IEEE Circuits and Systems Magazine - Q4 2020 - 42

A closed-loop is established between the stimulator and the neural
recording circuits using the spike detection and sorting information.
Each spike-to-cluster association can be set to trigger a stimulation pattern
after a predetermined number of occurrences is found within a time window.
VIII. Experimental Validation In Vivo
The headstage was validated in vivo within experiments involving a freely moving mouse virally expressing ChR2-mCherry. It should be noted that the
ChR2-mCherry stimulation results in the activation of
the interneurons, hence an inhibitory effect is therefore expected. The recordings were carried out seven
months after the viral injection, so the animal was
able to fully recover from the surgical procedure and
to adopt some normal behaviors, like running, lifting
his head, lifting his body, etc. with the headstage in
place. The total weight on the head of the animal was
less than 5g (system, optrode and adapter). The experimental setup is depicted in Fig. 10. To reduce light
artifacts, we used blunt (opaque polyimide covered)
electrodes to prevent the direct light reaching the tip
of the electrode, and the electrode tips were placed in
a low illuminated region.
The CL algorithm was configured to trigger when
6 APs belonging to cluster 1 were found within a 150-ms
time window. This configuration was selected after manually looking at close successive APs belonging to the
same cluster. This simple approach was used to validate
the system in freely moving animals. However, further
analysis of the neural bursting activity may be required
to find the optimal configuration.
Fig. 11(a)(top) shows two reconstructed signals of
30 s after decompression, along with some generated
CL stimulation triggers (dark blue signal). Fig. 11(b)(bottom) shows two signal closeups in which the stimulation
pulses and the evoked silencing phases (light blue), due

Headstage
Animal Freely
Behaving During
the Experiment
Optrode
Mouse Expressing ChR2-mCherry (~25 g)
(a)

(b)

Figure 10. (a) The headstage is connected to the optrode
and installed on the head of the mouse. (b) The freely-behaving animal during a CL experiment.
42â	

to inhibitory neuron activation, are visible. The right
and left closeup shows two APs belonging to cluster 2
and 3 respectively within the 150-ms time window, thus
triggering no stimulation, while 6 APs belonging to cluster 1 within each time window triggers a stimulation
pulse and a silencing phase. These results show that the
proposed CL system works well within an in vivo freely
moving experimental setting, since it was able to induce
silencing phases by stimulation, as expected.
After this experiment, three other validation experiments were performed. The first validation involved
the same animal and a commercial Fi-Wi optogenetic
system from Doric Lenses, Canada. Fig. 11(b) shows a
sample signal recorded using the Fi-Wi system during
stimulation. We can see that silencing phases are also
observed with the commercial system after each stimulation light pulse. Fig. 11(c) shows a confocal image of
mPFC sections from the mouse used in the in vivo experiments 11 months post viral injection. The cells expressing the ChR2-mCherry (red) construct and the nuclei
labeled with DAPI (blue) are visible, demonstrating that
the ChR2 was expressed at the time of the experiment.
These validation experiments show that the silencing
phases shown in Fig. 11(a)-(c) were most likely caused
by inhibitory neuron activation.
Table 1 compares the electro-optic platform with
recently published systems and other experimental settings intended either for CL optogenetics, optogenetics
and neural recording in parallel or CL electrical stimulation. These systems have various applications, such
as controlling the neuronal firing rate [13], studying
epilepsy [54], repairing damaged neural pathways [31]
or for studying the brain in general [12], [29], [30], [32].
The presented prototype calculates its CL optogenetic
triggers based on the AP classification, as in [12], and
also on the AP bursting [13], which are two essential
features for performing neural decoding [27], [28],
[55]. This contrasts with simple trigger solutions based
on AP detection [29], [30] or PID controller based on
the LFP energy [32]. This paper has demonstrated an
electro-optic prototype utilized for performing CL optogenetics in vivo with freely moving laboratory animals.
This area is attracting a growing interest as shown by
the several recent publications presented ICs for similar purposes [29], and results in vivo with anesthetized
[12], [13], [29]-[32], [54] or freely behaving animals [15].

IEEE CIRCUITS AND SYSTEMS MAGAZINE 		

FOURTH QUARTER 2020
IEEE Circuits and Systems Magazine - Q4 2020

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