Introduction
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.
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.
Sensation is part of bottom-up processing where our senses send information to our brain.
Perception is part of top-down processing where our brain assembles the info to make sense of the impulses being sent to it.
Selective attention
We sense a lot of information. Scientists estimate we observe 11,000,000 bits of info per second. But we weed out all but 40 bits.
The ability to screen out sensory information and focus on only a small portion of it is called selective attention. Think of 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.
The cocktail party effect is 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 attention is seen in car crashes. People that talk or text on the phone are distracted by the phone and are much more likely to crash (4 times more in one study for talking, 23 times more for texting). More specifically than "being distracted", they selectively put their attention on one task at a time, driving or texting, and the other suffers.
Despite people's claim of "multi-tasking", as humans, we focus on one thing at a time.
On the flip side is selective inattention or inattentional blindness. This is the ability to purposefully block out all but one bit of sensory input - to focus on one thing only.
This was seen 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.
Similarly, there is change blindness which is where people won't notice a change in "scenery" after a brief interruption.
This was seen in a scene where 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.
In choice blindness, people are unaware of the choices or preferences they make.
This was seen in an experiment where women were shown two pictures and asked to choose the most attractive person. Then they were tricked and shown the one they'd rejected and asked, "Why'd you choose this person as more attractive."
Only 13% caught the switcheroo. They explained why they chose that person.
When asked if they'd recognize a switcheroo, 84% said they'd catch a switch. This became known as choice blindness blindness – they can't see that they're choice blind.
In pop-out, 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.
Thresholds
We sense only a sliver of the info coming at us. We can't see everything (like X-rays or radio waves) or hear everything (the family dog can hear much more than us).
Take sound for example, at some point there is a point where we can't hear a frequency (but the dog still can). This cut-off point to sensation is called the absolute threshold. It's defined as the minimum stimulation needed to detect light, a sound, a pressure, taste or odor 50% of the time.
As an example, people lose the ability to hear high-pitched as they grow older.
Teens use this to set "mosquito" ringtones that adults likely won't hear. Businesses use they to shoo away loitering teens.
Signal detection theory is 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.
People in life-or-death situations, like war, often have heightened signal detection.
Subliminal stimulation (kin to "subliminal perception") 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.
In one study, subliminal stimuli can prime or prepare responses. This means a subliminal stimulus prepared people for a response to a second stimulus.
A difference threshold or just noticeable difference (JND) is the minimum difference between two stimuli that can be detected at least 50% of the time.
For example, imagine picking up a 20 pound weight and then a 20 pound 1 ounce 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 says the difference between two stimuli must differ by a constant proportion, not necessarily a constant amount.
Think about weight, weight must differ by 2% for a person to notice the difference.
Think about sound, two tones must differ by 0.3% for a person to notice the difference.
Sensory adaptation
Sensory adaption is a person's diminishing sensitivity to a sensory stimulus. In other words, if a stimulus persists, you get used to it.
Think of a person spilling a bit of perfume in class. You smell it strongly at first, but by the end of the class, you don't really notice it. As soon as the next class walks in, students say, "What's that smell?"
This also works for vision. Normally our eyes scan everything to "take it all in". If you were forced to look at the same thing over and over, you'd eventually stop seeing it.
The stimulus input: light energy
The process where our eyes sense light energy and change it into neural messages that our brain can handle is called transduction.
Visual light makes up a very small portion of the electromagnetic spectrum. But, that's the part that we see.
Visible light has two important characteristics...
Frequency – Frequency refers to the wavelength of the light wave, or like waves on the beach, how frequently they hit. Think of "FM" (frequency modulation) on the radio where a radio wave's wavelength is changed.
Amplitude – Amplitude refers to the intensity of the light wave, or like waves on the beach, how strongly they hit. Think of "AM" (amplitude modulation) on the radio where a radio wave's amplitude is changed.
