1 6: Other Senses Cognitive Neuroscience David Eagleman Jonathan Downar

2 Chapter Outline Detecting Data from the World HearingThe Somatosensory System Chemical Senses The Brain is Multisensory Time Perception

3 Detecting Data from the WorldWe use five senses to interact with the outside world, as traditionally defined. There are many subdivisions of those five, plus sensations from inside the body. For all senses, there are specialized receptors to transduce the stimulus. All senses project to primary sensory cortex, and have some form of mapping.

4 Hearing The Outer and Middle EarConverting Mechanical Information into Electrical Signals: The Inner Ear The Auditory Nerve and Primary Auditory Cortex The Hierarchy of Sound Processing Sound Localization Balance

5 The Outer and Middle EarSounds are vibrations carried through air or water as waves We interpret the frequency (measured in Hz) of the vibration as the pitch of the sound. The amplitude of the vibration is interpreted as the loudness of the sound.

6 The Outer and Middle EarFIGURE 6.2 Pressure changes in the air are interpreted as sound by the brain. Amplitude is the size of the pressure change, and frequency (hz) refers to the number of waves that occur per second.

7 The Outer and Middle EarThe pinna collects and amplifies certain frequencies of sound and directs that sound down the ear canal. The sound energy strikes the tympanic membrane and causes it to vibrate at the same frequency as the sound wave.

8 The Outer and Middle EarFIGURE 6.3 Anatomy of the ear. The pinna of the external ear captures sound and reflects it down the auditory canal. The pressure waves vibrate the tympanic membrane. These vibrations are passed by the bones of the middle ear (malleus, incus, stapes) to the cochlea, a part of the inner ear.

9 The Outer and Middle EarIn the middle ear, vibrations of the tympanic membrane cause movement of the three bones of the middle ear. Malleus (Hammer) Incus (Anvil) Stapes (Stirrup) Movement of these bones causes movement of the oval window.

10 Converting Mechanical Information into Electrical SignalsThe oval window is part of the cochlea, which is the inner ear. The cochlea contains three fluid-filled tubes, wound around a central axis, like a snail shell. Inside the cochlea is the basilar membrane, which vibrates in time with the sound wave.

11 Converting Mechanical Information into Electrical SignalsThe basilar membrane is tighter at one end (the base) and looser at the other end (the apex). This difference means there is a tonotopic map of frequencies along the basilar membrane High frequencies near the base. Low frequencies near the apex.

12 Converting Mechanical Information into Electrical SignalsFIGURE 6.4 The cochlea. In the inner ear, the cochlea is coiled up much like a snail’s shell. If you could unroll it, the tube would be about 35 mm long for humans. As shown here, the basilar membrane of the cochlea maps frequencies, with higher tones near the oval window.

13 Converting Mechanical Information into Electrical SignalsFIGURE 6.5 The hair cells of the inner ear. here you see a transverse cut across all three canals of the cochlea, showing inner and outer hair cells and the overlying tectorial membrane.

14 Converting Mechanical Information into Electrical SignalsInner hair cells along the basilar membrane transduce sound into electrical signals. The vibration of the membrane causes the stereocilia to flex closer together or further apart.

15 Converting Mechanical Information into Electrical SignalsTip links on the stereocilia cause ion channels to be pulled open, depolarizing the cell, when the cilia move one direction. When cilia move the other direction, the channels close and the cell is hyperpolarized.

16 Converting Mechanical Information into Electrical SignalsFIGURE 6.6 Structure of an individual hair cell. (a) The stereocilia and the kinocilium of the hair cell, connected by the tip links at the top of the stereocilia. (b) Movement of the hair cells can increase or decrease the force on the tip links, causing ion channels to open or close. Opening the channels depolarizes the cell, whereas closing the channels hyperpolarizes the cell.

17 Converting Mechanical Information into Electrical SignalsThe outer hair cells help to amplify and sharpen the incoming sound. The basilar membrane can break apart a complex sound into the component frequencies.

18 The Auditory Nerve and Primary Auditory CortexThe auditory (cochlear) nerve carries information from the inner hair cells to the cochlear nucleus of the brainstem. Each fiber is a labeled line, carrying information about only one frequency. Information travels from the cochlear nucleus through a number of nuclei to the primary auditory cortex in the temporal lobe.

19 The Auditory Nerve and Primary Auditory CortexFIGURE 6.8 Pathways from the ear to the primary auditory cortex. From the auditory nerve, sound information travels to the cochlear nuclei in the brainstem, which relay information to the olivary nuclei (olive) on both sides of the brainstem. From the olive, auditory information passes over the lateral lemnisci to the inferior colliculi and on to the medial geniculate nucleus of the thalamus, which relays the information to the primary auditory cortex.

20 The Hierarchy of Sound ProcessingDeafness can result from damage to the outer, middle, or inner ear. Damage that occurs to the auditory pathway after the inner ear typically results in damage to the ability to process sounds. Higher auditory areas are involved in the interpretation of sounds.

21 The Hierarchy of Sound ProcessingFIGURE 6.9 Hierarchical organization of the human auditory cortex. Primary area A1 is responsive only to basic sounds and their modulation; surrounding areas become activated only by intelligible speech.

22 Sound Localization We can localize sounds that occur around us based on characteristics of the sound and interaural differences. Interaural timing differences are used to identify the location of a sharp, brief sound. Interaural phase differences are used to localize a continuous sound.

23 Sound Localization FIGURE 6.11 Interaural differences enable us to locate sounds. (a) Differences in the timing of when a sound reaches the two ears can be used to locate a sound source. Noise from a source at position A would strike the right ear first and, later, strike the left ear. The signal from the right ear would travel farther along the neurons, whereas the signal from the left ear would travel less far. These signals would combine by spatial summation at coincidence detector A in the olive, enabling you to locate the source of the sound. (b) Interaural differences in the phase of the sound wave are important for localizing the source of a continuous sound. For a lower-frequency sound, the sound is at a different phase of the wave when it strikes the left ear from when it strikes the right ear. This phase difference enables you to identify the location of the sound.

24 Balance The vestibular system, with three semicircular canals and two otolith organs, provides information about orientation. The semicircular canals detect head rotation and angular acceleration. The otolith organs detect linear acceleration. Both systems detect movement by the displacement of hair cells, similar to the auditory system.

25 Balance FIGURE 6.12 The vestibular system. Showing the relative locations of the otolith organs and the semicircular canals.

26 Balance FIGURE 6.13 Otolith organs detect acceleration by changes in the position of the otoliths. The otoliths are not connected to the vestibular system except by the hair cells. (a) You can see a person standing still and the otolith is centered over the hair cells. (b) The person has just started moving. The otolith is left behind by inertia, causing the hair cells to bend. This opens ion channels, just like in the auditory system, causing the cells to depolarize or hyperpolarize.

27 The Somatosensory SystemTouch Temperature Pain Proprioception Interoception The Somatosensory Pathway

28 Touch The somatosensory system tells us about the external world, where our limbs are in space, and about our internal world. Receptors are found all over our skin and within our internal organs.

29 Touch Touch receptors are mechanoreceptors, which respond to stretching or bending. Meissner’s corpuscles and Merkel’s disks are located close to the surface of the skin and have small receptive fields. Pacinian corpuscles and Ruffini’s endings are located deeper in the skin and have large receptive fields.

30 Touch FIGURE 6.14 Somatosensory receptors and their receptive fields. (a) There are a number of different somatosensory receptors within the skin. These include the Meissner’s corpuscles and Merkel’s disks near the surface and Ruffini’s endings and Pacinian corpuscles further below the surface. There are also warm and cool thermoreceptors and nociceptors. (b) The receptive fields of somatosensory neurons vary in size. Those receptors close to the surface have a small receptive field, whereas those farther from the surface have a larger receptive field.

31 Temperature Thermoreceptors are mechanoreceptors that convey temperature information. These receptors carry information about how the stimulus differs from the temperature of the skin. One population carries information about stimuli warmer than the skin and a separate population carries information about stimuli cooler than the skin.

32 Pain The perception of pain by nociceptors is necessary for our survival. There are three types of nociceptors. Mechanical nociceptors are activated by physical damage. Thermal nociceptors are activated by very high or very low temperatures. Chemical nociceptors are activated by particular chemicals.

33 Pain Some nociceptors are responsive to more than one type of stimuli and are called polymodal. Silent nociceptors respond to the body’s own chemical signals are can play a role in the increased sensitivity to stimulation following injury.

34 Pain Nociceptors appear to be free nerve endings.Different nociceptors transmit their signals at different rates. C fibers are small and unmyelinated, carrying the signal slowly. A delta fibers are myelinated and carry mechanical and thermal pain signals quickly.

35 Proprioception Proprioception is the sense of position and movement of our own body. Muscle spindles detect the length of the muscle and the speed of stretching. Golgi tendon organs provide information about muscle tension.

36 Proprioception FIGURE 6.15 The muscle spindle and Golgi tendon organ. (a) The muscle spindle, embedded in the body of the muscle, reports on muscle length and stretch. (b) The Golgi tendon organ is interwoven with the fibers of the tendon and reports on the amount of tension in the muscle.

37 Interoception Interoception is our ability to perceive the internal state of our body, such as hunger, thirst, and mood. Receptors include stretch receptors and nociceptors.

38 Interoception FIGURE 6.16 Brain areas involved in perceiving the emotional and cognitive components of pain.

39 Interoception Gate control theory describes why we sometimes notice pain and sometimes do not, depending on the situation. If information from the interoceptors arrives at the central nervous system at the same time as nociceptive information, this can overwhelm the CNS, and the pain signals are blocked.

40 The Somatosensory PathwayInformation from the head and face is carried by the trigeminal cranial nerve. Information from each dermatome of the body is carried into the dorsal horn of the spinal cord. Different types of somatosensory information follow different pathways to the brain.

41 The Somatosensory PathwayFIGURE 6.18 Somatosensory input from the body. (a) Somato-sensory input from the face travels over the three branches of the trigeminal nerve. (b) Somatosensory input from the rest of the body enters the spinal cord via the dorsal root ganglion. (c) The entire body is divided up into dermatomes, based on the receptive fields of the somatosensory neurons that enter each dorsal root ganglion.

42 The Somatosensory PathwaySomatosensory information decussates and is relayed by the thalamus to the primary somatosensory cortex (S1) S1 is in the parietal lobe, immediately posterior to the central sulcus. There is a somatotopic map of the body, known as the homunculus.

43 The Somatosensory PathwayFIGURE 6.19 Somatosensory pathway to the brain. (a) The pathway from the periphery to the primary somatosensory cortex, on the contralateral side of the brain. (b) The somatosensory homunculus, or somatotopic map, of the body within the primary somatosensory cortex.

44 Chemical Senses Taste Smell The Sense of Flavor Pheromones

45 Taste For both taste and smell, molecules bind to receptors and trigger the release of neurotransmitters. Taste receptors are found on the tongue, palate, pharynx, epiglottis, and esophagus. Taste cells are grouped into taste buds, which are grouped into papillae.

46 Taste FIGURE 6.20 Taste receptor cells on the tongue and around the mouth. Taste buds contain taste receptor cells and are found on the tongue, palate, pharanyx, epiglottis, and esophagus. The microvilli of the taste receptor cells contain receptors to detect chemical stimuli.

47 Taste There are currently five basic tastes: Sweet, Sour, Bitter, Salty, Umami Salty and sour trigger ionotropic receptors. Sweet, bitter, and umami mostly trigger metabotropic receptors, but activate some ionotropic receptors as well.

48 Taste Most taste receptors appear to have a preferred type of taste they respond to, but can respond to other tastes in high concentration. Gustatory afferent neurons relay taste information to the brainstem and on to the frontal operculum.

49 Taste FIGURE 6.22 Gustatory pathway from tongue to primary gustatory cortex. Information travels over cranial nerves VII, IX, and X to the brainstem and then on to the thalamus and the gustatory cortex.

50 Smell The olfactory epithelium is found in the back of the nasal cavity. Odorants dissolve in the mucus covering the epithelium and bind to receptors on the cilia of the olfactory receptor cells. Each olfactory receptor cell expresses only one type of receptor. Receptors are all GPCRs.

51 Smell FIGURE 6.23 The olfactory epithelium. Olfactory sensory neurons detect odorant molecules in the olfactory epithelium.

52 Smell Olfactory receptor cells project to the glomeruli in the olfactory bulb. FIGURE 6.24 The olfactory bulb. The olfactory sensory neurons project through the cribriform plate to the glomeruli, located within the olfactory bulb. Tufted and mitral cells surround the glomeruli within the olfactory bulb.

53 Smell Olfactory pathway travels from the olfactory bulb to the primary olfactory cortex in the rhinencephalon. FIGURE 6.25 The rhinencephalon. The olfactory bulb projects to the primary olfactory cortex, or rhinencephalon.

54 The Sense of Flavor Flavor is a combination of taste, smell, temperature, and texture. Higher-level processing within the brain evokes memories of past experiences.

55 Pheromones Pheromones are chemicals released to transmit information to and influence another member of the same species. Pheromones are detected by the vomeronasal organ. At this time, it is not clear to what extent humans use pheromones.

56 The Brain is MultisensorySynesthesia Combining Sensory Information The Binding Problem The Internal Model of the World

57 Synesthesia The brain needs to bring together information from different senses. Synesthesia is a condition where different senses are integrated inappropriately, such as when a letter is associated with a particular color. It appears to be due to heightened connectivity between brain regions.

58 Synesthesia FIGURE 6.26 Synesthesia. In this model of synesthesia, neural populations coding for graphemes (right inferior temporal gyrus) interconnect on those coding for colors (area V4). As a result of these connections, activity triggered by a grapheme inappropriately triggers V4 activity and hence a color experience.

59 Combining Sensory InformationMany neurons in the brain are more responsive when presented with more than one sensation at a time. In the McGurk effect, what a subject hears can be influenced by what they see at the same time.

60 Combining Sensory InformationFIGURE 6.27 Some brain areas show greater activity when stimulated by more than one sensory modality at a time. When a person both hears and sees a bell, the researchers showed that some areas prefer auditory stimulation (blue), some areas prefer visual stimulation (pink), and some areas respond preferentially to both stimuli at the same time (green).

61 The Binding Problem We do not perceive a stimulus as having separate visual and auditory components, but as a unified stimulus. How the sensory signals are integrated is known as the binding problem. Recurrent connections between the sensory systems likely are important for solving the binding problem.

62 The Binding Problem FIGURE 6.29 The binding problem. (a) Although it’s easy to imagine signals climbing up an assembly line of processing, (b) the real anatomical fabric is characterized by interconnections between areas.

63 The Internal Model of the WorldOur final perception does not rely only on stimuli from the outside world, but also on expectations from past experiences. In anosognosia, a patient is apparently unaware of their own physical limitations.

64 Time Perception Time involves input from all of the sensory systems.Time perception is a construct of the brain. Time appears to slow down in crucial situations because the amygdala encodes memories more thoroughly. These more dense memories make it seem like the event took longer.