Reacción a "A study in mice shows that brain stimulation using contact lenses is effective against depression"

This study presents a novel and promising system for treating depression based on non-invasive physical stimulation delivered through the eye.

The authors have developed a soft, flexible contact lens that delivers an electrical current through the cornea (transcorneal electrical stimulation via temporal interference, or TI-TES). This current reaches the retina and, from there, travels along the visual pathways to deep brain structures.

The key lies in how the stimulus is generated: the technique is called temporal interference. It works like this: two high-frequency electrical signals, very similar to each other (2,000 Hz and 2,020 Hz), are emitted. Neither of them, on its own, activates the neurons, because they oscillate too quickly. But when they cross in the tissue, they generate an “enveloping wave” at a much lower frequency—in this case, 20 Hz—which is what actually stimulates the neurons. This allows deep structures to be reached without the need for surgery or implanted electrodes.

Before evaluating its efficacy, the researchers verified in mice that the device was safe: it caused no structural, functional, cellular, or inflammatory damage to the cornea or retina, and was biocompatible even with prolonged use.

Next, they verified that the external stimulation effectively activated retinal cells and that this signal reached the cerebral cortex, where they recorded the corresponding evoked potentials.

To induce a depressive state, they administered corticosterone (the primary stress hormone in rodents) to a group of mice. They then compared those receiving stimulation with those who did not, testing different durations and frequencies. A significant, though partial, improvement was observed on four levels:

• Behavior: the treated mice showed a reduction in typical signs of depression—they regained some of their social behavior, reduced anxiety, showed less hopelessness, and regained mobility—without achieving complete normalization.
• Brain connectivity: communication between the hippocampus and prefrontal cortex improved, a pathway typically inhibited in depression.
• Synaptic microstructure: connections between neurons increased, and part of the synapses’ plastic and regenerative capacity was restored.
• Inflammation: inflammatory markers decreased in brain structures, a known abnormality in depression.

To gauge the extent of the effect, they compared it with a group treated with fluoxetine (a common antidepressant): rTMS showed comparable efficacy. In other words, this is not a complete reversal of symptoms, but an improvement on par with that offered by a commonly used antidepressant.

It is worth recalling the context. The tools available to treat depression fall into three categories: psychotherapeutic, pharmacological (antidepressants), and physical treatments. Among the physical treatments, the following stand out:

• Electroconvulsive therapy (formerly known by the stigmatizing name “electroshock”): highly effective, but requires general anesthesia and can cause reversible amnesia associated with the sessions.
• Transcranial magnetic stimulation (TMS), which is gaining ground in our country, with increasing evidence of its usefulness and few adverse effects. It works by applying a pulsed magnetic field from a coil placed on the scalp; this field induces, through electromagnetic induction, small electrical currents in the most superficial layer of the cerebral cortex. Its main limitation is twofold: on the one hand, its reach is limited to relatively superficial brain structures; on the other, it is logistical—currently, patients must visit a specialized center and use specific equipment.
• Deep brain stimulation, which requires implanting an electrode in deep brain structures via neurosurgery and is reserved for cases of treatment-resistant depression.

TI-TES would fall into this latter group, that of neurostimulation treatments, but with two important differences. The first: unlike TMS, which reaches only the superficial cortex, temporal interference would allow for the stimulation of deep brain structures without surgery. The second: it could be administered via a contact lens, in an outpatient setting and following a simple ocular procedure. In the study, adverse effects appear to be nonexistent (something that, logically, must be confirmed in humans).

It is worth emphasizing the most important point: the results are from mice, and the observed improvement is partial, not a cure. The leap to humans is never direct, and in the field of psychiatry we have seen many promising therapies in animal models that have not shown the same effect in people. The group sizes in this study are small (around 10 mice per group), although the observed signal is consistent and of notable magnitude. The authors themselves also note that there are technical challenges yet to be resolved, such as the wireless transmission of the signal to the contact lens.

Even so, this work opens a door that was hard to imagine just a few years ago: stimulating the deep brain using electrical currents delivered through contact lenses. And, if the technology matures, it could be applied to other brain-related disorders and diseases. One might even consider its potential as a cognitive enhancer in the society we live in and are moving toward—with all the ethical questions that would raise.