Experimental Psychology PSY 433 Chapter 7 Perception (Cont.)

DVs in Perception Experiments Verbal descriptions of experience. Imprecise. Not immediately verifiable. Reaction times. Reports that can be verified: What did you see? Confidence ratings – how sure are you that you are correct?

IVs in Perception Experiments Physical characteristics of stimuli: Visual: size, shape, background, perspective. Auditory: pitch, intensity, waveform (timbre), complexity, relation of sounds to each other. Time course – brief presentation. Sensory degradation or deprivation.

Control Variables Physical aspects of the stimuli that are not being investigated (manipulated). Emotional and motivational aspects of the task. Hungry people see food-related objects. Decision aspects of the task.

Verbal Reports Since we cannot know the phenomenological experience of a person viewing a stimulus, what do verbal reports mean? A verbal report is meaningful if there is a relationship between it and the characteristics of the preceding perceptual event. Most people respond in similar ways to the same stimuli. Because a relationship exists, we can infer the phenomenology from the verbal report given.

Redefining Perception If perception is the interpretation of sensation, how can phenomena such as “blindsight” occur? Blindsight – visual capacity in a blind spot while there is no awareness of perception. Someone’s performance shows they can “see” but they report not seeing anything. D.B.’s scotoma (blind spot) was related to brain injury to visual cortex resulting from surgery.

Sample Gratings D.B. was consciously aware of seeing stimulus A but did not see stimulus B, despite making correct responses to it at a rate greater than chance.

Can You Learn in Your Sleep? How about weight-loss during sleep? Are teenagers being influenced by satanic verses hidden in rock & roll songs? http://www.umich.edu/~onebook/pages/frames/legalF.html Do “subliminal” pictures of hot buttered popcorn increase the likelihood that people will eat popcorn in a movie theater?

Perception Without Awareness? Marcel (1983) used a primed Stroop task presented using a tachistoscope. A prime word (a color word or a neutral word) was presented, followed by a mask at: 400 ms (aware condition) A shorter interval allowing only 60% detection of the prime (unaware condition) A color patch followed the mask. Prime and patch were either congruent, incongruent, or neutral.

Marcel (1983) Results Regardless of whether subjects were aware or unaware of the prime, compared to control: Faster responses on congruent trials Slower responses on incongruent trials Conclusion: meaning CAN be perceived without awareness Conclusion: the prime’s meaning is processed mentally, despite subjects’ verbal reports that they never saw the prime.

What is a Threshold? Threshold -- an intensity value above which a person always perceives, and below which a person does not perceive. Think of a door – the higher the threshold, the more difficult it is to enter a room Threshold and sensitivity are inversely related. Higher threshold means less detection. Lower threshold means more detection.

Cheesman and Merikle (1984) Questioned Marcel’s results – different thresholds may be used by different cognitive processes: Detection threshold Verbal report threshold The threshold for conscious awareness of words may be higher than for responding differentially to their meanings. Marcel’s mask may have interfered with one threshold (consciousness) but not the other.

Cheesman & Merikle’s Method The prime-mask interval was set by forced-choice detection of the 4 color word primes at 25%, 55%, and 90% Subjects said they couldn’t see prime in all three conditions (25%, 55%, 90%). Otherwise, their method was similar to Marcel’s.

Verbally unaware V. Aware

Two Threshold Theory Based on their findings, Cheesman & Merikle suggest that there exist two kinds of thresholds: Objective threshold – where discriminative responding is at chance. Subjective threshold – where responding is above chance but subjects report being unaware of the stimuli. This defines the three regions.

Cheesman & Merikle (1986) Does unconscious processing differ from conscious processing? (Yes, it does) Varied % of congruent trials (33 vs. 66) Previous results: For conscious processing, the more congruent trials, the greater the difference between congruent/incongruent RTs Subjects are biased to respond “congruent” because they know it’s the most frequent trial Does this occur when they don’t know?