•Absolute threshold as the stimulus intensity detected 50% of the time.
Weighing the Differences: The JND
•Just Noticeable Difference (JND) –The smallest difference in the amount of
stimulation that a specific sense can detect.
•JND closely related to absolute threshold. Absolute Threshold is simply the JND
•Weber’s Law: States that the size of JND is a constant proportion of the size of the
initial stimulus. The constant proportion is called the Weber Fraction.
•Weber Fraction for weights: 1/30. This means you can only tell the difference
between 300 grams and 310 grams, not 300 grams and 305 grams.
•Scale – Judging the increase in sensory input.
•Fechner’s Law – The magnitude of a sensory experience is proportion to the number
of JNDs that the stimulus causing the experience is above the absolute threshold.
oEx: Being in a darkroom, and turning on one light bulb. Second light bulb you
notice a small difference. Third one you hardly notice anything different.
•Perception can’t be measured on absolute scales.
•Signal-detection theory – The detection of stimuli involves decision processes as well
as sensory processes, which are both influenced by a variety of factors besides
•Hits – Detecting signals when present. Misses – Failing to detect signals when
present. False Alarms – Detecting signals when not present. Correct Rejections
– Not detecting signals when they are absent.
•Noise – Irrelevant stimuli that interferes with your ability to pick up weak signals.
Perception without Awareness
•Subliminal Perception – Registration of sensory input without conscious awareness.
(Limen = Threshold, sub = below. Therefore Subliminal = Below Threshold.
•Sensory Adaption – Gradual decline in sensitivity due to prolonged stimulation.
The Visual System
The Stimulus Light