Grid Cell Inspired Scalar Encoder

[Casey]

@Gary_Gaulin

The “Heterogeneity in hippocampal place coding” paper mentions that before exposure to a novel environment cells that go on to form place fields are more prone to “burst” in response to current injection. Is this the same as (or evidence of) HTM minicolumn cell “bursting”?

I can only reply based on layer 5.

I’ve never seen bursting used in journal articles to mean minicolumn bursting. Usually, a cell requires stronger somatic injection to burst or somatic injection alongside distal apical injection. Since a temporal memory minicolumn bursts when none of its cells receive sufficient predictive input (on the distal basal dendrite), a minicolumn bursts when its cells are less stimulated, which is the opposite of the extra stimulation needed for rapid spiking bursting.
It probably depends on methods like anesthetic a lot, but some studies found L5 cells that burst repetitively with weaker somatic current injection but with stronger injection, they burst once and then switch to regular spiking. That’s a potential way to argue that bursting cells are receiving less input, not more input, but it seems like a stretch to connect that to minicolumn bursting.

Another thing to consider is whether or not hippocampus has minicolumns. If it doesn’t, that doesn’t mean it doesn’t function the same way, with a predicted cell inhibited some other cells, but it would be another thing to find evidence for.

One interesting thing is that bursting might be involved in synaptic plasticity, consistent with bursty cells learning to respond to the new locations.

I can link some articles about bursting in L5 if you want. Bursting might work similarly in all pyramidal cells, including in hipocampus, although I don’t know that for a fact.

@Gary_Gaulin

Before exposure to a novel environment, cells that go on to form place fields are more prone to burst in response to current injection and during exploration the firing thresholds of active cells are significantly lower than those of silent cells [63]. Perhaps the strongest argument against the simple summation hypothesis is that injection of uniform positive current can instantly convert silent cells into active place cells that are indistinguishable from typical place cells in their subthreshold membrane potential profiles [64] (Figure 3).

Maybe those cells just have lower thresholds in general, whether because some cells in CA1 have lower thresholds than others or because some cell classes have lower thresholds than others. Do they specify the pyramidal cell class or layer? Indistinguishable might just mean below the statistical significance cutoff. I worry that which cells become place fields isn’t arbitrary (like would be required for selecting cells for memory formation or place field formation), because they say the cells that go on to form place fields were more likely to burst beforehand. Depending on how long beforehand, it might be unlikely that those cells were already chosen to form place fields and more likely that they’re just a different group with a different role or just are more responsive because of random variations in neuron thresholds or burstiness.

One way around that is, maybe those bursty cells are the ones which haven’t been assigned a place field yet, so they are more responsive for a functional reason. (Not that some cells being more responsive than others isn’t useful. A distribution of SDR sparsities might allow better flexibility and initially learning things more coarsely with the less selective cells and over time learning things more detailed with the more selective cells.)