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Readings (For Lecture 9, 10, and 11)

Terms in this set (49)

Optic Chiasm
The first thing that happens on this journey of retina to brain is that the visual signals from both eyes leave the back of the eye in the optic nerve and meet at a location called the optic chiasm. The optic chiasm is an x-shaped bundle of fibers on the underside of the brain.
How is the visual field determined?
The visual field is determined based on where the person is fixating; anything to right of the point of central focus is the right visual field (processed by the left hemisphere), and anything to the left is the left visual field (processed by the right hemisphere). Importantly, both eyes can see both visual fields.
Where do the visual signals go after they reach the optic chiasm?
Approximately 90 percent of the signals from the retina proceed to the lateral geniculate nucleus (LGN), located in the thalamus of each hemisphere, while the other 10 percent of fibers travel to the superior colliculus, a structure involved in controlling eye movements. The thalamus is the relay station for sensations before reaching the receiving area of the cerebral cortex.
What are some proposed purposes/functions of the LGN?
One proposal of LGN function is based on the observation that the signal sent from the LGN to the cortex is smaller than the input the LGN receives from the retina. Decrease in the signal leaving the LGN has led to the suggestion that one of the purposes of the LGN is to regulate neural information as it flows from the retina to the cortex. Another important characteristic of the LGN is that it receives more signals from the cortex than from the retina, aiding in regulation of information.
Where do the visual signals travel from the LGN?
From the LGN, the visual signal then travels to the occipital lobe, which is the visual receiving area—the place where signals from the retina and LGN first reach the cortex. The visual receiving area is also called the striate cortex or V1.
What are simple cortical cells?
Cells with these side-by-side (as opposed to center-surround) receptive fields are called simple cortical cells. Found in the striate cortex. They respond best to vertical lines of specifically preferred orientations.
What kind of receptive fields are in the retina and LGN?
Center-surround cell receptive fields.
What determines the relationship between a cell's firing and orientation?
The Orientation Tuning Curve, which is determined by measuring the responses of a simple cortical cell to bars with different orientations.
What is a complex cell?
Also prefer bars of a particular orientation like simple cells. However, unlike simple cells, which respond to small spots of light or to stationary stimuli, most complex cells respond only when a correctly oriented bar of light moves across the entire receptive field. Further, many complex cells respond best to a particular direction of movement.
What is an end-stopped cell?
Cells that fire in response to moving lines of a specific length or to moving corners or angles.
Why was the discovery of different cortical cells important?
It extended the idea first proposed in connection with center-surround receptive fields that neurons respond to some patterns of light and not to others.
What is another term for simple, end-stopped, and complex cells?
Feature cells.
What is selective adaption?
Selective adaption is the idea that this cell firing causes neurons to eventually become fatigued, or adapt. This adaptation causes two physiological effects: (1) the neuron's firing rate decreases, and (2) the neuron fires less when that stimulus is immediately presented again. "Selective" because only neurons initially firing can adapt.
Why is the idea of selective adaption important?
The important result of this experiment is that our psycho-physical curve shows that adaptation selectively affects only some orientations, just as neurons selectively respond to only some orientations.
What is selective rearing?
The idea behind selective rearing is that if an animal is reared in an environment that contains only certain types of stimuli, then neurons that respond to these stimuli will become more prevalent.
What is the idea of selective rearing is derived from?
From a phenomenon called neural plasticity or experience-dependent plasticity—the idea that the response properties of neurons can be shaped by perceptual experience. According to this idea, rearing an animal in an environment that contains only vertical lines should result in the animal's visual cortex having simple cells that respond predominantly to verticals.
What is the difference between selective adaption and selective rearing?
Selective adaption is short-term. Selective rearing is longer-term with a "use it or lose it" effect.
Oblique Effect
The fact that people perceive vertical and horizontal lines better than slanted lines. What is important about the oblique effect is not only that people see horizontals and verticals better, but that the brain's response to detecting horizontals and verticals is larger than when detecting slanted lines.
Retinotopic Map
This shows that locations on the cortex correspond to locations on the retina. This electronic map of the retina on the cortex is called a retinotopic map. This organized spatial map means that two points that are close together on an object and on the retina will activate neurons that are close together in the brain.
Cortical Magnification
Even though the fovea accounts for only 0.01 percent of the retina's area, signals from the fovea account for 8-10% of the retinotopic map on the cortex. This apportioning of a large area on the cortex to the small fovea is called cortical magnification.
What is the size of cortical magnification referred to as?
The size of this magnification is called the cortical magnification factor. This is determined with brain imaging. Using a simple red dot test, researchers could see that shining a tiny red dot on the fovea shows great activity in the cortex compared to a blue dot on periphery vision.
Why does the fovea take up so much cortical activity?
It allows us to perceive in detail, when staring at your finger you do not perceive your finger as large but rather see all the fine details in your finger like your nails, cuticle texture, etc, much better than the rest of your hand.
Location Columns
Columns that are perpendicular to the surface of the cortex, so that all of the neurons within a location column have their receptive fields at the same location on the retina. They also all prefer stimuli with the same orientation.
Orientation Columns
Columns within the location column that contain neurons with the same orientation preference. The neurons' preferred orientations changed in an orderly fashion, so a column of cells that respond best to 90 degrees is right next to the column of cells that respond best to 85 degrees, etc.
What is the dimension of a location column and why does this serve visual perception well?
This 1-mm dimension for location columns means that one location column is large enough to contain orientation columns that cover all possible orientations. Thus, the location columns serve one location on the retina each (all the neurons in the column have their receptive fields at about the same place on the retina) and contains neurons that respond to all possible orientations.
Hypercolumn
A location column with all of its orientation columns spanning 180 degrees.
Tiling
The adjacent (and often overlapping) location columns working together to cover the entire visual field (similar to covering a floor with tiles).
What happens in the V1 (striate cortex)?
V1 neurons represent the basic elements or features of the visual scene (edges and lines).
Exstriate Cortex
Other visual areas in the occipital lobe and beyond— areas conveniently known as V2, V3, V4, and V5. As we move from the V1 to the V2, to the V3, etc, the receptive fields gradually increase.
Ablation
Refers to the destruction or removal of tissue in the nervous system. Used in experiments on animals to determine the function of specific parts of the visual cortex.
Object Discrimination Problem
A monkey was shown one object, such as a rectangular solid, and was then presented with a two-choice task, which included the "target" object (the rectangular solid) and another stimulus, such as the triangular solid. If the monkey was able to discriminate between the two objects and thus push aside the target object, it received the food reward hidden under.
Landmark Discrimination Problem
The monkey's task here was to remove the cover of the food well that was closest to the "landmark"—in this case, a tall cylinder.
Ventral (What) Pathway
The pathway that reaches the temporal lobes is responsible for determining an object's identity (object recognition). Therefore, the pathway leading from the striate cortex to the temporal lobe is called the "what" pathway. At the bottom of the brain.
Dorsal (Where) Pathway
The pathway that leads to the parietal lobe is responsible for determining an object's location and to possibly guide the movement towards or away from it. Therefore the pathway leading from the striate cortex to the parietal lobe is called the "where" pathway. At the top of the brain. Sometimes called the "how" and "action" pathway since it helps determine how a person carries out an action.
Though the dorsal and ventral pathways serve different functions what is similar about them?
(1) The pathways are not totally separated but have connections between them and (2) signals flow not only "up" the pathway from the occipital lobe toward the parietal and temporal lobes but "back" as well. This "back" information is called feedback. It makes sense that there would be communication between the pathways because in our everyday behavior we need to both identify and locate objects, and we routinely coordinate these two activities every time we identify something. Important for top-down processing. "That's a pen." -> "It's over there next to the computer."
How does brain damage affect the pathways?
Research showed perception of length of line (the act of pinching their fingers at the right length) was affected by the damage but not the physical task of grasping where they reached forward and grasped the ends of the lines. This supports the idea that perception and action are served by different mechanisms.
How does the blind painter, Bramblitt, paint?
He uses a variety of texture from oil paints to puffy paints, allowing him to feel the art he is creating. He uses viscosity of paint to determine what color being used and the feeling of painting with that color as well.
What is a unique trait of blind individuals?
Sensitivity to touch, even though only 10% of the blind population uses braille anymore, many of them still demonstrate elevated senses of touch.
Cross-Modal Plasticity
When faced with sensory loss, we often show compensatory enhancement of our other senses. For example, this explains the enhanced touch of blind individuals, whose visual cortex is often co-opted by the auditory and touch senses. This occurs no matter the sensory ability of the individual when faced with even temporary loss of a sense.
How would five days of blindfolding/braille training change how your brain interprets your senses?
Five days of blindfolding alone can enhance basic touch skills. Also, regardless of whether you had the Braille training, your brain would have changed over the five days of blindfolding. The last brain scan would reveal that when you now touch complex patterns, your visual brain is activated in a way similar to that of an individual who is truly blind. But for participants who were not blindfolded, these brain changes would not occur, even if they did have the intensive Braille training. Five days of visual deprivation are enough to establish visual brain involvement in touch tasks, as well as the task performance advantages that involvement provides. These effects are usually short-lived and even used in a non-experimental basis, "feeling for keys in the dark."
Somatosensory Cortex
Area at the front of the parietal lobes that registers and processes body touch and movement sensations.
Two-Point Sensitivity
Ability to recognize the presence of two points close together with touch.
What changes occur in a musicians sense of touch?
There is mapping of the touches at a "glance" of touch. Such as the left hand fingers of a violinist being more substantially mapped as they used the dexterity in that hand to produce notes, than the right hand, which simply grips the bow. This is similar to practiced braille readers more speedily reading at a "glance." This can happen to the average person even using a tool like tongs for 10 minutes, these tools simply become an extension of your hand.
Coactivation
An untrained subject is lightly tapped on the fingertip by a small (one-third-inch) circular probe tip. The tapping occurs with a random rhythm averaging about one tap per second, and continues over the course of hours or even days (the tapping device is portable and unobtrusive). While small, the one-third-inch size of the tap is large enough to co-activate multiple touch nerves on the fingertip, which turns out to be critical for the method to work. This increases two-point sensitivity, but for not long, as opposed to long-term change such as for musicians/braille readers.
Short-Term Plasticity
Increase strength of synaptic connections. Change in performance of cells. Touch/senses become stronger for a temporary amount of time, activating previously dormant neighbor cells. Not using these cells causes them to fall dormant once again.
Long-Term Plasticity
Persistent changes in the strength of synaptic connectivity lasting hours, days, or longer. This is the result of short-term plasticity being maintained, which makes these connections to dormant cells more easily accessed or previous cells "spouting" new connections to access these dormant cells. New growth establishes more permanent interaction.
Is neuroplasticity always positive?
Not always, such as in cases of amputees with phantom pains where their limbs used to be. Your brain remember what it used to be able to feel, for good or bad, and will rewire accordingly.
Does neuroplasticity always require touch?
Not always, such as in the case of those who were asked to practice a pattern on the piano compared to those who were asked to merely think of practicing the pattern. They displayed similar levels of neuroplasticity. Occurs when practicing non-contact meditative tai chi as well.
What is the rubber hand experiment?
Taking a divider, the participant puts their left hand on one side and a rubber hand is placed on the other. Two brushes are used by the experimenter to stroke each hand. Shockingly, your brain integrates the rubber hand to be an extension of your body, taking the visual cue and combining it with your own sensation of a hand being stroked. When the experimenter takes out a hammer and slams it on the rubber hand, the participant jolts in artificial pain as if their real hand was struck. This would not happen if the stroking of the hands was not in sync, or if the rubber hand was not positioned to look like an extension of your body.
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