Laboratory Exercise in Sensory Physiology
Student Lab Manual
Introduction
Sensory organs allow us to perceive our environment by converting energy sources in the environment, like light or sound, to nerve impulses which our brains can interpret. The process of converting environmental stimuli into nerve impulses is called transduction. The world, which we perceive, is entirely dependent on the transduction properties of our sense organs. These properties differ between animal species. The world to a human looks, tastes, sounds, smells, etc. completely different from that perceived by another animal.
Each sensory organ responds to a particular kind of input, and only to a relatively small range of possible stimuli within that modality. The visual system, for example, responds to light, which is a form of electromagnetic radiation. We can only see a small portion of the possible wavelengths of this radiation, because our eyes can only respond to certain wavelengths (see Table 1). Other insects, and birds, for example, can see ultraviolet light, which is invisible to humans. Boas and pit vipers can detect infrared radiation with special, non-visual, organs. Within the range of stimuli to which a sense organ responds, the organ will respond more readily to some stimuli than others. In other words, sensory organs act as filters -- only allowing us to perceive those stimuli which evolution has determined are critical for survival.
In this lab, you will use some of the methods employed by sensory physiologists to objectively quantify human perceptual capabilities. You will see how our perception is influenced and limited by the properties of our sense organs. You will examine three sensory modalities: (1) vision, (2) hearing, and (3) touch. In a number of the following experiments, you will determine thresholds. A threshold is the amplitude of intensity of a stimulus at which it is just barely detectable.
I. Vision
Vision involves the detection of light, often reflected off other objects. Light is a form of electromagnetic radiation. It travels in waves. Different wavelengths of light produce the sensation of different colors. The range of different wavelengths is called the spectrum (see Table 1). The visible spectrum is the familiar colors of the rainbow. Light is detected by the eyes.
Table 1. The Electromagnetic Spectrum
Wavelength (nm) / Description700-1000 / Infrared (detectable by vipers and boas using special pit organs)
425-700 / Visible spectrum (to humans)
Colors of the rainbow starting with violet at 425, ending with red at 700
300-425 / Ultraviolet (visible to some birds and insects)
A schematic diagram of the human eye is shown in Figure 1. Incoming light passes through the pupil, and is focused by the lens onto the retina. Light is actually detected by special receptor cells called rods and cones (see Figure 2). Rods are responsible for vision under low light conditions. Cones function at higher light levels, and are responsible for color vision (note that the first two letters of cones and color are the same). We will consider first how rods function.
Each rod is filled with a pigment called rhodopsin, which absorbs light. When a molecule of rhodopsin absorbs light, it breaks down into two smaller molecules. This reaction is coupled to a change in electrical potential in the cell. This electrical potential is passed on to a cell, called a bipolar cell, which is connected directly to a cell called a ganglion cell. Ganglion cells are nerve cells that make up the optic nerve. If the rod is sufficiently excited (i.e. absorbs enough light), a nerve spike is caused in the ganglion cell. The greater the excitation of the rod (i.e. the more light the pigment rhodopsin absorbs) the more nerve spikes are sent down the ganglion cell to the brain. The ganglion cell carries the message to the brain that light has been detected and the more nerve spikes travel down the ganglion cell, the brighter the perceived light.
Not all colors of the visible spectrum are equally good at exciting the rods. Rhodopsin is much better at absorbing some wavelengths than others. As a result, a much smaller intensity of green light, for example, is required to excite a rod, than of red light. Scientists have extracted rhodopsin from rods and measured its ability to absorb different wavelengths of light. The results are shown in Figure 3. Since only rods function under dim light conditions we can study how rods function affects our perception by analyzing our perception in dim light. As you will observe later in the lab, under dim light conditions all the wavelengths appear to lack color. If a colored light excites a rod to a certain degree, the brain cannot tell if the rod is being stimulated by a color to which the rod is very sensitive, or if it is being stimulated by a very intense light of a wavelength to which it is less sensitive. A graph of the minimum stimulus strength required to just be seen versus the wavelength of light is called a spectral sensitivity curve. The fact that rods are differentially sensitive to different wavelengths of light does not, in itself, allow for perception of colors. This is because there is only one type of rod. The response of rods changes in exactly the same way if we increase brightness, or if we change to a wavelength to which the rod is more sensitive. There is an ambiguity between color and brightness. Thus, a spectral sensitivity curve, which is the minimum intensity required to elicit vision versus the wavelength of the stimulus, is not the same as measuring the ability to distinguish different colors.
Cones are very similar to rods but there are some important differences. First, cones are much less sensitive than rods, and function only under bright light conditions. Second, there are three different types of cones. Each has a different type of pigment, with a different spectral absorption function (illustrated in Fig. 4). One cone type has peak sensitivity to red, one type to blue, and one type to green. There are two main advantages to having three types of cones. First it expands the overall range of wavelengths of light, which are visible. Second it allows us to perceive colors. This is because any given wavelength stimulus excites each of the three different types of cones to a different extent. The brain determines the color of the stimulation by comparing the ratio of the neural output from each of the tree types of cones (cones, like rods, become excited and then excite a ganglion cell). An increase in brightness will increase the neural output from all three cone types in equal amounts and the brain will still see the same color, because the ratio of the outputs will not change.
Another interesting aspect of the visual system is that it remains sensitive over a remarkably wide range of light intensities. In a typical day, the brightness from midday sun exceeds the brightness of evening starlight by a factor of 100,000,000. At any one time, each photoreceptor can only respond to a 100-fold change in brightness. Fortunately, large changes in environmental brightness usually occur relatively slowly, and the visual system can shift so that the intensity range of the photoreceptors is centered about the average intensity of light in the background. The visual system has several ways of adjusting to changes in ambient light intensity, two of which we will discuss here. First, as already mentioned, there are two types of photoreceptors, rods and cones. Rods respond only in dim light, cones only in bright light. Second, rods change their sensitivity with time, according to the prevailing light conditions. This phenomenon is known as dark adaptation. If you go from a lighted area to a very dark room, you will not be able to see. After about ten minutes, however, you begin to be able to see again. This occurs because the rhodopsin pigment, which gets broken down when it absorbs light, is continually re-synthesized in the rods. When there is very little light, very little rhodopsin gets broken down and the amount of rhodopsin gradually builds up, making the rod much more sensitive to dim light.
Another way in which sensitivity is improved in the rods involves the way in which they are hooked up to the ganglion cells. As illustrated in Figure 2, a typical cone connects to one bipolar cell, which connects to a single ganglion cell. In contrast, a number of rods pool their inputs to a single ganglion cell. By adding their inputs, each single rod needs to be excited by only a very small amount to produce a neural spike in the ganglion cell. The phenomenon is known as receptor convergence. There is a trade-off with this method of improving sensitivity. A ganglion cell, which receives input from a number of rods, spread out over the retina, cannot pinpoint the source of a light stimulus as well as a ganglion cell, which is stimulated by a single photoreceptor. In the case of a ganglion cell receiving the added input of several rods, light hitting any one of the rods can cause a neural spike in the ganglion cell, but the brain has no way to know which rod was excited. It is therefore harder to pinpoint the location of light stimuli on the retina when the stimulus is detected by rods (as compared to cones). The result is that our ability to see detail, or our resolving power, is considerably lower under dim light conditions than under bright light conditions.
II. Hearing
Sound consists of rapid changes in air pressure. These pressure changes travel through the air as waves. These waves of pressure change travel through the air and into our ears. Figure 5 shows a simplified picture of the human ear. The sound pressure waves enter the ear and cause the eardrum, or tympanum, to vibrate back and forth. The vibrations of the eardrum are passed through a series of three inner ear bones to an organ called the cochlea. The cochlea is a long thin organ, which, in humans, is coiled into a snail shell shape. Two membranes run the length of the cochlea, with tiny hairs suspended between them. One of these membranes, the basilar membrane, vibrates at the same rate as the eardrum, the other, the tectorial membrane, does not vibrate. This causes a shearing force in the hairs. This shearing force is converted to electrical potential, and causes nerve spikes to be sent, via the neurons of the auditory nerve, to the brain. The amplitude of the vibration in the cochlea varies along its length, depending on the frequency of the stimulation. Low frequency vibrations stimulate the far end of the cochlea, while high frequencies stimulate the near end. The brain determines the frequency of a sound by noting how far down the cochlea the nerve cell, which is sending signals, is located.
The ear is not equally sensitive to all sound frequencies. A greater amplitude is required to hear some sound frequencies than others, while some cannot be heard at all. In this lab, you will measure the relative sensitivity of the human ear to sounds of different frequency.
III. Touch
The sense of touch, as well as hot and cold, is mediated by a variety of sense organs located in the skin. Sense of pain and hot/cold is mediated by free nerve ending s near the surface of the skin. There are a large number of receptors, which consist of nerve endings wrapped around the base of hairs, which protrude from the skin and detect light touch. There are also pressure receptors called Pascinian corpuscles, which are located below the surface of the skin. These resemble tiny onions in shape.
The ability to precisely locate a stimulus on the skin depends on the local density of these receptors. You will examine this effect in the lab today.
Another phenomenon that can be readily observed with the sense of touch (although it occurs in all sensory modalities) is the phenomenon of adaptation, in which sensitivity to a stimulus declines rapidly with exposure time. This occurs in many sense organs. It allows the brain to direct attention to important stimuli and ignore those that are constantly present. In general, changes in stimulation are more critical for attention that is ongoing stimuli. Thus under constant stimulation, the receptor stops sending messages to the brain. This allows us, for example, to wear clothing without having our brain being constantly aware of the contact with our skin. If, however, someone touches us on the back, the touch receptors detect the change and send messages to the brain that the back is being touched.
Quantifying Sensory Stimuli
There is a variety of ways to quantify sensory perception. The most common method is to develop a graph of threshold (the magnitude of the stimulus which is barely detectable) versus stimulus quality (e.g. frequency). In many cases, sensory systems respond over an extremely wide range of stimulus values. Often this extreme range is achieved because the sensory systems respond only when a stimulus value increases by a multiple of its previous value. In order to graph such results we make use of graphs with logarithmic axes. In a logarithmic graph, each major division on the graph represents an increase by a factor of 10. Figure 6 illustrates how one reads a logarithmic scale. In some cases, only one axis is logarithmic. Such a graph is called a semi-log plot. When both axes are logarithmic, the graph is called a log-log plot.