Psychology Chapter 5 Sensory, Attentional And Perceptual Processes

Our sense organs provide us with first hand information about our external or internal world. Different sense organs deal with different forms of stimuli and serve different purposes. Each sense organ is highly specialised for dealing with a particular kind of information. However, all sense organs function with certain limitations. For example, our eyes cannot see things which are very dim or very bright. Similarly, our ears cannot hear very faint or very loud sounds. As human beings, we function within a limited range of stimulation. A stimulus, to be noticed by a sensory receptor, has to be of an optimal intensity or magnitude. In order to be noticed, a stimulus has to carry a minimum value or weight.

The minimum value of a stimulus required to activate a given sensory system is called absolute limen (AL). Absolute limen is not a fixed point, instead it varies considerably across individuals and situations depending on the people’s organic conditions and their motivational states. Hence, it is assessed on the basis of a number of trials. Further, it is not possible to differentiate between all stimuli. In order to notice two stimuli are different from each other. These has to be some minimum difference between the value of those stimuli. The smallest difference in the value of two stimuli that is necessary to notice them as different is termed as difference limen (DL). Thus, it is not possible to understand sensations without the AL and DL of different types of stimuli.

Besides the stimulus characteristics, sensory processes also depend on other characteristics. Sense organs and the neural pathways connecting them to various brain centres also play a vital role in this process. A sense organ receives the stimulus and encodes it as an electrical impulse. For being noticed this electrical impulse must reach the higher brain centres. Any structural or functional defect or damage in the receptor organs, its neural pathway, or the concerned brain area may lead to a partial or complete loss of sensation.

Light adaptation refers to the process of adjusting to bright light after exposure to dim light. This process takes nearly a minute or two. On the other hand, dark adaptation refers to the process of adjusting to a dimly illuminated environment after exposure to bright light. This may take half an hour or even longer depending on the previous level of exposure of the eye to light.

Light and dark adaptations occur due to certain photochemical processes. The rods have a photo-sensitive chemical substance, called rhodopsin or visual purple. By the action of light the molecules of this chemical substance get bleached or broken down. Under such conditions the light adaptation takes place in the eyes. On the other hand, the dark adaptation is achieved by the removal of light and thereby allowing for restorative processes to generate the pigment in the rods with the help of vitamin A. The regeneration of rhodopsin in rods is a time consuming process. That is why dark adaptation is a slower process than light adaptation.

It has been found that people who suffer from vitamin-A deficiency do not achieve dark adaptation at all, and find it really difficult to move in the dark. This condition is generally known as night blindness.

A person’s ability to distinguish different shades of colour is termed as colour vision. A person with normal colour vision can distinguish more than seven million different shades of colour.

There are three basic dimensions of colour-hue, saturation, and brightness. Hue is a property of chromatic colours. It refers to the name of the colour, e.g., red, blue, and green. Hue varies with wavelength, and each colour is identified with a specific wavelength. For example, blue has a wavelength of about 465 nm. and green of about 500 nm. Achromatic colours like black, white or grey are not characterised by hues.

Saturation is a psychological attribute that refers to the relative amount of hue of a surface or object. The light of single wavelength (monochromatic) appears to be highly saturated. As we mix different wavelengths, the saturation decreases. The colour grey is completely unsaturated.

Brightness is the perceived intensity of light. It varies across both chromatic and achromatic colours. White and black represent the top and bottom of the brightness dimension. White has the highest degree of brightness, whereas black has the lowest degree.

Auditory sensation begins when sound enters ear and stimulates chief organs of hearing.

The process through which certain stimuli are selected from a group of others is generally referred to as attention.

Attention has different properties such as selection, alertness, concentration, and search.

(i) Selection: A large number of stimuli impinge upon our sense organs simultaneously but we do not notice all of them at the same time. Only a selected few of them are noticed. For example, when a student enters his classroom he encounters several things in it, such as doors, walls, windows, paintings on walls, tables, chairs, students, school bags, etc., but he selectively focus only on one or two of them at one time.

(ii) Alertness: It refers to an individual’s readiness to deal with stimuli that appear before him/her. For example while participating in a race in school, a student can see the participants on the starting line in an alert state waiting for the whistle to blow in order to run.

(iii) Concentration: It refers to focusing of awareness on certain specific objects while excluding others for the moment. For example in the classroom, a student concentrates on the teacher’s lecture and ignores all sorts of noise coming from different corners of the school.

(iv) Search: In search an observer looks for some specified subject or object among a set of objects. Example : When you go to fetch your younger sister and brother from the school, you just look for them among many boys and girls. All these activities require some kind of effort on the part of the people.

Selective attention is concerned mainly with the selection of a limited number of stimuli or objects from a large number of stimuli.

There are a number of factors that determine the selective attention. These factors generally relate to the characteristics of stimuli and the characteristics of individuals. All these factors are classified as external and internal factors.

(i) External factors are related to the features of stimuli. The size, intensity, and motion of stimuli are considered as important determinants of attention. Large, bright, and moving stimuli easily catch attention. Stimuli, which are novel and moderately complex, also easily get into ones focus. It has been observed that human photographs are more likely to be attended to than the photographs of inanimate objects. Similarly, rhythmic auditory stimuli are more readily attended to than verbal narrations. Sudden and intense stimuli have a wonderful capacity to draw attention.

(ii) Internal factors lie within the individual. These may be divided into two main categories-motivational factors and cognitive factors.. Motivational factors relate to our biological or social needs. When a person is hungry, he notices even a faint smell of food. A student taking an examination is likely to focus on a teacher’s instructions more than other students. Cognitive factors include factor's lik)e interest, attitude, and preparatory set. Objects or events, which appear interesting, are readily attended by individuals, similarly one pays quick attention to certain objects or events to which he is favourably disposed. Preparatory set generates mental state to act in a certain way and readiness of the individual to respond to one kind of stimuli and not to others.

According to Gestalt psychologists, human beings perceive different stimuli not as discrete elements, but as an organised “whole” that carries a definite form. They believe that the form of an object lies in its whole, which is different from the sum of their parts. For example, a flower-pot with a bunch of flowers is a whole. If the flowers are removed, the flowerpot still remains a whole. It is the configuration of the flower pot that has changed. Flower pot with flowers is one configuration, without flowers it is another configuration.

The Gestalt psychologists also indicate that cerebral processes of human beings are always oriented towards the perception of a good figure or pragnan. That is the reason why human beings perceive everything in an organised form. The most primitive organisation takes place in the form of figure-ground segregation. When we look at a surface, certain aspects of the surface clearly stand out as separate entities, whereas others do not. For example, when we see words on a page, or a painting on a wall, or birds flying in the sky, the words, the painting, and the birds stand out from the background, and are perceived as figures, while the page, wall and sky stay behind the figure and are perceived as background.

The visual field or surface in which things exist, move or can be placed is called space. We perceive not only the spatial attributes (e.g., size, shape, direction) of various objects, but also the distance between the objects found in this space. While the images of objects projected on to our retina are flat and two dimensional (left, right, up, down) we still perceive three dimensions in the space. It occurs due to our ability to transfer a two dimensional retinal vision into a three dimensional perception.

Monocular cues of depth perception are effective when the objects are viewed with only one eye. These cues are often used by artists to induce depth in two dimensional paintings. Hence, they are also known as pictorial cues.

Role of binocular cues in the perception of depth: Some important cues to depth perception in three dimenstional space provided by binocular cues (both eyes). Some of them are as follows:

(i) Retinal or Binocular Disparity: Retinal disparity occurs because the two eyes have different locations in the head. They are separated from each other horizontally by a distance of about 6.5 centimeters. Because of this distance, the image formed on the retina of each eye of the same object is slightly different. This difference between the two images is called retinal disparity. The brain interprets a large retinal disparity to mean a close object and a small retinal disparity to mean a distant object, as a disparity is less for distant objects and more for the near objects.

(ii) Convergence: When we see a nearby object our eyes converge inward in order to bring the image on the fovea of each eye. A group of muscles send messages to the brain regarding the degree to which eyes are turning inward, and these messages are interpreted as cues to the perception of depth. The degree of convergence decreases as the object moves further away from the observer. One can experience convergence by holding a finger in front of his nose and slowly bringing it closer. The more his eyes turn inward or converge, the nearer the object appears in space.

(iii) Accommodation: Accommodation refers to a process by which we focus the image on the retina with the help of ciliary muscles. These muscles change the thickness of the lens of the eye. If the object gets away (more than 2 meters), the muscle is relaxed. As the object moves nearer, the muscle contracts and the thickness of the lens increases. The signal about the degree of contraction of the muscle is sent to the brain, which provides the cue for distance.

Perceptions of human beings are not always veridical. Sometimes he fails to interprete the sensory information correctly. This results in a mismatch between the physical stimuli and its perception. These misperceptions resulting from misinterpretation of information received by our sensory organs are generally termed as illusions. Illusion result from an external stimulus situation and generate the same kind of experience in each individual. Although illusions can be experienced by the stimulation of any of our senses, psychologists have studied them more commonly in the visual than in other senses modalities.

Sense organs of our body collect information from the external world as well as from our own body.

Sensation, attention, and perception.

All the mental activities associated with knowing, namely, perceiving, thinking, and remembering, etc. are called cognition.

All well-defined elements in the environment affecting the organism which may lead to an overt or a covert response is termed as stimulus.

The initial experience of a stimulus or an object registered by a particular sense organ is called sensation.

Psychophysics is a discipline in which the relationship between stimuli and the sensations they evoke has been studied.

The minimum value of a stimulus required to activate a given sensory system is called absolute threshold.

The smallest difference in the value of two stimuli that is necessary to notice them as different is called difference threshold or difference limen.

Vision.

Our eyes are sensitive to a spectrum of light, the wavelength of which ranges from 380 nm to 780 nm.

‘nm’ refers to nanometer, which is one billionth of a meter.

Cornea.

Choroid.

This process is regulated by ciliary muscles.

The sensory receptors for taste are located inside small bumps on the tongue, known as papillae.

Iris.

Iris controls the amount of light entering the eye by regulating pupil dilation.

Rods are the receptors for scotopic vision (night vision).

Cones.

Yellow spot.

Light adaptation refers to the process of adjusting to bright light after exposure to dim light.

Dark adaptation refers to the process of adjusting to a dimly illuminated environment after exposure to bright light.

‘Hue’ is a property of chromatic colours. It refers to the name of the colour, e.g., red, blue and green.

Saturation is a psychological attribute that refers to the relative amount of hue of a surface or object.

Red, green and blue are called primary colours.

Examples of complementary colours are red-green and yellow-blue.

Auditory meatus is a canal protected by hair and wax that carries sound waves from pinna to the tympanum or eardrum.

Organ of Corti is the main organ for hearing.

Loudness is determined by amplitude in (db) or decibel.

Frequency is reciprocal of wavelength. The range of hearing is 20 Hz to 20000 Hz.

Loudness is measured in decibel (dB).

Timbre refers to nature or quality of sound. Example of timbre is sound of a car engine.

It is an instrument which is used to meature the span of attention.

ADHD is Attention deficit hyperactive disorder.

Filter Theory was developed by Broadbent (1956). According to this theory, many stimuli simultaneously enter our receptors creating a kind of‘bottleneck’ situation. Moving through the short-term memory system, they enter the selective filter, which allows only one stimulus to pass through for higher levels of processing. Other stimuli are screened out at that moment of time. Thus, we become aware of only that stimulus, which gets access through the selective filter.

Multimode theory was developed by Johnston and Heinz (1978). This theory believes that attention is a flexible system that allows selection of a stimulus over others at three stages. At stage one the sensory representations (e.g., visual images) of stimuli are constructed; at stage two the semantic representations (e.g., names of objects) are constructed; and at stage three the sensory and semantic representations enter the consciousness. It is also suggested that more processing requires more mental effort. When the messages are selected on the basis of stage one processing (early selection), less mental effort is required than when the selection is based on stage three processing (late selection).

External factors are related to the features of stimuli. Other things held constant, the size, intensity, and motion of stimuli appear to be important determinants of attention. Large, bright, and moving stimuli easily catch our attention. Stimuli, which are novel and moderately complex, also easily get into our focus. It has been observed that human photographs are more likely to be attended to than the photographs of inanimate objects. Similarly, rhythmic auditory stimuli are more readily attended to than verbal narrations. Sudden and intense stimuli have a wonderful capacity to draw attention.

Filter-attenuation theory was developed by Triesman (1962) by modifying Broadbent’s theory. According to this theory the stimuli not getting access to the selective filter at a given moment of time are not completely blocked. The filter only attenuates their strength. Thus, some stimuli manage to escape through the selective filter to reach higher levels of processing. It is indicated that personally relevant stimuli can be noticed even at a very low level of sound. Such stimuli, even though fairly weak, may also generate response occasionally by slipping through the selective filter.

The Kinesthetic system: Its receptors are found primarily in joints, ligaments, and muscles. This system gives us information about the location of our body parts in relation to each other, and allows us to perform simple (e.g., touching one’s nose) and complex movements (e.g., dancing). Our visual system provides a great deal of help in this respect.

Colour is the sensational property of our sensory experiences. It is created when our brain interprets the information received from external world. Light is represent in the form of dolour net wavelength.

Sound waves cause due to successive pressure changes over time in the form of single repeating sinewaves. Sound waves vary in amplitude as well as wavelengths.

A complete change in pressure from compression to rarefraction and again compression makes a cycle of wave.

Motion Parallax: It is a kinetic monocular cue. It occurs when objects at different distances move at a different relative speed. The distant objects appear to move slowly than the objects that are close. The rate of an object’s movement provides cue to its distance. For example, when we travel in a bus, closer objects move ‘against’ the direction of the bus, whereas the farther objects move ‘with’ the direction of the bus.

Monocular cues or depth perceptions are effective only when the objects are viewed with only one eye. These cues are used by artists to induce depth in painting. Thus they are also called as pictorial cues. Some of the molecular cues are:

(i) Relative Size: The size of the retinal image allow us to judge distance based on our past and present experiences with similar objects. As the object gets away the retinal image become smaller and smaller.

(ii) Interposition or Overlapping: These cues occur when some portion of the object is covered by another object.

(iii) Linear Perspective: This reflects the phenomena by which distant object appear to be closer than nearer object.

Geometrical illusions: Muller-Lyer illusion has given illusion figures. In the figures given below all of us perceive line A as shorter than line B, although both the lines are equal. This illusion is experienced even by children. There are some studies that suggests that even animals experience this illusion more or less like us. Besides Muller-Lyer illusions, several other visual illusions are experienced by human beings (also birds and animals). In the vertical-horizontal illusion the illusion of vertical and horizontal lines can be seen. Although both the lines are equal, we perceive the vertical line as longer than the horizontal line.

Fig. Muller-Lyer Illusion

Fig. Structure of the Human Eye

Fig. : Structure of the Human ear

Visual sensation: Among all sense modalities, vision is the most highly developed in human beings. Various estimates indicate that we use it in approximately 80 per cent of our transactions with the external world. Audition and other senses also contribute significantly information gathering from the external world.

Visual sensation starts when light enters the eyes and stimulates our visual receptors. Our eyes are sensitive to a spectrum of light, the wavelength of which ranges from 380 nm to 780 nm (nm refers to nanometer, which is one billionth of a meter). No sensation is registered beyond this range of light.

The effect of a visual stimulus persists for some time even after the removal of that stimulus from the visual field. This effect is called ‘after image’. After images are positive and negative. Positive after images resemble the original stimulus in terms of hue, saturation and brightness. They usually occur after a brief intense stimulation of dark adapted eyes. On the other hand, negative after images appear in complementary colours. These images appear when a person stares at the patch of a particular colour for at least 30 seconds, and then transfers the gaze to a neutral background. For example - if the person looks at the blue colour the negative after image will appear in yellow.

Some important monocular eyes that in judging the distance and depth in two dimensional surfaces are as follows:

(i) Relative Size: The size of retinal images allows to judge distance based on past and present experiences with similar objects. As the objects gets away, the retinal image becomes smaller and smaller. One tends to perceive an object farther away when it appears small, and closer when it appears bigger.

(ii) Interposition or Overlapping: These cues occur when some portion of the object is covered by another object. The overlapped object is considered farther away, whereas the object that covers it appears nearer.

(iii) Linear Perspective: This reflects a phenomenon by which distant objects appear to be closer together than the nearer objects. For example, parallel lines, such as rail tracks, appear to converge with increasing distance with a vanishing point at the horizon. The more the lines converge, the farther away they appear.

(iv) Aerial Perspective: The air contains microscopic particles of dust and moisture that make distant objects look hazy or blurry. This effect is called aerial perspective. For example, distant mountains appear blue due to the scattering of blue light in the atmosphere, whereas the same mountains are perceived to be closer when the atmosphere is clear.

(v) Light and shade: In the light some parts of the object get highlighted, whereas some parts become darker. Highlights and shadow provide us with information about an object’s distance.

(vi) Relative Height: Larger objects are perceived as being closer to the viewer and smaller objects as being farther away. When we expect two objects to be the same size and they are not, the larger of the two will appear closen and the smaller will appear farther away.

(vii) Texture Gradient: It represents a phenomenon by which the visual field having more density of elements is seen farther away in the figure given below the density of stones increases as we look farther away.

Fig. Texture Gradient

(viii) Motion Paraller: It is a kinetic monocular cue, and hence not considered as a pictorial cue. It occurs when objects at different distances move at a different relative speed. The distant objects appear to move slowly than the objects that the close. The rate of an object’s movement provides a cue to its distance. For example, when we travel in a bus, closer objects move “against” the direction of the bus, whereas the farther objects move “with” the direction of the bus.

D.

Cochlea

B.

the cornea and the lens

C.

Difference limen

A.

Vitreous humour

D.

All of the above

A.

The Vestibular system

A.

Retina

A.

rods

D.

Blue, red, green

D.

Cochlea

C.

Organ of Corti

B.

C.

Pitch

C.

Johnston and Heinz

A.

the principle of similarity

B.

the principle of symmetry

D.

seven plus and minus four

A.

the two eyes have different locations in our head