* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Gamma Band Oscillation
Activity-dependent plasticity wikipedia , lookup
Biochemistry of Alzheimer's disease wikipedia , lookup
Brain Rules wikipedia , lookup
Neuroeconomics wikipedia , lookup
Synaptic gating wikipedia , lookup
Premovement neuronal activity wikipedia , lookup
Signal transduction wikipedia , lookup
Neuroplasticity wikipedia , lookup
Cognitive neuroscience of music wikipedia , lookup
Single-unit recording wikipedia , lookup
Psychophysics wikipedia , lookup
Aging brain wikipedia , lookup
Embodied cognitive science wikipedia , lookup
Neuroanatomy wikipedia , lookup
Development of the nervous system wikipedia , lookup
Nervous system network models wikipedia , lookup
Spike-and-wave wikipedia , lookup
Holonomic brain theory wikipedia , lookup
C1 and P1 (neuroscience) wikipedia , lookup
Neuropsychopharmacology wikipedia , lookup
Stimulus (physiology) wikipedia , lookup
Optogenetics wikipedia , lookup
Neural coding wikipedia , lookup
Neuroesthetics wikipedia , lookup
Channelrhodopsin wikipedia , lookup
Neural correlates of consciousness wikipedia , lookup
Clinical neurochemistry wikipedia , lookup
Efficient coding hypothesis wikipedia , lookup
Neural oscillation wikipedia , lookup
Inferior temporal gyrus wikipedia , lookup
Feature detection (nervous system) wikipedia , lookup
Time perception wikipedia , lookup
Metastability in the brain wikipedia , lookup
Gamma Band Oscillation The Binding Problem & Binding by Synchronization 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) 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 Where do we find Gamma Oscillations? Gamma frequency oscillations are present during waking, slowwave 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) 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. 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. 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 aren’t enough neurons for there to be dedicated high-order structures for each concept or object. “Combinatorial Explosion” 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) 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. 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. 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. 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) 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) 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. 1. 2. 3. 4. 5. 6. 7. 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 higherlevel 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? Criticism of the Temporal Binding Hypothesis 8. 9. 10. 11. 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. 1. 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? 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.