Archive for January, 2013

This is the second of my post on basic concepts in brain science.

Synchrony is to do with oscillations in the brain. I posted about what oscillations are here. To recap, a population of neurons that repeatedly, fire together (burst) and then go quiet and then fire together again, are said to oscillate. The speed of the oscillation is called the frequency and is measured in Hertz (Hz). When two distinct populations oscillate at different frequencies they are desynchronised, but when they both oscillate at the same frequency they are said to be synchronous.

Synchrony is a different concept to resonance. Resonance is where one thing is oscillating and a second thing is not, but then the second starts to oscillate at the same frequency as the first. Resonance is therefore when one thing oscillates in sympathy with another. The reason the second oscillates in sympathy is due to some connection between the two. For example, one object oscillating may be causally linked to another by the gas in our atmosphere and these vibrations my effect the second so it too starts oscillating.

Synchrony is also caused by a connection between two objects. Unlike resonance, where one object is originally oscillating and one is not, with synchrony both objects are originally oscillating. The key is that the frequencies at which each is originally oscillating are different. When they synchronise they may synchronise to a frequency that is different from either of the original frequencies. So for example, you may have two pendulums connected together by the beam they are both hung upon. One may be swinging at 20 Hz and the other at 40 Hz. The beam connecting the two creates a causal interaction. After a while and much interaction both may end up oscillating at 30 Hz. Both are synchronised to the same frequency, but at a different frequency than either was at originally. The reason they may have ended up at a different frequency is that the causal interaction is going both ways. The oscillation from one is effecting the oscillation of the other, and vice versa. With resonance the causal effect is one way, hence the second object oscillating in sympathy at the frequency of the first.

Now back to synchrony in the brain. In the brain you may have one population of neurons oscillating at one frequency and another population oscillating at another frequency. The neurons in one population are causally connected to the other by synapses, and vice versa. Over time the oscillation in each synchronises so that the bursts of firing in each population are at the same frequency. This can be likened two two people hitting a drum at the same tempo. Not only that but also they may have same phase. The phase refers to when the beats happen. Imagine two people drumming at the same tempo. Even though each is at the same tempo one person may hit the drum when the other person is quiet, and vice versa. If this is so they are said to be completely out of phase. If they hit the drum at the same time and therefore quiet moments also happen at the same time they are said to be in phase. Similarly if the two neural populations are synchronous and the burst of firing and moments of quiet occur at the same time then they are in phase.

Gamma-band oscillations (a population of neurons firing together at the rate of 30-80 Hz) can emerge in a population of excitatory and inhibitory neurons. The inhibition causes the moments of quiet in the oscillation. This provides windows for interaction at the moment inhibition wears off and there is a burst of firing. Excitatory signals from a different oscillating population can then take advantage of this because gamma band oscillations are sufficiently regular to allow prediction of the next burst. As long as the travelling time from the sending to the receiving group is also reliable, their communication windows for input and output are open at the same times (i.e. when the bursts occur). Packages of spiking signals from one population of neurons can therefore arrive at the other neuronal group in precise synchronization and enhance their impact. In short, synchronisation between two populations allows two populations to work together and provides the optimal conditions for transferring information. Pascal Fries discusses the mechanistic consequences of neuronal oscillations and calls this hypothesis ‘communication through coherence’. You can read a more technical report by him here.

I am going to write a few post on basic concepts in brain science. This first one is about oscillations.

A group of neurons that are close together is referred to as a population or cluster. A population will have a specific role, e.g. responding to a particular stimulus such as for example a cat.

When the neurons in a population fire at roughly the same time, then go quiet, and then fire again and repeat this process this is called an oscillation. The time when they fire is called a burst of firing. The number of bursts in a second is the frequency of the oscillation. A frequency of 1 Hertz or for short ‘Hz’ is 1 oscillation a second, which means that there will be one burst of firings and one period of silence. 10 Hz is 10 oscillations a second, 50 Hz is 50 oscillations a second etc.

Different names are given to different ranges of the frequency (Hz) of the oscillation (also called rhythms). The delta band rhythm ranges from 0.1−3.5 Hz. Theta rhythm ranges from 4−7.5 Hz. Alpha band is 8−13 Hz. Beta is 14−30 Hz, and gamma is 30-80 Hz.

The amplitude or power of the oscillation/rhythm is dictated by the number of neurons in a population that fire during a burst. If there is a population of 200 neurons and 10 fire in the burst that will have a lower power than if 150 neurons fire. 200 neurons firing during the burst in a population of 200 neurons will have the maximum possible amplitude/power.

The various rhythms have diverse associations. Thalamocortical networks display increased delta band power during deep sleep. Theta activity is increased during memory encoding and retrieval. Alpha band changes are associated with attentional demands. Beta oscillations have been related to the sensorimotor system. Of all the frequency bands the role of gamma is thought to be most extensive and is hypothesized to provide a mechanism that underlies many cognitive functions such as: attention, associative learning, working memory, the formation of episodic memory, visual perception, and sensory selection.

So for example, a population that responds to a cat with a gamma oscillation of very high power may indicate that you attending to a very strong visual perception of a cat.

Having studied western philosophy at Uni, I had a lot of respect for Buddhism when I read about it. The majority of what it states is very sound and in line with modern thinking. In fact Buddhism is in many ways 2500 years ahead of western thought which is quite incredible.

The basic underlying principle of the way Buddhism sees the world is about causality and impermanence. This can be interpreted in line with modern materialism and dynamical systems theory. Put simply we can view the world as a dynamical system with a set of variables, say for example atoms. Some of these come together to form a thing. However, the things that are formed are impermanent. For example all the atoms in a persons body change many times over their life time until eventually it completely disintergrates. This is an example of impermanence. What we believe is a thing is really the presence of form over time not stuff such as matter. The dynamical system also by definition exhibits causality.

Buddhism defines the self as having five aspects: matter, sensation, perception (i.e. pattern recognition/concepts), mind (i.e. thoughts), and consciousness. These aspects should not naively be viewed as components that plug in together but rather qualities on the self with which there may be overlap. It’s pretty hard to define such a thing as self and the Buddha made a pretty good job here I think. The self in Buddhism doesn’t exist but is illusionary due to the fact there are no real things just perceived form over time.

The self is dukkha which is often inappropriately translated as suffering but is better described as thirst, the continual striving and wanting to hold on to things. Dukkha therefore makes life unsatisfactory as we cant hold on to stuff because it is impermanent.

Here is where is starts to get sketchy. In order to get rid of the bad effects of dukkha one should let go of holding on to things, conceptualisation, and of the self. Now although there is no objective reason to say that a life through intellectual conceptualisation is more right or wrong than a life without, we may concede that following a Buddhist approach may lead to a more content life. Where it now turns into mumbo jumbo is that they say that the there is a causal effect of having a self focused life which continues after death. Although original Buddhism doesn’t say anything about reincarnation, this causal effect is not simply say a social effect that you have had on people continuing, but rather an actual physical causal effect and it is the negative effect of causing more dukkha.

The method of achieving Buddhist goals is through lifestyle and meditation. Through this one reaches a nirvanic state where conceptualisation and self do not appear. One might argue that the seeming attainment of nirvana through meditation may be illusionary and just a high state similar to taking psychotropic drugs. However, neurological studies using brain scans on people who practice meditation have shown amazing results, such as gamma power 30 times hight than normal, heighten activity in brain areas that have responses to things being meditated on etc. So it may be possible to control your brain to such a degree that you suppress lots of conceptual features.

Jill Bolte Taylor is a neuroanatomist who suffered from a stroke. Whilst this was happening she was aware of certain high level brain functions shutting down (such as language and internal monologue). She found herself in a nirvana kind of state. You can watch a TED video where she talks about her experience here.

The coolest toy I’ve seen lately. Webots is a robot simulator that runs using the ODE physics engine. There are lots of real robot models included such as Kheperas and even the iCub. You can also build your own robots to simulate. Robot controllers can be coded in lots of languages and you can even link directly to matlab. How cool is that!! Only costs £215/$342 for the normal version.

Lots of videos on the webots simulator site.

I want one 🙂

Hi All,

I havent posted on this blog for almost 2 years. I have been snowed under with the PhD and to be frank couldn’t think about anything else. I am going to try to start posting again. As a sign of good will I have set up a nice new look. Hope you like it 🙂