Stimulus modality, also called sensory modality, is one aspect of a stimulus or what is perceived after a stimulus. For example, the temperature modality is registered after heat or cold stimulate a receptor. Some sensory modalities include: light, sound, temperature, taste, pressure, and smell. The type and location of the sensory receptor activated by the stimulus plays the primary role in coding the sensation. All sensory modalities work together to heighten stimuli sensation when necessary. In fact, the hypothesis of having a centralized multisensory region is receiving continually more speculation, as several regions previously uninvestigated are now considered multimodal. The reasons behind this are currently being investigated by several research groups, but it is now understood to approach these issues from a decentralized theoretical perspective. Moreover, several labs using invertebrate model organisms will provide invaluable information to the community as these are more easily studied and are considered to have decentralized nervous systems.
Lip reading
Lip reading is a multimodal process for humans. Specific inhibitory responses that take place in the visual cortex help create a visual focus on a specific point rather than the entire surrounding.
Perception
To perceive a light stimulus, the eye must first refract the light so that it directly hits the retina. Refraction in the eye is completed through the combined efforts of the cornea, lens and iris. The transduction of light into neural activity occurs via the photoreceptor cells in the retina. When there is no light, Vitamin A in the body attaches itself to another molecule and becomes a protein. The entire structure consisting of the two molecules becomes a photopigment. When a particle of light hits the photoreceptors of the eye, the two molecules come apart from each other and a chain of chemical reactions occurs. The chemical reaction begins with the photoreceptor sending a message to a neuron called the bipolar cell through the use of an action potential, or nerve impulse. Finally, a message is sent to the ganglion cell and then finally the brain.
Adaptation
The eye is able to detect a visual stimulus when the photons (light packets) cause a photopigment molecule, primarily rhodopsin, to come apart. Rhodopsin, which is usually pink, becomes bleached in the process. At high levels of light, photopigments are broken apart faster than can be regenerated. Because a low number of photopigments have been regenerated, the eyes are not sensitive to light. When entering a dark room after being in a well lit area, the eyes require time for a good quantity of rhodopsin to regenerate. As more time passes, there is a higher chance that the photons will split an unbleached photopigment because the rate of regeneration will have surpassed the rate of bleaching. This is called adaptation.
Tests
Some common tests that measure visual health include visual acuity tests, refraction tests, visual field tests and colour vision tests. Visual acuity tests are the most common tests and they measure the ability to bring details into focus at different distances. Usually this test is conducted by having participants read a map of letters or symbols while one eye is covered. Refraction tests measure the eye's need for glasses or corrective lenses. This test is able to detect whether a person may be nearsighted or farsighted. These conditions occur when the light rays entering the eye are unable to converge on a single spot on the retina. Both refractive errors require corrective lenses in order to cure blurriness of vision. Visual field tests detect any gaps in peripheral vision. In healthy normal vision, an individual should be able to partially perceive objects to the left or right of their field of view using both eyes at one time. The center field of vision is seen in most detail. Colour vision tests are used to measure one's ability to distinguish colours. It is used to diagnose colour blindness. This test is also used as an important step in some job screening processes as the ability to see colour in such jobs may be crucial. Examples include military work or law enforcement.
Sound modality
thumb|Diagram of the human ear.
Description
The stimulus modality for hearing is sound. Sound is created through changes in the pressure of the air. As an object vibrates, it compresses the surrounding molecules of air as it moves towards a given point and expands the molecules as it moves away from the point. Periodicity in sound waves is measured in hertz. Humans, on average, are able to detect sounds as pitched when they contain periodic or quasi-periodic variations that fall between the range of 30 to 20000 hertz. In a series of 214 tests conducted on 7 pregnant women, a reliable increase in fetal movement was detected in the minute directly following the application of a sound stimulus to the abdomen of the mother with a frequency of 120 per second.
During a whispered speech test, the participant is asked to cover the opening of one ear with a finger. The tester will then step back 1 to 2 feet behind the participant and say a series of words in a soft whisper. The participant is then asked to repeat what is heard. If the participant is unable to distinguish the word, the tester will speak progressively louder until the participant is able to understand what is being said. The other ear is then tested.
Taste modality in flies and mammals
In insect and mammalian taste, receptor cells changes into attractive or aversive stimulus. The number of taste receptors in a mammalian tongue and on the tongue of the fly (labellum) is same in amount. Most of the receptors are dedicated to detect repulsive ligand.
Integration of taste and smell modality
Impression of both taste and smell occurs in heteromodal regions of the limbic and paralimbic brain. Taste–odor
integration occurs at earlier stages of processing. By life experience, factors such as the physiological significance of a given stimulus is perceived. Learning and affective processing are the primary functions of limbic and paralimbic brain. Taste perception is a combination of oral somatosensation and retronasal olfaction.
Temperature modality
Description
Temperature modality excites or elicits a symptom through cold or hot temperature. Different mammalian species have different temperature modality.
Perception
The cutaneous somatosensory system detects changes in temperature. The perception begins when thermal stimuli from a homeostatic set-point excite temperature specific sensory nerves in the skin. Then with the help of sensing range, specific thermosensory fibers respond to warmth and to cold. Then specific cutaneous cold and warm receptors conduct units that exhibit a discharge at constant skin temperature.
Nerve fibers for temperature
Warm and cold sensitive nerve fibers differ in structure and function. The cold-sensitive and warm-sensitive nerve fibers are underneath the skin surface. Terminals of each temperature-sensitive fiber do not branch away to different organs in the body. They form a small sensitive point which are unique from neighboring fibers. Skin used by the single receptor ending of a temperature-sensitive nerve fiber is small. There are 20 cold points per square centimeter in the lips, 4 in the finger, and less than 1 cold point per square centimeter in trunk areas. There are 5 times as many cold sensitive points as warm sensitive points. This will give a sense of what is being perceived, and give information about size, shape, weight, temperature, and material. Tactile stimulation can be direct in the form of bodily contact, or indirect through the use of a tool or probe. Direct and indirect send different types messages to the brain, but both provide information regarding roughness, hardness, stickiness, and warmth. The use of a probe elicits a response based on the vibrations in the instrument rather than direct environmental information. Tactual perception gives information regarding cutaneous stimuli (pressure, vibration, and temperature), kinaesthetic stimuli (limb movement), and proprioceptive stimuli (position of the body). There are varying degrees of tactual sensitivity and thresholds, both between individuals and between different time periods in an individual's life. It has been observed that individuals have differing levels of tactile sensitivity between each hand. This may be due to callouses forming on the skin of the most used hand, creating a buffer between the stimulus and the receptor. Alternately, the difference in sensitivity may be due to a difference in the cerebral functions or ability of the left and right hemisphere. Tests have also shown that deaf children have a greater degree of tactile sensitivity than that of children with normal hearing ability, and that girls generally have a greater degree of sensitivity than that of boys.
Tactile information is often used as additional stimuli to resolve a sensory ambiguity. For example, a surface can be seen as rough, but this inference can only be proven through touching the material. When sensory information from each modality involved corresponds, the ambiguity is resolved.
Somatosensory information
Touch messages, in comparison to other sensory stimuli, have a large distance to travel to get to the brain. Tactile perception is achieved through the response of mechanoreceptors (cutaneous receptors) in the skin that detect physical stimuli. The response from a mechanoreceptor detecting pressure can be experienced as a touch, discomfort, or pain. Mechanoreceptors are situated in highly vascularized skin, and appear in both glabrous and hairy skin. Each mechanoreceptor is tuned to a different sensitivity, and will fire its action potential only when there is enough energy. The axons of these single tactile receptors will converge into a single nerve trunk, and the signal is then sent to the spinal cord where the message makes its way to the somatosensory system in the brain.
Mechanoreceptors
There are four types of mechanoreceptors: Meissner corpuscles and merkel cell neurite complexes, located between the epidermis and dermis, and Pacinian corpuscles and Ruffini endings, located deep within the dermis and subcutaneous tissue. Mechanoreceptors are classified in terms of their adaptation rate and the size of their receptive field. Specific mechanoreceptors and their functions include:
- Thermoreceptors that detect changes in skin temperature.
- Kinesthetic receptors detect movements of the body, and the position of the limbs.
- Nociceptors that have bare nerve endings that detect tissue damage and give the sensation of pain.
Tests
A common test used to measure the sensitivity of a person to tactile stimuli is measuring their two-point touch threshold. This is the smallest separation of two points at which two distinct points of contact can be sensed rather than one. Different parts of the body have different degrees of tactile acuity, with extremities such as the fingers, face, and toes being the most sensitive. When two distinct points are perceived, it means that your brain receives two different signals. The differences of acuity for different parts of the body are the result of differences in the concentration of receptors.
Smell modality
Sensation
The sense of smell is called olfaction. All materials constantly shed molecules, which float into the nose or are sucked in through breathing. Inside the nasal chambers is the neuroepithelium, a lining deep within the nostrils that contains the receptors responsible for detecting molecules that are small enough to smell. These receptor neurons then synapse at the olfactory cranial nerve (CN I), which sends the information to the olfactory bulbs in the brain for initial processing. The signal is then sent to the remaining olfactory cortex for more complex processing.
Odors
An olfactory sensation is called an odor. For a molecule to trigger olfactory receptor neurons, it must have specific properties. The molecule must be:
- volatile (able to float through the air)
- small (less than 5.8 x 10-22 grams)
- hydrophobic (repellant to water)
However, humans do not detect or process the smell of various common molecules such as nitrogen or water vapor.
Olfactory ability can vary due to different conditions. For example, olfactory detection thresholds can change due to molecules with differing lengths of carbon chains. A molecule with a longer carbon chain is easier to detect, and has a lower detection threshold. Additionally, women generally have lower olfactory thresholds than men, and this effect is magnified during a woman's ovulatory period.
Tests
A common psychophysical test of olfactory ability is the triangle test. In this test, the participant is given three odors to smell. Of these three odors, two are the same and one is different, and the participant must choose which odor is the unique one. To test the sensitivity of olfaction, the staircase method is often used. In this method, the odor's concentration is increased until the participant is able to sense it, and subsequently decreased until the participant reports no sensation.