COGS 17 - Midterm 2

What are visual receptors?

Rods & Cones containing Photopigment molecules

What is isomerization?

Converting light into a neural signal

What is the outer segment of receptors composed of?

Lamellae with embedded Photopigment molecules

What are the two components of Photopigment molecules?

Opsin and Retinal

What happens when 11-Cis Retinal absorbs a photon of light?

It changes shape and detaches from Opsin

What happens when Opsin changes color from reddish-purple to pale yellow?

It activates Second Messengers in Receptor that lead to ion gates closing, modifying graded release of NT

What is Rhodopsin?

Photopigment molecules in Rods

What is Photopigment Regeneration?

Recombination of Retinal & Opsin using enzymes from Pigment Epithelium

What is the Retina composed of?

Neurons (except Pigment Epithelium), multi-layered, covering rear, inner wall of eyeball

What are the two types of visual receptors?

Rods and Cones

What is the Fovea?

Small central area of high concentration of Cones only; for high detail resolution

What are Bipolars?

Postsynaptic to Receptors, show Spontaneous firing, Graded Potentials, release Excitatory NT

What are Ganglions?

Postsynaptic to Bipolars, Show Action Potentials; Axons of the Ganglion Cells form the Optic Nerve \=\> brain

What is the Blind Spot or Optic Disk?

Where Optic Nerve leaves eye & blood vessels enter/leave; No Receptors there

What is the function of the Optic Nerve?

To transmit visual information from the retina to the brain

What are interneurons?

Neurons that influence interactions between other neurons.

What are horizontals?

Neurons that modify the interface of receptors and bipolars through graded potentials and mostly inhibitory neurotransmitters.

What are amacrines?

Neurons that modify the interface of bipolars and ganglions through graded potentials and mostly inhibitory neurotransmitters. There are many kinds of amacrines.

What is the pigment epithelium?

The rearmost layer of non-neural cells that feeds and recycles from receptors and helps reflect and maximize light.

What happens when light hits receptor cells?

Light turns receptor cells off (down), while darkness turns them on (up).

What are the characteristics of receptors?

Receptors show spontaneous firing, graded potentials, and release inhibitory neurotransmitters.

What happens in the absence of stimulation for receptors?

In the absence of stimulation, the receptor's Na+ gates open, Na+ flows in and out, and Ca++ gates also open, causing continuing release of neurotransmitters. Ca++ is actively pumped out so the cycle can repeat.

What happens when photopigments are isomerized?

When photopigments are isomerized, Na+ and Ca+ gates close, increasing the receptor's polarity and decreasing neurotransmitter release.

What happens in the dark?

In the dark, receptors release enough inhibitory neurotransmitter to prevent bipolars from triggering ganglions. So ganglions, by not firing, in effect, report to the brain: 'No light'.

What happens in bright light?

In bright light, receptors are shut down and do not inhibit bipolars. Bipolars spontaneously release enough excitatory neurotransmitter to pass ganglion's threshold for firing, so ganglion sends message: 'Bright light!'

What happens in dim light?

In dim light, receptors' spontaneous release of neurotransmitter is decreased (a graded reaction, more light, less neurotransmitter), so bipolars are only somewhat inhibited and release enough excitatory neurotransmitter to sometimes trigger ganglions.

What is the rods convergence?

High convergence, avg. 120:1 ganglion

What is the cones convergence?

Low convergence, avg. 6:1 ganglion, in fovea very low, often only 1:1 midget ganglion

How does convergence help account acuity?

For acuity (detail resolution) differences between rods & cones

Where do rods receive light from?

From 3 environmental points falls on 3 rods, but if all 3 rods converge (via bipolar) on 1 ganglion, that 1 ganglion can only send the message "something's out there"

What is Sensitivity Convergence?

Accounts for sensitivity differences between rods and cones

Where do cones receive light from?

From 3 environmental points falls on 3 Cones, each communicates (via Bipolars) with just 1 Ganglions. Those 3 Ganglions send message: "3 things out there" \= High Acuity

What happens to rods in dim light?

Very little light reaches photopigments in receptors

What happens to cones in dim light?

Little change in inhibitory NT released from each cone, little excitatory NT released from each bipolar, may be insufficient to trigger AP from each ganglion, so no info sent to brain (i.e. no light detected)

What happens to rods in dim light?

Little change in inhibitory NT released from each rod, little excitatory NT released from each bipolar, but since many converge on one ganglion, sum of NT is sufficient to trigger AP from ganglion, so info sent to brain (i.e. even dim light detected)

What is Receptive Field (RF)?

Set of receptors influencing a target cell

What determines the size and type of a target's RF?

Patterns of convergence and lateral influences

What is the difference in RF size between a ganglion along the path from converging rods and a ganglion along the path from cones?

The former has a large RF while the latter has a small RF

What is the relationship between small receptive fields and high acuity perception?

Cells with small receptive fields are often involved in high acuity perception

What are some cells that have Center-Surround RFs?

Some Ganglions, LGN, and V1 cells have Center-Surround RFs per pattern of Excitation, Inhibition and Convergence

What happens when a ganglion cell with an Excitatory Center-Inhibitory Surround RF is exposed to light in the center of its RF?

The ganglion is excited (by activity of Bipolars from that area)

What happens when a ganglion cell with an Excitatory Center-Inhibitory Surround RF is exposed to light in the surround of its RF?

The ganglion is inhibited (by activity of Amacrines from that area)

What happens when a ganglion cell with an Excitatory Center-Inhibitory Surround RF is exposed to more light in the center than in the surround of its RF?

The ganglion is more likely to reach threshold or to fire at higher rate

What happens when a ganglion cell with an Excitatory Center-Inhibitory Surround RF is exposed to more light in the surround than in the center of its RF?

The ganglion is less likely to reach threshold or to fire at lower rate

What is Lateral Inhibition?

Connectivity pattern throughout brain to create Center-Surround RFs

What is the function of Lateral Inhibition?

Exaggerate differences to highlight edges of objects

What is an example of Lateral Inhibition?

Amacrine sends inhibition to Ganglions nearby

How does Amacrine respond to Bipolar excitation?

Graded response, more excitation leads to more inhibition sent to neighbors

What is Simultaneous Contrast?

An illusion created by Lateral Inhibition

What is an example of Simultaneous Contrast?

Medium 1 appears darker than Medium 2 despite being physically the same

Why does Medium 1 appear darker than Medium 2?

Bipolars are more excited by the light gray border, leading to more Lateral Inhibition in the center region

What are Direction-Sensitive Cells?

Motion detectors

What type of Lateral Inhibition is involved in Direction-Sensitive Cells?

Uni-Directional Lateral Inhibition

What is the optic nerve?

A division that divides and goes to both sides of the brain

What is the Right Visual Field?

Stimuli to the right of the fixation point

What is the Left Visual Field?

Stimuli to the left of the fixation point

What is the Optic Chiasm?

Point where optic nerves cross over

What is the LGN?

Lateral geniculate nucleus, a relay center in the thalamus for visual information

What is the Striate Cortex?

Primary visual cortex, where visual information is processed

What is the Corpus Callosum?

Bundle of nerve fibers connecting the two hemispheres of the brain

What happens to info from LVF?

Processed in the right hemisphere of the brain

What happens to info from RVF?

Processed in the left hemisphere of the brain

What is the columnal organization in the visual cortex?

All cells within a given column show the same preferred stimulus.

What is the receptive field in the visual cortex?

All cells within a given hypercolumn have the same receptive field.

What is a hypercolumn in the visual cortex?

A set of columns that includes columns for one full set of orientations and blobs for color processing from the same area of the retina.

What is the retinotopic map in the visual cortex?

It reproduces the retinal layout such that adjacent columns of cortical cells have adjacent receptive fields in the retina.

What does the topological map in the visual cortex preserve?

It preserves spatial relationships (area A next to B next to C etc. in both the retina and the cortex).

What is the magnification factor in the visual cortex?

Cortical cells with small receptive fields fill a disproportionally large area of the visual projection areas.

What happens when you imagine something in the visual cortex?

Imagery (in absence of actual visual input) activates many of the same areas of visual cortex as actual input.

What is the orientation preference of cells in the visual cortex?

Cells in each column respond to lines oriented in one particular orientation.

What are blobs in the visual cortex?

They are areas in a hypercolumn that are responsible for color processing.

What is the relationship between the retina and the cortex in the visual cortex?

The retinotopic map reproduces the retinal layout such that adjacent columns of cortical cells have adjacent receptive fields in the retina.

What is the Ventral Stream?

Identifies stimuli, specialized for color and detail, integrated with language centers.

What is the Dorsal Stream?

For visio-spatial mapping, detects motion, locates objects, navigates and manipulates environment.

What is the basic pathway of the Dorsal Stream?

Rods and cones, Y ganglions, superior colliculus, LGN, V1, V2, MT, MST, posterior parietal cortex.

What is the Parvocellular Pathway?

Small ganglions, sustained response, for color and detail.

What is the Magnocellular Pathway?

Large ganglions, transient response, for visio-spatial mapping.

What is the function of the Ventral Stream?

Identifies stimuli.

What is the function of the Dorsal Stream?

Visio-spatial mapping.

What is the Ventral Stream/Parvocellular Pathway?

Pathway for determining shape and texture

What do Simple Cells in V1 respond best to?

Lines of particular Orientation

What do Complex Cells in V2 respond best to?

Moving lines of particular Orientation

What do Complex Cells in V3 respond best to?

Combinations of orientations

What do Sine Wave Gradients represent?

Gradually changing bands of dark & light

What is Spatial Frequency?

\# dark-light changes / degree of visual angle

What are High SFs used for?

Detail

What are Low SFs used for?

Gross outline

How is the visual scene coded?

Simultaneous activity along multiple SF channels for diff orientations

What do some cells in the Fusiform Gyrus respond best to?

Faces

What is Prosopagnosia?

Inability to recognize familiar faces

What do other cells in IT react to?

Objects of which you are an expert discriminator

What is the range of wavelengths for visible light?

350nm to 700nm

What is color constancy?

Ability to perceive colors consistently under different illuminations.

What is the Dorsal Stream pathway?

The WHERE/HOW pathway for motion and depth perception.

What is the role of Uni-Directional Lateral Inhibition in the Dorsal Stream pathway?

It is responsible for the direction sensitivity of cells in the pathway.

What are the receptive fields of cells in the Dorsal Stream pathway primarily composed of?

Rods, which are sensitive to movement due to their convergence and clustering.

What is Optic Flow?

It is the perception of the whole environment streaming toward/from the observer as they move forward/backward.

Where is motion represented in the cortex besides the Dorsal Stream pathway?

In the STS (Superior Temporal Sulcus) in the lateral Temporal lobes.

What is Biological Motion?

It is the observed motion of walking, head movements, facial expressions, gestures, etc.

What is Binocular Disparity?

It is the difference in the location of a point on the two retinas when both eyes focus on one environmental point.