PSYCH 202 (Paul's content)

Systems neuroscience

Sensory systems:

• Energy → action potentials

Motor systems:

• Action potentials → energy

In CNS, all membrane potentials (graded + action)

Sensory coding

We can only sense those aspects of the world for which we have receptors → specialised neurons that transduce energy into action potentials

Perceiving the world

Distal stimulus: external world

Proximal stimulus: pattern of light on the retina

• Only info we have

• based on energy we can detect

Stimulus: properties of visible light

• electromagnetic radiation

• travels in photons

• each photon has a wavelength (390-700nm)

• all photons of the same wavelength are identical

Visible light

Visible light spectrum neatly aligns with transmission through water

A photon can be

• Reflected (blue reflects off blue)

• absorbed (red absorbs green)

• transmitted (red passes through red)

Two conditions of light

• Low intensity/scotopic (different wavelength, low intensity → nighttime)

• High intensity/photopic (different wavelength, high intensity → daytime)

Scotopic vision

Intensity of photons is the same (low), no sense of hue or change in brightness

• 490nm is brightest

• 640nm is dimmest

Change in brightness w/o change in intensity??

Photopic vision

Intensity of photons in the same (high), can see hue

• 540nm brightest

• 420/640nm dimmest

Combining coloured light

• 540nm (Gr. Yellow) + 640nm (red) = yellow (identical to 580nm)

• 490nm (blue) + 580nm (yellow) = white

• Therefore 490nm + 540nm + 640nm = white

Any given wavelength simulated by superimposing diff. wavelengths

Additive colour mixing

Adding photons to create colour (light-based)

Overlap = white

Subtractive colour mixing

Pigments rely on absorption + reflection of light

Overlap = black

Colour vision anomalies

• Protanopia/maly

• Deuteranopia/maly

• Tritanopia/maly

• Monochromacy (occurs after brain injury)

• Achromatopsia

Basic colour theory

• Additive colour mixing: blue, red, green

• Ewald Hering: Blue, Red, Yellow, Green

• → opponent afterimages

Negative afterimages

• Gradual adaptation to image

• Changes zero point so white looks different

• Look at yellow → system leans more blue to stop responding to yellow

• When look at white again, see blue

Foveal pit

One region where vision is least distorted → small patch of acute vision + saccades to create a whole image

Optic nerve

Creates blind spot in vision

Rods

• 120 million in each retina

• Distributed all over retina except fovea

• Contain rhodopsin → bleaches when exposed to light, hyperpolarises photoreceptor → depolarised bipolar cell → stimulates ganglion cell

• Respond to very low light levels

• Respond differentially to wavelength

Rod Physiology

Dark current → Na+ channels opened, rod is partially depolarised → releases glutamate into synaptic cleft (excitatory during darkness)

Light transduction → Na+ channels close, rod becomes hyperpolarised → glu release terminates (inhibitory AP generated)

Rod response to light

Similar peak as in scotopic vision (peak around 500nm).

Why do low intensity lights vary in brightness?

Rhodopsin absorption varies in wavelength (peaking at 500nm) → peak efficiency at this wavelength therefore relatively blind to other wavelengths

Frequency coding

The intensity of a stimulus is often coded by the frequency of firing (action potentials) in cells that respond to that stimulus (MORE not LARGER)

Principle of Univariance

A given receptor can be excited by multiple attributes (wavelength AND intensity), but its output (firing rates) varies in only one dimension (i.e., univariate) → so it can only code a single dimension and cannot distinguish between stimulus attributes

Cone responses (trichromats)

Three cone types

• Short

• Medium

• Long

Not just generated via AP → distinguish light based on ratio of activity in each cone

Coarse coding

Neurons respond to a broad range of stimuli, with a GRADED response depending on the match to a preferred stimulus → short cones respond most to blue, less to green, least to red

Population coding

Integrating the responses from a number of differently tuned neurons enables precise coding → e.g. large response from all cones = white light, no response from any cones = dark

Protanopia

No L cones

Deuteranopia

No M cones

Tritanopia

No S cones

Opponent-process theory of colour vision

Hering noted colours seemed to form opponent pairs (red/green), (blue/yellow)

Hurvich and Jameson proposed that neurons beyond the photoreceptors could implement opponent processing

• Cell can only code either red/green but not both, same from blue/yellow

Nonopponent RGC

• Excitatory input from L and M cones

• No understanding of hue

• Greyscale

L+M- opponent RGC

• Excited by short wavelength (L cone peaks slightly)

• Inhibited by medium wavelength (M cone peaks)

• Excited by long wavelength (L cone peaks)

S+ (L&M) -

• Excited by short wavelength (coded by s cone)

• inhibited by medium and long wavelength (shows as yellow)

Opponent processing

Many neurons that encode some dimension do so in an opponent fashion, so that excitation and inhibition of the neuron have opposite interpretations in the nervous system

Colour coding in RGC

L+M+, L+M-, S+(L&M)- combine to create visual light rainbow spectrum

• appearance of each monochromatic light (hue and brightness) can be understood in terms of the responses across these RGC types

• Opponent RGC responses define a colour space like that proposed by hering and hurvich & jamison

Retina to Cortex

• Optic nerve (RGC axons)

• Optic chiasm (crossover of axons from nasal hemiretina)

• Optic tract (still RGC axons)

• Lateral geniculate nucleus (optic radiations)

• Striate cortex (LGN projects to here)

Retinal Ganglion Cells (RGCs)

Transmit action potentials from retina to brain.

• Parasol (M-projecting) → 10% of retina, large dendrites, large axons, magnocellular level (1&2) of LGN

• Midget (P-projecting) → 80% of retina, small axons, level parvocellular level (3-6) of LGN

Receptive fields

Coarse coding of visual space

• parasol cells in periphery, less overlap = low visual acuity

• midget cells in centre, closely packed = high visual acuity

• Multiple cone signals converge into one RGC →

Centre-surround opponency

Found in midget RGCs → stimulus in centre of RF can excite or inhibit

• On-centre

• Off centre

• Colour opponent → m-cones opposed by L cones, s cones opposed by l and m cones

On-centre

Stimulus in the periphery inhibits the cell

Off-centre

A stimulus in the periphery excites the cell

Mach bands

Uniform grey bands appear to have a gradient from left (brighter) to right (dimmer)

• Uniform stimulation in RF = minimal response

• More stimulation in centre = cell is excited

• More stimulation in surround = cell is inhibited

RGC images

Emphasise boundaries between objects

• creates outlines

• respond to areas of changing stimulation instead of uniform

• tracing areas of VF that change stimulation

LGN response latency

Parvocellular: Tonic response

• Stimulation continues in order to get a good look → cell still responding to stimulus after intial showing

Magnocellular: Phasic response

• Large receptors → quick AP, detecting change → lose interest once things stay stable

LGN Physiology

Magnocellular (1-2) → Large RF, fast latency, phasic (responsive only to change), L+M+, L-M- (non-opponent, greyscale)

Parvocellular (3-6) → small RF, slow latency, tonic (detail-oriented, even when stimuli is stable), L+M-, L-M+, S+(L&M)- (opponent RGC, centre surround)

Parallel processing

Different populations of neurons perform different functions. Complex problems broken up into relatively simpler ones (colour, motion, form)

Visual cortex electrophysiology

Cat thing where theres the crackling and the line with the x’s

Simple cells

LGN layer 4C → midget cells combine to a larger simple cell (forms a line)

Cytochrome oxidase blobs

Staining sample of PVC (V1) with cytochrome oxidase = blobs

• Neurons in the blobs are more active (dark areas) than interblobs (lighter between dark areas)

• dark areas more metabolically active