3 ms·

Each cortical area has six layers, but most behaviors require interactions between many cortical areas, so "input" passes through many more than six layers befo

Each cortical area has six layers, but most behaviors require interactions between many cortical areas, so "input" passes through many more than six layers before it produces an output. Felleman and Van Essen is a classic paper on the organization of the visual system. Figure 2 (p. 4) might give you good sense for how much of the brain it occupies and Figure 4 (p. 30) is the well-known "wiring diagram. In the 30 years since that paper was written, we've found a few more boxes and a lot more wires! We've also come to appreciate that there are lots of recurrent loops. V1 -> V2 is one of the biggest connections in the brain, V2 -> V1 is a near runner up. https://cogsci.ucsd.edu/~sereno/201/readings/04.03-MacaqueAreas.pdf https://cogsci.ucsd.edu/~sereno/201/readings/04.03-MacaqueAr...

verdverm 5y ago
Indeed, there are many "horizontal" and recurrent connections and simply thinking of it as a 6 layer feed forward network is a gross oversimplification. It's more like a complex network of complex networks... of the spiking variety
- mattkrause 5y ago
  Yup, and that’s just the “classical” synaptic transmission. Mixed in with that, there is also slower signaling via neuromodulators (dopamine, norepinephrine etc), neuroendocrine system, and God only knows whatever the astrocytes are doing. Every neuron has its own internal dynamics too, over scales ranging from milliseconds (channel inactivation) to hours or days (receptor internalization). There’s even the possibility of “ephaptic coupling”, wherein the electric fields produced by some neurons affect the activity of others, without making any sort of direct contact. We’ve collected some of the stronger data in favor of that possibility and yet I remain firmly in denial because it would make the brain so absurdly complicated.
  - verdverm 5y ago
    Did you see the FPGA which had an "ineffectual" circuit created while using Evolution Algo to optimize design for calculating integrals? Turns out it wasn't ineffectual but had EM effects between circuits that the calculation required! I would not be surprised if the brain did similar things, especially given that light and magnets are causing effects in various studies.
    - mattkrause 5y ago
      Yes! It's wild! That's sorta what might be happening in the brain. I do brain stimulation experiments where we generate electric fields in the brain by passing currents across the scalp: 0.5 V/m or so (which you get by putting 1-2 mA across the scalp) is enough to do some really wild things to neurons, even in a big primate head. This is cool, and potentially useful for various kinds of rehab/neuroprosthetics/etc. The crazier thing is that the neurons themselves generate much stronger electric fields themselves (~10x stronger, in some parts of the brain). Whether those fields are actively used or merely "tolerated" is hard to say but....woah.

verdverm 5y ago

Indeed, there are many "horizontal" and recurrent connections and simply thinking of it as a 6 layer feed forward network is a gross oversimplification. It's more like a complex network of complex networks... of the spiking variety

mattkrause 5y ago
Yup, and that’s just the “classical” synaptic transmission. Mixed in with that, there is also slower signaling via neuromodulators (dopamine, norepinephrine etc), neuroendocrine system, and God only knows whatever the astrocytes are doing. Every neuron has its own internal dynamics too, over scales ranging from milliseconds (channel inactivation) to hours or days (receptor internalization). There’s even the possibility of “ephaptic coupling”, wherein the electric fields produced by some neurons affect the activity of others, without making any sort of direct contact. We’ve collected some of the stronger data in favor of that possibility and yet I remain firmly in denial because it would make the brain so absurdly complicated.
- verdverm 5y ago
  Did you see the FPGA which had an "ineffectual" circuit created while using Evolution Algo to optimize design for calculating integrals? Turns out it wasn't ineffectual but had EM effects between circuits that the calculation required! I would not be surprised if the brain did similar things, especially given that light and magnets are causing effects in various studies.
  - mattkrause 5y ago
    Yes! It's wild! That's sorta what might be happening in the brain. I do brain stimulation experiments where we generate electric fields in the brain by passing currents across the scalp: 0.5 V/m or so (which you get by putting 1-2 mA across the scalp) is enough to do some really wild things to neurons, even in a big primate head. This is cool, and potentially useful for various kinds of rehab/neuroprosthetics/etc. The crazier thing is that the neurons themselves generate much stronger electric fields themselves (~10x stronger, in some parts of the brain). Whether those fields are actively used or merely "tolerated" is hard to say but....woah.