How Do We Sense and Perceive the World?

Sensation, Transduction and Perception

Sensation is the process of receiving stimulus energy from the external environment. We detect stimulus energy through specialized receptor cells, called sensory receptors, found in our sense organs. For example, we see things through our eyes, hear sounds through our ears, and feel touch through our skin. Stimulus energies detected by our sense organs are then transduced (or decoded) and passed on by afferent nerves to the brain. Transduction is the process of transforming external stimulus energy into electrical energy, thus triggering action potential. Action potential, as stated in the previous article about neurons and neurotransmitters, is the process of converting electricity into chemical code. Electrochemical transmission is the process by which nerves in the body communicate with each other. When sensory information reaches the brain, it then organizes, interprets and gives meaning to it, a process called perception. Sensory information thus travels from sensory receptors, to afferent nerves, and then to the brain. A part of the brain, called the thalamus, directs the path of sensory information to the different sensory areas in the cerebral cortex (the convoluted cap of the brain). According to David More (1982), perception is the process of representing external sensory information internally. Although sensation and perception may seem different from each other, they are actually inseparable, forming part of a unified information processing system.

Sensory information may travel upwards or downwards. The process of sensation and perception is shown to move in two pathways. Bottom-Up Processing (or Inductive Processing) is the common way of sensing and perceiving. For example, while you're reading this article, visual information is decoded electrochemically until it reaches the brain, where it gets its meaning. Top-Down Processing (or Deductive Processing), on the other hand, happens when we think of something and then miraculously "sense" it. For example, in the middle of a conversation, you might remember something funny that happened just this morning. You see your dog pooped over your mother's head even if it is not there! This is because you pulled the sensory information from memory, and momentarily saw the event right in front of your eyes. Top-down processing also happens during sleep, especially when we dream. How many times have you felt like running and then tripped over while lying on your bed?

The importance of sensation and perception, according to the evolutionary perspective, is survivability. Aside from knowing what food to eat, what sound comes from predators and preys, and what smells are dangerous or not, the struggle for survival has also shaped the way we sense and perceive the world. For example, Anablepis microlepis is a 4-eyed fish believed to have developed its two eyes poked above sea surface to warn itself of predators lurking above the sea.

Sensory Receptors

As previously mentioned, sensory receptors are cells in the body that receive sensory information from the external environment. The characteristics of sensory receptors are:

Selectivity. Sensory receptors only allow certain types of energy. For example, visual sensory receptors cannot process auditory information.
Specialization. Because sensory receptors are selective, they also connect to specific neural pathways. For example, visual and auditory information do not travel on the same afferent nerves and route towards the brain.
Adaptation. Sensory receptors respond to changes based on the average level of sensory stimulation. For example, it takes approximately 45 minutes for the human eye to completely adjust the contrast between darkness and light.

Because sensory receptors are selective and specialized, and adapt differently, sensory receptors may be classified mainly into 3 types:

Photoreceptive Cells include the visual receptor cells found in the eyes;
Mechanoreceptive Cells include the auditory, cutaneous (pressure, temperature and pain), kinesthetic and vestibular sensory receptors found in the joints and the ears; and lastly,
Chemoreceptive Cells include the taste and olfactory sensory receptors found in the tongue and in the nose.

Threshold

Psychophysics is the scientific study of the link between the physical properties of stimuli and the experience of them. Threshold, a core concept in psychophysics, is also central to the study of sensation and perception. Thresholds are established when 50% of the population report any change in the level of sensory stimulation. Ideally, psychophysicists try to eliminate noise, or any irrelevant and competing stimuli, when measuring thresholds.

There are two kinds of thresholds - absolute and difference thresholds. Absolute Threshold is the minimum amount of energy that can be detected. Although it varies across individuals, approximate absolute threshold for the five senses are: seeing candle light in total darkness from 30 miles, hearing a ticking watch in total silence from 10 feet, smelling perfume drop across 3 rooms, tasting a teaspoon of sugar in 2 gallons of water, and feeling a fly wing touch the cheek from a distance of 1 centimeter. In contrast to absolute threshold, Difference Threshold, also known as "just noticeable difference", is the minimum amount of energy needed to notice difference in magnitude. According to German physiologist E.H. Weber (1850s), there is a constant minimum percentage increase required to notice a difference - 1.67% for brightness, 3% for pitch, 20% for taste, and 25% for smell. Also known as Weber's Law, this constant minimum percentage is found to be generally true across the different senses.

The Role of Attention

Three general characteristics of attention are central to the study of sensation and perception. These are selectivity, shiftability and simultaneous monitoring.

Selectivity is the ability to focus and ignore other stimuli. One good example is the cocktail party effect, a phenomenon wherein the conversation between two people dominates the louder background sounds in the party. This is why you can still hear your friend talk amidst a crowd of talking people. An important aspect of selectivity is decision-making. We basically decide what stimuli to attend to. One theory that elaborates the function of selectivity is the Signal Detection Theory. It basically states that we select what stimuli to attend to and what to ignore based on our own established criterion. Oftentimes applied in medical diagnosis for brain tumor, the Signal Detection Theory enlists four possible outcomes from identifying the presence or absence of a stimulus - hit, miss, false alarm and correct rejection. Using the example of the cocktail party effect, you might hit or miss some of your friend's words, think it was your friend talking, or correctly identify that a certain phrase did not come from your friend.

Shiftability is the ability to attend from one stimulus feature to another. For example, we are especially attuned towards hearing our own names, no matter who says it. This is because we know exactly the collective features (the timbre) of the sound of our names. Another example is a common kindergarten problem: Identify how many apples are in the basket and what their colors are. It would be easy to say "four red apples". However, there are times when we are unable to easily shift from one stimulus feature to another. One popular example is the Stroop Effect, which demonstrates our inclination to read words than to identify their colors. (Try it: Tell me the text colors of these words - red, blue. You probably read the words first before identifying their colors. Now imagine if I asked you to identify the text colors of 30 words in 30 seconds. The chance of you getting the items wrong is significantly higher than getting them right.) Shiftability is often difficult during a perceptual set, or the tendency or predisposition to perceive something in a particular way, that is, in the stroop effect, identifying meaning dominates over identifying color.

Simultaneous Monitoring is the subconscious ability to engage in parallel processing. What the cocktail party effect and the stroop effect also show is our ability to monitor and attend to many stimuli at once, although in varying degrees.

Besides our abilities to select, shift and simultaneously attend to different stimuli, attention is also guided by many other factors, such as motivation, interests (which involves top-down processing), and the stimulus' features (which involve bottom-up processing). Generally, we are turned on the novelty, size, color and movement of a stimulus.

Awareness and Subliminal Perception

Although the above discussions emphasize how aware we are of our sensations and perceptions, there are also times when we are actually unaware of them. Subliminal Perception is our ability to sense and perceive without knowing (or without being aware of knowing). Carol Fowler, et al. (1981) demonstrated this phenomenon when they flashed the word "lodge" for less than a fraction of a second, which is too fast to be read, before flashing the words "hotel" and "book". The participants were asked to identify which of the two words is more related to the previous "unknown" word. Surprisingly, a significant majority of the participants correctly identified "hotel".

The concept of subliminal perception has been applied in different areas. It was allegedly used by the Republicans in a TV commercial where the word "RATS" was flashed 1/30 of a second before the statement "The Gore Prescription Plan: Bureaucrats Decide." Also, it was later unveiled that Tiger Woods used subliminal motivational tapes when he was young. Lastly, in the world of rock music, Mosey Crue's "Shout at the Devil" album was controversially shown to say "Backward mask, where are you, oh. Lost in error, Satan," when played backwards.

Although the implications of subliminal perception caught the attention of popular media, many psychologists still criticize the extent of its effect. For one, clear messages can influence behavior better than subliminal messages. Furthermore, it was shown that those who expect to hear a message from a subliminal tape indeed heard more than those who don't. Clearly, factors, such as message clarity and expectations, play significant roles in communication.