In this issue of Ask a Neuroscientist, graduate student Becca Krock answers a question about whether our brains encode stimuli that we can't consciously perceive.
Interesting question! I love the unconscious.
As you say, not all stimuli that reach the human body are consciously perceived: there is a threshold for intensity below which you can’t detect touch (which is also known as somatosensation; since ‘soma’ means ‘body’, this simply means ‘body sensation’.) It’s the same idea for sight, hearing, and the other senses. If an insect’s movement on your arm triggers sensations above the detection threshold, you definitely know it’s there. But we’ve all had the experience of being informed, to our great surprise and chagrin, that there’s a spider hanging out on our back undetected. So what’s happening in that case, when the stimulus intensity is below the detection threshold?
The first step in somatosensation happens in the peripheral nervous system, in the sensory nerve endings in our skin, muscle, and joints. These nerve endings detect sensations like pressure or movement, and are responsible for informing the brain about the situation. If these nerve endings don’t detect the bug on your back, no information about it is going to reach the brain, and if you put that person in an fMRI scanner you won’t see any relevant neural activity.
But what if the peripheral nerve endings do detect the insect, send action potentials up the spine and into the parts of the brain responsible for processing touch? Then is it still possible you wouldn’t be consciously aware of the stimulus?
The answer is yes. It is in fact possible for a person’s brain to have a representation of a somatosensory stimulus that they insist they don’t know about. This result was actually first observed in 1967 (1).
A little context for that 1967 result: Sensory stimuli, as a general rule, are processed by the brain in a hierarchical way. Raw peripheral sensory signals reach an early brain area that processes the basic features of the stimulus, passes the new information along to one or more new areas that extract some more complicated features, and so on and so forth until you somehow recognize the stimulus, select the useful bits and re-aggregate this information in a helpful way for decision-making and action-taking. This is all, of course, an extraordinarily general schema that’s sure to be wrong in untold ways, but it’s the way many people look at it.
Touch information comes first to the thalamus, a subcortical relay station for all sensory processing. The first cortical area to receive touch information is primary somatosensory cortex, or SI. It extracts basic features of the stimulus - like how intense it is and its location on the body - and if it's damaged, the patient will struggle to perceive touch in these basic ways. In 1967, using sensitive electrodes directly on the surface of the brains of human subjects, scientists observed the patterns of brain activity in SI that occurred when they applied a subtle somatosensory stimulus to the skin. Even when the subject wasn’t able to detect the stimulus, they still had activity in SI (1).
Now, since the 60’s, research has gone much further in understanding touch awareness. Let’s say you deliver a shock to a person that’s sitting right on their detection threshold. On some trials of your experiment, they will be able to tell you “Yes, I felt it”, and on some trials they won’t. What’s different in the brain during these two types of trials? That difference might tell you how touch awareness comes about.
It's clear that the earliest touch-related cortical activity, in SI, is by itself not enough to explain conscious awareness of touch. As long as peripheral receptors are capable of detecting the stimulus as it's happening, SI shows corresponding activity, whether you become aware of it or not (2,3,4). But is this true in the areas of the brain that deal with more complex features of touch sensation?
The secondary somatosensory cortex, or SII, takes the basic touch information encoded by SI, and extracts more complex features of the stimulus. Unlike SI, SII does not clearly represent simple features like stimulus intensity (2). Instead, it represents features like the texture or the size of an object.
Just last year, a study using fMRI showed that out of all areas that responded to vibrations on the skin, SII activity correlated the best with the subject's awareness of whether the stimulation occurred on each trial (4). Keep in mind, as always, that correlation is not causation. But this certainly suggests that SII may be where conscious touch perception starts.
A final note—you’ve suggested a neuroscientific definition of unconscious perception: when a stimulus registers in our brain activity, but we don’t take note of it. But unconscious percepts can and do turn up unannounced in our everyday behavior as well. This fascinating and clear review highlights many studies showing how features of our environment can affect our behavior even if we aren’t aware that they’re doing so—nay, even if we aren’t aware we have seen them at all. This is sometimes called “priming.” It can be achieved with overt messages you simply ignore or forget about: in one study the review cites, for example, reading words related to cooperation led people to work together more on economic games. You can see how watching and hearing the same awful commercials over and over might have an insidious effect even if you really, truly feel, deep down in your bones, that they’re awful.
And perhaps more amazingly, priming can also caused by subliminal stimulation, messages presented at such a low intensity or duration that people can’t even consciously detect them. In one such study, people drank more after receiving subliminal messages with words related to drinking (5). Fascinating, and terrifying.
1. Libet, B., Alberts, W. W., Wright Jr. E.W., and Feinstein, B. Responses of Human Somatosensory Cortex to Stimuli below Threshold for Conscious Sensation. Science, 22 December 1967: 158 (3808), 1597-1600.
2. Porro, C.A., Lui, F., Facchin, P., Maieron, M., and Baraldi, P. Percept-related activity in the human somatosensory system: functional magnetic resonance imaging studies. Magnetic Resonance Imaging, December 2004: 22 (10), 1539-1548.
3. Schubert, R., Blankenburg, F., Lemm, S., Villringer, A. and Curio, G. (2006), Now you feel it—now you don't: ERP correlates of somatosensory awareness. Psychophysiology, 43: 31–40.
4. Moore, C.I., Crosier, E., Greve, D.N., Savoy, R., Merzenich, M.M., and Dale, A.M. Neocortical Correlates of Vibrotactile Detection in Humans. Journal of Cognitive Neuroscience, 2013: 25 (1), 49-61
5. Custers, R. and Aarts, H. The Unconscious Will: How the Pursuit of Goals Operates Outside of Conscious Awareness. Science, 2 July 2010: 329 (5987), 47-50.