18.2 Thresholds

18-3 How do absolute thresholds and difference thresholds differ, and what effect, if any, do stimuli below the absolute threshold have on us?

At this moment, we are being struck by X-rays and radio waves, ultraviolet and infrared light, and sound waves of very high and very low frequencies. To all of these we are blind and deaf. Other animals with differing needs detect a world that lies beyond our experience. Migrating birds stay on course aided by an internal magnetic compass. Bats and dolphins locate prey using sonar, bouncing echoing sound off objects. Bees navigate on cloudy days by detecting invisible (to us) polarized light.

The shades on our own senses are open just a crack, allowing us a restricted awareness of this vast sea of energy. But for our needs, this is enough.

Absolute Thresholds

To some kinds of stimuli we are exquisitely sensitive. Standing atop a mountain on an utterly dark, clear night, most of us could see a candle flame atop another mountain 30 miles away. We could feel the wing of a bee falling on our cheek. We could smell a single drop of perfume in a three-room apartment (Galanter, 1962).

German scientist and philosopher Gustav Fechner (1801–1887) studied our awareness of these faint stimuli and called them our absolute thresholds—the minimum stimulation necessary to detect a particular light, sound, pressure, taste, or odor 50 percent of the time. To test your absolute threshold for sounds, a hearing specialist would expose each of your ears to varying sound levels (FIGURE 18.2). For each tone, the test would define where half the time you could detect the sound and half the time you could not. That 50-50 point would define your absolute threshold.

Figure 18.2
Absolute threshold Can I detect this sound? An absolute threshold is the intensity at which a person can detect a stimulus half the time. Hearing tests locate these thresholds for various frequencies.

Detecting a weak stimulus, or signal (such as a hearing-test tone), depends not only on its strength but also on our psychological state—our experience, expectations, motivation, and alertness. Signal detection theory predicts when we will detect weak signals (measured as our ratio of “hits” to “false alarms”). Lonely, anxious people at speed-dating events tend to respond with a low threshold, and thus can be unselective in reaching out to potential dates (McClure et al., 2010). Signal detection theorists seek to understand why people respond differently to the same stimuli, and why the same person’s reactions vary as circumstances change.

Try out this old riddle on a couple of friends. “You’re driving a bus with 12 passengers. At your first stop, 6 passengers get off. At the second stop, 3 get off. At the third stop, 2 more get off but 3 new people get on. What color are the bus driver’s eyes?” Do your friends detect the signal—who is the bus driver?—amid the accompanying noise?

Stimuli you cannot detect 50 percent of the time are subliminal—below your absolute threshold (see Figure 18.2). Under certain conditions, you can be affected by stimuli so weak that you don’t consciously notice them. An unnoticed image or word can reach your visual cortex and briefly prime your response to a later question. In a typical experiment, the image or word is quickly flashed, then replaced by a masking stimulus that interrupts the brain’s processing before conscious perception (Herring et al., 2013; Van den Bussche et al., 2009). In one such experiment, researchers monitored brain activity as they primed people with either unperceived action words (such as go and start) or inaction words (such as still or stop). Without any conscious awareness, the inaction words automatically evoked brain activity associated with inhibiting behavior (Hepler & Albarracin, 2013).

Another priming experiment illustrated the deep reality of sexual orientation. As people gazed at the center of a screen, a photo of a nude person was flashed on one side and a scrambled version of the photo on the other side (Jiang et al., 2006). Because the nude images were immediately masked by a colored checkerboard, viewers saw nothing but flashes of color and were unable to state on which side the nude had appeared. To test whether this unseen image unconsciously attracted their attention, the experimenters then flashed a geometric figure to one side or the other. This, too, was quickly followed by a masking stimulus. When asked to give the figure’s angle, straight men guessed more accurately when it appeared where a nude woman had been a moment earlier (FIGURE 18.3). Gay men (and straight women) guessed more accurately when the geometric figure replaced a nude man. As other experiments confirm, we can evaluate a stimulus even when we are not aware of it—and even when we are unaware of our evaluation (Ferguson & Zayas, 2009).

Figure 18.3
The hidden mind After an image of a nude man or woman was flashed to one side or another, then masked before being perceived, people’s attention was unconsciously drawn to images in a way that reflected their sexual orientation (Jiang et al., 2006).

“The heart has its reasons which reason does not know.”

Pascal, Pensées, 1670

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How can we feel or respond to what we do not know and cannot describe? An imperceptibly brief stimulus often triggers a weak response that can be detected by brain scanning (Blankenburg et al., 2003; Haynes & Rees, 2005, 2006). The stimulus may reach consciousness only when it triggers synchronized activity in multiple brain areas (Dehaene, 2009, 2014). Such experiments reveal the dual-track mind at work: Much of our information processing occurs automatically, out of sight, off the radar screen of our conscious mind. Our conscious minds are upstairs executives who delegate routine tasks to downstairs mental butlers.

So can we be controlled by subliminal messages? For more on that question, see Thinking Critically About: Subliminal Persuasion.

Difference Thresholds

To function effectively, we need absolute thresholds low enough to allow us to detect important sights, sounds, textures, tastes, and smells. We also need to detect small differences among stimuli. A musician must detect minute discrepancies when tuning an instrument. Parents must detect the sound of their own child’s voice amid other children’s voices. While living two years in Scotland, I [DM] noticed that sheep baa’s all sound alike to my ears. But not to those of ewes, who, after shearing, will streak directly to the baa of their lamb amid the chorus of other distressed lambs.

The difference threshold (or the just noticeable difference [jnd]) is the minimum difference a person can detect between any two stimuli half the time. That difference threshold increases with the size of the stimulus. If we listen to our music at 40 decibels, we might detect an added 5 decibels. But if we increase the volume to 110 decibels, we probably won’t detect a 5 decibel change.

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THINKING  CRITICALLY  ABOUT

THINKING CRITICALLY ABOUT: Subliminal Persuasion

18-4 Does subliminal sensation enable subliminal persuasion?

Hoping to penetrate our unconscious, entrepreneurs offer audio and video programs to help us lose weight, stop smoking, or improve our memories. Soothing ocean sounds may mask messages we cannot consciously hear: “I am thin”; “Smoke tastes bad”; or “I do well on tests—I have total recall of information.” Such claims make two assumptions: (1) We can unconsciously sense subliminal (literally, “below threshold”) stimuli. (2) Without our awareness, these stimuli have extraordinary suggestive powers. Can we? Do they?

As we have seen, subliminal sensation is a fact. Remember that an “absolute” threshold is merely the point at which we can detect a stimulus half the time. At or slightly below this threshold, we will still detect the stimulus some of the time.

But does this mean that claims of subliminal persuasion are also facts? The near-consensus among researchers is No. The laboratory research reveals a subtle, fleeting effect. Priming thirsty people with the subliminal word thirst might therefore, for a moment, make a thirst-quenching beverage ad more persuasive (Strahan et al., 2002). Likewise, priming thirsty people with Lipton Iced Tea may increase their choosing the primed brand (Karremans et al., 2006; Veltkamp et al., 2011; Verwijmeren et al., 2011a,b). But the subliminal-message hucksters claim something different: a powerful, enduring effect on behavior.

To test whether subliminal recordings have this enduring effect, Anthony Greenwald and his colleagues (1991, 1992) randomly assigned university students to listen daily for five weeks to commercial subliminal messages claiming to improve either self-esteem or memory. But the researchers played a practical joke and switched half the labels. Some students who thought they were receiving affirmations of self-esteem were actually hearing the memory-enhancement message. Others got the self-esteem message but thought their memory was being recharged.

Subliminal persuasion? Although subliminally presented stimuli can subtly influence people, experiments discount attempts at subliminal advertising and self-improvement. (The playful message here is not actually subliminal—because you can easily perceive it.)

Were the recordings effective? Students’ test scores for self-esteem and memory, taken before and after the five weeks, revealed no changes. Yet the students perceived themselves receiving the benefits they expected. Those who thought they had heard a memory recording believed their memories had improved. Those who thought they had heard a self-esteem recording believed their self-esteem had grown. (Reading this research, one hears echoes of the testimonies that ooze from ads for such products. Some customers, having bought what is not supposed to be heard (and having indeed not heard it!) offer testimonials like, “I really know that your recordings were invaluable in reprogramming my mind.”)

Over a decade, Greenwald conducted 16 double-blind experiments evaluating subliminal self-help recordings. His results were uniform: No recording helped more than a placebo, which works only because of our belief in it (Greenwald, 1992).

In the late 1800s, Ernst Weber noted something so simple and so widely applicable that we still refer to it as Weber’s law. This law states that for an average person to perceive a difference, two stimuli must differ by a constant minimum percentage (not a constant amount). The exact proportion varies, depending on the stimulus. Two lights, for example, must differ in intensity by 8 percent. Two objects must differ in weight by 2 percent. And two tones must differ in frequency by only 0.3 percent (Teghtsoonian, 1971).

The difference threshold In this computer-generated copy of the Twenty-third Psalm, each line of the typeface increases slightly. How many lines are required for you to experience a just noticeable difference?

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RETRIEVAL PRACTICE

  • Using sound as your example, explain how these concepts differ: absolute threshold, subliminal stimulation, and difference threshold.

Absolute threshold is the minimum stimulation needed to detect a particular sound (such as an approaching bike on the sidewalk behind us) 50 percent of the time. Subliminal stimulation happens when, without our awareness, our sensory system processes the sound of the approaching bike (when it is below our absolute threshold). A difference threshold is the minimum difference needed to distinguish between two sounds (such as the familiar hum of a friend’s bike and the unfamiliar sound of another bike).