Vision Notes

The Eye

• Key components of the eye:

  • Ligament

  • Iris

  • Pupil

  • Lens

  • Cornea

  • Eye muscle

  • Ciliary muscle

  • Sclera (the white part of the eye)

  • Retina

  • Fovea

  • Optic nerve

  • Blind spot

Receptive Fields

• A receptive field is the specific location of a visual image where a stimulus can influence the activity of a given cell.

• Photoreceptor receptive fields are determined by their position in the retina and the eye's optics.

Neural Integration: Spatial Summation

• Spatial summation is the process by which EPSPs (Excitatory Postsynaptic Potentials) are added up and IPSPs (Inhibitory Postsynaptic Potentials) are subtracted on a neuron.

• This integration occurs at different sites on the same neuron, specifically at the axon hillock.

Center-Surround Receptive Fields

• Downstream of photoreceptors, in the retina and brain, receptive fields result from the addition and subtraction of photoreceptor receptive fields.

Types of Retinal Ganglion Cells

• On-center retinal ganglion cell:

  • Light in the center increases the firing rate.

  • Light in the surround decreases the firing rate.

• Off-center retinal ganglion cell:

  • Light in the center decreases the firing rate.

  • Light in the surround increases the firing rate.

Visual Field Mapping

• The receptive fields of many neurons together form a map of the visual field.

• Both eyes see both sides of the visual field.

• Both retinas project to both sides of the brain.

• Axons segregate to ensure that visual brain areas map the contralateral visual field.

Maps as Representations

• Maps are representations that preserve neighbor relationships.

• The quote "The map is not the territory" emphasizes that a map cannot be a perfect copy; it must distill important elements while omitting others to be useful (Jorge Luis Borges).

• A map takes in whats important and leaves out was is not

Topographic Mapping

• Topographic mapping is a general principle across sensory systems.

• The somatosensory and motor cortex are organized as maps of the body surface.

• Auditory cortex is organized as a map of sound frequency.

  • The apex of the cochlea corresponds to lower frequencies (e.g., 500 Hz).

  • The base of the cochlea corresponds to higher frequencies (e.g., 16,000 Hz).

Retinotopic Mapping and Cortical Magnification

• Visual cortex is organized as a map of the visual field through retinotopic mapping.

• Receptive fields of neurons are topographic, meaning nearby neurons respond to nearby parts of the visual field.

• Cortical magnification refers to the disproportionate representation of the fovea in the cortex.

• The binocular zone is the part of the visual field seen by both eyes.

Visual Cortex Lesions and Scotoma

• There's a topographic relationship between visual cortex lesions and the location of scotomas (blind spots).

Calcium Imaging

• Calcium imaging uses sensors that change fluorescence intensity when Ca^{2+} enters the cell.

• The sensor is based on a gene from jellyfish called GFP (Green Fluorescent Protein).

Topographic Mapping of Visual Cortex Activity

• Visual cortex activity and stimulus location have a topographic relationship.

• The visual cortex is organized as a map of the visual field.

Neural Integration

• Neural integration involves the spatial summation of EPSPs and IPSPs that occur at different sites on the same neuron.

  • Two simultaneous EPSPs sum to produce a greater EPSP.

  • Two simultaneous IPSPs sum to produce a greater IPSP.

  • A simultaneous IPSP and EPSP can cancel each other out.

Center-Surround Receptive Fields Formation

• Center-surround receptive fields are formed by adding and subtracting photoreceptor receptive fields through horizontal cells.

• This process involves bipolar cells.

Retinal Ganglion Cells

• On-center retinal ganglion cells:

  • Light in the center increases the firing rate.

  • Light in the surround decreases the firing rate.

• Off-center retinal ganglion cells:

  • Light in the center decreases the firing rate.

  • Light in the surround increases the firing rate.

Simple Cell Receptive Fields

• Adding up circular receptive fields can create a simple cell with an oriented line receptive field.

Simple Cells in Primary Visual Cortex

• Simple cells in the primary visual cortex respond best to bars with a preferred orientation.

• The diagram shows how lateral geniculate cells contribute to the receptive field of a simple cell.

• 'On' and 'Off' areas determine the cell's response to light.

Complex Cells in Primary Visual Cortex

• Complex cells respond best to moving bars with a preferred orientation.

Visual Cortex Devotion

• A significant portion of the cortex is dedicated to vision.

Visual Cortex Specialization

• In blind individuals, the visual cortex can be repurposed for other functions like spoken language, math, and touch.

Modularity in Visual Cortex

• The visual system is divided into two pathways: the "what" and "where" pathways.

• Different regions of extrastriate cortex process different aspects of the visual image (Van Essen et al., 1992).

"What" and "Where" Pathways

• "What" pathway (ventral stream):

  • Located ventrally, spanning the occipital and temporal lobes.

  • Neurons are sensitive to object properties like shape and color.

  • Damage impairs conscious perception of objects.

• "Where" pathway (dorsal stream):

  • Located dorsally, spanning the occipital and parietal lobes.

  • Neurons are sensitive to spatial properties/motion.

  • Damage impairs visually-guided behaviors (e.g., reaching, grasping).

Visual Pathways

• Ventral (temporal) pathway:

  • Includes areas like AIT, CIT, PIT, and V4.

• Dorsal (parietal) pathway:

  • Includes areas like 7a, LIP, MST, MT, and VIP.

• Parvo and Magno pathways:

  • P cells form the P pathway.

  • M cells form the M pathway.

Color and Motion Selectivity

• Area MT neurons mostly respond to moving stimuli.

• Area V4 neurons mostly respond to the color of stimuli.

Visual Cortex Specialization and Deficits

• Damage to individual areas causes specific deficits:

  • Achromatopsia (inability to see color) – area V4.

  • Akinetopsia (inability to see motion) – area MT.

Random Dot Motion Task

• Monkeys discriminate dot stimuli based on the direction of motion.

• Discrimination is easy when the % correlation is high and gets worse as the dots become more random (Britten et al., 1992).

MT Activity and Motion Coherence

• Activity in MT correlates with the coherence of the motion stimulus (Britten et al., 1992).

Neuron Perception

• Individual neurons can be as perceptive as the whole monkey (Britten et al., 1992).

MT Activity and Behavioral Choice

• Activity in MT correlates with behavioral choice (Britten et al., 1996).

MT Stimulation

• Stimulation of MT influences the perception of motion (Salzmann et al., 1990).

Adding Inputs and Vision

• Adding and subtracting photoreceptor inputs produce a center-surround receptive field.

• Adding and subtracting center-surround receptive field inputs produces a simple cell receptive field (oriented bar).

Limitations of Simple Addition Models

• It suggests a need for a large number of cells, potentially one for each face.

• It raises concerns about the loss of specific representations if a single cell dies.

• It necessitates invariant representation of faces.

Jennifer Aniston Cell

• The concept of a Jennifer Aniston cell raises questions about how the brain recognizes specific individuals and objects.

Invariant Object Recognition

• Invariant object recognition is the ability to recognize objects regardless of changes in viewpoint, lighting, or other variations.

Specialization of Ventral Visual Areas

• Ventral visual areas are specialized.

• Responses to faces and houses show distinct patterns (Kanwisher et al., 97-99; Tong et al.; Sergent et al., 92; Haxby et al., 91, 94, 99; Puce et al., 95, 96; McCarthy et al., 97; Halgren et al., 99).

Specialized Visual Areas

• Fusiform Face Area (FFA): Specialized for face recognition.

• Parahippocampal Place Area (PPA): Specialized for recognizing places.

• LOC: Processes information about things.

• Body Area: Processes information about bodies (Downing et al., 01).

Stanford Program

• Stanford Human Intracranial Cognitive Electrophysiology Program studies cognitive functions using intracranial recordings.

Biological Motion

• Biological motion involves the simultaneous processing of what and where.

• Posterior STS processes biological motion vs. scrambled motion.

• EBA processes headless bodies vs. objects (stationary).

• LOCd processes faces & objects vs. texture.

• FFA & OFA process faces vs. objects.