NE 203 LEC 10 CH 9

ON and OFF pathways in the retina.

• ON bipolar cells:

• OFF bipolar cells:

Antagonistic center-surround arrangement of their receptive fields.

OFF-center retinal ganglion cell responses to stimulation of different regions of their receptive fields —>Antagonistic center-surround arrangement of their receptive fields.

ON CENTER ganglion cell fires action potentials when illuminated in the receptive field.
OFF-center ganglion cell decreases firing action potentials when light is decreased in receptive field; fires action potentials when not illuminated (light is turned off)

ON/OFF ganglion cells respond transiently to both increases and decreases in light.

depolarization/ hyperpolarization of ON and OFF pathways in retina

• ON bipolar cells: depolarized in response to photoreceptor – induced activation by light; express mGluR6 -> hyperpolarize.

• OFF bipolar cells: hyperpolarized in response to photoreceptor-induced activation by light; express AMPA kainate glutamate receptors -> depolarize

effect of ON and OFF pathways to light

These different types of glutamate receptors allow these cells to respond to increments and decrements in light levels.

Lateral interactions in the retina: Circuitry responsible for the receptive field surround of an ON-center retinal ganglion cell

Horizontal cells: Use GABA.

- Amacrine cells: Use GABA, glycine, acetylcholine, dopamine, and glutamate. Dendrites of both cells extend laterally. Enables ganglion cells to be sensitive to contrast via their antagonistic center-surround receptive fields

Retinal Ganglion Cells Carry Feature Information to the Brain: Types of retinal ganglion cells.

- Many different types.

- All generate parallel pathways: retino-recipient brain regions (process image-forming information).

- Retina -> lateral geniculate nucleus -> visual cortex.

- Retina -> superior colliculus.

- Retino-no-recipient brain regions (process non-image forming information): suprachiasmatic nucleus and olivary pretectal nucleus.

Retinal Ganglion Cells Carry Feature Information to the Brain: Parallel and retinotopic projections in the retino-geniculo-cortical pathway.

• Thalamocortical relays: In lateral geniculate (LGN)

• Retino-geniculo-cortical pathway: Projects to Layer 4 of primary visual cortex

  • sensory input always landing in layer 4

Damage (e.g., stroke, tumor) to the retino-geniculo-cortical pathway can lead to visual field deficits
Describe deficits and where

• Damage (e.g., stroke, tumor) to the retino-geniculo-cortical pathway can lead to visual field deficits

• Damage (e.g., stroke, tumor) to the retino-geniculo-cortical pathway can lead to visual field deficits

inferior vs superior visual field

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Parallel and retinotopic projections in the retino-geniculo-cortical pathway:

• Midget (P cells): Project to 4 parvocellular layers of lateral geniculate.

• Parasol (M cells): Project to 2 magnocellular layers of lateral geniculate.

• The LGN is a nucleus in the thalamus!

• THE THALAMUS IS THE GATEWAY TO THE CORTEX

• Striate cortex=visual cortex or V1

P Cells: Parallel and retinotopic projections in the retino-geniculo-cortical pathway.

receptive field size: Small receptive fields
Duration of response: Sustained

Sensitivity to color: Transmit information about color
Ability to detect low-contrast stimuli: Cannot detect low-contrast stimuli

M cells: Parallel and retinotopic projections in the retino-geniculo-cortical pathway.

• Receptive field size: Large receptive fields

• Duration of response: Transiently

• Sensitivity to color: No information about color transmitted

• Ability to detect low-contrast stimuli: Can detect low-contrast stimuli

Retinotopic map:

Spatial relationships among retinal ganglion cells are maintained throughout the retino-geniculo-cortical pathway.

• ensures that the orderly representations in the retina are transferred to the visual centers in the brain.

Distorted allocation from retina to cortex

Although fovea ~1% of retinal surface area, it supplies ~50% of visual information to the visual cortex.
takes up have of cortical space at occipital lobe

ocular dominance columns (layer 4 of visual cortex)

where Lateral geniculate axons terminate (eye-dependent)

Eye-specific projections

Lateral geniculate receives input from the retina based on both cell type (M and P) and which eye (ipsilateral vs. contralateral)

Stereopsis

binocular depth perception

Cerebral cortex and visual percepts.

Much of the cerebral cortex engages in higher-level processing by integrating the information arriving at the primary visual cortex.

Cellular and columnar organization of primary visual cortex. (layers)

• 6 cellular layers.

• pyramidal neurons: Layers 2, 3, 5, 6

  • release glutamate to subcortical and other cortical targets.

• spiny stellate cells: layer 4

  • are primary target of extremely dense projecting axons from lateral geniculate; appears as stripes(striate cortex)

• columnar arrangement

  • primary visual cortex

Cellular and columnar organization of primary visual cortex.

Neurons within the same cortical column have the same or similar receptive field properties and response preferences.

Edge detection and orientation preferences in primary visual cortex —> light

Preferred orientation: the orientation of a stimulus to which a cortical neuron maximally responds. - Also called the peak of the tuning curve

• Orientation tuning curve for a neuron in primary visual cortex. In this example, the highest rate of action potential discharge occurs for vertical edges—the neuron’s “preferred” orientation

• Neurons in the primary visual cortex typically respond vigorously to a bar of light oriented at a particular angle and less strongly—or not at all—to other orientation

Neurons in the primary visual cortex respond selectively to oriented edges  (Part-DE

there is little variation in the orientation preference of the neurons

Neurons displaced along the tangential axis of the cortex (i.e., in different cortical columns) exhibit orientation preferences (right panel) that shift in an orderly progressive fashion.

Visual Centers in the Cerebral Cortex Detect Increasingly Complex Visual Features

From detection to object recognition and visually guided behaviors.

-Extrastriate visual areas: other cortical areas devoted to visual processing of information from the primary visual cortex -> hierarchical processing.

-Ventral stream: recognizes the object/scene; starts at primary visual cortex -> inferior temporal cortex; “What?”

-Dorsal stream: analysis of motion and spatial relationships; starts at primary visual cortex -> medial temporal area; “Where?”

Outside the occipital lobe, visual areas are broadly organized into two pathways: a ventral pathway that leads to the temporal lobe (object recognition), and a dorsal pathway that leads to the parietal lobe (spatial vision).

Illustrates the location of primary visual cortex (V1)

Additional visual areas V2, V3, V3a, VP (ventral posterior area)

V4, and MT (middle temporal area)

Circuitry responsible for the pupillary light reflex

Accommodation and the “near response”.

optic nerve → ciliary ganglion → pretectum

-Ciliary muscles thicken and round up the lens when viewing something near vs. far.

-Zonule fibers exert tension vs. lens elasticity determine the shape of the lens.

• muscles accomodate less

-Accommodation also under control of Edinger-Westphal nucleus.

-Near-response: When viewing a close object, accommodation, convergence, and constriction are required for optimal acuity.

Optimizing Vision and Visual Behaviors Requires Autonomic, Motor, and Cognitive Control

Maintaining the image of a visual target on the fovea.

-Extraocular eye muscles redirect focus.

-Conjugate eye movements: coordinated movements by both eyes by cooperating extraocular eye muscles so that binocular fixation is maintained.

Maintaining the image of a visual target on the fovea.

2 eye movements for redirecting gaze

• i. Smooth pursuit: both eyes shift their positions in a slow, smooth, continuous motion to track a moving object.

• ii. Saccadic eye movements: rapid, sudden change in eye position to redirect focus from one area to another.

• Saccadic suppression: prevents a blurring of the visual percept as an object image sweeps across the retina; inability to detect  a visual percept during saccadic eye movement.

  • blurriness no more

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