Why don't humans have an "eighth sense"?

For more than a century, neuroscientists have been searching for the physical traces of memory—the mysterious neural patterns known as "engrams," which are thought to be the basis for encoding experiences in the brain. Now, researchers may have discovered something even more fascinating: a mathematical reason that might explain why human perception and memory seem to be "tuned" to a specific limit.

In a new study published in Scientific Reports, researchers from King's College London, Loughborough University, and the Skolkovo Institute of Science and Technology proposed a theoretical model showing that the brain's ability to form and retain memories may depend on the number of dimensions in the "sensory world" it perceives.

The results indicate that there is an "optimal number of sensory dimensions"—around 7— at which the efficiency of memory storage peaks and begins to decline at higher dimensions.

The researchers wrote, "An interesting result of the model is that it implies the existence of an 'optimal number of senses' in the evolution of nervous systems or neural-like systems. When the number of senses equals 7, the capacity of the conceptual space is maximized—that is, the perception of the external world is the richest, and the number of different concepts that can be preserved also reaches its maximum."

The core question of this study is: How many senses does an intelligent system need to remember the most information about its environment?

To explore this question, the researchers developed what they call the "engram dynamics model". These are mathematical representations used to show how memories form, change, and fade in a "conceptual space"—which can be understood as a multi-dimensional map of experiences. In this model, each engram is almost like a "living" object that expands, contracts, or merges depending on the frequency of stimulation (such as visual, auditory, tactile, etc.).

The researchers wrote, "If each feature corresponds to a sense, then this critical dimension means that for a system to retain the most different concepts, the optimal number of senses is precisely this dimension."

In other words, if each sense represents a dimension in perception, then there seems to be a natural limit—seven dimensions—beyond which the brain's ability to distinguish different concepts declines.

This study is based on the concept of "engrams" proposed by German biologist Richard Semon in 1904. For more than a century, neuroscientists have tried to find these neural "memory traces" in the brain through brain imaging and optogenetic experiments and identify specific groups of neurons that are reactivated during recall. However, although these studies have revealed where memories might be located, they cannot explain how memories evolve and compete with each other over time.

This new study fills this gap with mathematics. The researchers used Monte Carlo simulations and analytical solutions to model the dynamic behavior of engrams under continuous stimulation.

In the simulation, memories form when multiple sensory impressions converge and become stronger under repeated stimulation; if there is a lack of stimulation, they gradually spread and become blurred—like metaphorical "forgetting".

This tug-of-war between memory and forgetting ultimately reaches a balance. However, when the research team simulated engrams in conceptual spaces of different dimensions, they found surprising results.

As the number of dimensions increases, the number of unique memories also rises—up to a point. After the seventh dimension, memory capacity begins to decline because the overlap and interference between different engrams reduce the system's efficiency.

Nikolai Brilliantov, a professor at the Skolkovo Institute of Science and Technology and a co-author of the paper, pointed out in a press release, "When we considered the maximum capacity of the conceptual space in a given dimension, we unexpectedly found that in the steady state, the number of different engrams stored in memory reaches its maximum at seven dimensions. Therefore, we call it the 'seven-sense hypothesis'."

This "critical dimension" not only reveals the memory mechanism but may also explain why biological sensory systems have evolved the way they have.

Traditionally, humans are thought to have five senses—vision, hearing, smell, taste, and touch—but neuroscience now also recognizes several additional senses, such as proprioception (the sense of body position) and equilibrioception (the sense of balance).

The idea that cognition peaks at around seven inputs is not new. Psychologist George A. Miller proposed in 1956 that humans can, on average, hold about "seven plus or minus two" units of information in working memory. This new model provides a potential physical and mathematical basis for this long-observed cognitive limit.

The researchers pointed out that the balance between sensitivity and precision—the trade-off between being open to new experiences and maintaining clear memories—may reflect a general principle. Highly sensitive systems tend to form blurred, overlapping memories; while overly selective systems may miss out on new experiences.

The researchers wrote, "The higher the sensitivity, the less clear the learned concepts." They compared this tension to the "bias-variance trade-off" in machine learning, where the system must balance generalization ability and overfitting.

From a biological perspective, this discovery suggests that evolution may have "tuned" human sensory abilities to an optimal point—where the efficiency of perception, learning, and memory is maximized. Adding more senses or sensory dimensions does not necessarily improve cognition; instead, it may overload the brain's conceptual space and cause interference between memories.

This "engram dynamics framework" may also inspire the fields of artificial intelligence and neural-like computing—in these fields, mimicking the biological brain's memory system must also balance the flexibility of learning and the stability of information. AI systems with too many input channels may, like brains beyond the optimal sensory dimension, experience "information saturation" and confusion.

The researchers wrote, "In addition to revealing and proving the existence of a critical dimension in the conceptual space, the proposed engram dynamics model may also provide new interpretations for existing and future empirical data. For example, the mechanisms of engram fusion and fission can be tested experimentally."

They suggested that by presenting a series of sensory stimuli to subjects and measuring the distinguishability of their engrams, scientists can estimate the model variables in reality, such as the learning rate and forgetting rate.

Ultimately, this study provides an intriguing perspective: the structure of human perception—the way we see, hear, touch, and balance—may not only be the product of biological evolution but also reflect the deep mathematical laws governing the memory system.

Dr. Brilliantov added, "Our conclusions are still speculative when applied to human senses, but who knows? Future humans may really evolve the ability to perceive radiation or magnetic fields."

References

Otieno, W., Tyukin, I.Y. & Brilliantov, N. The critical dimension of memory engrams and an optimal number of senses. Sci Rep 15, 29972 (2025). https://doi.org/10.1038/s41598-025-11244-y

Original Article:

https://thedebrief.org/forget-the-sixth-sense-new-study-says-the-human-brain-may-be-wired-for-seven-senses

This article is from the WeChat official account "Neural Reality" (ID: neureality), author: Tim McMillan, translator: EY, published by 36Kr with permission.