<p>The different types of connections can be histologically identified and separated on the basis of their connectivity in the cortex.</p>
<p>Recurrent synapses in the cortex can be defined as <a href="https://core.ac.uk/download/pdf/82111916.pdf" rel="nofollow noreferrer">(Douglas &amp; Martin, 1995)</a>:</p>
<blockquote>
<p>[...] connections between layer 2 and 3 pyramidal cells, in which a target neuron projects
back to its source neuron in a tight positive feedback loop</p>
</blockquote>
<p>Feedforward and feedback neurons can be histologically defined as <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3219532/" rel="nofollow noreferrer">(Berezovskii <em>et al</em>., 2011)</a>:</p>
<blockquote>
<p>In general, feedforward (FF) projections originate in the superficial layers of the cortex and terminate in layer 4, while feedback (FB) connections originate in the superficial and deep layers, and their axon terminals tend to avoid layer 4.</p>
</blockquote>
<p>The cortical layers and their functions are schematically depicted in Fig. 1.</p>
<p><a href="https://i.sstatic.net/MKdRo.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/MKdRo.png" alt="cortex" /></a><br />
<sup>Fig. 1. Schematic of the cortex showing recurrent connections (rc) in layer 2/3 and feedforward and feedback connections that avoid layer 4. source: <a href="https://core.ac.uk/download/pdf/156785162.pdf" rel="nofollow noreferrer">Rolls &amp; Mills (2017)</a></sup></p>
<p><sub><strong>References</strong><br />
<strong>-</strong> <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3219532/" rel="nofollow noreferrer">Berezovskii <em>et al</em>., <em>J Comp Neurol</em> (2011); <strong>519</strong>(18): 3672–83</a><br />
<strong>-</strong> <a href="https://core.ac.uk/download/pdf/156785162.pdf" rel="nofollow noreferrer">Rolls &amp; Mills, <em>Neurobiol Learn Mem</em> (2017); <strong>145</strong>: 205–21</a></sub></p>

CC BY-SA 4.0
2021-03-02T08:49:02.820Z
2021-03-02T13:07:07.880Z
2021-03-02T13:07:07.880Z
AliceD
<p>In the paper proposing recurrent convolutional neural networks (RCNN), &quot;<a href="https://openaccess.thecvf.com/content_cvpr_2015/papers/Liang_Recurrent_Convolutional_Neural_2015_CVPR_paper.pdf" rel="nofollow noreferrer">Recurrent Convolutional Neural Network for Object Recognition</a>&quot;, it is stated that &quot;recurrent synapses typically outnumber feed-forward and top-down (or feedback) synapses&quot;.</p>
<p>Up to this point, my understanding was that top-down processing and recurrent connections refer to the same thing... What is the difference between the two?</p>

<neuroscience><terminology><vision><neurophysiology>
What is the difference between recurrent and feedback synapses?
David Cian
# What is the difference between recurrent and feedback synapses?

(Asked by: username_0 on 2021-03-02)

In the paper proposing recurrent convolutional neural networks (RCNN), "[Recurrent Convolutional Neural Network for Object Recognition](https://openaccess.thecvf.com/content_cvpr_2015/papers/Liang_Recurrent_Convolutional_Neural_2015_CVPR_paper.pdf)", it is stated that "recurrent synapses typically outnumber feed-forward and top-down (or feedback) synapses".

Up to this point, my understanding was that top-down processing and recurrent connections refer to the same thing... What is the difference between the two?

**Tags:** neuroscience, terminology, vision, neurophysiology


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## 1 answers

### Answer by username_1 on 2021-03-02. Score: 2 | Accepted answer

The different types of connections can be histologically identified and separated on the basis of their connectivity in the cortex.

Recurrent synapses in the cortex can be defined as [(Douglas & Martin, 1995)](https://core.ac.uk/download/pdf/82111916.pdf):

> [...] connections between layer 2 and 3 pyramidal cells, in which a target neuron projects
> back to its source neuron in a tight positive feedback loop

Feedforward and feedback neurons can be histologically defined as [(Berezovskii *et al*., 2011)](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3219532/):

> In general, feedforward (FF) projections originate in the superficial layers of the cortex and terminate in layer 4, while feedback (FB) connections originate in the superficial and deep layers, and their axon terminals tend to avoid layer 4.

The cortical layers and their functions are schematically depicted in Fig. 1.

[![cortex](https://i.sstatic.net/MKdRo.png)](https://i.sstatic.net/MKdRo.png)  
Fig. 1. Schematic of the cortex showing recurrent connections (rc) in layer 2/3 and feedforward and feedback connections that avoid layer 4. source: [Rolls & Mills (2017)](https://core.ac.uk/download/pdf/156785162.pdf)

**References**  
**-** [Berezovskii *et al*., *J Comp Neurol* (2011); **519**(18): 3672–83](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3219532/)  
**-** [Rolls & Mills, *Neurobiol Learn Mem* (2017); **145**: 205–21](https://core.ac.uk/download/pdf/156785162.pdf)


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