S. D. V. Stephens

notes / Basic Neuroscience and Geometry of Neuroscience / chapter 3: The Senses

9 Touch, Taste, and Smell

The objectives for this lecture were to understand the typical and atypical body mapping of the brain, to describe the function and role of different types of tactile (touch) stimulation, and to analyze the higher-level processing involved in olfactory (smell) and gustatory (taste) sensory information.

Phantom limbs and brain maps

Dr. V. S. Ramachandran

Dr. Vilayanur S. Ramachandran (born 1951) is a neurologist and psychologist at the University of California, San Diego (UCSD). He is a pioneer in the study of phantom limb pain, synesthesia (perceiving one sense through another) and Capgras syndrome (the belief that someone close to you has been replaced by an impostor). His original training was in visual perception and cognition.

Phantom limbs

Definition 9.1 (Phantom limb). A phantom limb is a limb that is still felt after it has been lost.

  • Phantom limbs were first described by a physician attending wounded soldiers from the Battle of Gettysburg. The soldiers often reported that their limbs had come back to haunt them.
  • Sometimes the phantoms are very active: picking up the phone, waving.
  • Doctors believed the phantom was due to irritation of the nerve endings in the stump.
  • Dr. Ramachandran, however, thought neural plasticity might be involved in producing phantom limbs.

The homunculus

The body is mapped onto the cortex in order: this is the topographical organization of the sensory and motor cortices. The somatosensory cortex runs as a strip along the central sulcus, with the motor cortex right next to it. The slide’s drawing lifts the strip out of the brain and labels it. Starting at the top of the brain, near the midline, it runs through the genitalia, toes, feet, ankle, knee, leg, hip, trunk, shoulder, elbow, wrist, hand, the digits (V, IV, III, II) and the thumb, then the neck, brow, eyelids, eyeballs, face, lips, jaw, tongue and throat as it goes down the side of the brain, with areas for mastication (chewing) and salivation at the bottom.

The areas are not in proportion to the body. The hand and the lips have huge areas; the trunk and legs have small ones. The homunculus (“little man”) draws a body with the proportions the cortex gives it: a small, thin body with enormous hands, lips and tongue.

The body map along the cortical strip, unrolled from the top of the brain to its side: genitalia, toes, feet, ankle, knee, leg, hip, trunk, shoulder, elbow and wrist in small areas, then a large hand region (hand, digits V to II, thumb), then neck, brow, eyelids and eyeballs, then a large face-and-lips region, then jaw, tongue, throat, mastication and salivation.

Note that the hand’s part of the map and the face’s part lie close together.

Example 9.2 (Tom Sorensen: a demonstration of brain map plasticity). Tom Sorensen lost his arm in a car accident, but he suffered from an itch on his phantom limb that he could not scratch.

Dr. Ramachandran stroked different parts of Tom’s face with a Q-tip and found that in certain places, Tom reported also feeling the Q-tip on areas of his phantom limb.

After scanning Tom’s brain, Dr. Ramachandran found that, as he had predicted, the maps for the face and the hand had blurred together.

With the arm gone, the hand’s area of the map stopped receiving its own input, and input from the face, its neighbor on the map, spread into it. So touching Tom’s face also activated the hand’s area, and he felt the touch in his phantom hand. The maps for phantom limbs are always changing: they shrink and stop working properly.

Pain

Pain is a serious problem:

  • Chronic pain can lead to depression, helplessness and suicide.
  • About one in five American adults, some 50 million people, experience chronic pain.
  • Pain is believed to cost 80 billion dollars a year in the U.S., through consultations with health professionals, lost work and litigation.

Can pain be beneficial? The lecturer’s answer: “Obviously.”

Pain versus other touch sensations

There are different types of receptors for different types of touch. The slide sets a photo of a woman clutching her sore neck beside one of a dog being petted on the head.

Definition 9.3 (Nociceptors). Pain receptors are called nociceptors. They respond to mechanical, thermal and chemical stimuli.

The receptors that detect the other types of touch respond only to mechanical stimulation.

Separate sensory systems

The slide’s drawing points to a single spot on a fingertip where four different sensations are picked up: pain, nonpainful temperature, painful temperature and touch.

  • There are different receptors for the different types of touch sensation.
  • Each “sense” is conveyed to the brain by different neurons.
  • Each is processed in a different sensory region of the brain. (The lecturer’s challenge: think about where in the brain each type of sensation might be processed, and how those places differ.)

The gate control theory: neural inhibition of pain

During stress, endogenous opioids (opioids made by the body itself) are released from inhibitory neurons within the midbrain, and they inhibit pain neurotransmission. This is why the rush of a stressful, adrenaline-filled moment really can shut down pain (the slide’s photo shows two soldiers on patrol): stress releases endogenous opioids that block pain signals from reaching the brain.

The slide’s diagram shows how. Neuron 1 carries the pain signal from the skin; its cell body sits in a swelling just outside the spinal cord, and it enters the back of the cord and synapses onto neuron 2. Neuron 2 crosses to the other side of the cord and carries the signal up to the brain (to the somatosensory cortex). A neuron coming down from the periaqueductal gray (PAG) of the midbrain, which releases opioids, makes an inhibitory () connection right where neuron 1 meets neuron 2. When it is active, the pain signal is blocked in the spinal cord, before action potentials from the pain receptors can reach the somatosensory cortex.

Cross-section of the spinal cord: neuron 1 carries a pain signal from the skin, past its cell body outside the cord, into the back of the cord, where it synapses onto neuron 2, which crosses the cord and ascends to the somatosensory cortex; a neuron descending from the periaqueductal gray, which releases opioids, makes an inhibitory connection at that synapse.

Affective touch

Definition 9.4 (Affective touch). Affective touch is a type of touch associated with social interaction.

A unique set of receptors responds to affective touch, and it is processed in areas associated with stress, including the prefrontal cortex and the amygdala. Affective touch can help inhibit stress responses, and it tends to be quite good for our mental health.

Dr. Cynthia Breazeal (pictured with a small white tabletop robot) built the “Huggable” therapy robot, a robotic teddy bear. (The slide’s photo shows the bear beside its robotic frame.) It is fitted with sensors across its body and camera sensors in its eyes, so it can respond to the touch and movements of the patient. It decreases patient stress and anxiety.

Smell

Why do we smell?

The slide asks the question over four photos: a hamster eating a plate of fruit and vegetables, a snake flicking out its tongue, a mother mouse with her newborn pups, and two foxes nuzzling. Smell helps animals find food, detect other animals, and recognize their young and their companions.

Olfaction

Olfaction is the sense of smell.

  • Smells are detected by receptors in the olfactory epithelium, the lining at the top of the nasal cavity.
  • Odor molecules bind to the receptors, triggering action potentials that project to the brain.

The slide’s drawing is a side view of the head. The olfactory nerves branch over the roof of the nasal cavity, pass up through the cribriform plate (a thin, perforated plate of bone), and reach the olfactory bulb, from which the olfactory tract runs back into the brain. The lecturer described this slide as a general overview, not something to memorize.

Olfactory processing

The slide’s diagram, a view of the brain from below, follows the olfactory tract to five places:

  • the olfactory cortex of the temporal lobe: conscious perception of smell;
  • the hippocampus: olfactory memory;
  • the amygdala: emotional responses;
  • the hypothalamus;
  • the reticular formation in the brainstem: visceral responses to smell (gut-level bodily reactions).
Olfactory pathway: the olfactory epithelium feeds the olfactory bulb and the olfactory tract, which projects to the olfactory cortex of the temporal lobe (conscious perception of smell), the hippocampus (olfactory memory), the amygdala (emotional responses), the hypothalamus, and the reticular formation (visceral responses to smell).

Other species have an accessory olfactory bulb that specifically detects pheromones (chemical signals sent between members of the same species).

The lecturer asked the class to note all the other brain regions smell projects to: why does it go to each of these, and do other senses do that too? (The discussion question is in “Check yourself.”)

Taste

  • There are 5,000–10,000 taste buds on the tongue and palate.
  • Each taste bud contains 50–100 specialized sensory cells, stimulated by chemicals such as sugars, salts or acids.
  • Taste signals project to the brainstem, thalamus and cerebral cortex to detect taste sensation.

In the lecturer’s words, taste buds are just bundles of receptors for the specific chemical compositions that make up “sweet,” “salty,” “sour” and so on, and smell honestly does most of the heavy lifting for taste.

The slide’s drawing of the tongue shows its bumps, the papillae, which hold the taste buds: a row of large circumvallate papillae across the back, foliate papillae in folds along the sides, and small fungiform papillae scattered over the surface. Beside it is the old “tongue map,” which gives each taste its own zone: bitter at the back, sour along the sides, sweet/umami at the tip and salty at the front edges. In fact receptors for every taste are spread across the whole tongue, as the drawing on the next slide shows.

Taste what you smell!

Flavor perception combines taste and smell sensory information. The tongue can only distinguish 5 qualities of taste (sweet, salty, sour, bitter and umami, a savory taste), while the nose can detect thousands of smells.

The slide’s drawing shows four tongues, one each for salty, bitter, sour and sweet, with that taste’s receptors dotted all over the tongue. Beside them is a cross-section of the head showing two ways odors reach the smell receptors at the top of the nasal cavity: in through the nostrils, and up from the mouth through the back of the throat, called retronasal olfaction. Retronasal olfaction is how the smell of food in your mouth reaches the smell receptors.

As the lecturer put it, taste is basically just smell. That is why COVID made people unable to taste: the olfactory epithelium became inflamed.

Check yourself

Question 9.5 (Hebbian principles and sensory substitution (memory test)). Hebbian principles state that synapses are strengthened and weakened based on experience. Use this basis to describe how one sense can be used to detect another.

Show solution

Hebb’s principles come from Lecture 4. In sensory substitution, input from one sense (touch on the tongue) is repeatedly active at the same time as the activity the lost sense would have produced, as the person practices with the device. “Neurons that fire together wire together”: the synapses that link the tongue’s pathway to the balance pathway in the medulla are used and strengthened (“use it to improve it”), while the silent inputs from the damaged inner ear weaken (“use it or lose it”). Over time the tongue’s signal drives the balance areas.

Question 9.6 (Altering what we sense (memory check)). How can it be that we can alter our detection of our senses?

  1. The brain is re-wired through experience
  2. Nearby brain regions “overlap”
  3. The brain is re-wired through lack of use
  4. All of the above
  5. This isn’t a thing, this is a trick question m8
Show solution

4, all of the above. Sensory substitution re-wires the brain through experience (1); in Tom Sorensen, the hand’s area of the map fell out of use (3) and the neighboring face area spread into it, so the two maps overlapped (2).

Question 9.7 (Processing touch (memory check)). Which best describes how the brain receives and processes tactile (touch) information?

  1. One receptor & brain region for all sensations
  2. One receptor, but multiple brain regions for all sensations
  3. Different receptors, but one brain region for all sensations
  4. Different receptors & brain regions for different sensations
Show solution
  1. Pain, temperature and touch each have their own receptors (nociceptors for pain, mechanical receptors for touch), each sense is carried to the brain by different neurons, and each is processed in a different sensory region.

Question 9.8 (Smell, memory and emotion (discussion)). Some olfactory information is sent to the amygdala, hippocampus and hypothalamus. Why might these brain regions be involved in olfaction?

Show solution

The slide’s diagram labels their jobs: the hippocampus handles olfactory memory and the amygdala emotional responses, so smells can call up memories and feelings; the hypothalamus (with the reticular formation, which handles visceral responses to smell) ties smell to the body’s reactions. These fit what smell is for: finding food, detecting other animals, and recognizing offspring and companions.

Question 9.9 (Beyond the primary sensory cortices (exit discussion)). Sensory signals are processed in several different brain regions. While we often think of each sense as processed in its corresponding sensory cortex, other regions are involved as well. What other brain regions process sensory information in addition to the primary sensory cortices? Why?

Show solution

From this lecture and the last: smell goes to the hippocampus (memory), amygdala (emotion), hypothalamus and reticular formation (bodily responses) as well as the olfactory cortex; taste goes through the brainstem to the thalamus, hypothalamus and amygdala as well as the cortex; affective touch is processed in the prefrontal cortex and amygdala, areas associated with stress; and balance goes to the cerebellum (and on to the eye and head muscles). Sensory information reaches these regions so that it can be remembered, given emotional weight, and used to guide the body’s responses and movements.