We’ve all rated our pain at the doctor’s office, read an eye chart for a licence test, or told a friend the truth about their cooking. Despite the commonly held view that we have just five senses, we actually have many more, and we depend upon their accuracy and reliability for our day-to-day livelihood. The loss of sensation, for example in blindness or spinal cord injury, shows how important the senses are for how we interact with our surrounds.
Who measures sensation and why?
There are many reasons that one might want to measure sensation in humans, scientists trying to find out how it works, clinicians testing for signs of medical conditions, engineers building neuroprosthetics1, or coaches optimising the training of athletes. Each of these professions will have different requirements for measurement accuracy and precision, equipment costs, invasiveness of the measurement, and how much training is required to do the testing. Some senses can be easier to measure than others, and with recent technological advances it is becoming possible to learn more about all the senses.
Clinicians often trade-off between the cost and accuracy of the test, as well as minimising any potential harm to the patient. Sometimes they can find a commercially available test that meets their needs. On top of these demands, good scientific practice often requires customised hardware and software, tools that are just not available to many scientists. Sometimes scientists need to precisely control the how the sense is stimulated (e.g. making a pure sound), record how a person responds, and record neural activity (what the brain and nervous system are doing), all with very high accuracy and precision. Given the difficulty of the problem, scientists with sufficient funding employ engineers and programmers in their labs to build custom rigs to learn more about sensation2,3.
How precise is sensation?
We often take for granted the everyday things that we do, for instance feeling textures at a department store, quickly identifying the smell of burning toast, picking a friend’s voice out from strangers at a bar. In doing so we don’t often think about just how precise sensation is. For instance, we can use touch to perceive differences in textures as small as nanometers4,5.
How do we measure sensation?
Measuring sensation is not always as simple as providing a stimulus, and recording a response, We can measure different aspects of sensation depending on the context. For example, an audiologist6 testing a patient complaining of hearing loss might first begin by measuring the quietest sound that the patient can hear. The audiologist might find that the patient actually has normal hearing in a quiet room. So then they probe further, and measure the patient’s hearing with background noise and voices, finding that patient his not good at picking out signal from noise. Without the tools to carefully control the sound in the room, the audiologist could not have done these hearing tests.
This is similar for measuring all of the senses, without the ability to provide a range of carefully controlled stimuli in a controlled environment it is not possible to fully understand sensation.
To further complicate things, sensation is not always easily explained by a number, making some sensations even harder to measure. Pain is often measured by having patients rate it on a scale, but this is fraught with problems. A 4 out of 10 for one person might mean be 9 out of 10 for another, because life experiences shape each individual’s experience of pain. Alternatively, pain can be measured descriptively, such as “it felt like a sharp knife”, or “on fire”. Figuring out how these descriptive and individual sensations of pain can be measured quantitatively (using numbers) is an unresolved problem.
Do we have the tools to understand sensation?
To really examine and understand the senses requires highly specialised and customised tools for stimulating the sense being tested, and measuring the patients response. Generally, it’s quite hard to buy tools that can do this, and if you can those tools often do not work right off the bat. There are many parameters of the stimulus that require fine control, its timing, strength, location in space, and repeatability. Similarly, there are many features of the response that require fine precision to record. At the time of writing, physiology and medical engineering are the fields to look to for specialization in building these tools and making these measurements7.
- A device intended to replace or augment a function of the nervous system, or connect to it. For example, a bionic eye, or a prosthetic hand that connects the nerves to be controlled by the brain.
- John O’Keefe was awarded a nobel prize for the “discovery of cells that constitute a positioning system in the brain”. Together with his colleagues, they developed methods for working out which neural signals moved the body and separating them from those that tell the brain where we are in our environment.
- Andrew Schwartz and colleagues translate activity recorded from brain cells into movements of a bionic hand. This achievement requires sensitive and robust recording electrodes, high power computing, programming to interpret and command the intended movement, and a robotic hand with multi-dimensional control.
- See Skegung et al. (2013). Feeling small: exploring the tactile perception limits. Scientific reports, 3, 2617.
- One nanometer is one billionth of a metre.
- An audiologist is a trained professional specialising in the diagnosis and treatment of hearing (and sometimes balance) problems.
- Platypus Technical specialises in building tools to make these measurements.
Jack Brooks, PhD, is a neurophysiologist investigating proprioception and the sensorimotor control of movement. He works at the University of Chicago, USA.