1 PS1000 John Beech – School of Psychology. 2 Perception Light, the eye & its pathways Sound and...

transcript

PS1000

John Beech – School of Psychology

Perception

Light, the eye & its pathways

Sound and the ear

The organisation of perception

Properties of light

The properties of light (as a prelude to examining the eye)

• Heating an object creates electromagnetic radiation; its electrons vibrate and this creates the radiation.

• Radiation moves in straight lines and varies in wavelength (between peaks) from several miles to very tiny (e.g. a billionth of a metre) distances.

Properties of light

The human eye is only sensitive to a narrow band of this radiation, which we call light.

Properties of light

Electromagnetic Spectrum

Properties of light

• Light is measured according to its intensity, expressed as the number of photons that are emitted.

• These photons radiate in all directions from a light source, such as a candle. When they strike a surface they are reflected off in a different direction. Most light is indirect, or reflected

Properties of light

Reflectance• Objects in the way

of light absorb the quanta (ie. quantity of electromagnetic radiation), and reflect or refract (bend) them. The table shows reflectance according to the quality of the surface

Type of surface

mirror white black

reflectance Almost perfect

high low

Light and the human eye

How does the eye capture this light?

In the pinhole camera the small aperture has no lens in it, just a pinhole. Light is projected so that the image from outside the box is upside down and in focus.

Light and the human eye

How does the eye capture this light?

When a lens is introduced the hole can be larger and the effect of the lens is to sharpen the image, even though still upside down. Think of the lens as a pair of prisms.

Light passes through this lens and is projected to the back of the retina for processing.

The human eye• The aqueous humour (in the

anterior chamber) changes every 4 hours. Spots before the eye are due to impurities in the eye.

• On the cornea light bends on most of the surface. It has no blood supply and gets nutriment from the aqueous humour.

• The lens has thin layers and is suspended by the zonula, a thin membrane.

• The zonula controls focusing of the eye. By the age of 50 focusing has declined almost completely.

The human eyeThe iris opens to allow in more light. Its range is 16:1. It

tends to be most variable in dim conditions of light, but is hardly affected over higher intensities.

The human eyeThe retina has interconnected

nerve cells, including rods and cones. When we look at something, such as a dot, the dot can be seen sharply. If we look at another point, keeping our head still, our eyes have moved a few degrees of visual angle. This refers to the angle that the eyes would have to move to fixate to the new position.

The light of the dot one fixates on is projected to the fovea.

The blind spot is several degrees of visual angle away. This has no receptors.

The human eye

There are 120 million rod cells and 7 million cone cells in the human eye.

The centre of the fovea covers an area of 1o and consists only of cone cells. These fall away in number rapidly from the centre.

Cones function in daylight and analyse colour.

Rods work in low illumination and give tones of grey. This is known as photopic vision and during twighlight it is called scotopic.

The visual pathways

The optic nerve has one million axons passing from the retina past the underneath of the front of the brain.

A few inches above the mouth, the two optic nerves cross at the optic chiasm.

The axons from the left half of the right eye cross over and those in the right half of the left eye cross over at the optic chiasm.

Brodmann (1914): the information from the two eyes is processed in areas 17, 18 and 19 of the visual cortex. The top picture is the brain from the side and the bottom is a slice through the middle.

Studies have examined individual cells operating here. There is direct topological (point for point) mapping. Thus points in a particular region of the retina register in a corresponding region in the cortex.

The visual cortex has 100 million neurones.

The visual cortex

The topology of the visual cortex was discovered by examining war wounds of the cortex and finding scotoma (dark spots) in vision.

The patient fixates in the centre of a hemisphere and a small light is moved to different points and the patient reports whether it can still be seen.

This produces a map of the size and scale of the scotoma.

Simple cells in the visual cortex

Hubel and Wiesel (1962) monitored single cell activity in area 17 of cats. Then they flashed stimuli on a screen.

They found that simple cells, responded only to white bars on a black background, at a certain angle and location. These are line detectors.

Hubel & Wiesel: The simple cell

The figure shows simple cell receptive fields. The blue lines in the picture are time traces that plot the onset and offset of stimulation. The black vertical lines below them indicate individual nerve impulses.

The most effective stimulus for this particular receptive field is one that puts a lot of light in the excitatory region. It must have the right orientation, the right position, and the right size.

Complex cells in the visual cortex

More complicated cells lie in areas 18 and 19. Complex cells respond only when the bar moves, but location is not so important.

Hypercomplex cells respond to orientation, direction, length, width and shape, in varying degrees.

This suggest a hierarchical mode of processing within the visual cortex, moving from simple analysis in area 17 to more complex analysis deeper back.

Properties of sound

• Sound is similar to mechanical pressure. It is produced by the molecules in air knocking against each other. These produce waves outwards from the sound source.

• Tin tapped: molecules on side jolt and push against those next to them to produce chain reaction. Then after has moved forwards, bounces back. Although wave set in motion, each molecule involved in wave only moves short distance. Sound wave travels through any elastic medium (e.g. air, water).

The tuning fork

Sound wavesThe air close to a

tuning fork produces a sinusoidal change in pressure, or a sine wave.

There is a period of compression followed by rarefaction (= lessening of density).

Sound waves

The wavelength (lamda) is the distance travelled as the wave goes through one harmonic cycle (crest to crest).

Sound wavesAnother important parameter is frequency, or the number of

cycles/sec. (or Hertz Hz) e.g. 1000 Hz is 1000 cycles/sec.

Boyle (1660) showed that sound does not travel through a vacuum. He pumped air out of a jar and a watch ringing inside could no longer be heard.

Most sounds are not a simple sinusoidal function. E.g. guitar string: if entire length produces frequency f, it also divides in two and these produce tone of 2f, this is the second harmonic; the third will be the third harmonic, and so on. So a tone is combination of different harmonics.

SoundAmplitude or loudness of sound corresponds to the force of a

sound on the vibrating surface. This pressure is measured in dynes/square centimetre. Possible to convert to decibels which has effect of transforming to a log. scale.

Most of the noise we hear is in range 60-90 db.

Magnitude of sound Situation

100 db Pneumatic drill at 10 feet away

60 db In a supermarket

25 db Whispering at 5 feet

The ear

Structure of the ear

The ear has 3 sections: outer, middle and inner ear.

Outer can be seen outside skull, esp the pinna. Reflects sounds to auditory canal.

At the end is the ear drum or tympanic membrane. This vibrates with sound.

Behind ear drum is middle ear (filled with air at about 2 cc in vol.).

The inner earWithin are 3 tiny bones (or

ossicles) called the malleus (hammer), the incus (anvil) and the stapes (stirrup).

They are held in place by two muscles: the tensor tympanic and the stapedius. Their function is to transmit vibrations of the ear drum to the inner ear.

The air pressure is equal on both sides of the eardrum. This is done via the eustachian (pron. “U-station”) tubes which open at back of throat. They open when we swallow.

The inner ear

The inner ear: the stapes rests next to a thin membrane called the oval window.

The effect is that the sound waves are amplified 120 times by time oval window reached. When sound within the normal range, stapes has direct contact onto oval window. But if intensive, axis rotates and the effect is reduced.

The ear

Inside cochlea are fluids that produce waves when stapes vibrates. It contains hair cells which convert into nerve impulses.

These impulses are transmitted via auditory nerve to hearing centres.

23,500 hair cells: these analyse pitch and amplitude.

These hair cells are connected to 28k nerve fibres, each of which connected to many hair cells.

Ear bashing

Any sound louder than 85 decibels (dB) will damage your hearing.

1. If you yell at the kids (80dB), this is OK, but in a restaurant it’s (90dB) or if they use a hair dryer (85dB), or make them use a motor mower (98dB) this might be grounds for suing later!

2. Music in gyms (110dB) is as damaging as at a nightclub.

3. A football crowd can reach 123dB.

4. Driving in city traffic with windows down is 100dB.

5. Damage for the rave generation is probably the same as for WW2 veterans.

(Source: The Times; 6-11-2004)

[The information on this slide will NOT be tested by MCQ!]

• Figure-ground perception• The Gestalt psychologists and their laws• Perception of distance• Binocular vision• Size and other constancies• Visual illusions

Figure-ground perception

We have impression of seeing objects against backgrounds.

This impression seems to persist even if what we are looking at is quite complex.

This segregation between objects of figures from their background or ground or perceptual field is the starting off point for the study of organisation in perception.

The outline of the figures is known as its contour. See illustration by M.C. Escher

Figure-ground perception

Rubin (1921) studied figure-ground organisation and suggested that background may have form or shape to a certain extent, but these properties of the background are generally weaker compared to the shapes of the objects.

The organisation in perception is controlled to a certain extent by our expectations and experience.

The gestalt psychologists and their laws

The gestalt psychologists have made an important contribution to our understanding of the laws of organisation.

The word 'gestalt' comes from a German word meaning 'form' or 'whole'.

The three main gestalt psychologists were Max Wertheimer, Kurt Koffka and Wolfgang Kohler.

The gestalt psychologists proposed various laws of perception as follows…

The gestalt psychologists and their laws

• Proximity• Similarity• Closure• Good Continuation• Prägnanz• Figure-ground

The law of proximity

On the left, there appears to be three horizontal rows, while on the right, the grouping appears to be columns.

The law of similarity

There seems to be a triangle in the square.

The law of closure

• Humans tend to enclose a space by completing a contour and ignoring gaps in the figure.

The law of good continuation

Law of Prägnanz • The figure appears to the eye as a square

overlapping triangle, not a combination of several complicated shapes.

The Law of Figure-ground

Perception of distance

Refers to ability to tell how far objects are away from us. It is possible to pick up information concerning depth viewing our world through only one eye.

This is referred to as using monocular cues.

Monocular cues to judge distance are very useful. With one eye can still tell which objects are closest.

Perception of distance

MonocularRelative size

Gradient

Overlap

Height in plane

Others (e.g. motion)

Binocular

Monocular cues

Relative size

Monocular cues

Gradient of texture When things are closer one can see them in much more detail. As things stretch into the distance they become less distinct. This is sometimes a difficult lesson for the beginning artist who tries to put too much detail for things in the distance.

Monocular cues

Overlap: when the first object is placed over a second object, the first object appears closer than the second,

which is partially blocked

Monocular cues

Height in the plane: objects that are higher in our visual field are assumed to be further away. These two objects are the same size, however it appears that the object on the top is larger.

Monocular cues

Other cues are:

perspective: where parallel lines appear to converge in the distance;

shadowing gives details on angles or curves and can show that some parts are closer;

relative motion: when you sway from side to side, nearby objects seem to move more than those in distance.

Monocular cues

All these experiences are forged by our experiences with cues as we have moved round our environment. We learn to represent the two-dimensional image on our retina as 3D.

Binocular cues

The main advantage of seeing with two eyes is that one is viewing from two slightly different angles at same time.

If one looks through a device called a stereoscope one gets quite a compelling sense of depth. In the stereoscope the picture of scene is taken from two slightly different angles.

Each eye sees slightly a different photograph and the viewer consequently has a vivid sense of depth.

Binocular cues

• A binocular cue is convergence. If one looks at object that's 8 metres (about 25 feet) away, eyes must converge to perceive as one object in focus.

Binocular cues

Convergence of eyes produced by creating tension in eye muscles. You've learned from exerting degree of tension on these muscles when focusing on an image, that object must be a certain distance away.

However, beyond about 8 metres convergence is not a factor that operates to determine depth. Thus beyond 8 metres the eyes are focused at infinity.

Size and other constancies

• Size constancy• Emmert’s law• A clinical case (SB)• Anthropological example• Development of constancy• Other constancies

– Shape– Brightness– Location

Size constancy

Perception is affected by stimulus variables. A simple example is the size of an object and its distance.

When object moves further away from us it diminishes in size. E.g. if you are looking at car and walking towards it, your retinal image of car will expand. Thus you inference that the size of car is constant.

To put this slightly differently, when one is examining this object it has a particular size and shape projected onto our retinal wall.

Size constancy

We also perceive that the object is a certain distance away from us. Can we work out accurately the actual size of the object in front of us based on these two variables: retinal size and the perceived distance of the object?

Constancy and Emmert’s law

In a few moments you are going to steadily fixate on a black light bulb for thirty seconds or more. Try not to avert your gaze.

When instructed to do so turn your gaze to the white region on the right adjoining the bulb (or a blank white sheet of paper if you’re sitting near the back).

Constancy and Emmert’s law

Notice that if the afterimage is viewed on a nearby sheet of white paper, it appears relatively small.

If it is viewed on a distant wall, however, it appears to be much larger, even though the size and shape of the image on your retina remains the same.

The perceived size of the afterimage varies directly with the distance of the surface on which it is viewed.

The actual ‘visual angle’ of the afterimage remains exactly the same. So when your brain believes that this afterimage is further away it is thought as larger and if it is thought of as nearer it is thought of as smaller.

This relation is an instance of a more general perceptual relation known as Emmert's law.

Cases of limited size constancy

Some appear not to have size constancy.

There was the case of SB (Gregory 1977) – a 53-year-old patient who had had a cataract operation. Prior to the operation he had been blind. When sight restored there can be substantial problems of adjustment.

At one point he tried to climb out of his hospital window which was 4 storeys up. He thought the ground below was actually quite close. He didn’t at this stage have size constancy.

Cases of limited size constancy

Pygmies living in dense rain forests of the Congo river valley appear to lack size constancy. E.g. Turnbull (1961) describes the experience of a Pygmy called Kenge.

Examples of limited size constancy

Size constancy, at least in the distance within our reach, develops at about 6 months. In one experiment (Yonas, Pettersen & Granrud, 1982) infants were tested with a patch over one eye to eliminate any depth cues.

Infants were 5 or 7 months. Shown photo of large and small face at same time. So the only distance cue to location of face was its size. If they had size constancy the large face would appear near enough to touch, but the small face would seem too far away.

Examples of limited size constancy

5-month-old infants reached out to touch both faces equal amount of time.

But 7-month-old infants, by contrast, reached much more often for larger face.

These children had developed size constancy within this near distance so that they had inferred that the small face was out of reach.

Other constancies

Other constancies develop as the result of experience as well.

Shape constancy: Door appears rectangular when closed, trapezoid when partially open. But we don't think of it as changing shape.

Brightness Constancy

• Brightness with transparency

• Brightness without transparency

Same shade

Brightness constancy refers to objects being perceived as having same brightness independent of prevailing light. E.g. 2 sheets of paper, one grey other white.

Suppose grey put under bright light and white put in shady area, so that grey paper is actually brighter than the white paper. Despite this, white paper still perceived to be brighter.

Thus white is judged always to be bright and grey to be dimmer, no matter actual reflected light.

Location Constancy

Location constancy. When we look at a large object that's relatively close, our eyes move round it. But even though this results in continuous change of the images on our retina, we don't perceive object as moving round.

When employing the single-cell recording technique, researchers have found cells in the brain that only respond when an external stimulus moves. But this same cell does not fire when the eyes move over a static stimulus (Robinson & Wurtz, 1976).

Thus visual system is aware that eyes are moving, but takes this account when monitoring outside world.

Constancy theory can be used to explain some visual illusions.

Visual illusions

An illusion refers to perception of something that's false or distorted. One must guard against mixing this up with illusions that are optical illusions. Some examples of optical illusions: stick appears to bend in water, a distorted mirror. These due to a change in the stimulus itself before it reaches our perceptual system.

Psychologists find perceptual illusions interesting. Geometrical illusions are a large class of perceptual illusions. These are line drawings that appear distorted in some way, e.g., length or curvature of lines.

Visual illusions

• Ponzo• Muller-Lyer• Poggendorff

Visual illusions

Most illusions induced because they resemble three-dimensional scenes. The lines and curves are providing cues to size and distance. E.g. one with the slanting lines and the two horizontal lines is called the Ponzo illusion. Slanted lines probably viewed like railway lines and horizontals as sleepers. Thus there's a cue that the top horizontal line is further away. It's perceived to be larger.

Visual illusions

The figure below is an adaption of the Muller-Lyer illusion. You probably experience the horizontal line apparently changing its length, when it actually remains the same.

Visual illusions

In the case of Muller-Lyer illusion, these can be imagined to be solid surfaces. In the top one, with the arrows pointing outwards, it has been argued that represents inside of something (e.g. a room). But in bottom one surfaces suggests looking on outside of a shape

Visual illusions

This perspective suggests that the one with outward pointing arrows is further away and other one is nearer. Again, using constancy principles, the two lines are actually equal in length, but because one is perceived as being as further away, this is perceived as being larger than the other.

Visual illusions

Muller-Lyer: more research has been done on this illusion than for other illusions.

In one experiment: image of arrows fixed on retina by photo flash. The illusion remained, with one stem appearing to be larger than the other.

This experiment tested if the difference in perceived size was produced by changes in eye movement patterns across the two types of arrow. However, as the illusion still remained when the image was stabilised on the retina, this particular theory was ruled out.

Visual illusions

The Poggendorff illusion is more difficult to see as 3D. Some have argued that it can be viewed as part of something like a chair. And if this were a real chair the slanting lines could never meet. But the slanting lines are actually aligned with each other. In this left-hand picture below – the chair is very difficult to imagine.

Visual illusions: conclusion

Thus summarising principle that seems to emerge from the illusions: suppose that something is perceived to be further away and is of the same size as another object or line. It is seen to be larger than it would be normally.

Learning probably has an important influence on the perception of constancy. E.g. if we keep looking at illusions, we gradually learn that we are being deceived and they lose effect.

1 PS1000 John Beech – School of Psychology. 2 Perception Light, the eye & its pathways Sound and...

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