Test #4 - Neurophysiology

What was Ibn al-Haytham’s evidence that we see only because light enters the eyes, not by sending out sight rays?

First - You can see distant objects such as stars far faster than we could imagine any sight rays reaching them.

Second - When light strikes an object, we see only the light rays that reflect off the object and into the eyes.

What is the Law of Specific Nerve Energies (Johannes Müller)?

Statement that whatever excites a particular nerve always sends the same kind of information to the brain.

Think of the demonstration:

"If you rub your eyes, you may see spots or flashes of light even in a totally dark room.”

Why?

You applied *mechanical pressure*, which *excited visual receptors* in your eyes. Anything that excites those receptors is *perceived as light* .

If someone electrically stimulated the auditory receptors in your ear, what would you perceive?

Because of the law of specific nerve energies, you would perceive it as sound, not as shock (of course, a strong enough shock might spread far enough to excite pain receptors also).

If it were possible to flip your entire brain upside down, without breaking any of the connections to sense organs or muscles, what would happen to your perceptions of what you see, hear, and so forth?

Your perceptions would not change.

The way visual or auditory information is coded in the brain does not depend on the physical location within the brain.

Seeing something as “on top” or “to the left” depends on which neurons are active but does not depend on the physical location of those neurons.

What is the Pupil?

An opening in the center of the iris where light enters.

The pupils are focused by the ______ (which are _______) and the _______ (which are _____________).

the lens (adjustable)

the cornea (not adjustable)

What are Bipolar Cells?

Type of neuron in the retina that receives input directly from the receptors

Where are the Bipolar Cells located?

Closer to the center of the eye.

What do Bipolar cells send their messages to?

Ganglion Cells

What are Ganglion Cells?

Type of neuron in the Retina that receives input from from the Bipolar Cells.

Where are Ganglion Cells located?

Located closer to the center of the eye.

What are Amacrine Cells?

They get information from Bipolar Cells and send it to other Bipolar, Amacrine, and Ganglion cells.

The Retina has more _______ cells than _______ cells.

Ganglion cells

Bipolar Cells.

What is the Optic Nerve?

Ganglion cell axons that exit through the back of the eye and continue to the brain.

What is the Blind Spot?

Area at the back of the Retina where the optic nerve exits; there are no receptors here.

Why do we never notice our Blindspot?

1. Your brain fills in the gap.


1. Anything in the blind spot of one eye is visible in the other eye.

What makes the blind spot of the Retina blind?

It has no receptors because it is occupied by exiting axons and blood vessels.

An object in the visual field produces an ____ image on the Retina. The optic nerve exits the eyeball on the ________ side.

inverted

nasal side (the side closer to the nose)

What is the Fovea?

A tiny area specialized for acute, detailed vision.

What are Midget Ganglion Cells?

The ganglion cells in the fovea of humans and other primates; each one is small and responds to just a single cone.

What is the visual path of the eye?

Receptors send their messages to bipolar and horizontal cells, which in turn send messages to amacrine and ganglion cells. The axons of the ganglion cells form the optic nerve which exits the eye at the blind spot and continues to the brain.

Foveal vision has better ________ and peripheral vision has better _________.

acuity (sensitivity to detail).

sensitivity to dim light.

What are the 2 types of receptors in the Retina?

1. Rods 2. Cones

What are Rods?

Respond to faint light but are not useful in the daylight because bright light bleaches them.

* Abundant in the periphery of the human retina.

What are Cones?

Less active in dim light but are more useful in bright light and are essential for color vision.

* abundant in and near the fovea.

What is the ratio of Rods to Cones?

20:1

Cones provide about ____ percent of the brain’s input.

90%

What are Photopigments?

Chemicals that release energy when struck by light.

You sometimes find that you can see a faint star on a dark night better if you look slightly to the side of the star instead of straight at it. Why?

If you look slightly to the side, the light falls on an area of the retina with more rods and more convergence of input.

If you found a species with a high ratio of cones to rods in its retina, what would you predict about its way of life?

We should expect this species to be highly active during the day and seldom active at night.

What is the Trichromatic (Young-Helmholtz) Theory?

We perceive color through the relative rates of response by three kinds of cones, each one maximally sensitive to a different set of wavelengths.

Why did Helmholtz decide on the number 3?

He found that people could match any color by mixing appropriate amounts of just three wavelengths.

What are the 3 receptors in Helmholtz theory?

1. Red → Long wavelengths
2. Blue → Short wavelengths
3. Green → Medium wavelengths

What is your Visual Field?

The part of the world that you see.

What is Negative Color Afterimage?

Result of staring at a colored object for a prolonged length of time and then looking at a white surface, the image is seen as a negative image, with the replacement of red with green, green with red, yellow and blue with each other, and black and white with each other.

What is the Opponent-Processing Theory?

We perceive color in terms of opposites.

* In other words: The brain has a mechanism that perceives color on a continuum from…
* Red → Green
* Yellow → Blue
* White → Black

According to the Opponent-Process Theory, Under what circumstance would you perceive a white object as blue?

If you stared at a bright yellow object for a minute or so and then looked at a similar white object, it would appear blue.

What is Color Constancy?

The ability to recognize colors despite changes in lighting.

What is the Retinex Theory?

The cortex compares information from various parts of the retina to determine the brightness and color for each area.

When a television is off, its screen appears gray. When you watch a television program, parts of the screen appear black, even though more light is actually showing on the screen than when the set was off. What accounts for the black perception?

The black experience arises by contrast with the other brighter areas. The contrast occurs by comparison within the cerebral cortex, as in the Retinex theory of color vision.

What is Color Vision Deficiency?

Inability to perceive color differences.

Why is color vision deficiency a better term than color blindness?

Very few people see the world entirely in black and white. The more common condition is difficulty discriminating red from green.

What happens when you see something?

Light rays reflect off the object and strike your retina.

What are Horizontal Cells?

Type of cell that receives input from receptors and delivers inhibitory input to bipolar cells.

What is the lateral geniculate nucleus?

Thalamic nucleus that receives incoming visual information.

Where does the optic nerve start and where does it end?

It starts with the ganglion cells in the retina. Most of its axons go to the lateral geniculate nucleus of the thalamus; some go to the hypothalamus and superior colliculus.

What is Lateral Inhibition?

The reduction of activity in one neuron by activity in neighboring neurons.

When light strikes a receptor, does the receptor excite or inhibit the bipolar cells?

What effect does it have on horizontal cells?

What effect does the horizontal cell have on bipolar cells?

The receptor excites both the bipolar cells and the horizontal cell. The horizontal cell inhibits the same bipolar cell that was excited plus additional bipolar cells in the surrounding area.

If light strikes only one receptor, what is the net effect (excitatory or inhibitory) on the nearest bipolar cell that is directly connected to that receptor?

What is the effect on bipolar cells to the sides?

What causes that effect?

It produces more excitation than inhibition for the nearest bipolar cell. For surrounding bipolar cells, it produces only inhibition. The reason is that the receptor excites a horizontal cell, which inhibits all bipolar cells in the area.

What is the receptive field?

The area of the visual field that excites or inhibits it.

→ receptors have a tiny one of these and later cells have progressively ones.

Primate ganglion cells fall into what three categories?

1. Parvocellular.
2. Magnocellular.
3. Koniocellular.

What are Parvocellular Neurons?

Small bodies, small receptive fields.

Mostly in or near the fovea.

What are Magnocellular Neurons?

Larger cell bodies and receptive fields.

Distributed evenly throughout the retina.

What are Koniocellular Neurons?

Small cell bodies (similar to Parvocellular).

They occur throughout the Retina.

As we progress from bipolar cells to ganglion cells to later cells in the visual system, are receptive fields ordinarily larger, smaller, or the same size? Why?

They become larger because each cell’s receptive field is made by inputs meeting at an earlier level.

What are the differences between the magnocellular and parvocellular systems?

Neurons of the parvocellular system have small cell bodies with small receptive field, are located mostly in and near the fovea, and are specialized for detailed and color vision.

Neurons of the magnocellular system have large cell bodies with large receptive fields, are located in all parts of the retina, and are specialized for perception of large patterns and movement.

If you were in a darkened room and researchers wanted to “read your mind” just enough to know whether you were having visual fantasies, what could they do?

Researchers could use fMRI, EEG, or other recording methods to see whether activity increased in your primary visual cortex.

What is an example of an unconscious response to visual information?

In blindsight, someone can point toward an object or move the eyes toward the object, despite insisting that he or she sees nothing.

What is the primary visual cortex (area V1/striate cortex)?

Area of the cortex responsible for the first stage of visual processing.

What is the phenomenon called Blindsight?

The ability to respond in limited ways to visual information without perceiving it consciously.

what is a simple cell?

Type of visual cortex cell that has a receptive field with fixed excitatory and inhibitory zones.

What are complex cells?

Type of visual cortex cell located in areas V1 and V2 that responds to a pattern of light in a particular orientation anywhere within its large receptive field.

What are end-stopped (or hypercomplex) cells?

type of visual cortex cell that resembles complex cells; responds best to stimuli of a precisely limited type, anywhere in a large receptive field, with a strong inhibitory field at one end of its field.

How could a researcher determine whether a given neuron in the visual cortex is simple or complex?

First, identify a stimulus, such as a horizontal line, that stimulates the cell.

Then present the stimulus in several locations.

If the cell responds strongly in only one location, it is a simple cell.

If it responds in several locations, it is a complex cell.

What do cells within a column of the visual cortex have in common?

They respond best to lines in the same orientation. Also, they are similar in their preference for one eye or the other, or both equally.

What are feature detectors?

Neurons whose responses indicate the presence of a particular feature (such as shape or direction).

What is binocular input?

stimulation from both eyes.

What is a sensitive period?

When experiences have a particularly strong and enduring influence.

What is the effect of closing one eye early in life? What is the effect of closing both eyes?

If one eye is closed during early development, the cortex becomes unresponsive to it.

If both eyes are closed, cortical cells remain somewhat responsive for several weeks and then gradually become sluggish and unselective in their responses.

What is retinal disparity?

The discrepancy between what the left and right eyes see.

What is Strabismus (or Strabismic Amblyopia)?

“lazy eye”

Condition in which the eyes do not point in the same direction.

What early experience would cause a kitten or human child to lose stereoscopic depth perception?

If the eye muscles cannot keep both eyes focused in the same direction, the developing brain loses the ability for any neuron in the visual cortex to respond to input from both eyes. Instead, each neuron responds to one eye or the other. Stereoscopic depth perception requires cells that compare the input from the two eyes.

What is astigmatism?

a blurring of vision for lines in one direction.

caused by an asymmetric curvature of the eyes.

What causes astigmatism?

The eyeball is not quite spherical - as a result, the person sees one direction of lines more clearly than the other.

If an infant is born with dense cataracts on both eyes and they are surgically removed years later, how well does the child see at first?

The child sees well enough to identify whether two objects are the same or different, but the child doesn’t understand what the visual information means. In particular, the child cannot answer which visual display matches something the child touches. However, understanding of vision improves with practice.

What is the secondary visual cortex (area V2)?

Receives information from are V1, processes the information further, then transmits it to additional areas.

What is the Ventral Stream?

visual paths in the temporal cortex that are specialized for identifying an recognizing objects.

* the “what pathway.

What is the Dorsal Stream?

Visual path in the parietal cortex that helps the motor system locate objects.

* the “where” path.

Suppose someone can describe an object in detail but stumbles and fumbles when trying to walk toward it and pick it up. What is probably damaged, the dorsal path or the ventral path?

The inability to guide movement based on vision implies damage to the dorsal path.

What is the inferior temporal cortex?

Portion of the cortex where neurons are highly sensitive to complex aspects of the shape of visual stimuli within very large receptive fields.

What is visual agnosia?

An inability to recognize objects despite otherwise satisfactory vision.

What is the fusiform gyrus?

A brain area of the inferior temporal cortex that recognizes faces.

What is prosopagnosia?

Impaired ability to recognize faces.

The brain has no specialized areas for perceiving flowers, clothes, or food. For what items does it have specialized areas?

The temporal cortex has specialized areas for perceiving places, faces, and bodies, including bodies in motion.

The ability to recognize faces correlates with the strength of connections between which brain areas?

Ability to recognize faces correlates with the strength of connections between the occipital face area and the fusiform gyrus.

Area V4 is important for color constancy. What is color constancy?

It is the ability to recognize the color of an object despite changes in the lighting.

What two areas are especially important for motion perception?

1. area MT (middle temporal cortex), aka area V5
2. area MST (medial superior temporal cortex)

What is motion blindness?

ability to see objects but impairment at seeing whether they are moving or, if so, which direction and how fast.

What is saccades?

Voluntary eye movements.

When you move your eyes, why does it not seem as if the world is moving?

Neurons in areas MT and MST respond strongly when and object moves relative to the background, and not when the object and background move in the same direction and speed.

Under what circumstances does someone with an intact brain become motion blind, and what accounts for the motion blindness?

People become motion blind shortly before and during a saccade, because of suppressed activity in area MT.

What is Amplitude?

The INTENSITY of a sound wave.

What is Frequency?

The NUMBER OF COMPRESSIONS PER SECOND of a sound.

* measured in hertz.

What is Pitch?

The aspect of auditory perception related to the frequency of a sound.

What is Timbre?

Tone quality or tone complexity.

What is the Pinna?

* Outer ear.
* The familiar structure of flesh and cartilage attached to each side of the head.

What is the Tympanic Membrane?

aka., The Eardrum.

* Middle ear

What is the Oval Window?

A membrane of the inner ear.

What is the Cochlea?

A structure in the inner ear containing auditory receptors.

* a “snail-shaped” structure.