Chapter 8: Sensation

Sensation

• The process by which our sensory receptors and nervous system receive and represent stimuli from our environment.

• Sensation involves “bottom-up” processing because first, sense receptors detect a stimulus, and then the brain integrates the sensory information.

Perception

• The process of organizing and interpreting sensory information, enabling us to recognize meaningful objects and events.

• Perception involves “top-down” processing because when we construct perceptions we draw on or experience and expectations. This involves “higher level” mental processes conducted in the brain.

Our 5 Senses: Seeing, Hearing, Smelling, Tasting,

Touching• Sensory systems enable organisms to

obtain needed information.

• Animals’ sensory abilities suit their survival needs.

Psychophysics

• The study of relationships between the physical characteristics of stimuli, such as their intensity, and our psychological experience of them.

• What stimuli can we detect?

• At what intensity?

• How sensitive are we to changing stimulation?

Absolute Threshold• The minimum stimulation needed to detect a particular

stimulus 50% of the time.• Absolute threshold for sight: Seeing a candle flame on a dark

night from 30 miles away.• For smell: Smelling a single drop of perfume in a 3 room

house• For hearing: Hearing a ticking watch from about 20 feet away.• For taste: Tasting about one teaspoon of sugar dissolved in 2

gallons of water.• For touch: Feeling the wing of a bee falling on the cheek from

about 1 cm away.

Signal Detection Theory

• According to this theory the threshold for detecting a signal depends not only on the properties of the stimulus itself, such as its intensity- the loudness of a sound, for example- but also on the level of background stimulation (noise), and on the biological and psychological characteristics of the perceiver.

Subliminal Stimulation

• Stimuli that are below one’s absolute threshold for conscious awareness.

• Can we sense stimuli below our absolute thresholds?– The Answer is Yes. The effects of

subliminal perception appear to be subtle and to depend on very precise experimental conditions.

Difference Threshold

• Also called “just noticeable difference” or “jnd”

• The minimal difference in the magnitude of energy needed for people to detect a difference between two stimuli.

• Jnd’s apply to each of our senses… they are the minimal differences between two stimuli that people can reliably detect.

Weber’s Law

• The principle that the amount of change in a stimulus needed to detect a difference is given by a constant ratio or fraction, called a constant, of the original stimulus.

• According to this law, the amount you must change a stimulus to detect a difference is given by a constant fraction or proportion of the original stimulus.

Examples of Weber’s Constants

• Saltiness of food= 1/5 or 20%

• Pressure on skin= 1/7 or 14%

• Loudness of sounds= 1/10 or 10%

• Odor= 1/20 or 5 %

• Heaviness of weights= 1/50 or 2%

• Brightness of lights= 1/60 or 1.7%

• Pitch of sounds= 1/333 or .3%

Sensory Adaptation• The process by which sensory systems

adapt to constant stimuli by becoming less sensitive to them.

• Our sensory systems deal with repeated exposure to the same stimuli by becoming less sensitive to them.

• Sensory adaptation may not occur when we are repeatedly exposed to certain strong stimuli, such as the loud wail of a car alarm.