Sensory Systems
General vs. Special Senses
Sensory systems fall into two broad categories based on where their receptors are located and what kind of information they detect.
General senses (also called somatosensation) include touch, pressure, temperature, pain, and proprioception (your awareness of body position and movement). Receptors for these senses are distributed widely throughout the body in skin, muscles, tendons, and joints. That's why you can feel a hot coffee mug in your hand, sense the texture of fabric between your fingers, or notice muscle soreness after a workout.
Special senses include vision, hearing, taste, smell, and equilibrium (balance). Unlike general senses, these receptors are concentrated in specific organs: eyes, ears, tongue, nose, and the vestibular system of the inner ear. Each special sense has its own type of specialized receptor and dedicated neural pathway to the brain.
Stages of Sensory Perception
Turning a stimulus from the environment into a conscious experience involves five stages. Each one builds on the last.
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Stimulus detection: Sensory receptors pick up environmental stimuli such as light, sound, chemical molecules, or pressure. The receptor converts (transduces) that stimulus into an electrical signal called a receptor potential. For example, photoreceptors in the retina detect light entering the eye.
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Sensory transduction: This is the actual conversion of stimulus energy into changes in the membrane potential of the receptor cell. In the ear, for instance, sound waves vibrate the basilar membrane, which causes hair cells to generate receptor potentials.
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Sensory nerve activation: If the receptor potential is strong enough, it triggers action potentials in sensory neurons. The frequency of those action potentials is proportional to stimulus intensity. Stronger pressure on the skin produces a higher frequency of action potentials.
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Signal transmission: Action potentials travel along sensory nerve fibers to the central nervous system (CNS). Signals may be processed or modified in the spinal cord or brainstem along the way. Sensory signals from your hand, for example, pass through the spinal cord and brainstem before reaching the somatosensory cortex.
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Brain processing: Signals arrive at the appropriate region of the cerebral cortex. The brain interprets the information by integrating it with memories, emotions, and input from other senses. This is what produces conscious perception and drives your response. The visual cortex, for instance, processes signals from the eyes so you can recognize objects and faces.
Sensory Processing and Integration
Sensory processing is how the brain organizes and interprets incoming information. Different sensory pathways carry signals from receptors to specific brain regions, with sensory neurons relaying the information along the way.
Sensory integration takes this a step further by combining information from multiple senses into a single coherent perception. This happens in several brain regions, including the thalamus and cortex. A familiar example: you locate a barking dog by combining what you see (the dog) with what you hear (the bark and the direction it's coming from).
Just-Noticeable Difference and Thresholds
- Absolute threshold is the minimum stimulus intensity needed for detection 50% of the time. It sets the lower limit of what you can perceive. Think of the faintest sound you can hear in a completely quiet room.
- Just-noticeable difference (JND), also called the difference threshold, is the smallest change in a stimulus you can detect 50% of the time. For example, it's the smallest weight difference between two objects that you can reliably tell apart when holding them.
- Weber's Law states that the JND is a constant fraction of the original stimulus intensity. The Weber fraction is:
where is the JND and is the initial stimulus intensity. This means detecting a change requires a larger absolute difference when the starting intensity is high. To notice a change in brightness in a brightly lit room, you need a bigger increase than you would in a dim room. The Weber fraction itself varies by sense (it's different for hearing than for vision, for example).
Sensory Adaptation
Sensory adaptation is the decrease in responsiveness to a constant, unchanging stimulus over time. This keeps your sensory systems tuned to changes in the environment rather than being overwhelmed by background stimulation.
Some senses adapt quickly (touch, smell), while others adapt slowly or barely at all (pain, which makes sense from a survival standpoint). A classic example: when you walk into a room with a strong odor, the smell hits you immediately but fades within minutes as your olfactory receptors adapt.