For brain research and treatment, there are many forms of electrical brain stimulation (EBS) including transcranial magnetic stimulation, and direct current stimulation. According to Wikipedia:
A comprehensive review of EBS research compiled a list of many
different acute impacts of stimulation depending on the brain region
targeted. Following are some examples of the effects documented:
- Sensory: Feelings of body tingling, swaying, movement, suffocation, burning, shock, warmth, paresthesia, feeling of falling,
oscillopsia, dysesthesia, levitation, sounds, phosphenes,
hallucinations, micropsia, diplopia, etc.
- Motor: Eye movements, locomotion, speech arrest, automatisms, laughter, palilalia, chewing, urge to move, crying without feeling
- Autonomic: Blushing, mydriasis, change in blood pressure and breathing, apnea, nausea, tachycardia, sweating, etc.
- Emotional: Anxiety, mirth, feeling of unreality, fear, happiness, anger, sadness, transient acute depression, hypomania, etc.
- Cognitive: Acalculia, paraphasia, anomic aphasia, recalling memories, "going into a trance", "out of this world", conduction
aphasia, hemispatial neglect, alexia, déjà vu, reliving past
experiences, agraphia, apraxia, etc.
EBS in face-sensitive regions of the fusiform gyrus caused a patient
to report that the faces of the people in the room with him had
"metamorphosed" and became distorted: "Your nose got saggy, went to
the left. [...] Only your face changed, everything else was the
Note that electrical brain stimulation relies on the patient wearing some type of device (electrodes, coils, a helmet, etc...) and then targeting very specific areas of the brain, sometimes through trial and error. I find it difficult to believe that any type of ranged transmitter device would have either the strength or the accuracy to consistently and accurately produce a specific effect.
In addition to directly stimulating the brain through electromagnetic means, there is also a more straightforward means of getting "frequencies" into the brain. The eyes detect light frequencies (within the visible range) and the ears detect auditory frequencies (in the audible range). This suggests that messages can be implanted via audio-visual means, which, of course, is the case. In fact, it is far more likely that these means would be used, as they provide a far richer palette of options. Indeed, advertisers, film-makers, and politicians already use much of this. I'm going to break this up into a couple of categories.
Linguistic: Control people's actions through words, either spoken or written. Includes rhetorical devices, used by politicians, and pretty much anyone that says (or writes) anything. Imagine giant billboards telling you to be a compliant citizen, or public speakers broadcasting pro-government messages. Could also be done through control of nes stations, social media, websites, educational system, rewriting history, controlling the "narrative", creating new derogatory words for one's opponents, etc...
Symbolic: Control people's actions more subtly through non-linguistic cues, such as colors, shapes, sound palettes, etc... It may sound silly, but how many little boys in a toy store would dare to go down the "pink aisle"? Advertisers realize that we never grow out of this, and package similar products separately for men and women, allowing them to perform "gendered pricing". This would also include playing soothing music at train stops, which one of the commenter suggested. Or composing the soundtrack, and adjusting the composition and color balance of films. There's a lot audio-visual meaning that can be conveyed without language. Even gestures and facial expressions could fall into this category.
Subliminal Effects: This can be done either linguistically or symbolically, but it involves sending messages that are below the threshold of conscious detection, so that the target is unaware of them. This could also include more traditional hypnotic suggestions.
Physiological Effects: This could include things like bright flashing lights, or loud noises, being used to daze and confuse an individual (like a flash bang grenade). In some cases, this may also cause seizures.