“With the smartphone, we’re starting to reach the limits of what we can do,” said Doug Clinton, the founder of Loup Ventures, a new venture capital firm that has invested in Neurable. “These companies are the next step.”

The Neurable prototype shows what is possible today. Using electroencephalography, or EEG — a means of measuring electrical brain activity that has been around for decades — the company can provide simple ways of mentally interacting with a game. Some companies hope to go much further, and want to build ways of performing nearly any computing task with the mind. Imagine a brain interface for rapidly typing on a smartphone.

Even for Silicon Valley entrepreneurs like Mr. Musk, setting that goal pushes technological optimism to new heights. Some efforts seem particularly quixotic. Mr. Musk said in one interview that Neuralink planned to develop ways of implanting hardware in the skulls of completely healthy people.

At Neurable, which is based in Boston, Mr. Alcaide and the members of his team are pushing the limits of EEG headsets. Although sensors can read electrical brain activity from outside the skull, it is very difficult to separate the signal from the noise. Using computer algorithms based on research that Mr. Alcaide originally published as a doctoral student at the University of Michigan, Neurable works to read activity with a speed and accuracy that is not usually possible.

The algorithms learn from your behavior. Before playing the game, you train them to recognize when you are focusing your attention on an object. A pulse of light bounces around the virtual room, and each time it hits a small colored ball in front of you, you think about the ball. At that moment, when you focus on the light and it stimulates your brain, the system reads the electrical spikes of your brain activity.

After you do this for a few minutes, the game learns to recognize when you are concentrating on an item. “We look at specific brain signals,” Mr. Alcaide said, “and once we understand them, we can use them.”

When you play the game, the same light bounces around the virtual room. When it hits the item you are thinking about, the system can identify the increase in brain activity.