Physiology of Vision: Spatial & Temporal Summation

• Lecture is about Physiology of Vision, specifically L 9: Spatial & Temporal Summation

Spatial and Temporal Summation

• Neurons receive impulses from thousands of other neurons.
• An action potential depends on the summation of these inputs.
• Temporal and spatial summations are the two types of summation.
• Spatial summations are signals from multiple simultaneous input areas.
• Temporal summation comes from repeated inputs within a time frame.
• The threshold voltage must be reached to achieve an action potential.
• Temporal and spatial summation determine threshold voltage.

Neurotransmitters

• Glutamate (excitatory) neurotransmitter causes depolarization.
• Depolarization makes the neuron becomes more positively charged, this means neuron -excited
• GABA (inhibitory) neurotransmitters causes hyperpolarization.
• Hyperpolarization makes the neuron becomes more negatively charged, this means neuron -inhibited.
• Neurons can affect one another through excitation or inhibition.

Spatial Summation Defined

• Spatial summation happens when inputs from multiple neurons trigger an action potential
• Spatial summation is determined when inputs are added together
• The greater the number of excitatory postsynaptic potentials, increases chances of eliciting an action potential
• The greater the number of inhibitory postsynaptic potentials, lowers chances of generating an action potential
• Spatial summations are be signals from multiple different simultaneous inputs.

Temporal Summation Definition

• Temporal summation is achieved when high frequency inputs from one presynaptic neuron summated to produce an action potential
• Temporal summation comes from repeated inputs from the same neuron

Stimulus Visibility

• Visibility is dependent on size and duration of the stimulus
• Small objects presented briefly are hard to see
• These effects are most pronounced for the absolute threshold (AT).
• Large/long duration stimuli appear more visible, so AT decreases because less light required
• Sensitivity increases with large/long stimulus
• Sensitivity increases as threshold reduce

Vision type

• Contrast sensitivity is higher under photopic conditions
• Weber fraction is 0.015 for photopic vision, and 0.14 for scotopic vision.
• Visual Acuity (VA) and Contrast Sensitivity (CS) are not the full story
• Think about dark adaptation

Spatial Summation

• Ability to detect a stimulus is superior under scotopic conditions.
• Absolute sensitivity is superior under scotopic conditions.
• A 420 nm stimulus requires 3 log units more intensity to be detected under photopic conditions compared to scotopic conditions.
• Visual resolution and visual sensitivity balanced by how rods and cones connect to post-receptoral elements in the retina.

Rods & Cone Connections

• Convergence of inputs from rods onto large ganglion cells (periphery)
• Cone connections onto small ganglion cells (fovea) are one-to-one connections so it's different
• Convergence measures the number of neuron synapses on another neuron
• There are 120 million rods, 6 million cones and 1 million ganglion cells.
• Rods to cones convergence ratio is 400:1 in the periphery.
• Rods have higher convergence than Cones.

Spatial Summation Explained

• A ganglion cell requires 10 quantal absorptions to signal an event
• Rods connect to sum up information over space, resulting in greater sensitivity but poor resolution
• Cones connections maximise visual resolution at the expense of sensitivity.
• The scotopic system sums information over space which leads to greater spatial summation

Scotopic System

• Each photoreceptor absorbs just one quanta
• If quanta is delivered by two separate flashes then each flash contains 5 quanta
• Stimulus causes the scotopic system to reach threshold.
• The ganglion cell sums this information to produce a total of 10 quantal absorptions, detects a single light, not two lights
• Scotopic system offers higher sensitivity but poorer spatial resolution.

The Photopic System

• The photopic system has limited spatial summation
• A similar case, no stimulus seen because cones are attached to ganglion cells (5 photoreceptors connected to a single ganglion cell).
• A stimulus falls below threshold and no light is seen.
• Double light intensity results in 2 lights seen under photopic conditions; while scotopic shows one flash
• Has greater spatial resolution but poor spatial sensitivity in the photopic system

Spatial Summation: Ricco's Law

• Stimulus luminance measures intensity per unit area (e.g. cd/m²).
• Total light emitted from stimulus increases, if stimulus area increases
• A threshold number of quanta is found when an observer presented w/ small spot of light
• The experiment is repeated with spots of increasing ​​​​​​​size.
• The result is the graph of threshold number of quanta was plotted against spot diameter

Spatial Summation Graph Explained

• In the spatial summation graph critical area equals 10 min. arc.
• The total number of quanta is constant to reach a stimulus up to 10 min of arc in diameter
• Riccos Law for diameter of 10 min of arc which formula LAT x a = k
• or LAT =k/a
• From LAT a for < 10 min: complete summation
• Formula for Piper's Law is LAT ∞ 1/√ a for > 10 min which implies partial summation

Ricco's Law applied

• Quanta needed for detection is constant, up to 10 min of arc diameter (critical diameter)
• The scotopic system shows spatial summation within critical diameter.
• Very small stimuli (e.g. < 10' diameter) are imaged in the peripheral retina following LAT ∝ 1 / a (a = stimulus area, mm²)
• Which mean Lᴀᴛ x a = constant

Spatial Summation Summarized

• Trade-off between spatial summation of photopic and scotopic systems determined by critical diameters
• Critical diameter for photopic system smaller as it is smaller than scotopic reduces spatial summation capability.
• In the fovea, each cone photoreceptor synapses onto single bipolar cell, connecting to a single ganglion cell.
• The one-to-one connections preserves fine detail, avoids signal pooling -spatial summation hallmark
• In peripheral retina, many rods and cones converge onto fewer bipolar/ganglion cells leading to high spatial summation.
• Stimulus with Diameter greater than 10 threshold is independent area

Partial Summation

• Has an area > 10 min
• Usually partial summation occurs; the stimulus area doubles and it leads a small drop in the area
• Piper's law is valid since Lᴀᴛ ∝ 1/√a and where Lᴀᴛ x √a = constant and Lᴀᴛ represents lum. at abs. thresh.
• The area is measured in mm²
• Partial threshold exists for difference thresholds

Example of partial summation

• Ex. If Area of 9 mm2 , the LAT equals ​2 cd/m²
• Piper's is valid where LAT x √ a = constant
• 2cd/m² x √ 9 means constant equals 6
• area is assumed a stimulus as 9 ​​​​​​​mm² and is larger than critical diameter of 10 mins of arc]
• Now if area of 16 ​​​​​​​mm² what is what the LAT
• LAT = constant and LAT = constant / √ a
• Means 6 / √ 16 = and the LATequals 1.5 cd/m2
• Area nearly double still see LAT lowers slightly

Critical Diameter

• For peripheral targets, total spatial summation (i.e. Ricco's law) can happens for targets with the diameter of 10 min. arc.
• Target with < 10 min.arc means there a complete summation
• However, Larger targets have partial summation
• Targets that are extremely ​​​​​​​large ​​​​​​​show no summation
• Critical diameter determined by eccentricity
• Stimulus diameter > 10 min threshold shows independence

Spatial Summation Mechanism

• Rods display summation through "retinal convergence".
• At least 100 rods can send their information to each ganglion cell.
• Ganglion are cells more sensitive to large than small stimulus on photoreceptors
• Rods and cones communicate horizontal cells, bipolar cells, etc., before ganglion cells
• Ganglion cells pool info' to responds retina field
• The cell responds to "receptive filed"
• Receptive filed's size gauged based on critical diameter
• Receptive filed is gauged by direct electrical recording.
• Receptive field sizes vary with eccentricity, with RF size increases with distance from fovea

Temporal Summation Basics

• Stimulus visibility is dependent on duration
• Objects presented quickly hard to see
• Most pronounced effects is AT absolute threshold where AT decreases
• So long duration stimuli means increased light which means great sensitivity

Defined Temporal Summation

• Amount of light energy on the retina is determined by its duration.
• Luminance and Illuminance, this is used to measure flow of light- not total of light being measured
• Log for light emits light than short flashes
• There is a Blocks Law, that is equal to Ricco's Law

Blocks Law Explained

• Systems have different critical duration of photopic and scotopic
• Scotopic vision offers greater temporal summation where critical duration of 100ms
• Photopic vision shows limited summation with 10 - 50 msec of critical duration
• Scotopic system has information summated than photopic
• Blocks Laws shows equaliveness with Ricco's Law
• The stimulus presented is within the so-called value- the temporal summation is the value
• If Stimulus is seen one value, delivered this value

Complete Summation

• For absolute value and that is the measure to see short flashes=100 Ms
• Formula here equal 1/ T i equals value constant
• Show's law and the visual makes use of every value here

Quantum Counting

• This system this called accounting with soon light- 5 to 10 can be scene here
• To gain value = always constant- can easy be measured equal value value
• This means you double stimuli- the value means half

Partial Temporal Summation

• Is value is 0.1 sec then it be done like that
• Or value in can happen and that means time
• And can the the can you be here and have value
• In some parts it can be that in and difference can be shown.

Temporal and what it is!

The systems it has time value 100 of it = time that passed The short with short time period of 100/50 Two will make of that that happen the vision.