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 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). The sensitivity of our sensory receptors can be quite amazing. 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. Absolute threshold refers to the minimum amount of stimulus energy that must be present for the stimulus to be detected 50% of the time. The sensitivity of a given sensory system to the relevant stimuli can be expressed as an absolute threshold. 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). It turns out that this notion of five senses is oversimplified. You have probably known since elementary school that we have five senses: vision, hearing (audition), smell (olfaction), taste (gustation), and touch (somatosensation). The conversion from sensory stimulus energy to action potential is known as transduction. These cells relay messages, in the form of action potentials (as you learned when studying biopsychology), to the central nervous system. For example, light that enters the eye causes chemical changes in cells that line the back of the eye. When sensory information is detected by a sensory receptor, sensation has occurred. What does it mean to sense something? Sensory receptors are specialized neurons that respond to specific types of stimuli. The chapter will close with a discussion of a historically important theory of perception called Gestalt. Then we consider the physical properties of light and sound stimuli, along with an overview of the basic structure and function of the major sensory systems. We begin by learning the distinction between sensation and perception. This chapter will provide an overview of how sensory information is received and processed by the nervous system and how that affects our conscious experience of the world. We use this information to successfully navigate and interact with our environment so that we can find nourishment, seek shelter, maintain social relationships, and avoid potentially dangerous situations. We rely on our sensory systems to provide important information about our surroundings. You might be struck by movement everywhere as cars and people go about their business, by the sound of a street musician’s melody or a horn honking in the distance, by the smell of exhaust fumes or of food being sold by a nearby vendor, and by the sensation of hard pavement under your feet. Imagine standing on a city street corner.
(credit: modification of work by Cory Zanker) Figure 5.1 If you were standing in the midst of this street scene, you would be absorbing and processing numerous pieces of sensory input.
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