Weber's Law and Signal Detection Theory

• The just noticeable difference (JND) is small when the standard stimulus (S) is dim
• The JND is larger when S is bright
• For any sensory domain, the ratio between the JND and the standard stimulus is a constant value, except at extreme standard values
• Ernst Weber first noticed this relationship in 1834
• Gustav Fechner applied Weber's insight to psychophysics

Weber's Law

• States that for every sense domain, the change in a stimulus that is just noticeable is a constant proportion, despite variation in intensities
• Example: Noticing the difference between a 25-gram and 50-gram envelope versus not noticing the difference between a 10-kilogram package and one 25 grams heavier

Signal Detection Theory

• Measuring absolute and difference thresholds assumes a threshold exists
• Humans don't rapidly switch between perceiving and not perceiving
• The same stimulus may be perceived on some occasions but not others
• An absolute threshold is operationalized as perceiving the stimulus 50% of the time
• Accurate perception of a sensory stimulus can be haphazard

Noise

• Refers to all the other stimuli coming from the internal and external environments

Signal Detection Theory Defined

• Holds that the response to a stimulus depends on a person's sensitivity to the stimulus in the presence of noise and on a person's decision criterion
• Observers consider the sensory evidence evoked by the stimulus and compare it with an internal decision criterion
• If the sensory evidence exceeds the criterion, the observer responds, "Yes, I detected the stimulus."
• If it falls short of the criterion, the observer responds, "No, I did not detect the stimulus."

Practical Applications of Signal Detection Theory

• A radiologist deciding whether a mammogram shows breast cancer
• A strictly liberal criterion minimizes missing a true cancer but leads to many unnecessary biopsies
• A strictly conservative criterion cuts down on unnecessary biopsies but will miss some treatable cancers
• Different types of errors have to be weighed against each other in setting the decision criterion

The Real World: Multitasking

• Using a cell phone while driving makes having an accident four times more likely
• Selective attention is perceiving only what's currently relevant to you
• Perception is an active, moment-to-moment exploration for relevant or interesting information

Cell Phones While Driving

• Talking on a cell phone while driving demands juggling two independent sources of sensory input—vision and hearing—at the same time
• When attention is directed to hearing, activity in visual areas decreases
• Experienced drivers reacted significantly more slowly during phone conversations than during other tasks in a driving simulator
• Slower braking response translates into an increased stopping distance
• Whether the phone was handheld or hands free made little difference
• Laws requiring drivers to use hands-free phones may have little effect on reducing accidents
• Even after drivers had extensive practice at driving while using a hands-free cell phone in a simulator, the disruptive effects of cell phone use were still observed

Texting While Driving

• When either sending or receiving a text message in the simulator, drivers spent dramatically less time looking at the road, had a much harder time staying in their lane, missed numerous lane changes, and had greater difficulty maintaining an appropriate distance behind the car ahead of them
• The impairing effect of texting while driving is comparable with that of alcohol consumption and greater than that of smoking marijuana
• Browsing a Facebook page while driving is also detrimental to driving performance, but still not quite as harmful as texting

Impairments from Multitasking

• People who report that they multitask frequently have difficulty in laboratory tasks that require focusing attention in the face of distractions
• Lapses of attention in habitual multitaskers often result in memory failures
• Frequent multitasking in academic settings (via texting or Facebook use) can have a negative impact on learning