Brain and Behavior - University of Texas at Arlington - Chapter Five

Law of Specific Nerve Energies

Statement that whatever excites a particular nerve always sends the same kind of information to the brain. For example, if the auditory receptors in your ear were to be electrically stimulated, you would perceive the stimulus as sound.

Receptor Specificity

Each sensory receptor (sensor) is specialized to absorb one kind of energy (audio, visual, touch, etc) and transduce it into action potentials (coding) that are propagated to the brain

Coding

a frequency of response, i.e., how fast a neuron is firing. Usually, the intensity of a feeling is controlled by neuronal firing, like the difference in feeling from touch to pain.

The Form of Transmission

The law of specific nerve energy states that activity always conveys the same type of information by a particular nerve (pathway) in the form of impulses (action potentials) to the brain

Pupil

An opening in the center of the iris in which light enters the eye

Lens

An adjustable part of the eye that refracts light toward the retina. Specifically plays a large part in refocusing sight depending on the distance of an object. Relaxed and thin form allows for distant objects to be seen with better clarity, while contracted and thick form allows for closer objects to be seen with better clarity. Located past the pupil.

Cornea

A non-adjustable part of the eye that refracts light toward the retina. Dome shaped located near the surface of the eye.

Retina

Rear surface of the eye, where the light-sensitive photoreceptors (cones and rods) are located to sense the light.

Optics

Light is focused by the optics (the lens and cornea) to produce reversed (leftmost image light hits right of retina, etc) and inverted (topmost image light hits bottom of retina) visual image projections.

Macula

The round area at the center of your retina near the back of the eye. Responsible for central vision (things directly in front of you). Highly transparent because the blood vessels at this area are almost absent - scattering is minimized.

Fovea

Central portion of the macula that perceives acute, detailed vision. Contains high density of cones, highest resolution of visual process. Each receptor is directly connected to one bipolar cell which is connected to ganglion cells, giving the receptor a “direct line” to the brain, which allows the registering of the exact location of input.

Peripheral Region

Cannot discern fine detail, has a greater sensitivity to dim light, has a greater number of photoreceptors (mostly rods). Many receptors converged per bipolar cell, highly convergent.

Optic Nerve Head

The ganglion cells join together here to form the optic nerve and leave the eyes, also called the “blind spot”, because the blind spot contains no photoreceptors

Visual Path

Photoreceptors send messages to bipolar cells located closer to the center of the eye which then send messages to ganglion cells which are even closer to the center. The axons of ganglion cells join together to form the optic nerve that travels to the brains.

Amacrine Cells

A modulatory cell that receives information from bipolar cells and sends it to other bipolar, ganglion, or amacrine cells. They control the ability of the ganglion cells to respond to shapes, movements, or other specific aspects of visual stimuli.

Horizontal Cells

A modulatory and inhibitory cell. Makes connections between photoreceptors and bipolar cells, inhibiting perception of light based on varying brightness of objects, contributing to heightened contrast.

Photopigments

Located on the outer segment of rods and cones. Consist of a light sensitive chemical, 11-cis-retinal that is bound to optic proteins named opsins.

11-cis-retinal

Light energy is absorbed by quickly converting this chemical to all-trans-retinal. This activates the second messenger, cGMP, via g-protein within the photoreceptor.

cGMP

Activation leads to change in cGMP-gated ion channels which completes the visual signal transmission in the retina.

Opsins

Optic proteins located in cones that alter 11-cis-retinal to be sensitive to certain wavelengths. Equivalent in rods is rhodopsin.

Color Vision

Three different kinds of cones in the eye sense visible wavelengths. Discrimination among colors depends on the combined responses of different types of cones which are then decoded by different visual neurons in the visual cortices.

Visible Wavelength Range

Shortest visible wavelength ~400 nanometers (violet) and longest visible wavelength ~700 nanometers (red)

Trichromatic Theory

Posed by Young-Helmholtz 1802-1850. Color perception occurs through the ratios of response by 3 types of cones: short, medium, and long wavelengths. Color always mixed, no pure blue, green, or red.

Opponent-Process Theory

Posed by Ewald Hering in 1878. Also accepts 3 types of cones (short, medium, long) but proposes that we perceive color by the way of paired opposites. First pair is red-green, then blue-yellow, and lastly black-white. Color processing is done by bipolar cells but a bipolar cell cannot perceive two colors at once. Blue-yellow is inhibited by red-green and so on as one explanation for why two colors cannot be seen at once. On one rode, green color sensors can be tired out which makes us perceive the color red instead or red can be perceived when green-red sensors pick up on wavelengths below their baseline for green.

Color Constancy

The ability to recognize colors despite changes in lighting. Suggests that the way we interpret color is based on context. If we cannot compare something to its surroundings, we may not be able to accurately identify the color of that object.

Color Blindness

The inability to perceive color differences. Usually due to lack of cones (most common is red-green deficiency), abnormality in one type of cone, or complete lack of color vision.

Color Vision Weakness

Not complete loss or not completely insensitive to color differences.

Gene Deficits

Color blindness is sex-linked, located on the X chromosome. 8% of men are colorblind while less than 1% of women are.

Optic Chiasm

The place where the optic nerves from two eyes meet and cross to the other side of the brain.

Lateral Geniculate Nucleus (LGN)

A part of the thalamus specialized for visual perception. Most ganglion cells go through it and are directed to the visual cortex in the occipital lobe; a small amount are directed to the superior colliculus in the midbrain instead.

Visual Field

The total area in which objects can be seen in the side (peripheral) vision while you focus your eyes on a central point. An object at the left visual field is sensed by the left nasal retina and right temporal retina. An object at the right visual field is sensed by the right nasal retina and the left temporal retina.

Lateral Inhibition

When activity in one neuron is reduced by activity in its neighboring neurons. Done by horizontal cells. Allows the retina to filter information in order to extract meaningful data and ignore other data.

Contrast Vision

The net result of excitatory and inhibitory messages received by visual cells. Process emphasizes borders of objects.

Contrast Vision: One Photoreceptor

Photoreceptor 8 is stimulated by light which then stimulates the horizontal cell and bipolar cell 8. The horizontal cell inhibits bipolar cell 8 as well as other adjacent bipolar cells. Since the horizontal cell is stimulated via EPSPs, the inhibitory effect decays with distance. Bipolar cells 6-10 are inhibited with the strongest inhibition being on #8. Since the excitatory synapse on bipolar cell 8 (from the photoreceptor) delivers a stronger impulse than the inhibitory synapse (from the horizontal cell), bipolar cell 8 seems more excited. Since bipolar cells 7 and 9 are the next strongly inhibited bipolar cells not being excited, then contrast is created between 7and 8 and 8 and 9.

Contrast Vision: Multiple Photoreceptors

Photoreceptor 6-10 are stimulated by light which then stimulates the horizontal cell and bipolar cells 6-10. The horizontal cell inhibits bipolar cells 6-10 as well as other adjacent bipolar cells. Bipolar cells 6-10 are receiving the same amount of excitation while 7-9 are receiving stronger inhibition than bipolar cells 6 and 10 since they are being inhibited “by both sides”. Therefore, bipolar cells 6 and 10 get he greatest net excitation. Bipolar cells 5 and 11 are not being excited and are being inhibited the most in contrast to other adjacent bipolar cells; they are showing the least activity. So the cells just inside the border (6 and 10) are very excited in contrast to the lack of excitation on the cells outside the border (5 and 11). This is how heightened contrast occurs.

Receptive Field

The area that, when struck by light, excites or inhibits the correlating visual neuron. A photoreceptor has a small RF, just a dot, while a bipolar cell has a larger RF, comprised of the RFs of the photoreceptors that connect to it, ganglion cells have the biggest RF. For a ganglion cell, there’s typically a centermost part of the RF that is either inhibitory surrounded by excitatory parts of the RF or the inverse.

Parvocellular Ganglion Neurons

Small cell bodies located in or near the fovea. Small RF, responds well to visual details and color, and have synapses on small sized cells of the LGN

Magnocellular Ganglion Neurons

Larger cell bodies distributed evenly through the retina. Larger RF, responds best to movement, sensitive to brightness, important for depth perception, and synapses on larger cells of the LGN.

Konicellular Ganglion Neurons

small cell bodies found throughout the retina that have functions associated with color vision. synapses on LGN, parts of the thalamus, and superior colliculus

LGN Cells

RF similar to ganglion cells , with inhibitory or excitatory central portions connected to the same category of ganglion cells.

Bromann Area 17

Located in the occipital lobe; the primary visual cortex (or V1 or striate cortex). Visual information is recieved and processed here first.

Blindsight

Damage to the striate cortex which results in loss of “conscious perception”. Those who are blind in this way may still respond to objects due to intact photoreceptors, responses range from being able to identify where object is or color of object but not being able to consciously “see” it or differentiate it from other visual stimuli. This may be due to areas of the striate cortex remaining undamaged but still not able to form conscious perception or visual stimuli bypassing the striate cortex completely and going directly to other areas of the brain that process visual stimuli.

Cortical Cells

Categorized based on their RF by David Hubel/Torsten Wiesel in 1950s. Hypothesized these categories of neurons are firing action potentials by responding to the edge of the object with a bar shaped RF.

Simple Cells

Located only in the V1 are neurons fixed with excitatory and inhibitory zones and specific orientations in their RF. The more light shines in the excitatory zone, the more the cell rsponds, the more light shines in the inhibitory zone, the less the cell responds, this helps find the shape/edge of RF.

Complex Cells

Located in V1 or V2. Have a large RF that cant be mapped onto fixed excitatory or inhibitory zones. Respond to moving stimulus to track moving object. Strongly respond to light in a particular orientation, such as diagonal or horizontal orientation.

Hyper-Complex or End-Stopped Cells

Located in V1 or V2. Resemble complex cells but with an exact inhibitory area at one end of its bar shaped receptive field and larger. Respond to bar shaped pattern of light anywhere in its large RF provided bar does not extend past inhibitory zone. These features enable cortical cells to produce basic imaging and image moving objects.

Columns in The Visual Cortex

Groups of cells that are perpendicular to the surface and organized/clustered together according to their responsiveness to specific stimuli. Example, cells in a particular responding to stimulus only from one eye or both, or responding best to stimuli in a specific orientation.

Feature Detectors

Neurons whose responses indicate the presence of a particular feature. Visual neurons are thought to be feature detectors.

Binocular Vision

When each neuron of the V1 responds to areas in the two retinas that focus on the same point in space. If you stopped using one eye as a child, your sight would weaken in that eye until you eventually became nearly blind due to lack of new connections in the eye forming or strengthening of older connections. The same happens when both eyes are deprive of stimulus, with most cells being responsive to just one eye after a while. Binocular deficit develops impairment of binocular vision.

Binocular Vision Deficit

Prolonged lack of exposure to light stimulus would lead to weakening of the visual cortex, not completely blind, but not as reactive and with less defined RFs. Stimulus in two eyes can become uncorrelated.

Starbismus

A symptom of binocular vision deficit. A condition in which eyes do not move in the same direction and therefore do not point in the same focus on the same visual neuron — they become uncorrelated. Usually develops in childhood. Cortical cells choose to strengthen connections with one eye when both eyes carry unrelated messages. Brains ability to develop SDP is impaired.

Stereoscopic Depth Perception (SDP)

Ability to perceive depth and objects in a 3D space by comparing input from both eyes. Relies on slight visual discrepancies from both eyes to function. Shaped through vision experience.

Sensitive Period

Time early in development when experiences have a particularly strong and enduring influence. If you made a baby wear goggles that put horizontal stripes in their vision, after a while, a majority of its visual cortex will only be able to respond to horizontal lines until eventually the baby stops responding to vertical lines.

Astigmatism

A decreased kind of responsiveness to one kind of line or another, caused by an asymmetric curvature of the eyes. 70% of infants develop this after birth, but eventually growth of the lenses drops the rate of astigmatism to 10%. Blurred vision due to oversensitivity to one type of line.

Cataracts

Cloudy eye lenses

Parallel Processing

Divides visual stimulus into components of shape, depth, color, and motion. Happens in the V2 (secondary visual cortex).

Ventral Stream

Refers to the visual path that goes through the temporal cortex and ends in the inferior temporal area. Called the ‘what’ path because it is specialized for identifying and recognizing objects. Performs detailed analysis of shape and color.

The Dorsal Stream

Refers to the visual path in the parietal and middle/superior temporal cortices. Its called the ‘where’ path because it helps the motor system to find objects and move towards them.

Ventral Stream Damage

A person with this type of damage can see where objects are and grab them but cannot make sense of a television program and color vision because they have trouble identifying what things are.

Dorsal Stream Damage

They can read, recognize, and describe objects in detail (including color vision). They know what things are but don’t know where they are and are unable to trace moving objects, i.e., cannot accurately reach out to grab an object.

Shape Perception

Inferior temporal cortex contains cells that respond selectively to complex shape stimuli from objects, which requires exchange of information with the prefrontal cortex. Recognizing an object and being familiar with it starts with experience of sight of the object. After enough information is collected, when you see the object you will be able to recall different perspectives/angles of that object. This is known as shape constancy and it is a mechanism of shape perception.

Recognizing Faces

Mostly done by the fusiform gyrus located in the inferior temporal cortex. Also activated to identify details of other objects such as differences between types of bird or models of cars. Babies are born predisposed to pay more attention to faces. The fusiform responds strongly to memories of a face, line drawings of a face, or anything that resembles a face.

Visual Agnosia

Inability to recognize objects including faces despite otherwise normal vision. Can still recognize things through audio or touch.

Prosopagnosia

Inability to recognize faces, may be caused by damage to the fusiform gyrus

Color Perception

Mainly done by V4 area which is important in processing color information that is picked and sensed by cones. Particularly critical in response to color changes as a result of changes in background brightness. This is known as color constancy and is achieved through comparison of other objects in a similar context and our memories/experiences of similar objects. Color perception done by parvocellular pathways and brightness is mediate by magnocellular cells which send their output to areas V2, V4, and the posterior inferior temporal cortex. Damage to V4 results not in color blindness, but loss of color constancy ability.

Motion Perception

Done by two areas in the dorsal stream, V5 (middle temporal cortex or MT) and the adjacent area, the medial superior temporal cortex (MST) which are important for motion detection. Both receive input from the magnocellular path which is color-insensitive. The MT or V5 responds to stimulus moving in a particular direction, acceleration, deceleration, and slightly to still pictures that imply movement. Dorsal MST cells respond to complex stimuli such as the expansion, contraction, or rotation of a large visual scene, as well as if an observer moves forward or backward or tilts their head (which from their perspective, causes the previously stated changes to the visual scene). Ventral MST cells respond to objects that move relative to their background, which can include objects that move while everything else around them stays still, or objects that stay still while everything else around them moves.

Motion Blindness

Usually caused by damage to MT or MST. People keep the ability to see and recognize objects but impair the ability to see whether the objects are moving or, if they’re moving, in what direction and how fast. May lead to behavior such as difficulty navigating traffic (one moment the car/person is far, the next they’re near with no perception of the steps in between) or difficulty pouring liquid (perceived signifier that cup is full is overflow).