Neural binding
Neural binding is the neuroscientific aspect of what is commonly known as the binding problem: the interdisciplinary difficulty of creating a comprehensive and verifiable model for the unity of consciousness. "Binding" refers to the integration of highly diverse neural information in the forming of one's cohesive experience. The neural binding hypothesis states that neural signals are paired through synchronized oscillations of neuronal activity that combine and recombine to allow for a wide variety of responses to context-dependent stimuli. These dynamic neural networks are thought to account for the flexibility and nuanced response of the brain to various situations.[1] The coupling of these networks is transient, on the order of milliseconds, and allows for rapid activity.[2]
A viable mechanism for this phenomenon must address (1) the difficulties of reconciling the global nature of the participating (exogenous) signals and their relevant (endogenous) associations, (2) the interface between lower perceptual processes and higher cognitive processes, (3) the identification of signals (sometimes referred to as “tagging”) as they are processed and routed throughout the brain, and (4) the emergence of a unity of consciousness.
Proposed adaptive functions of neural binding have included the avoidance of hallucinatory phenomena generated by endogenous patterns alone as well as the avoidance of behavior driven by involuntary action alone.[3]
There are several difficulties that must be addressed in this model. First, it must provide a mechanism for the integration of signals across different brain regions (both cortical and subcortical). It must also be able to explain the simultaneous processing of unrelated signals that are held separate from one another and integrated signals that must be viewed as a whole.[1]
Test cases[edit]
Sensory[edit]
Much of the experimental evidence for neural binding has traditionally revolved around sensory awareness. Sensory awareness is accomplished by integrating things together by cognitively perceiving them and then segmenting them so that, in total, there is an image created. Since there can be an infinite number of possibilities in the perception of an object, this has been a unique area of study. The way the brain then collectively pieces certain things together via networking is important not only in the global way of perceiving but also in segmentation. Much of sensory awareness has to do with the taking of a single piece of an object's makeup and then binding its total characteristics so that the brain perceives the object in its final form. Much of the research for the understanding of segmentation and how the brain perceives an object has been done by studying cats. A major finding of this research has to do with the understanding of gamma waves oscillating at 40 Hz. The information was extracted from a study using the cat visual cortex. It was shown that the cortical neurons responded differently to spatially different objects. These firings of neurons ranged from 40–60 Hz in measure and when observed showed that they fired synchronously when observing different parts of the object. Such coherent responses point to the fact that the brain is doing a kind of coding where it is piecing certain neurons together in the works of making the form of an object. Since the brain is putting these segmented pieces together unsupervised, a significant consonance is found with many philosophers (like Sigmund Freud) who theorize an underlying subconscious that helps to form every aspect of our conscious thought processes.[28]
In order for these multiple firings from multiple areas to be combined into a specific extrinsic event, there must be help from the dorsal thalamus. It is not proven whether the dorsal thalamus is the primary organizer, but it does fit the specific profile for collecting neuronal activity and rapidly coordinating between what is happening in the brain and outside of it. The space in and around the dorsal thalamus, the thalomocortical area, is able to generate fast voltage-dependent membrane potential oscillations which allow it to react quickly to received messages. The types of channels that cover this area are presumed to be GABAergic. Since sensory awareness needs to be quick, the threshold for sodium and potassium in this area is quite low.[28]
Cognitive[edit]
Francis Crick and Christof Koch proposed that specific neural activity can stimulate short-term memory, forming a continuous and dynamic consciousness.
Cognitive binding is associated with the different states of human consciousness. Two of the most studied states of consciousness are the wakefulness and REM sleep. There have been multiple studies showing, electrophysiologically, that these two states are quite similar in nature. This has led some neural binding theorists to study the modes of cognitive awareness in each state. Certain observations have even led these scientists to hypothesize that since there is little cognition going on during REM sleep, the increased thalamocortical responses show the action of processing in the waking preconscious.[29]
The thalamus and cortex are important anatomical features in cognitive and sensory awareness. The understanding of how these neurons fire and relate to one other in each of these states (REM and Waking) is paramount to understanding awareness and its relation to neural binding.
In the waking state, neuronal activity in animals is subject to changes based on the current environment. Changes in environment act as a form of stress on the brain so that when sensory neurons are then fired synchronously, they acclimate to the new state. This new state can then be moved to the hippocampus where it can be stored for later use. In the words of James Newman and Anthony A. Grace in their article, "Binding Across Time" this idea is put forth: "The hippocampus is the primary recipient of inferotemporal outputs and is known to be the substrate for the consolidation of working memories to long term, episodic memories."[30]
The logging of "episodes" is then used for "streaming", which can mediate by the selective gating of certain information reentering sensory awareness. Streaming and building of episodic memories would not be possible if neural binding did not unconsciously connect the two synchronous oscillations.
The pairing of these oscillations can then help input the correct sensory material. If these paired oscillations are not new, then cognitively these firings will be easily understood. If there are new firings, the brain will have to acclimate to the new understanding.[30]
In REM sleep, the only extreme difference from the waking state is that the brain does not have the actual waking amount of sensory firings, so cognitively, there is not as much awareness here, although the activity of the "brain’s eye" is still quite significant and very similar to the waking state. Studies have shown that during sleep there are still 40 Hz Oscillation firings. These firings are due to the perceived stimuli happening in dreams. "[30]
Learning/memory[edit]
Opitz argues that the binding of different brain areas is mediated by the hippocampus. Relational bindings, or relationships between separate objects, concepts, and memories, are very flexible because the targets can be combined in so many different ways to deal with the present situation, and the hippocampus ensures that these parts are arranged into a coherent whole.[31] Particularly, the hippocampus has been implicated in binding that is involved with episodic memory, working memory, and language acquisition. According to Opitz, this is viable because the hippocampus meets all criteria to be suitable for the regulation relational binding and, based on its patterns of activity, it is likely that it is involved.