<p>I've read evidence for single-neuron, two-neuron, and larger loops/cycles throughout the cortex, including intralaminar, interlaminar, and interareal neural loops. But it would take me far too long to back that statement up.</p>

<p>Instead, I offer a list of papers to get you started. I've read all these papers, and they all provide evidence for neural loops in the cortex, especially two-neuron loops. Within these papers, search for "loop", "reciprocal", and, if you're interested in single-neuron loops, "autapse".</p>

<h3>References</h3>

<blockquote>
  <ul>
  <li>Alonso, J.-M. (2002) Neural Connections and Receptive Field Properties in the Primary Visual Cortex. The Neuroscientist. 8(5):443-456</li>
  <li>Feldmeyer, D. (2012) Excitatory Neuronal Connectivity in the Barrel Cortex. Frontiers in Neuroanatomy. 6(24):1-22</li>
  <li>Rockland, K.S. and Knutson, T. (2000) Feedback Connections from Area MT of the Squirrel Monkey to Areas V1 and V2. The Journal of Comparative Neurology. 425:345-368</li>
  <li>Thomson, A.M., West, D.C., Wang, Y., and Bannnister, A.P. (2002) Synaptic Connections and Small Circuits Involving Excitatory and Inhibitory Neurons in Layers 2-5 of Adult Rat and Cat Neocortex: Triple Intracellular Recordings and Biocytin Labelling In Vitro. Cerebral Cortex. 12:936-953</li>
  </ul>
</blockquote>

CC BY-SA 3.0
2016-10-02T09:48:49.773Z
Would these loops classify as _dynamic circuit motifs_ (Womelsdorf et al., 2014)? https://www.ncbi.nlm.nih.gov/pubmed/25065440
noumenal
CC BY-SA 3.0
2016-10-06T03:48:28.953Z
Thank you John. I didnt have time to read all. But the first ones got me to read this one:

Reliable synaptic connections between pairs of excitatory layer 4 neurones within a single 'barrel' of developing rat somatosensory cortex.
Feldmeyer D1, Egger V, Lubke J, Sakmann B.
J Physiol. 1999 Nov 15;521 Pt 1:169-90.
that finds that 31% of connected spiny stellate cells in the L4 studied are "bi-directional"  Which are the 2-neurones loops i was talking about.
So it's full of loops in the L4 and it's very interesting.
ceillac
CC BY-SA 3.0
2016-09-30T05:07:23.197Z
2016-09-30T05:07:23.197Z
John Pick
<p>Yes, absolutely there are loops, and they can occur on a very small spatial scale.</p>

<p><strong>Do we have a proof of local loops in the cortex?</strong></p>

<p>Here are a few papers that describe experimental quantification of small networks (2+ neurons) with recurrent connections in various patterns:</p>

<blockquote>
  <ul>
  <li>Sporns O, Kötter R. 2004. Motifs in Brain Networks. PLoS Biology 2:e369</li>
  <li>Song S, Sjöström PJ, Reigl M, Nelson S, Chklovskii DB. 2005. Highly Nonrandom Features of Synaptic Local Connectivity in Local Cortical Circuits. Public Library of Science Biology 3:0507–0519</li>
  <li>Perin R, Berger TK, Markram H. 2011. A Synaptic Organizing Principle for Cortical Neuronal Groups. Proceedings of the National Academy of Sciences 108:5419–5424</li>
  </ul>
</blockquote>

<p><strong>How large a piece of cortex has to be to contain a loop?</strong></p>

<p>Judging from the experimental evidence I cited, loops of connections can occur on the scale of neighbouring neurons. That means, the smallest piece of cortex that contains a few neurons can contain a loop.</p>

<p><strong>What are these loops?</strong></p>

<p>I'm not sure what you're getting at here; if you are asking <em>why</em> there are loops, then we can only speculate. But even if connections were made randomly, loops would exist. A better question is <em>why are there more bidirectional / motif connections than we expect by chance?</em> </p>

CC BY-SA 3.0
2016-10-16T10:32:45.750Z
great! Actually if you look at my comment to John's answer I found a paper saying the same things. 
 What is interesting to me is how my question got so much negative feedback , got almost censured and was not understood. 
I will take a look at your papers as soon as i can. Thanks!
ceillac
CC BY-SA 3.0
2016-10-14T14:07:52.403Z
2016-10-14T14:07:52.403Z
Dylan Richard Muir
<p>What you're calling "loops" are usually referred to as <em>Recurrent Neural Networks</em> (as opposed to <em>Feedforward Neural Networks</em>).</p>

<p>Scholarpedia has an article on them, with a bibliography:</p>

<blockquote>
  <p>Typically... reviews consider RNNs that are artificial neural
  networks (aRNN) useful in technological applications. To complement
  these contributions, the present summary focuses on <strong>biological
  recurrent neural networks (bRNN) that are found in the brain</strong>. Since
  feedback is ubiquitous in the brain, this task, in full generality,
  could include most of the brain's dynamics. The current review divides
  bRNNS into those in which feedback signals occur in neurons within a
  single processing layer,  which occurs in networks for such diverse
  functional roles as storing spatial patterns in short-term memory,
  winner-take-all decision making, contrast enhancement and
  normalization, hill climbing, oscillations of multiple types
  (synchronous, traveling waves, chaotic), storing temporal sequences of
  events in working memory, and serial learning of lists; and those in
  which feedback signals occur between multiple processing layers, such
  as occurs when bottom-up adaptive filters activate learned recognition
  categories and top-down learned expectations focus attention on
  expected patterns of critical features and thereby modulate both types
  of learning.</p>
</blockquote>

<p>At least in this article (i just skimmed it) it looks like most of the presumed biological neural networks are inferred to exist from models, rather than having actually been found with a microscope.</p>

<p><a href="https://cogsci.stackexchange.com/questions/15932/are-there-neural-loops-within-a-column-or-an-area-of-the-cortex/15982#15982">John's</a> answer lists some papers which sounds like they probably provide more direct evidence.</p>

CC BY-SA 3.0
2016-09-30T08:04:48.537Z
2016-09-30T08:04:48.537Z
2017-04-13T12:56:16.167Z
David Bahry
<p><a href="https://i.sstatic.net/rXrMc.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/rXrMc.jpg" alt="examples"></a>Neuronal networks can make loops because a neuron has a direction (from dendrite to axon).</p>

<p>What's the smallest area in the cortex where we can find a loop and what are these loops?</p>

<p>I understand there are loops between different cortical areas through the white matter.</p>

<p>Experiments, like this <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3415278/" rel="nofollow noreferrer">one</a>, show there are interactions between cortical layers. But it doesn't necessary mean there are loops.</p>

<p>Other experiments, like this <a href="https://www.ncbi.nlm.nih.gov/pubmed/7973662" rel="nofollow noreferrer">one</a>,   looking at signals propagation in pieces of cortex with a  cut showed the propagation could  somehow circumvent the cut and keep propagating. Again it shows the existence of some local circuits but not necessarily loops.</p>

<p>Do we have a proof of local loops in the cortex? 
How large a piece of cortex has to be to contain a loop? 
What are these loops?</p>

CC BY-SA 3.0
2016-09-20T16:04:35.697Z
What do you mean with loop? Feedback loops? Sulci and gyri?
AliceD
CC BY-SA 3.0
2016-09-21T03:57:55.307Z
Any loop: you follow a neuron and after few connections it comes back to the same neuron. Excitatory or inhibitory.
ceillac
CC BY-SA 3.0
2016-09-21T04:47:05.557Z
I think you want to be looking at this. 'cycles' are the technical term you're looking for. https://www.researchgate.net/profile/Olaf_Sporns/publication/8391536_The_small_world_of_the_cerebral_cortex/links/0fcfd5112a65aa62c5000000.pdf
Russell Richie
CC BY-SA 3.0
2016-09-21T05:16:12.053Z
what's wrong with calling a loop a loop?
ceillac
CC BY-SA 3.0
2016-09-21T20:33:43.897Z
Nothing -- it's just that in graph theory, which has been applied to biological neural networks, they're called cycles.
Russell Richie
CC BY-SA 3.0
2016-09-22T12:40:52.797Z
Must say I find this question unclear. If you call a loop a loop, please mention a source or a framework. It is too general.
AliceD
CC BY-SA 3.0
2016-09-23T00:56:14.087Z
neuronal networks can make loops because a neuron has a direction (from dendrite to axon). My question is very clear: what's the smallest area in the cortex where we can find a loop? what are these loops?
ceillac
CC BY-SA 3.0
2016-09-23T20:04:56.573Z
For what it's worth, in the context of ANNs the term "recurrent" is usually used instead of cycles or loops.
SQLServerSteve
CC BY-SA 3.0
2016-09-23T20:59:25.847Z
I think "recurrent" is used for larger scales, like "reentrant". I believe there are loops in the cortical layers that maintain signal propagation locally
ceillac
CC BY-SA 3.0
2016-09-30T14:33:12.107Z
Your question is not that clear.
nachosan
CC BY-SA 3.0
2016-09-30T14:38:52.793Z
Firstly: Would you consider an autapse a loop? Would you consider reciprocal connection a 2 neuron loop? Would you consider a 25 neuron loop a loop? Secondly: Does it have to be to the specific single neurons or are you referring between neuron types? Thirdly: You say that you understand that there are loops between different cortical areas through white matter? Is this between specific neurons like your portray? Fourth: What type of evidence are you looking for?
nachosan
CC BY-SA 3.0
2016-09-30T19:16:14.243Z
Is your question specific to cortex or brain in general?
Justas
<neuroscience><theoretical-neuroscience><neural-network>
Are there neural loops within a column or an area of the cortex?
ceillac
# Are there neural loops within a column or an area of the cortex?

(Asked by: username_0 on 2016-09-20)

[![examples](https://i.sstatic.net/rXrMc.jpg)](https://i.sstatic.net/rXrMc.jpg)Neuronal networks can make loops because a neuron has a direction (from dendrite to axon).

What's the smallest area in the cortex where we can find a loop and what are these loops?

I understand there are loops between different cortical areas through the white matter.

Experiments, like this [one](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3415278/), show there are interactions between cortical layers. But it doesn't necessary mean there are loops.

Other experiments, like this [one](https://www.ncbi.nlm.nih.gov/pubmed/7973662), looking at signals propagation in pieces of cortex with a cut showed the propagation could somehow circumvent the cut and keep propagating. Again it shows the existence of some local circuits but not necessarily loops.

Do we have a proof of local loops in the cortex?
How large a piece of cortex has to be to contain a loop?
What are these loops?

**Tags:** neuroscience, theoretical-neuroscience, neural-network

> **Comments:**
> **username_1**: What do you mean with loop? Feedback loops? Sulci and gyri?  
> **username_0**: Any loop: you follow a neuron and after few connections it comes back to the same neuron. Excitatory or inhibitory.  
> **username_2**: I think you want to be looking at this. 'cycles' are the technical term you're looking for. https://www.researchgate.net/profile/Olaf_Sporns/publication/8391536_The_small_world_of_the_cerebral_cortex/links/0fcfd5112a65aa62c5000000.pdf  
> **username_0**: what's wrong with calling a loop a loop?  
> **username_2**: Nothing -- it's just that in graph theory, which has been applied to biological neural networks, they're called cycles.  
> **username_1**: Must say I find this question unclear. If you call a loop a loop, please mention a source or a framework. It is too general.  
> **username_0**: neuronal networks can make loops because a neuron has a direction (from dendrite to axon). My question is very clear: what's the smallest area in the cortex where we can find a loop? what are these loops?  
> **username_3**: For what it's worth, in the context of ANNs the term "recurrent" is usually used instead of cycles or loops.  
> **username_0**: I think "recurrent" is used for larger scales, like "reentrant". I believe there are loops in the cortical layers that maintain signal propagation locally  
> **username_4**: Your question is not that clear.  
> **username_4**: Firstly: Would you consider an autapse a loop? Would you consider reciprocal connection a 2 neuron loop? Would you consider a 25 neuron loop a loop? Secondly: Does it have to be to the specific single neurons or are you referring between neuron types? Thirdly: You say that you understand that there are loops between different cortical areas through white matter? Is this between specific neurons like your portray? Fourth: What type of evidence are you looking for?  
> **username_5**: Is your question specific to cortex or brain in general?  

---

## 3 answers

### Answer by username_6 on 2016-09-30. Score: 8

I've read evidence for single-neuron, two-neuron, and larger loops/cycles throughout the cortex, including intralaminar, interlaminar, and interareal neural loops. But it would take me far too long to back that statement up.

Instead, I offer a list of papers to get you started. I've read all these papers, and they all provide evidence for neural loops in the cortex, especially two-neuron loops. Within these papers, search for "loop", "reciprocal", and, if you're interested in single-neuron loops, "autapse".

### References

> * Alonso, J.-M. (2002) Neural Connections and Receptive Field Properties in the Primary Visual Cortex. The Neuroscientist. 8(5):443-456
> * Feldmeyer, D. (2012) Excitatory Neuronal Connectivity in the Barrel Cortex. Frontiers in Neuroanatomy. 6(24):1-22
> * Rockland, K.S. and Knutson, T. (2000) Feedback Connections from Area MT of the Squirrel Monkey to Areas V1 and V2. The Journal of Comparative Neurology. 425:345-368
> * Thomson, A.M., West, D.C., Wang, Y., and Bannnister, A.P. (2002) Synaptic Connections and Small Circuits Involving Excitatory and Inhibitory Neurons in Layers 2-5 of Adult Rat and Cat Neocortex: Triple Intracellular Recordings and Biocytin Labelling In Vitro. Cerebral Cortex. 12:936-953

> **Comments:**
> **username_7**: Would these loops classify as _dynamic circuit motifs_ (Womelsdorf et al., 2014)? https://www.ncbi.nlm.nih.gov/pubmed/25065440  
> **username_0**: Thank you John. I didnt have time to read all. But the first ones got me to read this one:  Reliable synaptic connections between pairs of excitatory layer 4 neurones within a single 'barrel' of developing rat somatosensory cortex. Feldmeyer D1, Egger V, Lubke J, Sakmann B. J Physiol. 1999 Nov 15;521 Pt 1:169-90. that finds that 31% of connected spiny stellate cells in the L4 studied are "bi-directional"  Which are the 2-neurones loops i was talking about. So it's full of loops in the L4 and it's very interesting.  

---

### Answer by username_8 on 2016-10-14. Score: 4

Yes, absolutely there are loops, and they can occur on a very small spatial scale.

**Do we have a proof of local loops in the cortex?**

Here are a few papers that describe experimental quantification of small networks (2+ neurons) with recurrent connections in various patterns:

> * Sporns O, Kötter R. 2004. Motifs in Brain Networks. PLoS Biology 2:e369
> * Song S, Sjöström PJ, Reigl M, Nelson S, Chklovskii DB. 2005. Highly Nonrandom Features of Synaptic Local Connectivity in Local Cortical Circuits. Public Library of Science Biology 3:0507–0519
> * Perin R, Berger TK, Markram H. 2011. A Synaptic Organizing Principle for Cortical Neuronal Groups. Proceedings of the National Academy of Sciences 108:5419–5424

**How large a piece of cortex has to be to contain a loop?**

Judging from the experimental evidence I cited, loops of connections can occur on the scale of neighbouring neurons. That means, the smallest piece of cortex that contains a few neurons can contain a loop.

**What are these loops?**

I'm not sure what you're getting at here; if you are asking *why* there are loops, then we can only speculate. But even if connections were made randomly, loops would exist. A better question is *why are there more bidirectional / motif connections than we expect by chance?*

> **Comments:**
> **username_0**: great! Actually if you look at my comment to John's answer I found a paper saying the same things.   What is interesting to me is how my question got so much negative feedback , got almost censured and was not understood.  I will take a look at your papers as soon as i can. Thanks!  

---

### Answer by username_9 on 2016-09-30. Score: 2

What you're calling "loops" are usually referred to as *Recurrent Neural Networks* (as opposed to *Feedforward Neural Networks*).

Scholarpedia has an article on them, with a bibliography:

> Typically... reviews consider RNNs that are artificial neural
> networks (aRNN) useful in technological applications. To complement
> these contributions, the present summary focuses on **biological
> recurrent neural networks (bRNN) that are found in the brain**. Since
> feedback is ubiquitous in the brain, this task, in full generality,
> could include most of the brain's dynamics. The current review divides
> bRNNS into those in which feedback signals occur in neurons within a
> single processing layer, which occurs in networks for such diverse
> functional roles as storing spatial patterns in short-term memory,
> winner-take-all decision making, contrast enhancement and
> normalization, hill climbing, oscillations of multiple types
> (synchronous, traveling waves, chaotic), storing temporal sequences of
> events in working memory, and serial learning of lists; and those in
> which feedback signals occur between multiple processing layers, such
> as occurs when bottom-up adaptive filters activate learned recognition
> categories and top-down learned expectations focus attention on
> expected patterns of critical features and thereby modulate both types
> of learning.

At least in this article (i just skimmed it) it looks like most of the presumed biological neural networks are inferred to exist from models, rather than having actually been found with a microscope.

[John's](https://cogsci.stackexchange.com/questions/15932/are-there-neural-loops-within-a-column-or-an-area-of-the-cortex/15982#15982) answer lists some papers which sounds like they probably provide more direct evidence.


---

