C is for …Connectivity

connectome

C is for Connectivity or “No neuron is an island”.

Level of analysis is an important problem in cognitive science. We are studying phenomena that can be, at the same time, relevant to genetics, molecular chemistry, behavioural happenings and cultural movements that effect not just one but millions of people. Drawing parallels between these levels is something central to cognitive neuroscience, understanding the brain/behaviour interaction in the most complete way.

Part of understanding the relationships is knowing that the brain is a dynamic system. The activation of an individual neuron effects the surrounding cells  which go onto effect  further  surrounding cells and so on.

At the most basic level, the communication between two cells is achieved by a wave of electrochemical charge called a spike train, travelling down one neuron, releasing neurotransmitters at the terminal buttons which acts as messengers, activating action potential (change in electrical properties) in the receiving neurons. The process, called neuronal firing then repeats.

The adage ‘cells that fire together wire together’ is attributed to Carla Shatz and describes the basic tenets of Hebbian theory. This theory emphasises the causal and intrinsic relationship between two cells. That a muscle-like physical growth occurs when one cell repeatedly activates another. The bond between the two cells grows meaning that it takes less excitatory activity for cell B to be activated in the future.

Relationships build in the neural circuitry. This strengthens specialised subsystems and results in an increased likelihood for cells to activate each other. There are millions of individual neurons in the brain, wired together in an incredibly complex circuitry. For us to fully understand a region of a brain, a subsystem, or even the job of a particular neuron – it is necessary to understand its relationship to the surrounding cells and other parts of the brain.

A good example to draw upon here is the recently Nobel prize winning discovery of place cells. In the 70s Nadel and O’Keefe discovered that certain physical places in the environment triggered the activation of individual neurons in the hippocampus. This is a fairly remarkable discovery and has given way to a multitude of theories that use the isolated relationship between a neuron and a place to explain cognitive maps, long term memory storage and categorization, episodic encoding of information.

But these theories are just interpretations of possible relationships that this cell has to other parts of the brain and many have been criticised with being overly simplistic. More recently researchers have turned to computer modelling techniques that take into account the functions and properties of individual cells, and the neural networks that these are part of. One such theory takes into account the properties of so called pyramidal cells in the CA3 region of the hippocampus. These cells seem to build very strong networks and have connections to many spatial cells. It has been theorised that they are involved in something called continuous attractor networks which use hairpin activation to inform head direction, orientation and movement in familiar environments.

The hippocampal and spatial memory system can also be approached from a higher level of analysis. Elanor Maguire and many other researchers argue that as we currently lack a complete working mechanistic account of the hippocampus and so we should explore the behavioural and neuropsychological evidence that we have.

They believe that the hippocampus works by constructing coherent scenes for which our memories can be based, ‘synthesizing representations of the world beyond our immediate sensorium’. This ties a number of systems together, episodic and spatial memories as well as spatial navigation and orientation, goal planning and movement.

At a higher level still, global connectivity can be seen in the form of neuronal oscillation, more commonly called brain waves. Without going into too much detail, oscillations are the collective electro-chemical rhythm of mass neuronal activity and, are a good way to appreciate the interconnected brain.  In terms of the hippocampus, the hippocampal, theta rhythm is a distinctive oscillation that is thought to originate in the brainstem and medial temporal septum and connect to the hippocampus. The rhythm gives an abstracted representation of the overall activity between a number of interconnected brain areas its occurrence has been linked to motor activity and there are a number of theories explaining its purpose.

These levels of analysis exist partly due to our methods of enquiry. Even at our current technological level we only have access to a limited number of methods and technologies. Each of which are  limited in spatial and temporal scope. It is for us to fill in the gaps and draw the parallels in attempts to fully understand neural phenomenon.

The take home message for connectivity is that the brain is immensely complex. Even understanding complete neural architecture as we are starting to, with initiatives such as the connectome project, it is still very difficult, to appreciate the brain as a whole dynamic system.

With connectivity, each level of analysis should be taken into account. How one neuron effects its neighbours and the links one area of the brain has to its surrounding parts. We have been able to map out and attribute jobs and functions to many parts of the brain through neuro-imaging, rare cases of specific brain injuries, animal-lesion studies, etc but understanding the mechanistic interactions of these modalities and truly understanding a role of a brain region as part of the larger system – how its role is expressed as part of behaviour – is much more difficult.

 

Image at top of the page taken from http://jamesthornton.com/static/images/connectome.jpg depicting visualisation of the connectome project.

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