How do sensation and perception relate to each other? - Chapter 5

A patient PT had suffered a cerebral vascular accident, commonly known as a stroke. The unusual aspect was that he got really weird symptoms 4 months later. The most troubling was, he could not recognize the people around him. He had, for instance, no trouble seeing his wife, but when it came to identifying her, he was at a complete loss. He knew that her body parts - arms, legs, head - formed a person, but PT failed to see these parts as belonging to a specific individual. A striking feature was that PT impairment was that his inability to recognize objects and people was limited to the visual modality. As soon as his wife would speak a few words, he would recognize her. 

What are senses, sensation and perception?

Perception begins when a stimulus from the environment such as sound, light, or touch, stimulates one of the sense organs such as the ear, eye or skin. The sense organs transduces the input into neuronal activity, which then goes to the brain for processing. Sensation is this initial activation of the nervous system, the translation of information about the environment into patterns of neural activity. The mental representation of that original stimulus, whether it accurately reflects the stimulus or not, is called a percept. Thus perception is the process of constructing a percept. Our senses are our physiological capacities to provide input from the environment to our neurological system. Our sense of sight is our capacity to capture light waves on the retina, convert them into electrical signals, and ship them on for further processing.

What is common processing across the senses?

Each system begins with some sort of anatomical structure for collecting, filtering, and amplifying information from the environment. Each system has also specialized receptor cells that transduce the environmental stimulus, such as sound waves, light waves, or chemicals, into neuronal signals. These signals are then passed along specific sensory nerve pathways: the olfactory signals travel via the olfactory nerve, visual signals via the optic nerve, auditory signals via the cochlear nerve, taste via the facial and glossopharyngeal nerves, facial sensation via the trigeminal nerve, and sensation for the rest of the body via the sensory nerves that synapse in the dorsal roots of the spinal cord. These nerves terminate their monosynaptically or disynaptically in different parts of the thalamus. From the thalamus, neural connections from each of these pathways travel first to what are known as primary sensory regions of the cortex, and then to secondary sensory areas. 

What are the sensory receptors?

Across the senses, receptor cells share a few general properties. Receptor cells are limited in the range of stimuli they respond to, and as part of this limitation, their capability to transmit information has only a certain degree of precision. 

• Range: each sensory modality responds to a limited range of stimuli. This range is not the same for all species. As limited as our receptor cells may be, we do respond to a wide range of stimulus intensities. 

• Adaptation: this is the adjustment of the sensory system's sensitivity to the current environment and to important changes in the environment. Adaptation happens quickly in the olfactory system; one minute you can smell the freshly baked bread in the bakery, the next minute it is gone. 

• Acuity: Our sensory systems are tuned to respond to different sources of information in the environment. How well we can distinguish among stimuli within a sensory modality, or what we would call acuity, depends on a couple of factors. Our visual acuity is better than most animals, but not better than an eagle's. Our acuity is the best in the center of the visual field, because the central region of the retina, the fovea, is packed with photoreceptors. We have to move our eyes frequently in order to focus on different parts of the visual scene. These rapid eye movements are called saccades. 

What is the role of olfaction?

The ability to smell is essential for terrestrial mammals, helping them to recognize foods that are nutritious and safe. It now serves other important roles as well - for instance, knowing or detecting a hazard such as fire or airborne toxins. Olfaction also plays an important role in social communication: pheromones are excreted or secreted chemicals that trigger a social response in another individual of the same species when perceived by the olfactory system. Smell is the sensory experience that results from the transduction of odor molecules, or odorants, into neuronal signals sent to the olfactory cortex. These molecules enter the nasal cavity during the course of normal breathing or when taking a sniff.

The human sense of smell uses the input from odor receptors embedded in the mucous membrane of the roof of the nasal cavity to discriminate among odorants. There are tens of thousands of odorants and over a thousand types of receptors: most receptors respond to only a limited number of odorants, though a single odorant can bind to more than one type of receptor. 

Another hypothesis is that the molecular vibrations of groups of odorant molecules contribute to odor recognition. This model predicts that odorants with similar vibrational spectra should elicit similar olfactory reponses, and it explains why similarly shaped olfactory molecules with dissimilar vibrations have very different fragrances. 

The olfactory receptors are called bipolar neurons because appendages extend from opposite sides of their cell bodies. When odorant triggers a receptor, they send a signal to the glomeruli, the neurons in the olfactory bulb. The axons from the glomeruli then exit laterally from the olfactory bulb, forming the olfactory nerve. Their destination is the primary olfactory cortex, located at the ventral junction of the frontal and temporal cortices. The olfactory pathway is unique in two ways:

1. Most of the axons of the olfactory nerve project to the ipsilateral cortex. 

2. The olfactory nerve arrives at the primary olfactory cortex without passing through the thalamus. 

Also, research has shown that the primary olfactory cortex might be essential for detecting a change in the external odor and that the secondary olfactory cortex might be playing an important role in identifying the odor itself. 

What holds gustation?

The sense of taste depends greatly on the sense of smell. Since these two senses interpret the environment by discriminating between different chemicals, they are referred to as the chemical senses. Gustation begins with the tongue. Across the surface of the tongue are different kinds of papillae present. They serve multiple functions; some are concerned with gustation, some with sensation, and some with the secretion of lingual lipase, an enzyme that helps break down fats. Whereas the papillae in the anterior region of the tongue contain just a few taste buds, the papillae found near the back of the tongue have hundreds to thousands of taste buds. There are five basic tastes: salty, sour, bitter, sweet and umami - the savory taste you experience when you eat steak or other protein-rich substances. All five tastes are present across the tongue. 

The sensory transduction in the gustatory systems begins when a food molecule, or tastant, stimulates a taste receptor cell and causes it to depolarize. Each of the basic taste sensations has a different from of chemical signal transduction. Synapsing with the taste receptor cells in the taste buds are bipolar neurons. Their axons from a nerve that joints the other fibers to form the facial nerve. The next synapse in the gustatory system is on the ventral posterior medial nucleus (VPM) of the thalamus. Axons form the VPM synapse in the primary gustatory cortex. This is connected to secondary processing areas of the orbitofrontal cortex, providing an anatomical basis for the integration of tastes and smells. 

The tongue does more than taste. Some papillae contain nociceptive receptors, a type of pain receptor. The output joints the trigeminal nerve. This nerve not only carries pain information, but also signals position and temperature. A gustotopic map has recently been identified in the primary gustatory cortex of the mouse brain, with considerable segregation of areas responsive to the five basic tastes. The perception of more complex tastes arises from the combination of these fundamental tastes, perhaps in the secondary gustatory cortex within the ortbitofrontal cortex.

What is somatosensation?

Somatosensory perception is the perception of all mechanical stimuli that affect the body, including the interpretation of signals that indicate the position of our limbs and the position of our head, as well as our senses of temperature, pressure, touch and pain. Somatosensory receptors lie under the skin and at the musculoskeletal junctions.

Touch is signaled through: Meissner's corpuscles, Merkel's cells, Pacinian corpuscles, and Ruffini corpuscles. Pain is is signaled by nociceptors, the least differentiated of the skin's sensory receptors. They come in three flavors:

• Thermal receptors that respond to heat or cold

• Mechanical receptors that respond to heavy mechanical stimulation

• Multimodal receptors that respond to a wide range of noxious stimuli, such as heat, mechanical insults, and chemicals

The afferent pain neurons may be either myelinated or unmyelinated. The myelinated fibers quickly conduct information about pain, the activation of these cells usually produces immediate action. Specialized nerve cells provide information about the body's position, or what is called proprioception. This enables the sensory and motor systems to represent information about the state of the muscles and limbs. Somatosensory receptors have their cell bodies in the dorsal-root ganglia. They enter the spinal cord via the dorsal root, some synapse on motor neurons in the spinal cord from the reflex arcs. Other synapse on neurons send axons up the dorsal column of th spinal cord to the medulla. 

The initial cortical receiving area is called the primary somatosensory cortex, or S1. It contains a somatotopic representation of the body, called the sensory homunculus. The relative amount of cortical representation in the sensory homunculus corresponds to the relative importance of somatosensory information for that part of the body. For instance, the large representation of the hand is essential, given the great precision we need in using our fingers to manipulate objects and explore surfaces. 

The secondary somatosensory cortex, or S2, builds more complex representations. Regarding to touch, the S2 neurons may code information about object texture and size. 

Looking at the somatotopic maps may make you wonder just how much of that map is set in stone. If you work with numbers from quite some time, would you see changes in parts of the visual cortex that discriminate numbers? They did a research experiment using the somatosensory representation of the hand area of professional violin players. They found that the responses in the musicians' right hemisphere, which controls the left-hand fingers that manipulate the violin strings, were stronger than those observed in non musicians. So, you can say that somatosensory information exhibit plasticity, showing variation in extent and organization as a function of individual experience. 

What is the role of audition?

The sense hearing, or audition, plays an important role in our daily lives. How does the brain process sound waves? How does the nervous system figure out what and where the sound comes from? The complex structures of the peripheral auditory system - outer, middle and inner ear - provide the mechanisms for transforming sounds into neuronal signals. Sounds waves arriving at the outer ear enter the auditory canal. In the canal, the sound waves are amplified, that they travel to the far end of the canal where they hit the tympanic membrane/ eardrum and make it vibrate. These low-pressure vibrations then travel through the air-filled middle ear and rattle three tiny bones: malleus, incus and stapes, which cause the oval window, to vibrate. The oval window is the door to the fluid-filled cochlea. In the cochlea there are tiny haircells located along the inner surface of the basilar membrane. The location of the hair cells on the basilar membrane determines the frequency tuning, the sound frequency that it responds to. 

The spatial arrangement of the sound receptors is known as tonotopy, and the arrangement of the hair cells along the cochlear canal forms a tonotopic map. Natural sounds such as music or speech are made up of complex frequencies, thus a natural sound will activate a broad range of hair cells. 

The central auditory system contains several synapses between the hair cells and the cortex. The cochlear nerve projects to the cochlear nuclei in the medulla. Axons from the cochlear nuclei travel up to the pons and split to innervate the left and right olivary nucleus. From the midbrain, auditory information ascends to the medial geniculate nucleus (MGN) of the thalamus, which projects to the primary auditory cortex (A1) in the superior part of the temporal lobe. 

The computational goal of audition is to determine the identity (what) and location (where) of sounds. The brain must take the auditory signal and, using acoustic cues such as frequency and timbre, convert it into a perceptual representation that can be further combined with information from other systems, such as memory and language. We can discriminate between the sound of a banjo and that of a guitar, but we are still able to identify a 'G' from both as the same note, because the notes share the same base frequency. 

A second important function of audition is to localize sounds in space. In solving the 'where' problem the auditory system relies on integrating information from two ears. Barn owls rely on two cues to localize sounds:

• the difference in when a sound reaches each of the two ears (the interaural time)

• the difference in the sound's intensity at the two ears. 

What is vision?

Both audition and vision are important for perceiving information at distance, engaging in what is called remote sensing; we need not be in immediate contact with stimulus to process it. An organism can avoid a predator better when it can detect the predator at a distance. 

Visual information is contained in the light reflected from objects. To perceive objects, we need sensory detectors that respond to the reflected light. As light passes through the lens of the eye, the image is inverted and focused to project on the retina. The deepest layers are composed of millions of photoreceptors that  are protein molecules that are sensitive to light. They consist of rods and cones. The rods contain the photopigment rhodopsin, therefor they are more useful at night, when light energy is low. Cones contain photopsin, they require more intense levels of light, cones are therefor more active during daytime vision. There are three types of cones:

1. Cones that respond to shorter wavelengths, the 'blue' part of the spectrum

2. Cones that respond to medium wavelengths, the 'green' region

3. Cones that respond to the longer wavelengths, the 'red' part

Rods and cones are not distributed equally across the retina. The cones are densely packed near the fovea. Rods are distributed across the retina. The rods and cones are connected to bipolar neurons that synapse with ganglion cells, the output layer of the retina. The axons of these cells form a bundle, the optic nerve, that transmits information to the central nervous system. The axons that make up the medial half of each optic nerve cross to the opposite hemisphere and form an intersection at the optic chiasm. Axons in the optic nerve synapse on the lateral geniculate nucleus or LGN and from the LGN become the optic radiations that project to the primary visual cortex, or V1. 

The visual system identifies the what and where of objects. Because of the optics of the eye, light reflecting off objects in the environment strikes the eye in an orderly manner. Neurons in the visual system keep track of where objects are located in space by responding only when a stimulus is presented in a specific region of space, named the receptive field. Visual cells from an orderly mapping between spatial location and the neural representation of that dimension. These tonotopic representations are called the retinotopic maps in vision. 

The optimal stimulus becomes more complex as information moves through the system: cells in the retina and LGN respond best to small spots of light, while cells in V1 are sensitive to edges. Farther up in the system, the areas like V4 and TE, the optimal stimulus becomes much more complex, such as shapes or even faces. The visual cortex is made up of many distinct regions defined by their distinct retinotopic maps. The visual areas have functional differences that reflect the types of computations performed by cells within each area. For instance, cells in area V4 are more sensitive to color information, whereas the cells in V5 are sensitive to motion information. 

Why has the primate brain evolved so many visual areas? One possibility is that processing works hierarchical. Each area, representing the stimulus in a unique way, successively elaborates on the representation derives by processing in earlier areas. Successive elaboration culminates in formatting the representation of the stimulus so that it matches information in memory. But there is a problem, there is no such thing as hierarchy.

An alternative hypothesis is based on the idea that visual perception is an analytic process. Each visual area provides a map of external space, each map represents different types of information. This hypothesis suggests that neurons within an area not only code where an object is located in visual space, but also provide information about the object's attributes. 

What leads from sensation to perception?

At what stage of processing does this sensory stimulation become a percept, something we experience phenomenally? One way to study this question is to 'trick' our sensory processing systems with stimuli that cause us to form percepts that do not correspond to the true stimuli in the environment - to perceive illusions. Our percepts are more closely related to activity in higher visual areas that to activity in the primary visual cortex. A strong case for the hypothesis that perception is more closely linked to secondary sensory areas would require evidence showing that activity in these areas can be sufficient, and even predictive, of perception. 

Before the advent of neuroimaging, much of what we learned about processing in the human brain came from the study of patients with lesions, including those with disorders of perception. For instance, describing the loss of the ability to perceive colors in the right visual field of a patient. 

A rare disorder of color perception is a disorder that arises from disturbances of the central nervous system. These disorders are called achromatopsia. Individuals with these disorders are able to see and recognize objects color is not a necessary cue for shape perception. But, despite their relatively good visual recognition, achromatiopsia patients are likely to have some impairments in their ability to perceive shape, given that color-sensitive neurons show tuning for other properties as well, such as orientation. Lesions to areas in and around V4 can result in achromatopsia. 

Another rare disorder has to do with a selective loss of motion perception, is called akinetopsia. The patients view of the world was akin to viewing the world as a series of snapshots, rather than seeing things move continuously in space, the patient saw moving objects appear in one position and then another. When pouring a cup of tea, she saw the liquid frozen in the air, failed to notice the tea rising in the cup, and was surprised when it would overflow. The impairment can be very dramatic when V5 is damaged in both left and right hemispheres. 

What is multimodal perception?

Even though the information provided by each sense is distinct, the resulting representation of the surrounding world is not one of disjointed sensations, but of a unified multisensory experience. A particular powerful presentation of this distortion comes from the world of speech perception. Most people think of speech as an inherently auditory process: we decipher the sounds of language to identify phonemes, combining them into words, sentences, and phrases. However, the sounds we hear can be adjusted and influenced by visual cues. This is made clear in a compelling illusion called the McGurk effect, in which the perception of speech - what you believe you 'hear' - is influenced by the lip movements that your eyes see. 

Some areas of the brain, such as the superior colliculus and the superior temporal sulcus, process information from more than one sensory modality, integrating the multimodal information to increase perceptual sensitivity and accuracy. 

The patient JW experiences the world differently than most people. He tastes words, the word 'exactly' tastes like yogurt, and the word 'accept' tastes like eggs. Most conversations are pleasant tasting, but when JW is tending bar, he cringes whenever someone showsup because the name 'Derek' tastes like earwax. This phenomenon, in which the senses are mixed is called synesthesia. Tasting words is an extremely rare kind of synesthesia, more common kinds of synesthesia are when people hear words or music as colors, or see achromatic lettering (as in books or newspapers) as colored. Given that synesthesia is such a personal experience, researchers have had to come up with clever methods to verify and explore this unique phenomenon. That is why researchers came up with the Stroop-task. The stroop task requires the person to name the color of a written word. For instance, the word green is written with red ink, the participant is supposed to say 'red'. Synesthesia is associated with both abnormal activation patterns in functional imaging studies and abnormal patterns of connectivity in structural imaging studies.

What holds perceptual organization?

In 1949, Hebb thought that the brain was unchangeable after the early formative years. He suggested a theoretical framework for how functional reorganization, or cortical plasticity, might occur in the brain through remodeling of neuronal connections. 

Brain regions that are typically associated with a particular sensory system become reorganized in individuals who lack that sensory system. For example, regions usually involved in visual processing become responsive to auditory and tactile stimuli in blind individuals. Much of the work involving cortical reorganization has focused on individuals who have complete loss of one sensory system, usually audition or vision. 

Reorganization in the motor cortex has been found to depend on the level of gamma-aminobutyric acid (GABA), the principal inhibitory neurotransmitter. When GABA levels are high, activity in individual cortical neurons is relatively stable. If GABA levels are lower, the neurons respond to a wider range of stimuli. For example, a neuron that responds to the touch of one finger will respond to the touch of other fingers if GABA is blocked. 

What is the role of engineering for compensation?

Cochlear implants are designed to help people with severe hearing problems for whom typical hearing aids does not help. Permanent hearing loss is usually the result of damage or loss of the hair cells in the cochlea, often due to aging or frequent exposure to loud noise. Hearing aids facilitate hearing by amplifying the signals carries by sound waves and thus increasing the intensity of the stimulus arriving at the sensory transducers in the ear. The benefits of cochlear implants can take some time to become optimal, likely because the brain has to learn to interpret the modified auditory input. 

Retinal implants are designed for patients who are blind because of degenerative diseases that affect the photoreceptors, resulting in progressive vision loss. Even when the visual loss is very advanced, many cells in the retina remain intact. Therefor the researchers came up with a subretinal implant, which exploits the remaining photoreceptors, and a epiretinal implant, which bypasses the photoreceptor cells and directly stimulates the ganglion neurons on the retina.