Gamma Band Oscillation

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Gamma Band Oscillation

• The Binding Problem Binding by Synchronization

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Outline

• What are Gamma Oscillations
• Where are they found
• What is the Binding Problem
• Hierarchal Model
• The Temporal Binding Hypothesis and Binding by
  Synchrony
• Evidence and the role of Gamma Oscillations
• Criticisms or the Temporal Binding Hypothesis
• Future Research and Directions
• Sources
• Rhythms of the Brain György Buzsáki
• Synchrony Unbound A Critical Evaluation of the
  Temporal Binding Hypothesis Shadlen M.
  Movshon J.A.
• Neural synchrony in cortical networks history
  concept and current status - Uhlhass et al.
  (2009)

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Gamma Oscillations

• The fastest frequency band of neural oscillations
  
• 20 100 Hz (typically 40-60 Hz)
• ..Oscillations are not independent events that
  impose timing on neuronal spiking but rather are
  a reflection of self-organized interactions of
  those same neurons that detect transfer and
  store information. (Buzsáki p 259)
• Being fast and having a small amplitude Gamma
  band oscillations were hard to detect in early
  cell recording

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Where do we find Gamma Oscillations

• Gamma frequency oscillations are present during
  waking slow-wave and REM sleep.
• They are intrinsic to the neo-cortex and heavily
  rely on GABA-a receptors as it mediates the time
  constant of the decay of IPSPs which varies from
  10 25 ms (40 100 Hz)
• Most characteristic field pattern of the waking
  activated neocortex.. (Buzsáki p.259)

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The Binding Problem

• The problem can be found in both neuroscience and
  philosophy however they are unique in both
  cases.
• In neuroscience the question is how
  higher-order neural structures are able to
  segregate and integrate the proper inputs both
  from sensory organs and internal computations
• In areas such as V1 this is partly accounted for
  by the discovery of cortical columns consisting
  of simple complex and hypercomplex cells which
  are attuned to certain stimuli.
• However there remains the question of how we are
  able to perceive as unified objects stimuli in a
  robust manner regardless of out point of view
  size and lighting conditions. (Buzsáki p 232)
• Ex. Our ability to reliably recognize all dogs.

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The Binding Problem

• Our brains are able to take different features
  such as colour texture distance spatial
  position and smell which are processed in
  separate parts of the cortex by different sets of
  neurons and are bound into a complex
  representation in a matter of 200 milliseconds
  (Buzsáki p260)
• This type of mental reconstruction has been
  largely documented by Gestaltian psychologists
  showing that human regularly and systematically
  impose top-down rules on visual stimuli.

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Hierarchical Model

• An early solution to the Binding problem a
  feed-forward design where lower level information
  is projected to higher level neural structures
  and based on which structures are activated .a
  Gnostic cardinal cell activates. (also called a
  grandmother cell)
• This cell represents the object or concept which
  fed the sensory input if it is active then the
  mental representation for a particular stimulus
  is activated if it is not activated then there
  is no internal representation.
• There has to be a Gnostic cell for every object
  concept or referent in the environment.
• This is problematic because the model focuses on
  the role of excitatory cells while eliminating
  the need for inhibitory interneurons.
• Also there just arent enough neurons for there
  to be dedicated high-order structures for each
  concept or object. Combinatorial Explosion

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Hierarchical Model

• Because the number of neurons needed grows
  exponentially with the number of unique objects
  represented by their numerous features the
  brain so the story goes quickly runs out of
  neurons. (Buzsáki p. 236) Combinatorial
  Explosion
• The Hierarchical Model also does not specify the
  location or spatial relationship of the Gnostic
  units.
• If they are clustered in particular areas then
  they would be easily susceptible to damage in
  case of injury or brain damage. However this does
  not appear to be the case in patients with
  cortical column damage.
• If they are widely distributed then there
  remains the problem of how they communicate and
  what kind of specialized connectivity they would
  require.
• Finally this model lacks a temporal scale and
  it would therefore still need to be explained how
  it could effectively be used in real-world time
  and environments. (Buzsáki p 237)

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The Temporal Binding Hypothesis

• A solution to the Binding problem requires an
  explanation of how the various inputs and neural
  computations are differentiated so that the right
  bits of information can be compared and
  integrated.
• For this it is necessary to tag each visual
  neuron to signify the object to which its
  activity relates (Shadlen Movshon)
• In the Temporal Binding Hypothesis this tag is
  indicated by synchronous neural spiking.
• This offers an endless capacity for coding
  combinations.
• Synchronous spiking also allows for cross modal
  and long range communication.

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The Temporal Binding Hypothesis (examples)

• In Figure A. focusing on the blue lines in
  order for us to accurately perceive that there
  are two separate lines and the proper
  configuration we need to be able to label point
  y and point z in the retina and subsequent
  retinotopic visual areas so that higher-order
  areas properly segregate the stimulus and
  therefore allowing us to properly perceive the
  image.
• In Figure B. however the same two points in the
  field of vision need to be tagged as bound
  component in the visual scene in order for
  accurate perception.
• It is believed that the manner in which this
  labelling occurs is through neuronal
  oscillations.

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Evidence From Gray and Singer (1989) as shown
in Buzsáki (2006)

• Singer and Gray recorded both multi-unit activity
  and local field potentials (LFP) from single
  electrodes placed in the Primary visual cortex of
  anesthetised and paralyzed cats.
• Using a correlational analysis of cell activity
  and Fourier analysis they noticed that a
  significant proportion of the recordings showed
  Gamma frequency oscillations. (30-60 Hz)
• This oscillatory response was induced by visual
  stimulus consisting of moving bars.
• The cell activity was phase-locked to the trough
  of the field oscillations.
• These findings provided conclusive evidence that
  the oscillatory ensemble events emerged locally
  and were not directly related to the stimulus
  but were added on by the brain. (Buzsáki p.
  240)
• Synchrony between various locations occurred only
  when neurons at those locations responded to
  related visual features of the object.
  Furthermore the determining factor of the
  vigour of synchrony was the response features
  of the neurons.
• During stimulus-induced transient oscillations
  neurons several millimetres apart and even
  contralateral to each other synchronized.

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Evidence and the Role of Gamma Oscillations

• In experiments recording multiple neurons in two
  separate recording sites in the
  motion-sensitive MT area of waking monkeys
  experimenters stimulated both sites
  simultaneously using 2 bars moving in the
  preferred directions of the neurons.
• In these trials there was rarely oscillatory
  coupling!
• Experimenters then substituted the stimuli with a
  single bar which activated both neuronal sites.
• In these trials there was robust synchrony!
• Therefore the oscillatory synchrony was produced
  not by the simultaneous excitations of both
  recording sites but induced by the coherence of
  the stimulus.
• Gamma-frequency power has been shown in motor
  areas during and more typically prior to
  voluntary movement.
• Gamma oscillations are commonly induced between
  150-300 ms after stimulus onset approximately
  at the time when stimulus acquire meaning
  (Buzsáki p.244)

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Binding by Synchrony -Gamma Oscillations

• If Gamma oscillations are to tag certain signals
  then it should follow that they are only found in
  selective brain areas and are not entirely
  identical in fact Intracranial and Subdural
  recordings in human corroborate this prediction.
• Recording sites as close as 3-4 millimetres from
  each other in the visual cortex yielded quite
  different amplitudes of gamma oscillations.
  (Buzsáki p.245)
• Experiments in patients with many subdural
  electrodes showed that gamma power increased
  linearly with memory load (when memorizing
  strings of syllables) especially above the
  prefrontal cortex with power levels remaining
  high until retention was lost and working memory
  was relaxed.
• There is increasing evidence such as this
  suggesting that gamma oscillations are used in
  the brain for temporally segmenting
  representations of different items. (Buzsáki)

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Criticism of the Temporal Binding Hypothesis

• Michael N. Shadlen and J. Anthony Movshon in
  their 1999 Review entitled Synchrony Unbound A
  Critical Evaluation of the Temporal Binding
  Hypothesis brought up roughly a dozen critical
  concerns about the reality of Oscillatory
  Binding.
• The hypothesis is not a theory about how binding
  is computed it is a theory only of how binding
  is signalled.
• How does the visual system decide which elements
  are part of single objects and which belong to
  different objects
• Full image segmentation (and recognition)
  probably requires even higher-level analyses
  including the explicit inclusion of information
  from memory about the nature and structure of
  previously viewed objects and scenes.
• If binding is not computed in the primary visual
  cortex as this level of computation focuses on
  extremely particular features (i.e. edge/contrast
  detection and orientation) at a micro level why
  is synchrony to be expected there
• Proponents have stated that synchronized signals
  would be particularly effective in activating
  post-synaptic neurons that operate as coincidence
  detectors. But how would these coincidence
  detectors differ from Gnostic cells
• Oscillations are observed in the cortex which
  have nothing to do with perceptual binding as
  well there will always be asynchronous
  renegade spiking How do the postsynaptic
  neurons distinguish which is special synchrony
  that is suppose to convey additional information
• How is the brain supposed to distinguish the
  temporal modulation due to visual input from the
  temporal modulation produced intrinsically

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Criticism of the Temporal Binding Hypothesis

• Temporally precise visual activity is sufficient
  for binding but it is not necessary for binding
  and its disruption does not affect binding
  elicited by other cues. (In the segmentation of
  visual stimuli.)
• The prevalence of gamma oscillatory responses
  varies widely from laboratory to laboratory for
  unknown reason.
• While Singer Eckhorn and Livingstone find
  oscillatory responses in about half their
  recordings most others find their prevalence to
  be in about 2-5 of recording sites.
• Initial experiments from which the theory was
  developed were conducted on anesthetised animals.
  How much did this affect the results
• Since no perceptual judgements were made during
  the experiments evidence that the chosen
  stimulus configurations actually promoted
  perceptual binding was circumstantial. The
  experiments typically used stimuli that promoted
  binding-like effects in human observers but did
  not establish that experimental animals perceived
  the stimuli in the same way.
• So what does this mean for the Temporal Binding
  Hypothesis

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Future Research

• Research of the Temporal Binding Hypothesis has
  suggested that there may well be a solution to
  the Binding Problem.
• However experimental results are fairly
  heterogeneous and many researchers fail to
  observe the type of gamma band oscillations which
  are implicated in perceptual and conceptual
  binding as well as those which may be implicated
  in memory and consciousness.
• Shadlen and Movshon bring up many questions which
  still need to be address in order for Binding by
  Synchrony to become a complete theory.
• More experimentation needs to be done in order to
  answer some of these questions however finer
  grained analysis of local and long range
  oscillations are hard to record. Higher spatial
  and even temporal resolution is required in order
  to give researchers a better picture of the
  behaviour of gamma oscillations and its role in
  Mental Binding.
• Regardless of whether gamma band oscillations are
  relevant in perceptual binding the question
  still needs to be addressed Whether gamma
  oscillations play a role in our brain function
  or whether they are merely an epiphenomenon a
  by-product of the actual causal agents of our
  central nervous system.