Stephen Grossberg

Just went to a talk by Stephen Grossberg from Boston University. It always sucks when you realize that all your ideas have already been though of by someone else... But basically his talk was about laminar theories of cortical computation. He didn't dive too much into the details of the model, but he showed its predictions and tons of evidence that supported many of the model's predictions. It was also very clear how much of the stuff in this blog fits in with his model.

So just as an overview, basically the model is that Layer 2/3 is acting like a pattern-completion circuit. This circuit gets input from layer 4 and feedsback to layer 6. Layer 4 and 6 are together acting like a pattern-seperation/competition/decision circuit. So this idea of two types of microcircuits in cortex is more prevelant.

The general theory is based on what he calls "ART": Adaptive Resonance Theory. And he talked a lot about the balance of excitation and inhibition. Lateral competition. Gain control and shunting inhibition. I'm not sure how deeply biophysical his model gets, or how fully-functional he's made it.

He went into a big discussion about all the different visual areas and how they are cooperatively computing. Each area is essentially doing the same thing on an abstract level, but computing with different inputs and feeding-back in different ways to influence other cortical areas. There are some structures that are not cortical that are playing specific roles such as maintaining object and spatial representations while the eyes move.

He went into a discussion about how top-down control of cortex is done through the apical dendrites. He was mainly talking about attention. It was as if attentional processes feedback to cortex through Layer1, and activate the apical tufts of pyramidal cells. This can turn up the gain of a particular area. He mentioned the point that the top-down processes shouldn't be enough alone to cause activation - as then you'd be hallucinating. He made a lot of mentions about illusory contours. The illusory contour activations are due to lateral connections coming in from "both-sides" that lead to activation.

All this made me think of the bursting of pyramidal cells. Top-down influences on the apical tuft will lead to calcium spikes etc, but if the pyramdial cell isn't also receiving bottom-up/lateral activations, then it won't fire. This prevents top-down attentional processes from causing firing and thus causing hallucinations. However, if there is both types of inputs, then the pyramdial cell will be bursting, which turns up the contrast of the representation. Bursting is also known to be important for some types of learning rules (it is easier to get LTP if the cells are bursting), and Grossberg discussed how learning during top-down modulation is important for his model.

So, yeah, definitely need to read his papers...