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
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)
3 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
4 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)
5 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.
6 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.
7 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
8 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)
9 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.
10 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.
11 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.
12 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)
13 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)
14 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
15 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
16 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.
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