<p>As said in the comments, $U_4$ and $U_5$ are not used to compute the values of the output neurons. </p>

<p>However, in some cases one may allow the connections between neurons to change over time, in which case $U_4$ and $U_5$ may be later used to compute the values of the output neurons. Seanny123 gives biology as an example. For another example, you may want to look at the  literature on <a href="https://en.wikipedia.org/wiki/Neuroevolution" rel="nofollow">neuro-evolution</a>, i.e. optimization of neural architectures with evolutionary algorithms, which may allow changing the connections between neurons over generations (e.g. that's what I did for my master thesis:  <em>Franck Dernoncourt. "<a href="http://www.francky.me/doc/mrf2011-HEC-ISIR-ENS_en.pdf" rel="nofollow">The medial Reticular Formation: a neural substrate for action selection? An evaluation via evolutionary computation.</a>". Master's Thesis. École Normale
Supérieure Ulm. 2011.</em>, though I may have coded the mutation and crossover operators in such a way that dead-end neurons don't appear, I don't recall).</p>

CC BY-SA 3.0
2016-09-07T00:08:48.053Z
2016-09-07T00:13:53.027Z
2016-09-07T00:13:53.027Z
Franck Dernoncourt
<p>I was reading over some sources about ANNs, and <a href="https://www.doc.ic.ac.uk/~nd/surprise_96/journal/vol4/cs11/report.html" rel="nofollow noreferrer">here</a> I found this diagram of a Recurrent Neural Network (RNN, the source calls it a feedback neural network).
<a href="https://i.sstatic.net/c9yS7.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/c9yS7.jpg" alt="enter image description here"></a></p>

<p>I noticed that a few neurons in this diagram lead to other neurons that are dead ends (have no connections to other neurons). For example, neuron <em>U5</em> leads to no other neurons, and <em>U9</em> has a connection to <em>U4</em>, but <em>U4</em> doesn't have a connection to any other neurons.</p>

<p>Is there a purpose for <em>U5</em> and <em>U4</em> to even exist?</p>

<p>Or am I missing something bigger here?</p>

CC BY-SA 3.0
2016-04-02T00:28:50.663Z
I think it's just indicating that most often, biologically, input gets re-routed and re-used.
Seanny123
CC BY-SA 3.0
2016-09-06T21:12:01.327Z
To the best of my knowledge, dead end neurons do not appear in artificial neural networks that are actually used and they do not have a purpose since they can't contribute to the output of the network.
awakenting
CC BY-SA 3.0
2016-09-10T01:51:30.360Z
In this particular architecture they don't seem to contribute to the outputs, which carry the "b" label. It is possible though to have outputs at intermediate levels, in addition to the traditional final output layer. So some neural network designer might conceivably attach outputs to U5 and U4 that are presented to the end user before the B-layer outputs. Just food for thought & something to watch out for when interpreting similar diagrams, where the dead-end neurons might not be as dead as they appear at first glance.
SQLServerSteve
<neural-network>
What is the purpose of dead end neurons in this diagram?
tzengia
# What is the purpose of dead end neurons in this diagram?

(Asked by: username_0 on 2016-04-02)

I was reading over some sources about ANNs, and [here](https://www.doc.ic.ac.uk/~nd/surprise_96/journal/vol4/cs11/report.html) I found this diagram of a Recurrent Neural Network (RNN, the source calls it a feedback neural network).
[![enter image description here](https://i.sstatic.net/c9yS7.jpg)](https://i.sstatic.net/c9yS7.jpg)

I noticed that a few neurons in this diagram lead to other neurons that are dead ends (have no connections to other neurons). For example, neuron *U5* leads to no other neurons, and *U9* has a connection to *U4*, but *U4* doesn't have a connection to any other neurons.

Is there a purpose for *U5* and *U4* to even exist?

Or am I missing something bigger here?

**Tags:** neural-network

> **Comments:**
> **username_1**: I think it's just indicating that most often, biologically, input gets re-routed and re-used.  
> **username_2**: To the best of my knowledge, dead end neurons do not appear in artificial neural networks that are actually used and they do not have a purpose since they can't contribute to the output of the network.  
> **username_3**: In this particular architecture they don't seem to contribute to the outputs, which carry the "b" label. It is possible though to have outputs at intermediate levels, in addition to the traditional final output layer. So some neural network designer might conceivably attach outputs to U5 and U4 that are presented to the end user before the B-layer outputs. Just food for thought & something to watch out for when interpreting similar diagrams, where the dead-end neurons might not be as dead as they appear at first glance.  

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## 1 answers

### Answer by username_4 on 2016-09-07. Score: 3 | Accepted answer

As said in the comments, $U\_4$ and $U\_5$ are not used to compute the values of the output neurons.

However, in some cases one may allow the connections between neurons to change over time, in which case $U\_4$ and $U\_5$ may be later used to compute the values of the output neurons. username_1 gives biology as an example. For another example, you may want to look at the literature on [neuro-evolution](https://en.wikipedia.org/wiki/Neuroevolution), i.e. optimization of neural architectures with evolutionary algorithms, which may allow changing the connections between neurons over generations (e.g. that's what I did for my master thesis: *username_4. "[The medial Reticular Formation: a neural substrate for action selection? An evaluation via evolutionary computation.](http://www.francky.me/doc/mrf2011-HEC-ISIR-ENS_en.pdf)". Master's Thesis. École Normale
Supérieure Ulm. 2011.*, though I may have coded the mutation and crossover operators in such a way that dead-end neurons don't appear, I don't recall).


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