Sensation Perception = gathering information from the environment 2 stages: –Sensation = simple sensory experiences and translating physical energy from the environment into neural codes –Perception = organizing, identifying, and interpreting sensations to create mental representations of objects and events

Top-down and Bottom-up Processes in Perception Bottom-up –“data driven” –“stimulus driven” –Example: different wavelengths of light produce different color sensations Top-down –“conceptually driven” –“context driven” –Example: reading an exit sign more quickly because you know what it is likely to be

Topics in Sensation Thresholds Signal Detection Theory Sensory Memory

Thresholds Absolute threshold – the lowest intensity of a stimulus that can be detected Subliminal stimuli –“Eat Popcorn” (McConnell et al 1958) –Backward messages –Self-help tapes (Greenwald et al 1991) –Priming –So do subliminal stimuli have real effects?

Thresholds: Theoretical and Actual Functions Theoretical Threshold Functions (Step Threshold) Actual Threshold Functions (Threshold is a continuous function) Operational Definition of the Absolute Threshold –The stimulus intensity at which it is correctly identified 50% of the time –Is there anything else besides stimulus intensity that must be considered?

Signal Detection Theory 2 Components of detection judgments –Discriminability How much sensory evidence is there that a signal is present? Controlled by stimulus Bottom-up –Bias How much evidence does the perceiver require before saying “yes”? Controlled by a “pay-off matrix” (among other things) Top-down

Separating Discriminability From Bias: 4 types of responses HitFalse Alarm MissCorrect Rejection Stimulus Present Absent Response Yes No

Person A: Threshold with a Conservative Bias Loudness = 10 decibels Hit 50% False Alarm 20% Miss 50% Correct Rejection 80% Stimulus Present Absent Response Yes No

Person B: Threshold with a Liberal Bias Loudness = 10 decibels Hit 80% False Alarm 50% Miss 20% Correct Rejection 50% Stimulus Present Absent Response Yes No

Person C: Threshold with a Liberal Bias Loudness = 5 decibels Hit 70% False Alarm 60% Miss 30% Correct Rejection 40% Stimulus Present Absent Response Yes No

Class Data from Coglab*: Positive (Liberal) Bias Condition Hit 67% False Alarm 53% Miss 33% Correct Rejection 47% Stimulus Present Absent Response Yes No

Class Data from Coglab*: Negative (Conservative) Bias Condition Hit 63% False Alarm 43% Miss 37% Correct Rejection 57% Stimulus Present Absent Response Yes No

Discriminability and Bias Signal and Noise distributions –X axis: Magnitude of internal sensory process (amount of neural excitation) –Y axis: frequency (probability density function) –Noise: effects of noise alone –Signal: effects of signal plus noise Criterion –How much neural excitation does the perceiver require before saying “yes”? –Tutorial:

Signal Detection Measures of Discriminability and Bias Discriminability –d’ (d prime) distance between signal and noise distributions 0 = no discriminability; higher values = more Assumes that the signal and noise distributions are normal –Log(alpha) another measure of discriminability that does not assume normality 0 = no discriminability; 4.6 = perfect performance

Signal Detection Measures of Discriminability and Bias Bias –Beta 1 = neutral (when the criterion is through the point where the signal and noise distributions cross) < 1 = liberal (“yes”) bias > 1 = conservative (“no”) bias –C 0 = neutral < 0 = liberal > 0 = conservative

Positive (Liberal) Bias Negative (Conservative) Bias *.43* *-.06* Class Coglab Data Results 1 (from a previous class. Your data are HERE)HERE Hits False Alarms d prime Log-alpha C (bias)

Evidence for Signal Detection Theory Swets, Tannar, & Birdsall, 1961 –4 choice detection task (tones A, B, C, D) –After an incorrect answer, answered again –If the participants had no information about which tone it was when they were wrong, should be at chance on subsequent guesses –Results: after a missed guess, they were above chance on the next guess –Therefore, the threshold is a continuous function (as Signal Detection Theory predicts), not a step function.

The Problem of Bias How can researchers avoid mistakenly attributing changes in response bias to perceptual effects (discrimination)? –Use Signal Detection Theory measures of discriminability and bias to distinguish the two –Induce a strong conservative bias –Use a forced-choice task

Importance of SDT Stimulus detection is a function of not only how strong the signal is, but also the criterion that the perceiver chooses Even basic sensory processes involve both bottom-up and top-down processes Perception depends on contributions from both the stimulus and the perceiver Sub-threshold stimuli can still provide information (and thus influence cognitive processes)

Sensory Storage Sensory information is briefly stored at a low (relatively unprocessed) level Icon – brief storage of visual information Echo – brief storage of auditory information