1.5 Sensation, Attention and Perception


Ch 5: Sensation: concepts of threshold, absolute and difference thresholds, signal-detection and vigilance; Factors influencing attention including set and characteristics of stimulus; Definition and concept of perception, biological factors in perception; Perceptual organization-influence of past experiences, perceptual defence-factors influencing space and depth perception, size estimation and perceptual readiness; The plasticity of perception; Extrasensory perception; Culture and perception, Subliminal perception.

Sensation and Perception 

Some people can see perfectly, but cannot recognize members of their family.This person has what’s called prosopagnosia, AKA “face blindness.” This illustrates the difference between sensation and perception.

Sensation is the ability to see in this case, but includes hearing, touching, tasting, and smelling. Sensation is part of bottom-up processing where our senses send information to our brain.

Perception is how we put the impulses received from our senses together so they make sense. Although a person may see perfectly, they cannot perceive or correctly process the impulses so that they make sense. Perception is part of top-down processing where our brain assembles the info to make sense of the impulses being sent to it.

Psychophysics : Study of relationship b/w the stimuli and the sensations they evoke.

Selective attention : The ability to screen out sensory information and focus on only a small portion of it is called e.g. a housewife telling her husband the things that need to be done around the house while he’s watching a football game on TV oblivious to what’s she’s saying. He’s got selective attention.

Cocktail party effect : a person’s ability to single out one voice amidst many others, then to “change channels” to another voice. A person in a crowded, talking room can weed out other voices and converse with one person. Then the first person can single out another voice and “tune in” on that person if desired.

Selective inattention or inattentional blindness :ability to purposefully block out all but one bit of sensory input- to focus on one thing only. e.g.in an experiment where a person in a gorilla suit walked among people passing a basketball. The observers were to count basketball passes so they focused on that task. Most people never saw the gorilla.

Change Blindness : phenomena whenpeople won’t notice a change in “scenery” after a brief interruption. e.g. person A was getting directions from a bystander, then was interrupted by construction workers, then was replaced with person B. The bystander doesn’t notice the switch from person A to person B 40% of the time.

Choice Blindness : people are unaware of the choices or preferences they make. e.g :women were shown two pictures and asked to choose the most attractive person. Then they were tricked and shown the one they had rejected and asked, “Why’d you choose this person as more attractive.” Only 13% caught the change. Others explained why they chose that person. When asked if they’d recognize a trick, 84% said they’d catch a switch. This became known as choice blindness–blind i.e. they can’t see that they’re choice blind.

Pop-out : when something is noticeably different from the others and thus, pops out to the viewer. Imagine a picture of a hundred white cats and one black dog. The black dog would pop out to you.


Absolute threshold/Absolute Limen : the minimum stimulation needed to detect light, a sound, a pressure, taste or odor 50% of the time. e.g. minimum amount of sugar granules to be added to a glass of water to detect sweetness.

difference threshold or just noticeable difference (JND) is the minimum difference between two stimuli that can be detected at least 50% of the time.e.g.imagine picking up a 20 kg weight and then a 20 kg-10 gm weight, you likely would not notice a difference. If you kept increasing the weight, you’d eventually say, “This is heavier.” At some weight, you’d notice it 50% of the time, that’s the JND.

Weber’s Law/Weber-Fechner law:says the difference between two stimuli must differ by a constant proportion, not necessarily a constant amount  i.e. the change in a stimulus that will be just noticeable is a constant ratio of the original stimulus. However, it has been shown not to hold for extremes of stimulation. e.g. think about weight, weight must differ by 2% for a person to notice the difference.; two sound tones must differ by 0.3% for a person to notice the difference.

Signal detection theory : the idea that predicting whether or not we detect a stimulus depends not only on the stimulus, but also on our experience, expectations, motivation, and alertness.e.g. people in life-or-death situations, like war, often have heightened signal detection.

Subliminal perception (aka “subliminal stimulation”) is stimulation just below our level of consciousness. This occurs when we’re subjected to a stimulus, but we just aren’t aware of it. For instance, an image might be flashed so quickly we don’t even know we saw it. Can we sense these things? By definition, yes, because absolute threshold is 50% of the time. Thus, we might sense this 49% of the time. Can these unconscious stimuli impact our behavior? We’re not sure the answer to this question just yet.

Backward Masking : refers to a phenomenon wherein presenting one visual stimulus (a "mask" or "masking stimulus") immediately after another brief (≤ 50 ms) "target" visual stimulus leads to a failure to consciously perceive the first stimulus.

Forward Masking :  when a masking stimulus precedes a target stimulus rather than following it: this is known as forward masking. While not consciously perceived, the masked stimulus can nevertheless still have an effect on cognitive processes such as context interpretation. This phenomenon can notably be used for psychological manipulation 

Visual Priming : Or Priming in general is an implicit memory effect in which exposure to one stimulus (i.e., perceptual pattern) influences the response to another stimulus. The seminal experiments of Meyer and Schvaneveldt in the early 1970s led to the flowering of research on priming of many sorts. Their original work showed that people were faster in deciding that a string of letters is a word when the word followed an associatively or semantically related word. For example, NURSE is recognized more quickly following DOCTOR than following BREAD.  

Priming works best when the two stimuli are in the same modality. For example, visual priming works best with visual cues and verbal priming works best with verbal cues. But priming also occurs between modalities,or between semantically related words such as "doctor" and "nurse".

Positive and Negative Priming: refer to when priming affects the speed of processing. A positive prime speeds up processing, while a negative prime lowers the speed to slower than un-primed levels.

Perceptual and Conceptual Priming : they tell whether items with a similar form or items with a similar meaning are primed, respectively.

Applications :
In advertising and branding:  The core of Advertising is to make the consumer/ client ‘respond’ to  ideas.
For this , phrases like  “buy this” , “ drink this”  are used with posters or video clips.
Use of colour : e.g. consumers perceiving lemonade suddenly as sweeter when the logo of the drink is more saturated towards yellow
In classroom teaching : structure, routine, and predictability are enhanced by the use of priming in teaching. The teacher can  introduce new material before the lesson occurs. It is a way to prepare students for an activity with which they usually have difficulty. 
In Psychological research : For instance, good amount of research done on amnesia patients involving priming has helped find the brain areas which are responsible for specific types of amnesia.