Is there really a thing like the sixth sense?
There is a largely undisputed sixth sense, and then there are lots of others that are more controversial.
I believe the proprioceptive system is totally a sixth sense, though it is generally ignored because it doesn’t often reach consciousness. Proprioception is the sense of self-movement and body position. It is often considered just another part of the somatosensory complex, which primarily senses what happens around the body. Proprioception doesn’t sense anything outside the body, it senses only internal body state, and its primary destination is not the somatosensory cortex but the motor cortex.
While proprioceptive nerves go through the thalamus, like all afferent PNS neurons, they are not processed there (very much), and they go to the somatosensory cortex, where they are also not processed (very much), their primary destination being the motor cortex, where they are depended on to mediate muscular activity.
When everything is working properly, proprioceptive neurons are part of a closed loop feedback system, running under the principles of control theory, that allows fine control of most of the muscles of our body. Every motor neuron firing is balanced by a returning proprioceptive response that modulates the firing. Without proprioception, our muscles would continually over- or under-activate, and our human sense of physical competence would vanish.
Of course, things don’t always work properly. When a proprioceptive signal is too far out of range to be compensated adequately by changing muscular output, the extreme example being a severed limb, the motor cortex reports this anomaly as pain, and that’s when proprioception reaches consciousness. This is not the pain reported by a nociceptor, nor is it the pain (tickle?), reported from the thalamus for missed top down predictions of touch expectations. It is a pain from the motor cortex indicating that a muscular control loop is malfunctioning. This difference often makes proprioceptive pain much harder to treat.
The sense of “touch” is really the union of exteroception (sensing the outside of the body), interoception (sensing the internal organs and tissues), and enteroception (sensing the internal organs that are topologically part of the body exterior, called hollow organs). Whether these should all be considered one sense is more of a philosophical question than a biological one. They all work about the same and follow the same paradigm of being processed in the thalamus based on top down predictions. So you might consider them one sense (touch, already counted in the original five) or two new ones.
The argument for counting enteroception as another sixth sense is that we have learned recently how intimately our gut receptors are connected to the brain through the vagus nerve (in addition with endocrine hormones). We continue to find new ways that enteroceptive neurons from the gut affect behavior, but again this activity is below the level of consciousness, so it often gets less recognition.
Another candidate for a sixth sense is the sense of balance and equilibrium we get from the vestibules and semi-circular canals of the ear. Since they are in the ear, connected with the brain through the same nerves as the hearing part of the ear, and operate below the level of consciousness, they are usually not considered a sixth sense, often just lumped in the with the sense of hearing.
So far all the candidates for a sixth sense have involved sensing the world of the body. Most proposed “sixth senses” that look outside the body are controversial, but one is not.
Animals have non-visual photoreceptors that synchronize their circadian rhythms to the daily cycle of dawn and sunset. In early animals this photoreception was carried out by the pineal gland, which still runs the hormone factory that controls our circadian rhythms. Early animals and many animals today, like birds and reptiles, have thin enough skulls that the pineal gland can sense the sun directly through the skull. In humans, the pineal gland is guided by non-visual photoreceptors in the eyes. Again, this activity is entirely unconscious.
Another recently characterized form of sensing the external world is blindsight. While blindsight depends on the photoreceptors in the retina reporting their signals to the thalamus, beyond that it has little to do with our normal sense of sight. While we have known about the collicular visual cortex (CVC) for quite some time, until recently it was thought to be a silent offshoot of the normal visual pathway. Now it is recognized as its own visual system, getting visual signals directly from the thalamus (even before they reach the striate cortex, known as V1, since the CVC is closer to the thalamus relay point). The CVC seems to be more ancient than the normal visual pathway in animals.
Other candidates for a sixth sense are much more controversial, since they depend either on forms of physical communication with little or no scientific evidence, like other forms of electromagnetic radiation than visible light, or purely speculated forms of communication, like ESP or communication with a spiritual realm.
Of course, some animals and humans are able to use the known senses to interpret the world in ways that some of us can’t. Are those “sixth” senses or just exceptional uses of the ones we know about?
