MEA Meeting 2010, day two

I'm in Germany this week, in a sleepy little southern town called Reutlingen, attending the MEA2010 conference. I'm here to present our most recent work, talk to other MEA nerds and berate Multi Channel Systems employees until they implement basic spike sorting in MC_Rack. MEA stands for multielectrode array; the focus of the conference is multi-unit electrophysiology, mostly neural networks cultured on titanium arrays in glass dishes, but the whole spectrum of techniques and analysis methods is represented: yesterday we saw analysis of data from an electrode net the size of a small hand that goes right on the top of the brain, more on that in a bit.

German is frustratingly close to my native Swedish and yet impossible to understand. Almost. Spielplatz must mean playground, speil sounding like a variant of spel (game in Swedish) and platz like plats (ground in this context, though it really means location). Spielplatz = gameground. In Swedish we say lekplats, lek meaning play. It doesn't help, I'm still a tourist, and the heat is just as bad here as in England. People at the conference are mostly from Europe and East Asia, many languages spoken in the hallways and a lot of poor English (Globish, according to yesterday's Start The Week). There's maybe 200 people here in the big hall now, though it's still pretty early; leave it to the Germans to serve food and wine until 23:00 and then start talks at 8:30 the next morning. It's ok, my hotel is ten minutes away.

The opening lecture yesterday was by Pascal Fries and entitled 'Unravelling the brain-wide web of attention'. It was very good. Fries looks like he could be 25 years old but is a Prof., a P.I. and an M.D. He was showing us evidence that objects are held in visual attention by selective synchronization of distributed but functionally relevant brain regions in the gamma (feed forward, Granger = 0.02) and beta (feed back) bands (and a mystery band at 30 Hz). The data was recorded using a large net of a few hundred 1 mm diameter electrodes placed at 2 mm distance across almost the entire right hemisphere of two monkeys trained to respond to some visual stimuli and ignore others. The stimuli was known to hit specific regions in visual cortex, and activity in these region was subsequently compared with activity across the brain, using the net. The degree of synchrony in the gamma band predicted fast reaction time, so I'm wondering if synchrony in the molluskan buccal ganglia predict feeding rate. In question time I asked him what his thinking was on the mechanism by which the network associated with one stimulus becomes able to entrain others. The answer: given sufficient dopamine and noradrenaline tone, the network will establish coherence by activating interneurons in target regions. Signal-to-noise.

I actually asked him two questions, and having finished answering the first question he appeared ready to move to another questioner before catching himself and exclaiming "Oh, right, the second stimulus.. eh I mean second question". It's ok man, no need to speak humaneese, we're all neuroscientists here. Anyway, I should pay attention to the talks. Great wireless at a conference is both a blessing and a curse. Mostly blessing though.

Notes on electrical rhythms in the brain

I'm reading George Buszáki's Rhythms of the Brain at the moment. Here are some notes on what I've read so far. Please correct me if I've got something wrong.

Electrical fluctuations in the cortex are organized into rhytmic oscillations at different spatial and temporal scales. The resting cortex is characterized by oscillations primarily in the alpha band (8-12 Hz, the brain's 'default network'). The active (i.e. behaving, percieving) cortex is characterized by oscillations primarily in the gamma band (25-100 Hz). Buszáki and others argue that cortical neurons that synchronize their membrane oscillations in the gamma band 'bind' their respective functions (e.g. visual feature detection) together into cognitive processes (e.g. object perception). Such formations of neurons are called neuronal groups or assemblies. Particularly striking are neuronal groups in the gamma range emerging in the prefrontal cortex for the duration of time that human patients are asked to hold items in working memory.

(alpha)

(gamma)

Cortical neurons sponaneously synchronize their membrane oscillations in the gamma range and form transient neuronal groups even in the absence of stimuli. This is, at least in part, due to the time constants of GABA curents, synaptic delays and synaptic potentiation. Buszáki writes:

"If neurons are already engaged in internal synchronization, the external stimulus will compete with the central oscillator, and the coutcome depends on the relative timing and strenght of the external input and the propensity of the internal oscillator. The stimulus may be ignored, or it may enhance or quench the internal oscillation." p.255

In other words, the effect of a stimulus on cortical activity depends strongly on the prior state of the brain. This explains the significant variability in brain activity (e.g. on EEG/MEG/fMRI) seen within and between subjects in response to invariant stimuli. Buszáki laments the fact that this variability is usually averaged out and treated as 'noise'. Björn Brembs often makes a similar argument.

Stimuli interact with ongoing cortical activity in various ways. Whereas a weak stimulus may reset the phase of ongoing oscillations, a strong or salient stimulus may completely change the type and distribution of oscillations in the cortex. Several studies have found that strong ongoing oscillations in the gamma, theta or alpha ranges prior to stimulus presentation promote efficient memory encoding. A stimulus that resets a strong rhythm presumably has a larger impact on brain activity than one that resets a weak rhythm. The presence or absence of strong rhythms in the brain is directly related to states of attention and catecholamine concentrations.