16-3 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.

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 smell a single drop of perfume in a three-room apartment. We could feel the wing of a bee falling on our cheek (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 16.2). 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.

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 people at speed-dating events often respond to potential dates unselectively—with a low threshold (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.

Stimuli you cannot consciously detect 50 percent of the time are subliminal—below your absolute threshold (see FIGURE 16.2). Under certain conditions, you can still 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 and stop). Without any conscious awareness, the inaction words automatically evoked brain activity associated with inhibiting behavior (Hepler & Albarracin, 2013).

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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 consciously perceived nothing but flashes of color and so were unable to state on which side the nude had appeared. To test whether this unseen image had 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 16.3). 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 consciously aware of it—and even when we are unaware of our evaluation (Ferguson & Zayas, 2009).

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 top-level executives who delegate routine tasks to lower-level mental assistants.

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

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. I [DM] noticed while living two years in Scotland that sheep baas 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. In the late 1800s, Ernst Weber described this with a principle 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 percentage 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).

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