window.

IllusionsWhen we misperceive the true characteristics of an object or image. Help researchers understand how sensation and perception normally works

Mller-Lyer IllusionPerceptual psychologists have hypothesized that the top horizontal line looks longer because it also looks farther awaySpecifically, the inward pointing arrows signify that the horizontal line is closest to you, and the outward pointing arrows signify the opposite case

Mller-Lyer Illusion

Mller-Lyer IllusionMost people think segment AB equals BC. In reality AB is much longer than BC.

Mller-Lyer Illusion

Mller-Lyer Illusion:Click below to view a video of how this works:Muller-Lyer Illusion Explanation

Ponzo IllusionConverging lines indicate that top line is farther away than bottom line

The interplay between perceived size and distance (a) The monocular cues for distance make the pursuing monster look larger than the pursued. It isnt. (b) This visual trick, called the Ponzo illusion, is based on the same principle as the fleeing monsters. The two red bars cast identical-sized images on our retinas. But experience tells us that a more distant object can create the same-sized image as a nearer one only if it is actually larger. As a result, we perceive the bar that seems farther away as larger.

Moon IllusionMoon appears larger when it is on the horizon than when it is directly overhead. Objects on the horizon are perceived as farther away than those above us The moon appears to be behind those objects on the horizon. Since it is bigger than those object it is perceived as huge! (click on box below for explanation)Click Below to View an Explanation:Moon Illusion

Ames Room Illusion: Secret RevealedWe perceive the room to be as we are used to, a perfect square or rectangular. When in fact it is a trapezoid!

Poggendorff Illusion

Impossible FiguresThese grouping principles help us construct reality but perceptual contradictions can lead us astray See how this and others like it are done

More Impossible Figures

Eschers Impossible Scenes

ChrysanthemumIs this 3-D?

Heads or Houses?Water or Monks?

Rocks or Horses?In or out of the picture?

***********************Hockenbury powerpoint (Schulman)Key point here: The retinal image is the same for the near and distant objects.For this to happen the distant object must be larger. *****Hockenbury powerpoint (Schulman)

*****Hockenbury powerpoint (Schulman)

*******Hockenbury powerpoint (Schulman)

*******Hockenbury powerpoint (Schulman)

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Documents

The Visual System