Chapter 5: Sensation and Perception Flashcards
Achromatopsia
Book definition: “A selective disorder of color perception resulting from a lesion or lesions of the central nervous system, typically in the ventral pathway of the visual cortex. In achromatopsia, the deficit in color perception is disproportionately greater than that associated with form perception. Colors, if perceived at all, tend to be muted. (p. 201)”
Achromatopsia is not to be confused with color-blindness.
In contrast, color-blindness is the result of a gene causing abnormalities in the photoreceptor system. Dichromatic color-blindness is caused by the lack of a third photopigment, while anomalous trichromatic color-blindness is caused by abnormal sensitivity in one of the three photopigments.
Akinetopsia
Book definition: “A selective disorder of motion perception resulting from a lesion or lesions of the central nervous system. Patients with akinetopsia fail to perceive stimulus movement, created by either a moving object or their own motion, in a smooth manner. In severe cases, the patient may only infer motion by noting that the position of objects in the environment has changed over time, as if the patient were constructing dynamics through a series of successive static snapshots. (p. 204)”
Akinetopsia has often been linked to lesions in the middle temporal visual area (area MT or V5), which is thought to be involved in motion perception.
The disorder can take on several forms. In inconspicuous akinetopsia, patients describe seeing motion as a lagging cinema reel or as a series of overlaid pictures. In the much rarer case of gross akinetopsia, no motion is perceived whatsoever, resulting in seeing the world as a series of still images.
A patient suffering from gross akinetopsia noted her difficulties in pouring a liquid into a cup, as the liquid would simply appear frozen in midair until suddenly, the cup had overflowed. She also noted feeling uncomfortable being in a room with multiple people, as people would seem to blip in and out of existence whenever they moved.
Area MT (V5)
Book definition: “A region in the visual cortex containing cells that are highly responsive to motion. Area MT is part of the dorsal pathway, thought to play a role not only in motion perception but also in representing spatial information. (p. 190)”
The role of the middle temporal visual area (area MT or V5) in motion perception is highlighted in patients with lesions in the area. Such damage may result in akinetopsia, a disorder in which patients cannot properly perceive motion.
In the case of inconspicuous akinetopsia, patients describe seeing motion as a lagging cinema reel or as a series of overlaid pictures. In the rarer gross akinetopsia, no motion is perceived whatsoever, resulting in seeing the world as a series of still images.
Area V4
Book definition: “A region in the visual cortex containing cells that are thought to process color information. (p. 194)”
Lesions to area V4 have been linked to achromatopsia, a disorder where color vision is severely impaired.
In addition, V4 has been shown to play a part in higher order shape perception.
Chemical senses
Book definition: “The two senses that depend on environmental molecules for stimulation: taste and smell. (p. 176)”
Corpuscle
Book definition: “A globular mass of cells that are part of the somatosensory system. (p. 179)”
The sensation of touch is signaled by specialized receptors in the skin, including Meissner’s corpuscles, Pacinian corpuscles, Ruffini corpuscles and Merkel’s cells.
Corpuscles are a type of mechanoreceptor; a nerve ending in the skin converting vibrations and pressure into the feeling of touch. Different types of corpuscles have different specializations – e.g. Pacinian corpuscles can use vibration and pressure to detect surface textures, while Meissner’s corpuscles register light touch and are concentrated in areas such as the fingers and lips.
Corpuscles do not detect pain – this is instead signaled by nociceptors.
Cortical visual area
Book definition: “Regions of visual cortex that are identified on the basis of their distinct retinotopic maps. The areas are specialized to represent certain types of stimulus information, and through their integrated activity they provide the neural basis for visually based behavior. (p. 189)”
Extrastriate visual areas
Book definition: “Visual areas that lie outside the striate cortex (Brodmann area 17, the primary visual cortex) and are considered secondary visual areas because they receive input either directly or indirectly from the primary visual cortex. (p. 189)”
Fovea
Book definition: “The central region of the retina that is densely packed with cone cells and provides high-resolution visual information. (p. 186)”
Ganglion cell
Book definition: “A type of neuron in the retina. Ganglion cells receive input from the photoreceptors (rods and cones) and intermediate cells of the retina and send axons to the thalamus and other subcortical structures. (p. 186)”
Although there are about 120 million rod cells and 6 million cone cells in each retina, there are only around 0.7-1.5 million ganglion cells in each retina to transmit their information.
A ganglion cell may receive a large combined input from hundreds of rod cells, meaning it can activate even in low light conditions. In contrast, only a few cone cells connect to each ganglion cell, meaning more light is necessary for activation, but that the amount of information is much higher, resulting in a sharper image.
Glomerulus (pl. glomeruli)
Book definition: “The neurons of the olfactory bulb. (p. 173)”
When an odorant triggers a neuron, a signal is sent to the glomeruli of the olfactory bulb.
The olfactory bulb is highly connected. A single bipolar neuron may activate over 8,000 glomeruli, while each glomerulus may receive input from up to 750 receptors.
Axons from the glomeruli then exit laterally from the olfactory bulb, forming the olfactory nerve, which terminates in the primary olfactory cortex.
Hemianopia
Book definition: “A condition resulting from destruction of the primary visual cortex in one hemisphere. The patient is unaware of any visual stimulation presented in the side of space contralateral to the lesion. (p. 206)”
Hemianopia (also called hemianopsia) can take on several different forms, depending on where in the visual pathway a lesion occurs.
For instance, a lesion in the right optical nerve results in complete loss of vision in the right eye, while a lesion in the right optical tract (which is later in the system, after the crossing over of nerves in the optic chiasm) results in a loss of vision in the left visual field of both eyes.
Inferior colliculus
Book definition: “A part of the midbrain that is involved in auditory processing. Compare superior colliculus. (p. 169)”
Interaural time (difference)
Book definition: “The difference in time between when a sound reaches each of the two ears. This information is represented at various stages in the auditory pathway and provides an important cue for sound localization. (p. 171)”
If a sound reaches your right ear before your left ear, the difference in timing will provide information about the sound’s location (in this case, somewhere on your right).
This is also the reason why we repeatedly turn our heads when trying to locate the source of a sound, using a mix of sound intensity and interaural time difference to help us.
Lateral geniculate nucleus (LGN)
Book definition: “The thalamic nucleus that is the main target of axons of the optic tract. Output from the LGN is directed primarily to the primary visual cortex (Brodmann area 17). Compare medial geniculate nucleus. (p. 187)”
The lateral geniculate nucleus, located in the thalamus, receives visual information transmitted from the retina through the optic nerve in the retinogeniculate pathway. As the optic nerves cross in the optic chiasm, each LGN receives information from its contralateral retina.
The LGN then sends information on to the primary visual cortex (V1) via the geniculocortical pathway.
In addition, as the different nuclei of the thalamus are interconnected, visual information can be connected to auditory information, and so on. This provides an opportunity for multisensory integration, e.g. integrating the visual input of someone’s lips moving with the auditory input of their voice.
Medial geniculate nucleus (MGN)
Book definition: “A collection of cell bodies in the medial portion of the thalamus involved in processing auditory information. Output from the MGN is directed primarily to the primary auditory cortex. Compare lateral geniculate nucleus (LGN) (p. 167)”
In addition to output from the MGN to the primary auditory cortex A1, the different nuclei of the thalamus are also interconnected, meaning that auditory information can be connected to visual information, and so on. This provides an opportunity for multisensory integration, e.g. integrating the auditory input of someone’s voice with the visual input of their lips moving.
Odorant
Book definition: “A molecule conducted through the air that leads to activation of the olfactory receptors and may be perceived as having a smell when processed through the olfactory system. Compare tastant. (p. 173)”
Odorant molecules enter the nasal cavity either through the nose or the mouth, e.g. during consumption of food.
Although still unclear, there are several hypotheses as to how odorants are interpreted by olfactory receptors.
One hypothesis suggests that they attach to one or several of the over 1,000 types of odor receptors embedded in the mucous membrane of the roof of the nasal cavity. Another suggests that odor recognition is based on the molecular vibrations of groups of odor molecules.
The second hypothesis would also explain why molecules of similar shape but dissimilar vibrations have different fragrances – e.g. alcohols, which smell like alcohol, and thiols, which smell like rotten eggs.
Photoreceptor
Book definition: “A specialized cell in the retina that transduces light energy into changes in membrane potential. The photoreceptors are the interface for the visual system between the external world and the nervous system. (p. 185)”
The photoreceptors of the retina, called rods and cones, contain protein molecules called photopigments.
When exposed to light, these molecules become unstable and split apart. This causes a change in the membrane potential of the photoreceptors, which triggers an action potential in downstream neurons.
Rods contain the pigment rhodopsin, which is destabilized by low levels of light, making them essential to vision in low lighting. Cones contain the pigment photopsin, which is destabilized in higher levels of light.
In addition, cones come in three different variations, with sensitivity to different regions of the visible spectrum: short wavelengths (blue light), medium wavelengths (green light), and long wavelengths (red light).
Primary auditory cortex (A1)
Book definition: “The initial cortical processing area of the auditory system. (p. 169)”
Primary somatosensory cortex (S1)
Book definition: “The initial cortical processing area for somatosensation, including Brodmann areas 1, 2, and 3. This area of the brain contains a somatotopic representation of the body called the sensory homunculus. (p. 180)”
Primary visual cortex (V1)
Book definition: “The initial cortical processing area for vision, located in the most posterior portion of the occipital lobe, known as Brodmann area 17. (p. 187)”
The path from retina to primary visual cortex goes as follows:
- Visual information from the retina is transmitted by the ganglion cells, which make up the optic nerve.
- Via the retinogeniculate pathway, the optic nerves from each eye cross in the optic chiasm before arriving at the contralateral lateral geniculate nucleus of the thalamus.
- The LGN then projects information on to the primary visual cortex (V1) via the geniculocortical pathway.
When visual information reaches V1, it has already been processed by at least four distinct neurons: photoreceptors, bipolar cells, ganglion cells, and LGN cells.
From V1, visual information is sent up the dorsal stream (or where-pathway) for motion and spatial processing, or down the ventral stream (or what-pathway) for object recognition.
Studies have shown that the size of V1 is negatively correlated with a person’s sensitivity to visual illusions of size. One hypothesis suggests that a larger visual cortex results in a larger (and thus more precise) representation of visual stimuli.
Receptive field
Book definition: “The area of external space within which a stimulus that must be presented in order to activate a cell. For example, cells in the visual cortex respond to stimuli that appear within a restricted region of space. In addition to spatial position, the cells may be selective to other stimulus features, such as color or shape. Cells in the auditory cortex also have receptive fields. The cell’s firing rate increases when the sound comes from the region of space that defines its receptive field. (p. 187)”
Retina
Book definition: “A layer of neurons along the back surface of the eye. The retina contains a variety of cells, including photoreceptors (the cells that respond to light) and ganglion cells (the cells whose axons form the optic nerve). (p. 185)”
Retinotopic map
Book definition: “A topographic representation in the nervous system that reflects spatial properties of the environment in an eye-based reference frame. For example, primary visual cortex contains a retinotopic map of the contralateral side of space, relative to the center of gaze. (p. 187)”
Synesthesia
Book definition: “A mixing of senses whereby stimulation of one sense (e.g., touch) automatically causes an illusory perceptual experience in the same or another sense (e.g., vision). (p. 212)”
