• Define sensation
• Define perception

You have probably known from a young age that we have five senses: vision, hearing (audition), smell (olfaction), taste (gustation), and touch (somatosensation). It turns out that this notion of five senses is oversimplified—we also have sensory systems that provide information about balance (the vestibular sense), body position and movement (proprioception and kinesthesia), pain (nociception), and temperature (thermoception).

So what does it mean to sense something?

sensation

Sensory receptors are specialized neurons that respond to specific types of stimuli. When sensory information is detected by a sensory receptor, sensation has occurred.

 

This message is then relayed to the central nervous system via the action potentials of sensory neurons (as you learned when studying biopsychology). The conversion from sensory stimulus energy to action potential is known as transduction.

The sensitivity of a given sensory system to the relevant stimuli can be expressed as an absolute threshold.

absolute threshold

Absolute threshold refers to the minimum amount of stimulus energy that must be present for the stimulus to be detected 50% of the time. Another way to think about this is by asking how dim can a light be or how soft can a sound be and still be detected half of the time. A stimulus reaches a physiological threshold when it is strong enough to excite sensory receptors and send nerve impulses to the brain: this is an absolute threshold.

The sensitivity of our sensory receptors can be quite amazing. It has been estimated that on a clear night, the most sensitive sensory cells in the back of the eye can detect a candle flame 30 miles away (Okawa & Sampath, 2007). Under quiet conditions, the hair cells (the receptor cells of the inner ear) can detect the tick of a clock 20 feet away (Galanter, 1962).

It is also possible for us to receive messages that are presented below the threshold for conscious awareness—these are called subliminal messages.

subliminal messages

Subliminal messages register below the threshold—they are not strong enough to excite sensory receptors and send nerve impulses to the brain. The message is received, but we are not consciously aware of it, and for whatever reason, it has not been selected for processing in working or short-term memory.

Over the years there has been a great deal of speculation about the use of subliminal messages in advertising, rock music, and self-help audio programs. Research evidence shows that in laboratory settings, people can process and respond to information outside of awareness. However, this does not mean that we obey these messages like zombies; in fact, hidden messages have little effect on behavior outside the laboratory (Kunst-Wilson & Zajonc, 1980; Rensink, 2004; Nelson, 2008; Radel, Sarrazin, Legrain, & Gobancé, 2009; Loersch, Durso, & Petty, 2013).

Dig Deeper: Nonconscious^[1] Perception

These days, most scientific research on nonconscious processes (processing that occurs while we are awake, but unaware) is aimed at showing that people do not need consciousness for certain psychological processes or behaviors. One such example is attitude formation. The most basic process of attitude formation is through mere exposure (Zajonc, 1968). Merely perceiving a stimulus repeatedly, such as a brand on a billboard one passes every day or a song that is played on the radio frequently, makes people view it more positively. This is known as the mere exposure effect. Interestingly, mere exposure does not require conscious awareness of the object of an attitude; the effect can occur even when novel stimuli are presented subliminally for extremely brief durations (e.g., Kunst-Wilson & Zajonc, 1980). In such subliminal mere-exposure experiments, participants indicate a preference for, or a positive attitude towards, stimuli they do not consciously remember being exposed to.

Absolute thresholds are generally measured under incredibly controlled conditions in situations that are optimal for sensitivity. Sometimes, we are more interested in how much difference in stimuli is required to detect a difference between them.

just noticeable difference (jnd) or difference threshold

Just noticeable difference (jnd) or difference threshold is the difference in stimuli required to detect a difference between them. Unlike the absolute threshold, the difference threshold changes depending on the stimulus intensity. While there are estimations of humans’ ability to detect differences in stimuli, the jnd can differ between individuals.

As an example, imagine yourself in a very dark movie theater. If an audience member were to receive a text message on her cell phone which caused her screen to light up, chances are that many people would notice the change in illumination in the theater. However, if the same thing happened in a brightly lit arena during a basketball game, very few people would notice. The cell phone brightness does not change, but its ability to be detected as a change in illumination varies dramatically between the two contexts.

Ernst Weber proposed this theory of change in difference threshold in the 1830s, and it has become known as Weber’s law.

Weber’s law

Weber’s law states that the difference threshold is a constant fraction of the original stimulus; it is the idea that bigger stimuli require larger differences to be noticed. For example, it will be much harder for your friend to reliably tell the difference between 10 and 11 lbs. (or 5 versus 5.5 kg) than it is for 1 and 2 lbs.

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