Neurostimulation
Implantation and in-vivo recordings in visual cortex of macaques.
Electrode Implantation
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92 electrodes were succesfully inserted in the first experiment, 139 in the second.
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A third experiments in which 320 electrodes were succesfully implanted is ongoing right now.
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Electrodes were placed in the visual cortex to record neural spikes, capturing multi-unit activity.
Recordings from visual stimulation
Results
Spontaneous Activity: Neural spikes were successfully recorded without stimuli.
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Visual Stimuli Response: Clear multi-unit activity detected in response to visual stimuli, confirming that the electrodes effectively recorded neural reactions in the visual cortex.
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Receptive Fields: Visual field mapping showed which brain regions responded to stimuli. Receptive fields remained stable over 2.5 years, proving long-term stability of the recordings.
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Long-term Recordings: A small decrease in activity was observed over time but remained robust enough for consistent data collection.
Recordings from spontaneous activity
Behavioral results
When it was established that recording from the implanted electrodes worked flawless and they did not cause health risks for the macaques, the actual goal of the project can be tested:
Do the macaques see dots of light when electrodes inserted in their visual cortex are stimulated?
The functionality was proved in a trained thesus monkey, with following results:
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Eye movement recording: Saccade-memory task towards real and stimulation-induced light dots
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Electrical activity change in higher brain areas
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Stability over 2.5 years based on receptive fields
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Histology:
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Minimal tissue damage over time!
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The graph on the right shows how the monkey works:
First its behavior is assessed based on trials with real targets on the screen (target 1, 2, 4, 5). The monkey performs an eye saccade towards the target when the go cue goes off, which is a successful trial.
Next, an electrode in the monkey’s brain is stimulated. Based on the receptive field recordings we know where the monkey should perceive a light dot due to stimulation of that specific electrode, this target is shown in the black circle (target 3). The red stars are the end points of the monkey’s eye saccades for the trials on target three: the monkey saccades towards the target and does not seem to notice a difference between a real and a stimulated target!
A third kind of trial is a fixation trial: no target is shown/stimulated (central target). This is to make sure the monkey does not just guess where a target should be, proving that when stimulated, the monkey in fact does perceive a light dot at the expected location.
tDCS (Direct Current Stimulation)
Since lower stimulation intensities are preferred for safety purposes, new methods were researched to be able to decrease the stimulation intensity. One way is the use of tDCS, which would increase the reactivity of neurons, causing them to react more to lower stimulation intensities.
The tDCS electrodes are spirals located on the skull. The idea is to pre-stimulate the neurons with a low intensity, which does not cause them to fire action potentials but lowers the threshold: stimulation of the inserted electrodes with lower intensities will result in just as much firing as stimulation with higher intensities not using tDCS.
Histology
Histology at the end of at least 4 months of implantation reveals that out implants are very well tolerated by the brain. There is almost no scar left behind. The effected area is about 100x smaller than that of deep brain stimulation electrodes currently used in the clinic.
