Chapter 5

Three General Principles of Perception (explain)

1. Object emits energy which stimulates the receptors (i.e., cones and rods)

2. Information is encoded by the retinas and then transmitted along sensory nerves to the brain

3. In the brain cortex, the information is decoded in the respective region for that sensory information

Receptor Specificity (what is it?)

Each sensory receptor is specialized to ABSORB ONLY ONE KIND of energy and transduces (converted) it into action potentials (coding) that are propagated in the brain

Coding by Frequency (what is it?)

The frequency at which a neuron fires controls the intensity of a sensation or feeling

e.g., the increased rate of firing is the difference between a light touch and an intense pain

Form of Transmission (what is it?)

Activity always conveys the SAME type of information by a particular nerve in the form of action potentials to the brain

e.g., the eye will always send visual information to the brain despite the type of activation

Visual Processing Components (what are the three components?)

1. Eyes

2. Optic Nerves

3. Brain Cortex

The Optics (what is it?)

A process that focuses light using the cornea and lens and produces an inverted and reversed visual image projection

Inverted and Reverse Visual Projection (what is it?)

Light from one side of the world will strike the opposite side of the retina (bottom to top, left to right)

The Pupil (what is it?)

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

The Retina (what is it?)

The rear surface of the eye where light-sensitive photoreceptors are located to sense the light

The Retina (what two types of photoreceptors?)

1. Rods

2. Cones

The Macula (what is it?)

The CENTER of the RETINA that is responsible for CENTRAL, COLOR, and DETAILED vision. It is also HIGHLY TRANSPARENT

The Macula (why is it highly transparent?)

Due to the near absence of blood vessels

The Fovea (what is it?)

An area in the CENTRAL portion of the MACULA that perceives ACUTE (sharp), DETAILED vision

The Fovea (why does it specialize in acute vision?)

1. Scattering is minimized due to the MACULA being highly transparent

2. Contains the HIGHEST DENSITY of CONES

3. LESS or NO CONVERGENCE between CONES → bipolar cells → ganglion cells

The Peripheral Region (what is it?)

The region outside of the central area of the retina that is more sensitive to dim light and has a greater number of photoreceptors (mostly rods)

The Peripheral Region (TRUE/FALSE: can discern fine details)

False

The Optic Nerve (what is it?)

The point where the AXONS of GANGLION CELLS join together to form the OPTIC NERVE and LEAVES the eye, carrying visual information to the BRAIN

The “blind spot” (what is it?)

The area that contains no photoreceptors (The optic nerve head)

The Intra-Retinal Path (what is it?)

The pathway signals take in the retina before exiting through the optic nerve and sent to the brain

The Intra-Retinal Path (what are the three projecting cells; what do they do?)

1. Photoreceptors → sends messages to:

2. Bipolar Cells → sends messages to:

3. Ganglion Cells → axons form optic nerve

Modulatory Cells (what are they?)

1. Amacrine Cells

2. Horizontal Cells

Amacrine Cells (what are they?)

One of the two MODULATORY cells in the visual path that RECEIVES information from BIPOLAR CELLS and SENDS to other bipolar, ganglion, or amacrine cells

It also controls the ability of ganglion cells to respond to shapes, movement, etc.,

Horizontal Cells

One of the two MODULATORY cells in the visual path that make CONNECTIONS between PHOTORECEPTORS and BIPOLAR CELLS

It also contributes to CONTRAST vision

Inhibitory cell

Visual Path Process (within retina?)

Light excites photoreceptors → sends message to bipolar cells → sends message to ganglion cells

Visual Path Process (outside retina?)

Ganglion cell axons form the Optic Nerve → connects to the brain, specifically the Thalamus and the visual cortices

Visual Path Process (in fovea? why does fovea have acute vision?)

Each cone attaches to a single bipolar cell → makes less convergent ganglion cells

Each cone has a Direct Line to the brain which registers the Exact location of Input

Visual Path Process (in periphery? downside? benefit?)

A greater number of Rods converge into ganglion and bipolar cells

Downside: The high convergence leads to less detailed vision

Benefit: Allows for greater perception of much fainter light in peripheral vision

Foveal Vision (receptors? convergence of input? brightness sensitivity? Detail? Color vision?)

Receptors: cones only

Convergence of input: Single cone to a single ganglion cell

Brightness sensitivity: Distinguishes among bright lights, but responds poorly to dim light

Sensitivity to detail: Good detail vision; because of direct connection to brain

Color vision: good; because of many cones

Foveal Vision (receptors? convergence of input? brightness sensitivity? Detail? Color vision?)

Receptors: proportion of rods increases toward periphery

Convergence of input: many receptors to one ganglion

Brightness sensitivity: Responds to dim light; poor for distinguishing among bright light

Sensitivity to detail: Poor detail vision because many receptors converge their input onto a give ganglion cell

Color vision: poor; because of few cones

Photopigment

A molecule located in the outer segments of photoreceptors cells (rods and cones) of the retina, that consist of a light-sensitive chemical called 11-cis-retinal, and opsins; plays a role in converting light into electrical signals

Opsins

Optic proteins that determine the specific wavelength of light the photopigment can absorb

11-cis-retinal

A light sensitive chemical that releases energy when struck by light; bound to optic proteins

Visual Signal Transmission (what is the process?)

LIGHT ENERGY is ABSORBED by quickly CONVERTING 11-cis-retinal into all-trans-retinal

→ ACTIVATES cGMP (2nd messenger) via G-protein within the photoreceptor

→ This leads to CHANGES in cGMP-gated ion channels

Color Perception (how? requires?)

Done by detecting the wavelengths of light; ‘visible’ wavelengths are sensed by the cones

requires comparing the responses of three different kinds of cones.

discrimination depends on the combined responses of different types of cones and then decoded by different neurons in the visual cortices

Human Visible Wavelengths (shortest? longest?)

Shortest: about 400 nanometers (violet)

Longest: about 700 nanometers (red)

Two Major Interpretation of Color Vision

1. Trichromatic Theory

2. Opponent-process Theory

Color Perception (Trichromatic Theory; how?)

Occurs through the relative rates of response by 3 types of cones: 1) short 2) medium 3) long - wavelengths

Each type of cone is sensitive to a different set of wavelengths

Determined by the ratio of activity across these 3 types of cones

(in actual cases, color is mixed, no pure red, green, or blue and is dependent on the frequency of response in one cone relative to the frequency of other cones)

Color Perception (Opponent-process Theory; how?)

Accepts the 3 types of cones (short, medium, and long), but proposes that we perceive color by the way of paired opposites (i.e., the brain perceives color via three paired color opponents: from red - green, from yellow - blue, from white - black)

The key is that color perception is done by bipolar cells BUT, bipolar cells can only process one color at a time

Color Perception (Opponent-process Theory; hypothetical mechanism?)

When stimulated by light, a yellow-blue bipolar cell is excited by a blue cone and inhibited by a mixture of a red and a green cone; an increase in yellow-blue bipolar cell’s activity produces the experience of blue; experience of yellow occurs when the blue light stimulates the bipolar cell long enough that it becomes fatigued OR when blue light stimulation is reduced and the cell responds below its baseline level

Actual case: at any moment, the 3 pairs of opponents produce combinations of colors through the opponent process, which involves many types of bipolar cell activity, producing a different and mixed perception

Two Types of Color Vision Deficiency

1. Color Blindness

2. Color Vision Weakness

Color Blindness (what is it? three possible causes? what they mean?)

The inability to perceive color differences due to either of the three different issues:

1. loss of cones - lacking one or two of the three types of cones (mostly red-green deficiency)

2. abnormal functioning - three types of cones, but one is abnormal

3. complete loss of color vision - perception of only black and white (rarest case)

Color Vision Weakness (what is it?)

Not complete loss or not complete insensitive to red-green

Red-Green Color Deficiency (what is it? main reason for it?)

Trouble distinguishing red from green

Gene deficits - the gene causing the deficiency is on the X chromosome, so more men (8%) are affected than women (1%) because women have two X chromosomes meaning more of a chance to have the needed gene

Photoreceptors (synaptic contact: horizontal cells and bipolar cells?)

Photoreceptors synapse directly with bipolar cells and contact bipolar cells indirectly via horizontal cells

The Optic Chiasm (what is it?)

The pace where the optic nerve from two eyes meet and cross

The Optic Chiasm (TRUE/FALSE: in humans, half of the axons from each eye cross to the other side of the brain)

TRUE

Lateral Geniculate Nucleus (LGN)

A part of the thalamus specialized for visual perception, where ganglion cell axons go and then sends them to mostly the visual cortex in the occipital lobe

A smaller amount of ganglion axons travels to the superior colliculus in the midbrain

Visual Field (what is it?)

The total area in which objects can be seen in the side (peripheral) vision while you focus your eyes on a central point

Visual Field (sensed by? from what sides?)

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 (what is it? what does it produce? )

A process where activity in one neuron is reduced by the activity in its neighboring neurons - produces contrast vision

The retina only responds to a particular pattern of information to extract meaningful data and ignore other data (cones and rods send a huge number of visual messages to filter)

Contrast Vision (what is it? how is it produced?

The ability of the visual system that distinguishes objects from their background based on differences in light intensity or color

Produced by the net result of excitatory and inhibitory messages received (lateral inhibition)

Retina uses this to create contrasts by emphasizing the borders of objects

Contrast Vision (simplified mechanism?)

A photoreceptor excites both horizontal cell and bipolar cell → the horizontal cell then inhibits more bipolar cells; because the horizontal cell spreads widely, it’s excitation will inhibit the surrounding bipolar cells (decaying with distance) → results in different intensities of inhibitions with the strongest on the original → BUT that inhibition of the original is outweighed by the direct excitation due to light stimulus

Contrast Vision (actual case?)

Light excites multiple photoreceptors → which excites multiple bipolar cells and the horizontal cell. The bipolar cells are receiving the same amount of excitation but their inhibition intensity differs creating contrast (the middle cells are inhibited the most while the outer has the greatest net excitation)

The Receptive Field (what is it?)

An area where light striking it inhibits or excites a neuron in the visual system

The Receptive Field (level of: photoreceptor; bipolar cell; rods and cones; ganglion cells)

Photoreceptor: the point or dot in space from which light strikes it (tiny)

Bipolar cell: derived from the connections they receive from rods and cones; groups of rods and cones connect to bipolar cell making it have a larger field than its sum

Rods and Cones: have the smallest in space to which it is sensitive

Ganglion cells: larger because they converge the receptive fields of photoreceptors and bipolar cells

Contrast Vision (at the ganglion cell level)

Further processed at the level of ganglion cells

Light in the center of the receptive field of a ganglion cell might be excitatory (on-center), with the surround inhibitory, or opposite (off-center) creating contrast

Three Categories of Ganglion Cells

1. Parvocellular Ganglion Neurons

2. Magnocelluar Ganglion Neurons

3. Koniocellular Ganglion Neurons

Parvocellular Ganglion Neurons (what are they? where? receptive field size? responds best to? synapses on?)

Small cell bodies located in or near the fovea with 1) a small receptive field, 2) responds best to details and color and 3) synapses on small cells of the LGN cells

SMALL + DETAIL/COLOR

Magnocellular Ganglion Neurons (what are they? where? receptive field size? responds best to? sensitive to? important for? synapses on?)

Larger cell bodies distributed evenly through the retina with 1) a larger receptive field; 2) responds best to moving stimuli, 3) sensitive to brightness; 4) important for depth perception; and 5) synapses on larger cells of LGN

LARGE + MOVEMENT

Koniocellular Ganglion Neurons

Small cell bodies found throughout the retina that 1) has functions associated with color vision; 2) connect to the LGN, other parts of the thalamus and the superior colliculus

LGN Cells (receptive field

Has a receptive field that is excitatory in the central portion and inhibitory in the surrounding ring; similar to ganglion cells

Bromann Area 17

An area in the occipital lobe considered the primary visual cortex; the area where visual information is received and processed (also named the Striate Cortex, or V1)

Blindsight

A clinical syndrome where people who are cortically blind respond to visual stimuli due to lesions in V1, they don’t consciously see

Some patients are able to tell where an object is by responding to light from the object even though they can’t see it

Blindsight (how?)

Likely due to intact photoreceptors; 1) small islands of healthy tissue remain in an otherwise damaged visual cortex (not large enough to provide conscious perception); 2) visual information bypasses the V1 and is sent to several other brain areas (which are strengthened after V1 is damaged)

Three Types of Cortical Cells (what are they? what are they categorized on?)

1. Simple

2. Complex

3. Hyper-complex

Categorized by their receptive field; firing potentials by responding to the edge of the object with a bar-shaped receptive field

Two Functions of Cortical Cells

1. Produce basic imaging

2. Image moving objects

Simple Cells

Cells located ONLY in V1 that responds to primarily BARS or EDGES in a SPECIFIC orientation due to its BAR-SHAPED or edge-shaped receptive field; those bars have excitatory zones and inhibitory zones where the more light that shines in the area the more or less the cell responds depending on the zone (excitatory = more, inhibitory = less)

Complex Cells

Cells located in V1 or V2 that responds to bars and edges in specific orientations with a MEDIUM receptive field that CAN’T be mapped into fixed excitatory or inhibitory zones; they respond to MOVING stimulus to track objects moving in a particular direction

Hyper-complex (or end-stopped) Cells

Cells located in V1 or V2 with an exact inhibitory area at one end of its LARGE bar-shaped receptive field, with a STRONG inhibitory zone at one end (sensitive to the length or corners of an object

Columnar Organization of the Visual Cortex

Cells in the visual cortex are GROUPED together in columns that are perpendicular to the surface according to their responsiveness to specific stimulus (FUNCTION)

e.g., cells in a particular column may only respond to visual input from the left or right eye

QUESTION: do visual cortical neurons exactly cover all information of the object that is seen?

No, it is now proposed that visual neurons are FEATURE DETECTORS, so the shape and orientation are encoded to present a particular feature of an object

Early Development of Visual System (involves what two things?)

1. Differentiation

2. Synaptogenesis

QUESTION: is vision simply perception of what you are “seeing”? (what key things play a role?)

No, the brain develops both visual paths and visual experience to maintain and “fine-tune” its connections

1) Learning to recognize the world; and 2) Learning to make sense of what you saw

Binocular Vision

The ability to perceive a single, cohesive image using input from both eyes

Binocular Deficit (kitten example: one eye)

If a kitten is deprived of visual stimulation in one eye in the first 4-6 weeks after birth, it will become almost BLIND in that eye because synapses in the visual cortex poorly develop and gradually become unresponsive to input from the deprived eye

Binocular Deficit (kitten example: both eyes)

If both of a kitten’s eye were deprived of light stimulation for the first few weeks after birth, the kitten’s cortex remains responsive to visual input, BUT most cells become responsive to just one eye (not both)

Prolonged lack of visual experience either in one eye or both eventually leads to “Binocular vision deficit”

Binocular Deficit (uncorrelated stimulus?)

As a result of binocular deficit, a stimulus in two eyes becomes uncorrelated

Strabismus

A symptom of binocular vision deficit where the eyes don’t move in the same direction and therefore don’t point in the same focus on the same visual neuron (becomes uncorrelated)

Usually develops in CHILDHOOD; when two eyes carry unrelated messages, cortical cells strengthen connections with only one eye; also stereoscopic depth perception vision is IMPAIRED

Stereoscopic Depth Perception Vision (requires binocular vision?)

SDP relies on retinal disparity - which is the discrepancy between what the left and right eye see (i.e., when cortex perceives and compares slightly different inputs from the two retina)

The ability to detect retinal disparity is shaped through experience, which requires binocular vision

Astigmatism

Decreased responsiveness to one kind of line or more; leads to blurry vision (image curved abnormally in one direction)

from 70% of infants which reduces to about 10% during early childhood

Impaired Vision (infants)

If not fixed at an early age, there can be long-term consequences; this suggests that there is a sensitive period for the visual cortex where, after that period, the visual cortex won’t change much

Parallel Processing

An ability of the brain to process multiple aspects of visual information simultaneously

The primary visual cortex (V1) sends information to the secondary visual cortex (V2), (which is responsible for the second stage of visual processing), while the V1 is processing visual input

Two Types of Processing from V2

1. The Ventral Stream

2. The Dorsal Stream

The 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

The Dorsal Stream

Refers to the visual path in the parietal and middle/superior temporal cortices. Called the “Where” because it helps the motor system to find objects and move towards (tracing) them (also called spatial vision

The Ventral Stream (damage to?)

They can see where objects are and grab them but can’t make SENSE of a television program and color vision because they have trouble identifying what things are

The Dorsal Stream

They can read, recognize, and describe objects in detail; BUT they don’t know WHERE they are and are unable to trace moving objects (i.e., they can’t accurately reach out to grab an object)

As information travels from V1 to V2 and further ventrall, it becomes:

More complex and more specialized; different neurons respond specifically to circles, lines, edges, and colors

Shape Perception (where? how?)

The Inferior Temporal Cortex contains cells that respond selectively to complex shape stimuli from objects by the way to exchange information with the prefrontal cortex; starts with responding to the sight, then when enough experience is collected, it will respond from other viewpoints

Shape Constancy

The mechanism for recognizing an object’s shape, even though the object approaches, retreats, or rotates

Recognizing Faces (where?)

The Fusiform Gyrus in the inferior temporal cortex is largely specialized for this. It is also activated to identify the details of other objects, car model, bird species, etc.,

Recognizing Faces (when?)

Occurs relatively soon after birth; human newborns come into the world predisposed to pay more attention to faces (mom’s faces) than other stationary displays

Continues to develop maturely into adolescence

Visual Agnosia

The inability to visually recognize objects including faces despite otherwise normal vision; patients can still recognize objects by hearing or tactile feedback, etc.,

Prosopagnosia

Inability to recognize faces due to damage to the fusiform gyrus - form of visual agnosia specifically for faces

Color Perception (where?)

Belongs to the ventral stream; V4 is an important area in processing color information that is picked and sensed by cones

Color Perception (how?)

Percieved by the parvocellular pathway and brightness perception mediated by magnocellular cells → which sends their outputs to areas V2, V4, and the posterior inferior temporal cortex

Color Constancy

The ability to recognize color despite changes in brightness; however after removing context, this becomes more difficult (rubix cube example)

Color Constancy (damage to V4)

People don’t become colorblind, they can still pick a color with intact cones; however, they may not re-find this color if the context light has been changed

Motion Perception (where?)

By the dorsal stream system; in the dorsal stream, area MT (middle-temporal cortex, aka V5) and adjacent area MST (medial superior temporal cortex) are important for this

Motion Perception (how?)

The MT and the MST both receive input from the magnocellular path that is color-insensitive;

Cells in the MT respond selectively to a stimulus moving in a particular direction, acceleration (how still photographs can imply movement)

Cells in the dorsal part of MST responds best to complex stimuli such as the expansion, contraction, or rotation of a large visual scene (cells respond even if observer moves forward/backwards or tilts); Cells in the ventral part of MST respond to an object that moves relative to its background