Multisensory neuronal convergence

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Multisensory neuronal convergence
Neuroinformatics seminar presentation
Claire BRICHE
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Introduction

• Environmental events are converted by the nervous
  system to perceptions, memories and/or behaviors.
• Environmental events can be detected by two or
  more sensory systems (vision, hearing)
• Multisensory stimulation
• Neuronal convergence
• Study of the multisensory on the neuronal level.

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Plan

• Definition of neuronal convergence
• Definition of multisensory convergence
• Description of the superior colliculus
• Characteristics of the multisensory neuronal
  convergence
• Facilitation
• Interaction
• Inhibition
• Receptive field of multisensory neurons
• Time influence

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What does neuronal convergence mean?
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What does multisensory convergence mean? (1)

• Stimuli from differents modalities (vision,
  hearing)
• Convergence of the information coming from the
  different sensory systems onto individual neurons
• Response of the neuron (integration of the
  information given by the two modalities)

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What does multisensory convergence mean? (2)

• Different type of multisensory neurons
• Bimodal neurons neurons activated by 2
  modalities
• Trimodal neurons neurons activated by 3
  modalities.

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The superior colliculus

• Predominant model of multisensory convergence in
  mammals.
• Includes many multimodal neurons and few unimodal
  neurons.
• Important role in mammals orientation
• Reticular system (awakening), posterior and
  lateral thalamus contain multimodal neurons too.

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Characteristics of multisensory
convergence facilitation (1)
Enhancement of the multimodal neurons response
with more than one modality
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Characteristics of multisensory convergence
facilitation (2)

• One modality few spikes
• Two modalities enhancement of the response
  (Multimodalidity Convergence)
• Excitatory excitatory form of multisensory
  convergence.

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Characteristics of multisensory convergence
facilitation (3)

• Different mechanisms can explain that phenomenon
• Facilitation with spatial summation

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Characteristics of multisensory convergence
facilitation (4)

• Facilitation with temporal summation

Frequency of the action potentials.
Sum of the epsp (excitatory post-synaptic
potential) coming from neuron 1 and neuron 2 when
they arrive quasi simultaneously.
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Characteristics of multisensory convergence
facilitation (5)

• Pre-synaptic facilitation

• IPSP (Inhibitory post synaptic potential) for
  neuron 1
• Small hyperpolarization of the membrane
• Gain of energy for the action potentials
• Enhancement of the number of neurotransmitter in
  the synapse.

Gain of energy
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Characteristics of multisensory convergence
interaction (1)
Somatosensoriel stimulus (SS)
Auditive stimulus (A)
A SS

• The multimodal neurons response is superior
  when two modalities are stimulated.
• But the real sum of the two modalities response
  is inferior to the sum of the response of one
  modality the response of the other modality.

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Characteristics of multisensory convergence
interaction (2)

• Some explanation
• Occlusion

One of the two modalities is enough to generate
an action potential The addition of the other
modality will not change the response a lot
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Characteristics of multisensory convergence
interaction (3)

• Recurrent device

• Spatial summation of the EPSP and IPSP
  (Inhibitory post synaptic potential)
• IPSP attenuates EPSP effects.

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Characteristics of multisensory convergence
inhibition (1)

• With two modalities, the multisensory neurons
  response is attenuated compared with the response
  of one modality
• Excitatory inhibitory multisensory convergence

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Characteristics of multisensory convergence
inhibition (2)

• Explanations
• Direct synaptic inhibition

• One of the modalities activates the Neuron 2
• Sum of the IPSP and EPSP

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Characteristics of multisensory convergence
inhibition (3)

• Presynaptic inhibition

• Small depolarization of the membrane of the
  axone of the neuron 1
• Loss of energy for the action potential
• Decrease of the number of neurotransmitter
  liberated in the synapse.

Loss of energy
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Real multisensory convergence
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Comparison between unimodal and bimodal
convergence.
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Receptive fields of multisensory neurons (1)
Receptive field sum of the sensory receptors
of the neuron which can modify its activity.
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Receptive fields of multisensory neurons (2)
Position of the stimulus very important in the
receptive field of each modality.
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Time influence
After more than 200ms after the first stimulus, a
second stimulus coming from another modality is
taken into account (large time window)
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Conclusion

• Multisensory convergence on the neuronal level is
  not totally understood
• Some other unknown mechanisms
• How can the neuron differentiate information
  coming from one modality from information coming
  from two different modalities?