HUMAN INPUT-OUTPUT CHANNELS, VISUAL PERCEPTION

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Human Computer Interaction (CS408)

Lecture

Lecture 7. Human Input-Output Channels

Part I

Learning Goals

As the aim of this lecture is to introduce you the study of Human Computer

Interaction, so that after studying this you will be able to:

Understand role of input-output channels

Describe human eye physiology and

Discuss the visual perception

7.1 Input Output channels

A person's interaction with the outside world occurs through information being

received and sent: input and output. In an interaction with a computer the user

receives information that is output by the computer, and responds by providing input

to the computer the user's output become the computer's input and vice versa.

Consequently the use of the terms input and output may lead to confusion so we shall

blur the distinction somewhat and concentrate on the channels involved. This blurring

is appropriate since, although a particular channel may have a primary role as input or

output in the interaction, it is more than likely that it is also used in the other role. For

example, sight may be used primarily in receiving information from the computer, but

it can also be used to provide information to the computer, for example by fixating on

a particular screen point when using an eye gaze system.

Input in human is mainly though the senses and out put through the motor control of

the effectors. There are five major senses:

· Sight

· Hearing

· Touch

· Taste

· Smell

Of these first three are the most important to HCI. Taste and smell do not currently

play a significant role in HCI, and it is not clear whether they could be exploited at all

in general computer systems, although they could have a role to play in more

specialized systems or in augmented reality systems. However, vision hearing and

touch are central.

Similarly there are a number of effectors:

· Limbs

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Fingers

Eyes

Head

Vocal system.

In the interaction with computer, the fingers play the primary role, through typing or

mouse control, with some use of voice, and eye, head and body position.

Imagine using a personal computer with a mouse and a keyboard. The application you

are using has a graphical interface, with menus, icons and windows. In your

interaction with this system you receive information primarily by sight, from what

appears on the screen. However, you may also receive information by ear: for

example, the computer may `beep' at you if you make a mistake or to draw attention

to something, or there may be a voice commentary in a multimedia presentation.

Touch plays a part too in that you will feel the keys moving (also hearing the `click')

or the orientation of the mouse, which provides vital feedback about what you have

done. You yourself send information to the computer using your hands either by

hitting keys or moving the mouse. Sight and hearing do not play a direct role in

sending information in this example, although they may be used to receive

information from a third source (e.g., a book or the words of another person) which is

then transmitted to the computer.

7.2 Vision

Human vision is a highly complex activity with range of physical and perceptual

limitations, yet it is the primary source of information for the average person. We can

roughly divide visual perception into two stages:

· the physical reception of the stimulus from outside world, and

· the processing and interpretation of that stimulus.

On the one hand the physical properties of the eye and the visual system mean that

there are certain things that cannot be seen by the human; on the other interpretative

capabilities of visual processing allow images to be constructed from incomplete

information. We need to understand both stages as both influence what can and can

not be perceived visually by a human being, which is turn directly affect the way that

we design computer system. We will begin by looking at the eye as a physical

receptor, and then go onto consider the processing involved in basic vision.

The human eye

Vision begins with light. The eye is a mechanism for receiving light and transforming

it into electrical energy. Light is reflected from objects in the world and their image is

focused upside down on the back of the eye. The receptors in the eye transform it into

electrical signals, which are passed to brain.

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The eye has a number of important components as you can see in the figure. Let us

take a deeper look. The cornea and lens at the front of eye focus the light into a sharp

image on the back of the eye, the retina. The retina is light sensitive and contains two

types of photoreceptor: rods and cones.

Rods

Rods are highly sensitive to light and therefore allow us to see under a low level of

illumination. However, they are unable to resolve fine detail and are subject to light

saturation. This is the reason for the temporary blindness we get when moving from a

darkened room into sunlight: the rods have been active and are saturated by the

sudden light. The cones do not operate either as they are suppressed by the rods. We

are therefore temporarily unable to see at all. There are approximately 120 million

rods per eye, which are mainly situated towards the edges of the retina. Rods therefore

dominate peripheral vision.

Cones

Cones are the second type of receptor in the eye. They are less sensitive to light than

the rods and can therefore tolerate more light. There are three types of cone, each

sensitive to a different wavelength of light. This allows color vision. The eye has

approximately 6 million cones, mainly concentrated on the fovea.

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Fovea

Fovea is a small area of the retina on which images are fixated.

Blind spot

Blind spot is also situated at retina. Although the retina is mainly covered with

photoreceptors there is one blind spot where the optic nerve enter the eye. The blind

spot has no rods or cones, yet our visual system compensates for this so that in normal

circumstances we are unaware of it.

Nerve cells

The retina also has specialized nerve cells called ganglion cells. There are two types:

X-cells

These are concentrated in the fovea and are responsible for the early detection of

pattern.

Y-cells

These are more widely distributed in the retina and are responsible for the early

detection of movement. The distribution of these cells means that, while we may not

be able to detect changes in pattern in peripheral vision, we can perceive movement.

7.3 Visual perception

Understanding the basic construction of the eye goes some way to explaining the

physical mechanism of vision but visual perception is more than this. The information

received by the visual apparatus must be filtered and passed to processing elements

which allow us to recognize coherent scenes, disambiguate relative distances and

differentiate color. Let us see how we perceive size and depth, brightness and color,

each of which is crucial to the design of effective visual interfaces.

Perceiving size and depth

Imagine you are standing on a hilltop. Beside you on the summit you can see rocks,

sheep and a small tree. On the hillside is a farmhouse with outbuilding and farm

vehicles. Someone is on the track, walking toward the summit. Below in the valley is

a small market town.

Even in describing such a scene the notions of size and distance predominate. Our

visual system is easily able to interpret the images, which it receives to take account

of these things. We can identify similar objects regardless of the fact that they appear

to us to be vastly different sizes. In fact, we can use this information to judge distance.

So how does the eye perceive size, depth and relative distances? To understand this

we must consider how the image appears on the retina. As we mentioned, reflected

light from the object forms an upside-down image on the retina. The size of that

image is specified as visual angle. Figure illustrates how the visual angle is calculated.

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If were to draw a line from the top of the object to a central point on the front of the

eye and a second line from the bottom of the object to the same point, the visual angle

of the object is the angle between these two lines. Visual angle is affected by both the

size of the object and its distance from the eye. Therefore if two objects are at the

same distance, the larger one will have the larger visual angle. Similarly, if two

objects of the same size are placed at different distances from the eye, the furthest one

will have the smaller visual angle, as shown in figure.

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Visual angle indicates how much of the field of view is taken by the object. The

visual angle measurement is given in either degrees or minutes of arc, where 1 degree

is equivalent to 60 minutes of arc, and 1 minute of arc to 60 seconds of arc.

Visual acuity

So how does an object's visual angle affect our perception of its size? First, if the

visual angle of an object is too small we will be unable to perceive it at all. Visual

acuity is the ability of a person to perceive fine detail. A number of measurements

have been established to test visual acuity, most of which are included in standard eye

tests. For example, a person with normal vision can detect a single line if it has a

visual angle of 0.5 seconds of arc. Spaces between lines can be detected at 30 seconds

to 1 minute of visual arc. These represent the limits of human visual perception.

Law of size constancy

Assuming that we can perceive the object, does its visual angle affect our perception

of its size? Given that the visual angle of an object is reduced, as it gets further away,

we might expect that we would perceive the object as smaller. In fact, our perception

of an object's size remains constant even if its visual angel changes. So a person's

height I perceived as constant even if they move further from you. This is the law of

size constancy, and it indicated that our perception of size relies on factors other than

the visual angle.

One of these factors is our perception of depth. If we return to the hilltop scene there

are a number of cues, which can use to determine the relative positions and distances

of the objects, which we see. If objects overlap, the object that is partially covered is

perceived to be in the background, and therefore further away. Similarly, the size and

height of the object in our field of view provides a cue to its distance. A third cue is

familiarity: if we expect an object to be of a certain size then we can judge its distance

accordingly.

Perceiving brightness

A second step of visual perception is the perception of brightness. Brightness is in fact

a subjective reaction to level of light. It is affected by luminance, which is the amount

of light emitted by an object. The luminance of an object is dependent on the amount

of light falling on the object's surface and its reflective prosperities. Contrast is

related to luminance: it is a function of the luminance of an object and the luminance

of its background.

Although brightness is a subjective response, it can be described in terms of the

amount of luminance that gives a just noticeable difference in brightness. However,

the visual system itself also compensates for changes in brightness. In dim lighting,

the rods predominate vision. Since there are fewer rods on the fovea, object in low

lighting can be seen easily when fixated upon, and are more visible in peripheral

vision. In normal lighting, the cones take over.

Visual acuity increases with increased luminance. This may be an argument for using

high display luminance. However, as luminance increases, flicker also increases. The

eye will perceive a light switched on and off rapidly as constantly on. But if the speed

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of switching is less than 50 Hz then the light is perceived to flicker. In high luminance

flicker can be perceived at over 50 Hz. Flicker is also more noticeable in peripheral

vision. This means that the larger the display, the more it will appear to flicker.

Perceiving color

A third factor that we need to consider is perception of color. Color is usually

regarded as being made up of three components:

· hue

· intensity

· saturation

Hue

Hue is determined by the spectral wavelength of the light. Blues have short

wavelength, greens medium and reds long. Approximately 150 different hues can be

discriminated by the average person.

Intensity

Intensity is the brightness of the color.

Saturation

Saturation is the amount of whiteness in the colors.

By varying these two, we can perceive in the region of 7 million different colors.

However, the number of colors that can be identified by an individual without training

is far fewer.

The eye perceives color because the cones are sensitive to light of different

wavelengths. There are three different types of cone, each sensitive to a different

color (blue, green and red). Color vision is best in the fovea, and worst at the

periphery where rods predominate. It should also be noted that only 3-4 % of the

fovea is occupied by cones which are sensitive to blue light, making blue acuity

lower.

Finally, we should remember that around 8% of males and 1% of females suffer from

color blindness, most commonly being unable to discriminate between red and green.

The capabilities and limitations of visual processing

In considering the way in which we perceive images we have already encountered

some of the capabilities and limitations of the human visual processing system.

However, we have concentrated largely on low-level perception. Visual processing

involves the transformation and interpretation of a complete image, from the light that

is thrown onto the retina. As we have already noted, our expectations affect the way

an image is perceived. For example, if we know that an object is a particular size, we

will perceive it as that size no matter how far it is from us.

Visual processing compensates for the movement of the image on the retina which

occurs as we around and as the object which we see moves. Although the retinal

image is moving, the image that we perceive is stable. Similarly, color and brightness

of objects are perceived as constant, in spite of changes in luminance.

This ability to interpret and exploit our expectations can be used to resolve ambiguity.

For example consider the image shown in figure `a'. What do you perceive? Now

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consider figure `b' and `c'. the context in which the object appears allow our

expectations to clearly disambiguate the interpretation of the object, as either a B or

13.

Figure a

However, it can also create optical illusions. Consider figure `d'. Which line is

longer?

concave

Figure c

convex

the Muller Lyer illusion

Figure d

A similar illusion is the Ponzo illusion as shown in figure

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