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.
Multimodal perception Multimodal perception is the apparent ability of the mammalian nervous system to combine all of the different inputs of the sensory nervous system to result in an enhanced detection or identification of a particular stimulus. Combinations of all sensory modalities are done in cases where a single sensory modality results in an ambiguous and incomplete result.
Integration of all sensory modalities occurs when multimodal neurons receive sensory information which overlaps with different modalities. Multimodal neurons are found in the superior colliculus; they respond to the versatility of various sensory inputs. The multimodal neurons lead to change of behavior and assist in analyzing behavior responses to certain stimulus. Information from two or more senses is encountered. Multimodal perception is not limited to one area of the brain: many brain regions are activated when sensory information is perceived from the environment. 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. By watching movements of lips and face, humans get conditioned and practice lip reading. Silent lip reading activates the auditory cortex. When sounds are matched or mismatched with the movements of the lips, temporal sulcus of the left hemisphere becomes more active.
Integration effect Multimodal perception comes into effect when a unimodal stimulus fails to produce a response. Integration effect is applied when the brain detects weak unimodal signals and combines them to create a multimodal perception for the mammal. Integration effect is plausible when different stimuli are coincidental. This integration is depressed when multisensory information is not coincidentally presented.
Polymodality Polymodality is the feature of a single receptor of responding to multiple modalities, such as free nerve endings which can respond to temperature, mechanical stimuli (touch, pressure, stretch) or pain (nociception).
Light modality
Description The stimulus modality for vision is light; the human eye is able to access only a limited section of the electromagnetic spectrum, between 380 and 760 nanometres. 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.
Colour stimuli Humans are able to see an array of colours because light in the visible spectrum is made up of different wavelengths (from 380 to 760 nm). Our ability to see in colour is due to three different cone cells in the retina, containing three different photopigments. The three cones are each specialized to best pick up a certain wavelength (420, 530 and 560 nm or roughly the colours blue, green and red). The brain is able to distinguish the wavelength and colour in the field of vision by figuring out which cone has been stimulated. The physical dimensions of colour include wavelength, intensity and purity while the related perceptual dimensions include hue, brightness and saturation.
Primates are the only mammals with colour vision.
The Trichromatic theory was proposed in 1802 by Thomas Young. According to Young, the human visual system is able to create any colour through the collection of information from the three cones. The system will put together the information and systematize a new colour based on the amount of each hue that has been detected.
Subliminal visual stimuli Some studies show that subliminal stimuli can affect attitude. In a 1992 study Krosnick, Betz, Jussim and Lynn conducted a study where participants were shown a series of slides in which different people were going through normal everyday activities (i.e. going to the car, sitting in a restaurant). These slides were preceded by slides that caused either positive emotional arousal (i.e. bridal couple, a child with a Mickey Mouse doll) or negative emotional arousal (i.e. a bucket of snakes, a face on fire) for a period 13 milliseconds that participants consciously perceived as a sudden flash of light. None of the individuals were told of the subliminal images. The experiment found that during the questionnaire round, participants were more likely to assign positive personality traits to those in the pictures that were preceded by the positive subliminal images and negative personality traits to those in the pictures that were preceded by the negative subliminal images.
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
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.
Perception When there are vibrations in the air, the eardrum is stimulated. The eardrum collects these vibrations and sends them to receptor cells. The ossicles which are connected to the eardrum pass the vibrations to the fluid-filled cochlea. Once the vibrations reach the cochlea, the stirrup (part of the ossicles) puts pressure on the oval window. This opening allows the vibrations to move through the liquid in the cochlea where the receptive organ is able to sense it.
Pitch, loudness and timbre There are many different qualities in sound stimuli including loudness, pitch and timbre.
The human ear is able to detect differences in pitch through the movement of auditory hair cells found on the basilar membrane. High frequency sounds will stimulate the auditory hair cells at the base of the basilar membrane while medium frequency sounds cause vibrations of auditory hair cells located at the middle of the basilar membrane. For frequencies that are lower than 200 Hz, the tip of the basilar membrane vibrates in sync with the sound waves. In turn, neurons are fired at the same rate as the vibrations. The brain is able to measure the vibrations and is then aware of any low frequency pitches.
When a louder sound is heard, more hair cells are stimulated and the intensity of firing of axons in the cochlear nerve is increased. However, because the rate of firing also defines low pitch the brain has an alternate way of encoding for loudness of low frequency sounds. The number of hair cells that are stimulated is thought to communicate loudness in low pitch frequencies.
Aside from pitch and loudness, another quality that distinguishes sound stimuli is timbre. Timbre allows us to hear the difference between two instruments that are playing at the same frequency and loudness, for example. When two simple tones are put together they create a complex tone. The simple tones of an instrument are called harmonics or overtones. Timbre is created by putting the harmonics together with the fundamental frequency (a sound's basic pitch). When a complex sound is heard, it causes different parts in the basilar membrane to become simultaneously stimulated and flex. In this way, different timbres can be distinguished.
Sound stimuli and human fetuses A number of studies have shown that a human fetus will respond to sound stimuli coming from the outside world. 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.
Tests Hearing tests are administered to ensure optimal function of the ear and to observe whether or not sound stimuli is entering the ear drum and reaching the brain as should be. The most common hearing tests require the spoken response to words or tones. Some hearing tests include the whispered speech test, pure tone audiometry, the tuning fork test, speech reception and word recognition tests, otoacoustic emissions (OAE) test and auditory brainstem response (ABR) test.