1.2. Sensation Versus Perception

You have probably known since elementary school 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 a bit 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). Regardless, the process of sensing always begins with a stimulus in the environment (see Figure 1.1). A stimulus could be a chemical that we taste or smell, or something that we see, hear, or feel. Our sense organs (eyes, ears, nose, mouth, skin, etc.) contain specialized neurons called receptors. The job of these receptors is to detect a stimulus in the environment. For example, when light from an object (like a tree) enters the eye it causes chemical changes in the receptors that line the back of the eye. These cells then convert the light into neural signals. This process is known as transduction. The neural signals are then sent to other cells in the brain in the form of action potentials. These travel up to the primary visual cortex in the brain. This entire process is known as sensation (represented by the blue boxes in Figure 1.1).

Our sensory receptors are constantly collecting information from the environment, however, it is how we interpret the information that affects how we interact with the world.

Perception refers to the way sensory information is organized, interpreted, and consciously experienced. In the example in Figure 1.1, information is sent to multiple brain areas before we recognize that the object we are looking at is a tree. Perception depends on both bottom-up, and top-down processing (see Figure 1.2). Bottom-up processing refers to the fact that perceptions are built from sensory input. On the other hand, top down processing refers to using what we already know, our experiences (including culture), and our thoughts to interpret the information (Egeth & Yantis, 1997; Fine & Minnery, 2009; Yantis & Egeth, 1999). In the example in Figure 1.1, most people will recognize the picture as a tree, but some people might know the type of tree it is, if they have a lot of prior knowledge about trees and have seen pictures or actual trees that look like this one. Bottom-up processing is automatic, but top-down processing is often under our conscious control.

One way to think of the difference between sensation and perception is that sensation is a physical process, whereas perception is psychological. For example, imagine walking into a kitchen and smelling the aroma of sofrito sizzling, the delicious scent of onions, garlic, and bell peppers dancing in the pan, releasing their sweet fragrance. The sensation begins with the olfactory receptors in the nose detecting the chemicals in the food and sending that information to the brain, but the perception may be “Mmm, Abuela is cooking something delicious.”

Although our perceptions are built from sensations, not all sensations result in perception. Attention plays a significant role in determining what is sensed versus what is perceived. Imagine you are at a party full of music, chatter, and laughter. You get involved in an interesting conversation with a friend and tune out all the background noise. If someone interrupts you to ask what song had just finished playing, you would probably be unable to answer that question. In fact, we often are unaware of stimuli that remain relatively constant over long periods of time. This is known as sensory adaptation. Imagine entering a classroom with an old-fashioned clock that makes a loud ticking sound. When you first enter the room, you can hear the ticking of the clock; but as you begin to engage in conversation with classmates or listen to your professor greet the class, you are no longer aware of the ticking. The clock is still ticking and that information is still affecting sensory receptors of the auditory system. However, because you are no longer paying attention to it – you don’t hear the ticking. We will cover more about attention in an upcoming module.