NEUR2201 - Summary Notes (Module 2)

Neuroscience Fundamentals (University of New South Wales)

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NEUR2201 - Summary Notes (Module 2 - Cortical Maps and Organisation)

• REAL-TIME NEURAL MEASUREMENTS

Differentiate between direct and indirect measures of neuronal activity.

Explain what a real-time neural measurement is, and provide three examples including the temporal and spatial resolution of each of the measurements.

Real-Time Neural Measurement: measures neural activity at a rate that keeps up with the speed of the activity being monitored

Describe experimental circumstances where each would be an appropriate and informative recording technique.

• Microelectrode Recording: direct, one neuron, one action potential/less, invasive

  • Determine neural coding strategy

  • Determine if binding uses gamma waves

  • EEG Recording: direct, groups of neurons, one action potential/less, non-invasive (awake human subject)

    • Diagnose conditions - epilepsy, sleep disorders and brain tumours

    • Locating a sensory brain region

TRANSDUCTION AND CODING

Transduction.

Transduction: stimulus-alerting events wherein a physical stimulus is converted into an action potential, which is transmitted along axons towards the CNS for integration; part of sensory processing

1. Stimulus

2. Change in ionic permeability of receptor cell/afferent nerve ending

3. Change in membrane potential – receptor potential

4. Generation of action potentials in afferent nerve terminal

5. Propagation of action potentials to CNS

Describe the two broad classes of receptors that couple the environment to neural activity.

Briefly explain how photon capture causes a change in membrane potential in the photoreceptor. Photoreceptors: receptor cells that enable transduction; contain opsin and retinal

• In vision, retinal captures a photon and dissociates, activating opsin (GPCR)

  • Activated opsin interacts with phosphodiesterase → reduction in concentration of cGMP

Coding. Explain how we can design an experiment to study neural coding.

Neural Code: uncovering the meaning behind the activity of a given neuron → insight into cognition

• Time based codes contain MORE information than counting based codes

  • How to design an experiment that studies neural coding:

    • Add sensory input, record animal’s perception/behaviour, record neuronal activity

    • Measure three languages using real-time measurement

  • Can record at different levels of nervous system

1. Peripheral afferent neuron synapses in brain stem

2. Brain stem neuron synapses in thalamus

3. Thalamic neuron synapses in sensory cortex

Recoding & emergent properties. Explain how a neuron at a higher level in the nervous system can encode a stimulus property not represented in any single neuron at a lower level.

Orientation-Selectivity (in cortical neurons): emerges from their convergent inputs

• Retinal ganglion cells - respond to spots/rings of light (have concentric visual fields) – NO response that relates to the orientation of a bar of light

  • ON centre cell: excitatory center, inhibitory surround

  • OFF centre cell: inhibitory center, excitatory surround

  • Cortical neurons - sensitive to bar orientation – this is an emergent property of these cortical cells which is NOT present in their input neurons

MODULAR PROCESSING IN VISION

Explain how a brain area is defined and what hierarchical organisation means.

Brain has a modular organisation – composed of distinct sections defined by function/histology.

• Module: brain area in the hierarchy; can be organised into:

  • Hierarchical – each module performs DIFFERENT task but works on TOTAL scope of job

  • Parallel – each module performs SIMILAR tasks working on LIMITED scope of total job

  • Neural Cytoarchitecture: shape of neurons; identifies distinct cortical areas

    • Neuron density, soma size, spine count, spine density, dendritic tree size

  • Connection Patterns: determines hierarchies and regions

    • Sensory inputs arrive at V1 in layer 4 (granular layer)

      • Feedforward projections terminate in layer 4

      • Feedback projections originate from layer 5 and 6

    • Hierarchies - suggest increasingly specialised roles

    • Regions - identified functionally by reversals in topographic map

    • Topography: each area on map should only be represented once – the repeat of a mapped feature signals a new cortical region

Explain the difference in the information that flows on the M & P pathways from retina to cortex. Retinal Ganglion Cell: neuron located near inner surface (ganglion cell layer) of retina; receive visual information from photoreceptors via two intermediate neuron types: bipolar cells and amacrine cells

• Receptive fields of retinal ganglion neurons have two concentric regions – a center and an antagonistic surround

  • More photoreceptors that connect to a ganglion cell → larger receptive field

Lateral Geniculate Nucleus (LGN): visual nucleus of thalamus; relay station between retina and visual cortex; multilayered structure (6 layers in humans)

• 4 layers receiving input from P pathway; 2 layers receiving input from M pathway

  • Contralateral eye inputs to layers 1, 4 and 6

  • Ipsilateral eye inputs to layers 2, 3 and 5

  • Thalamic neurons reflect the properties of their input from retinal ganglion cells (M - large receptive fields, P - small receptive fields)

Visual deficits arising from LGN lesions:

Provide an example of an area in the dorsal and another in the ventral stream, and the information type each one processes.

Parallel pathways for different info arriving at V1: V1 separates info coming from LGN about different dimensions → dispatches it to different extrastriate regions each specialised for a particular kind of analysis (features, colour, movement, depth, texture)

Beyond V1:

• NOTE: increasing receptive field sizes beyond V1 reduce acuity but allow generalisability

  • IT (large receptive field) → V4 → V1

BUILDING CORTICAL MAPS

Overlaid maps. Describe how receptive field, eye dominance, orientation and colour of a visual stimulus are coded in an orderly manner across the cortex.

Overlaid Maps: make modules that encode all parameters, spread evenly across cortex

• EYE DOMINANCE – encoded by ocular dominance bands in V1

  • Ocular dominance bands: stripes of neurons across surface of V1 that respond preferentially to input from one eye or the other; span multiple cortical layers

• ORIENTATION – orientation preference is encoded by cortical columns in V1

  • V1 must represent lots of properties including line orientation

  • Needs to represent each orientation at each retinal location

  • Ordered distribution of orientations across topographical map – these are swirled together with colour and eye dominance patterns so all properties are represented within every few square mm of cortex

• COLOUR – encoded by blobs

  • Blobs: sections of the visual cortex where groups of neurons that are sensitive to colour assemble in cylindrical shapes

  • Colour signals to V1 terminate in blobs

  • Inter-blob zones: where neurons are not sensitive to colour (receive M pathway inputs)

Building Maps. Describe the role of activity on shaping cortical maps. Ocular Dominance Maps: develop in first few weeks after birth

• Need visual experience to form maps → drives formation of bands

• Activity dependent plasticity – neurons are dependent on trophic (growth) support from their inputs and stabilise their inputs

• Neighbouring neurons compete for input synapses → dynamic, ongoing process – disruptions to subset of inputs lead to cortical dominance by remaining subsets

Combining information. Describe the convergence of information from different senses in the superior colliculus and explain binding.

Superior Colliculus: a multisensory midbrain structure that integrates visual, auditory, and somatosensory spatial information

• Sensory maps of surrounding space are superimposed in SC

• Involved in eye movements and orienting responses

• Responsible for blindsight through its input to extrastriate cortex

• Blindsight is thought to occur when information can still reach cortex, but the V1/LGN loop is broken. The patient can't "see" but is able to do certain visual tasks.

Binding Problem: concerns how items that are encoded by distinct brain circuits can be combined for perception/decision/action; combining different sensory representations of one object and segregating activity of multiple distinct objects

• Activity is NOT processed in parallel

• Cannot be solved by spatial location

• We represent distinct objects/thoughts by having the involved neurons fire action potentials out- of-phase with different neurons representing another object at ~40 Hz

• Registering the maps across different sensory systems is critical to binding outside world into coherent percepts ← achieved by overlaying maps and timing neural activity

PRACTICAL / TUTORIAL

Describe, using examples from the practical class, the localization of sensory experience to two different levels of the nervous system.

Localisation of Sensory Experience: sensory systems use receptors to transduce an environmental signal → signal processed by layers of neurons at successive levels of nervous system

Afferent Pathway for tactile information

• Tactile afferents travel in the ipsilateral dorsal column

  • Decussate (form X shape) at medulla and form medial lemniscus

  • From here, they synapse in ventro-basal thalamus

  • These thalamic neurons project to somatosensory cortex

Visual Pathway determines effect of a lesion on visual field

• Axons originating from each eye separate before reaching thalamus → right visual field is sent to left thalamus, and vice versa

  • Inputs from each eye remain separate until reaching cortex

• Example 1: colour adaption after-effect

  • Adaption occurs at level of retina, thalamus and cortex before binocular convergence

  • Adaption in peripheral field is weak - no cones in comparison to center field

• Example 2: binocular rivalry

  • Reflects intra-cortical battles for dominance between two stimuli

  • Perception alternates between different images presented to each eye

  • Not an after-effect but an unstable percept from a stable stimulus

  • Occurs at a higher level than level of retina and thalamus – occurs in cortex after convergence

Describe how electroencephalograms (EEG) are recorded and what information they can provide. EEG (electroencephalography) recordings:

• Cerebral cortex contains large numbers of neurons

• Activity of these neurons is synchronised in regular firing rhythms (brain waves) to an extent

• Electrodes placed on scalp can pick up variations in electrical potential that derive from this underlying cortical activity

• EEG signals are affected by state of arousal of cerebral cortex and show characteristic changes in different changes of sleep

• Used in diagnosis of epilepsies and of brain death

• Records many channels of activity from multiple recording electrodes placed in an array over the head

Explain what is occurring in the brain in the case of a phantom limb.

Phantom Limb: a vivid perception that a limb that has been removed or amputated is still present in the body and performing its normal functions

• Amputees usually experience sensations including pain in the phantom limb

• Caused by reorganisation of wiring of sensorimotor cortex (processes sensory inputs and executes movements)

• Mismatch between movement and perception of that movement

• Thalamus: relay motor and sensory signals to the cerebral cortex

• Brain stem: acts as a relay center connecting the cerebrum and cerebellum to the spinal cord

Describe targeted muscle and sensory reinnervation.

Targeted muscle and sensory reinnervation (TMSR): rerouting motor and sensory nerves from residual limb towards intact muscles and skin regions

• Skin near or over the targeted muscle is denervated, then reinnervated with afferent fibers of the remaining hand nerves

• When this piece of skin is touched, it provides the amputee with a sense of the missing arm or hand being touche