Tfy-99.2710 Introduction to the Structure and Operation of the Human Brain, fall 2015 Exercise 5
1. The auditory system a.) What is the difference between conduction deafness and nerve deafness?
Conduction deafness is caused by a disturbance in the conductive path between the outer ear and the cochlea, e.g., blocked ear canal, ruptured of otherwise damaged tympanic membrane, stiffening or other pathological conditions of the ossicular chain. Most causes of conduction deafness can be treated surgically in comparatively minor procedures.
Nerve deafness is a result of damages at the neural level. This can mean the loss of neurons in the auditory nerve or hair cells in the cochlea. Common causes include exposure to high sound pressure levels over time (noise induced hearing loss), tumors affecting the inner ear and certain pharmaceutical products (quinine, some antibiotics.) The treatment of nerve deafness is less straightforward than in the case of conduction deafness; different treatments however are available, depending on the severity of the neural damage. In the case of partial hair cell loss, a hearing aid can be used to amplify the sound arriving at the middle ear. If the neural damage is bilateral and the auditory nerve is not damaged, cochlear implants can be used to make up for the absence of haircells. b.) Describe possible reasons, symptoms, and treatments for tinnitus.
Tinnitus can be a sympton of a number of neurological problems. Common causes include diseases of the inner ear and the auditory nerve. In addition, exposure to very high sound pressure levels, certain pharmaceutical products and aging can result in tinnitus. The earlier views held thought that the cause of tinnitus resided in the cochlea; more recent views see the condition as a result of pathologies in the central nervous system. The auditory sensation of tinnitus is thought to be due to changes in the central neural auditory structures such as the auditory cortex. Cochlear damage can result in changes in brain functioning such as downregulation of synaptic inhibition in the auditory areas, thus causing hyperactivity in the auditory neurons resulting in phantom auditory activity. Good tinnitus treatments should aim to reduce this hyperactivity and thus supress the tinnitus.
A number of possible treatment methods for tinnitus have been proposed and applied in practice. Many treatments are however designed to merely manage tinnitus rather than abolish it completely. Some treatment methods include the use of hearing aids, various forms of sound therapy, pharmacotherapy and brain stimulation (Transcranial direct current stimulation (TdCS) has been used mainly for the treatment of depression, but shows signs of being a promising treatment for tinnitus as well). Below I discuss different forms sound therapy in brief.
The most common form of sound therapy treatment for tinnitus makes use of the phenomenon of auditory masking, wherein the presence of one acoustic stimulus raises the threshold of audibility of another. In the case of tinnitus treatment, the idea is to present the patient with a continuous acoustic stimulus, that artificially raises the noise floor of the auditory system to a level high enough to swamp the tinnitus sensation. This stimulus can be presented via speakers placed around the living space of via headphones. Due to its simple and straightforward concept, several commercial products intended for this function have been commercially available for several years. A more interesting case of sound therapy is tailor-made notched music therapy (TMNMT). TMNMT utilizes brain plasticity and lateral inhibition (”contrast enhancement” applied to the spectrum of the auditory stimulus) of the neurons affected by the tinnitus. The basic idea is to first map out the frquency range of the patients tinnitus and design the treatment according to this subjective information (tailor-made). The treatment comprises of the patient listening to music that Tfy-99.2710 Introduction to the Structure and Operation of the Human Brain, fall 2015 Exercise 5 has been passed through a band-stop filter centered at the tinnitus frequency. The activity of neurons affected by tinnitus is inhibited by the neighboring neurons. Due to the neuronal plasticity of the brain, long term exposure to auditory stimuli filtered in this manner can decrease the activity in the hyper active neurons.
Illustration 1: Example of a STFT of TMNMT stimulus. The blue bars denote the energy in the FFT-bins of the treatment stimulus at the example time frame. The Orange bars denote the activity in the frequencies affected by the tinnitus. The notch in the stimulus spectrum leads to the neurons corresponding to the tinnitus frequencies being laterally inhibited by the relatively large activity of the neighboring neurons.
2. The somatosensory system a.) What kind of sensory receptors exist?
The somatosensory receptors of the human body are various, and distributed throughout the body. Most of the receptors in the somatic sensory system are mechanoreceptors sensitive to physical distortion e.g., bending and stretching. These are present throughout the body and responsible for monitoring skin contact, pressure in the heart and blood vesseld, stretching of the digestive organs and urinary bladder as well as force against the teeth. Several types of mechanoreceptors have been identified. These include: Merkel's disks, free nerve endings, meissners corpuscles, hair follicle receptors, Pacinian corpuscles and Ruffini's endings. These receptors respond to different kinds of physical stimuli with receptive fields of various sizes; the receptive fields of Pacinian corpuscles for example are much larger than those of Meissner's corpuscles, as shown on the right hand side of the figure below. Tfy-99.2710 Introduction to the Structure and Operation of the Human Brain, fall 2015 Exercise 5
What properties doe they have and how do these properties affect the type of information gathered and the way the brain processes this information? b.) What kind of sensory prostheses exist?
Several sensory prostheses employ the concept of using electrode arrays to electrically stimulate the nerves corresponding to a given sensory modality.
Cochlear implants. Auditory brainstem implants. Retinal prostheses. Spinal cord stimulators. Bladder control implants. Motor prostheses.
An interesting case of a sensory prostheses are those employing sensory substitution. In sensory substitution, the characteristics of one sensory modality are transformed into stimuli of another sensory modality auditory displays are probably one of the most common examples of these devices. A more esoteric example of such a prosthesis is the BrainPort. The device makes use of the large number of neurons dedicated to processing the somatosensory information in the tongue to allow blind individuals to see. The device consists of an a crude video camera mounted on glasses, a processor and an electrode array placed on the tongue. The processor maps the video captured by the glasses into a low resolution ”somatosensory image” presented on the ”pixels” of the tongue electrode array. The visual information is represented as electrical pulses varying in intensity, duration and location on the electrode array. This information is then picked up and processed by the highly sensitive receptors of the tongue. Similar
Describe the function of at least one sensory prosthesis.
Cochlear implants can provide a crude sense of hearing if the auditory pathway is intact beyond the cochlea. The core element of the prosthesis is an electrode array inserted into the (damaged) cochlea, that is used to artificially stimulate the auditory nerve. The signal pathway of a generic electrode array implant is shown in the figure below. An acoustic stimulis is picked up by a microphone intergrated into the prosthesis. This signal is then processed in frequency bands corresponding to those of human hearing (critical bands or ERBs), and the activity in each band is transmitted to the corresponding electrode in the cochlear array. The electrode array is arranged so that the individual electrodes are arranged so that the individual electrodes conform the tonotopic Tfy-99.2710 Introduction to the Structure and Operation of the Human Brain, fall 2015 Exercise 5 mapping of the basilar membrane. The processor analyzes the incoming acoustic stimulus and decides which electrodes to activate. High frequencies correspond to electrodes at the base of the cohclea and conversely low frequencies activate the apical electrodes.
Obviously the frequency resolution achievable by the electrode array is limited by both the number of electrodes in the array and their size. Ideally, at least one electrode should be available per critical band. However, studies have shown that basic speech comprehension can be achieved with as few as 8 electrodes placed at the critical bands of human hearing corresponding to the meaningful frequencies of human speech (formant frequencies). Tfy-99.2710 Introduction to the Structure and Operation of the Human Brain, fall 2015 Exercise 5
What kind of sensory prostheses could exist in the near future?
Touch sensitive prostheses, e.,g., prosthetic limbs that are not only controlled by the CNS but provide some form of tactile feedback, pain etc.
Self-charging implants fuelled by bioenergy.
Controlling devices with neural signals directly without an interface involving motor movements, e.g., control a computer without a mouse and keyboard etc.