# Setting neuron-neuron connections from a connectivity matrix of synapses

**URL:** <https://brian.discourse.group/t/setting-neuron-neuron-connections-from-a-connectivity-matrix-of-synapses/1180>\
**Category:** Support\
**Tags:** synapses\
**Created:** [28 March 2024 00:53 UTC](https://brian.discourse.group/t/setting-neuron-neuron-connections-from-a-connectivity-matrix-of-synapses/1180 "2024-03-28T00:53:47Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![itq](https://avatars.discourse-cdn.com/v4/letter/i/bc8723/32.png) [@itq](https://brian.discourse.group/u/itq)\
**Post date:** [28 March 2024 00:53 UTC](https://brian.discourse.group/t/setting-neuron-neuron-connections-from-a-connectivity-matrix-of-synapses/1180/1 "2024-03-28T00:53:47Z")

</div>

# Description of problem

I’ve got a connectivity matrix, where the entries describes the number of synapses connecting one neuron to another. So, given the following 3 x 3 matrix,

```python
 [
    [0, 5, 0],
    [0, 0, -2],
    [0, 0, 0],
]

```

we say that the 1st neuron connects to the second via 5 excitatory synapse connections. We then say that the 2nd neuron connects to the third via 2 inhibitory synapse connections. I think that my code accomplishes this (or is close to it, based on [Github issue - Setting Neuron Weights](https://github.com/brian-team/brian2/issues/1494)), but when I try to set the actual weights, I run into an error: about `DimensionMismatchError(error_message, dim1, dim2)` ?

I’ve looked around and stumbled on the [DimensionMismatchErrorClass](https://brian2.readthedocs.io/en/2.5.4/reference/brian2.units.fundamentalunits.DimensionMismatchError.html) page, but it’s not clear to me why the issue only pops up when I try to set the weights, not before. In my minimal code below, I specifically call out the line where the issue occurs.

# Minimal code to reproduce problem

```python
import numpy as np
from brian2 import Synapses, NeuronGroup, ms, mV, Hz

params = {
    'v_0' : -52 * mV, # resting potential
    'v_rst' : -52 * mV, # reset potential after spike
    'v_th' : -45 * mV, # threshold for spiking
    't_mbr' : 20 * ms, # membrane time scale (capacitance * resistance = .002 * uF * 10. * Mohm)
    't_rfc' : 2.2 * ms,
    'tau' : 5 * ms, # time constant
}

if __name__ == ' __main__':
    connectivity_matrix = np.asarray([
        [0, 5, 0],
        [0, 0, -2],
        [0, 0, 0],
    ])
    eqs = '''
    dv/dt = (v_0 - v + g) / t_mbr : volt (unless refractory)
    dg/dt = -g / tau : volt (unless refractory) 
    rfc : second
    '''

    neu = NeuronGroup( # create neurons
        N=len(connectivity_matrix),
        model=eqs,
        method='linear',
        threshold='v > v_th',
        reset='v = v_rst; w = 0; g = 0 * mV',
        refractory='rfc',
        name='default_neurons',
        namespace=params
    )
    neu.v =-52 * mV
    neu.g = 0
    neu.rfc = 2.2 * ms

    sources, targets = connectivity_matrix.nonzero()
    syn = Synapses(neu, neu, 'w : volt', on_pre='g += w', delay=1.8 * ms, name='default_synapses')
    syn.connect(i=sources, j=targets)

    
    syn.w[:] = connectivity_matrix[(sources, targets)] # <- the offending line
    print(syn)

```

# What you have aready tried

Adding a debugger to the line and stepping through. Having said that, I don’t really know what I’m looking for.

# Full traceback of error (if relevant)

```auto
  File "X/streamlined_model.py", line 38, in <module>
    syn.w[:] = connectivity_matrix[(sources, targets)]
  File "/opt/homebrew/anaconda3/envs/test/lib/python3.10/site-packages/brian2/core/variables.py", line 995, in __setitem__
    self.set_item(item, value, level=1)
  File "/opt/homebrew/anaconda3/envs/test/lib/python3.10/site-packages/brian2/core/variables.py", line 982, in set_item
    self.set_with_index_array(slice(None), value, check_units=check_units)
  File "/opt/homebrew/anaconda3/envs/test/lib/python3.10/site-packages/brian2/core/base.py", line 335, in device_override_decorated_function
    return func(*args, **kwds)
  File "/opt/homebrew/anaconda3/envs/test/lib/python3.10/site-packages/brian2/core/variables.py", line 1301, in set_with_index_array
    fail_for_dimension_mismatch(
  File "/opt/homebrew/anaconda3/envs/test/lib/python3.10/site-packages/brian2/units/fundamentalunits.py", line 260, in fail_for_dimension_mismatch
    raise DimensionMismatchError(error_message, dim1, dim2)
brian2.units.fundamentalunits.DimensionMismatchError: Incorrect unit for setting variable w (units are V and 1).

```

---

<div class="post-metadata">

**Author:** ![mstimberg](https://yyz2.discourse-cdn.com/free1/user_avatar/brian.discourse.group/mstimberg/32/11_2.png) [@mstimberg](https://brian.discourse.group/u/mstimberg)\
**Post date:** [28 March 2024 10:08 UTC](https://brian.discourse.group/t/setting-neuron-neuron-connections-from-a-connectivity-matrix-of-synapses/1180/2 "2024-03-28T10:08:39Z")

</div>

Hi @itq . In your model, the synaptic weight `w` has units of volts, but your connectivity matrix contains dimensionless values. When you set

```python
syn.w[:] = connectivity_matrix[(sources, targets)]

```

Brian therefore complains. You need to decide what the strength of each individual synapse should be, e.g. if it is one mV, then you could write:

```python
syn.w[:] = connectivity_matrix[(sources, targets)] * mV

```

A more general solution would be something like:

```python
weight_per_synapse = 0.5*mV
syn.w[:] = connectivity_matrix[(sources, targets)] * weight_per_synapse

```

Hope that helps!