The Eye & Ear Foundation’s July 27th webinar, “Hearing with the Brain” was presented by Philip Perez, MD, Assistant Professor in the Department of Otolaryngology specializing in Otology and Neurotology and Ross Williamson, PhD, Assistant Professor of Otolaryngology, Neurobiology, and Bioengineering.
What is a Cochlear Implant?
Dr. Perez started things off by describing a cochlear implant (CI). He called it an incredible piece of technology that really improves the lives of hundreds of thousands of people worldwide and one of the reasons he went into this field.
There are a few different versions of CIs. The typical one has a part behind the ear that looks kind of like a large hearing aid. It encompasses a microphone that receives sound and is a battery pack powering device that transmits to the coil that sits on a magnet behind the head. In turn, this makes contact with a magnet part of the device that is planted underneath the skin. The internal receiver sits underneath the scalp and the area behind the ear, with an electrode placed in the cochlea. The particular array shown has 22 different contact points, making contact with the cochlea to directly stimulate nerve endings that are sent to the intact auditory nerve.
Cochlear implants are discussed with patients when hearing loss is so poor that even the best hearing aids are not compensating. When there is nerve damage in the cochlea, it no longer can create usable signals sent to the auditory nerve. In most cases, however, the auditory nerve is still functional as well as the upstream circuits. “It is incredibly lucky that this works, that the brain receives the signals and is receptive to a plastic change that takes electrical pulses and turns them into something that’s meaningful hearing,” Dr. Perez said.
History of the CI
- 1957, Paris: Djourno and Eyries place electrode on stump of auditory nerve; patient can perceive sounds, not speech, for several weeks
- 1961, L.A.: Bill House implants several patients with 5-electrode wire in cochlea; sound awareness and some speech perception, but issues with device rejection and infection
- 1970s, SF: Michelson and Merzenich implant several patients with single=electrode implant usable outside of lab, and develop multi-electrode system
- Scientific and medical communities remain very skeptical of CI
- 1976: NIH commissions study at University of Pittsburgh led by Robert Bilger, PhD; improved speech understanding and quality of life for 13 subjects implanted in the US
- 1980: first children are implanted
- 1984: FDA approves 3M House Cochlear implant
- 1990s and 2000s: multi-channel devices with improved sound processing
Cochlear Implant Clinical Landscape
- Three companies with approved devices in US
- As of December 2019, about 736,900 registered devices have been implanted worldwide. In the US, about 118,100 devices implanted in adults and 65,000 in children
- At UPMC, one adult CI surgery a week (by Dr. Perez & Dr. Andy McCall)
- Three dedicated CI audiologists – help patients learn to hear, train the brain
- Expanding program to streamline access to CI across Western PA
- Continued broadening of candidacy for CI
- Early indications: adults deaf in both ears
- Current indications: (FDA expanded in 2019)
- Children at age 6 months
- Adults with some residual hearing, but poor speech understanding
- Single-sided deafness/asymmetric hearing
- Hearing preservation, sound localization, tinnitus suppression, music appreciation
Hearing Loss and Cognition
- Aging population with increasing prevalence of age-related hearing loss
- Hearing loss independently associated with negative outcomes in social, emotional, physical, and cognitive function
- Increasing research in CI for patients in their 70s, 80s, 90s show similar benefits to CI in younger patients
- Overall safe surgery with low complication rate
- Older adults are satisfied with their devices and 80% use device for more than 10 years (Choi et al 2014)
- Increasing age of implantation decreases speech recognition scores (Lin et al 2012)
- 2017 report from the Lancet Commission on Dementia Prevention, Intervention, and Care: up to 35% of dementia cases are potentially preventable, with hearing loss considered the largest modifiable risk factor
- Prospective longitudinal studies have identified improvements in attention, processing speed, and working memory after cochlear implantation
- When to implant?
The Brain’s Importance for Our Perception of the Sensory Environment
Sensory-guided behavior is necessarily a brain-wide endeavor, Dr. Williamson said. His lab is particularly interested in understanding how we use sounds to interact with the world around us. If you drive along the highway on your way to work and hear a police car behind you, the auditory information hitting your ears will result in a series of behavioral outcomes (whether to pull over or hit the gas, an emotional response, etc.).
From a neural perspective, the auditory system is not a hub of motor action or emotion. “If we want to understand how we use sensory information to interact with the world around us,” Dr. Williamson said, “we really need to understand how sensory information is propagated throughout the entire brain. We need to consider not the individual brain areas in isolation, but how everything is connected.”
Sound
Auditory brainstem neurons faithfully represent incoming sound. Sound transduced at the level of the cochlea jumps through multiple synapses, before reaching the primary auditory cortex. Sound consists of a single frequency and loudness of that sound is varying as a function of time. “If we look at the discharge patterns of a neuron in the auditory nerve, right next to the cochlea, you can see that the neuron is firing very reliably to the fine structure of this sound,” Dr. Williamson said.
Sound representations are reformatted at higher auditory stations. Cortical representations are not temporally precise and provide a rate-based abstraction of the source signal. One reason for this is modulation.
Philosophers have been thinking about modulation for millennia, Dr. Williamson said. He shared a couple of quotes:
“…persons do not perceive what is brought before their eyes, if they are the time deep in thought, or in a fright, or listening to some loud noise.” – Aristotle (350 BC)
“The soul can prevent itself from hearing a slight noise or feeling a slight pain by attending very closely to some other thing…” – Descartes (1649)
Pupil Diameter
Pupil diameter is a proxy for a behavioral state. Dr. Williamson showed a video of a cat following bubbles. At the start, the cat’s pupils are very small and constricted, but as bubbles appear, the diameter changes as the cat directs focus to bubbles around him.
From a biological perspective, pupil diameter is a direct readout of different neuromodulatory systems within the brain. Changes in pupil diameter reflect changes in cortical responses. Dr. Williamson showed an example of a recording session, in which the pupil diameter directly correlates with the discharge patterns of a representative cortical neuron.
Transmitting Information
Descending systems transmit information throughout the brain. The auditory cortex transmits sensory information to many locations in the brain. Some of these locations are not traditional “auditory” areas, and are heavily involved in emotional processing (amygdala) and reward (striatum).
“Modern technologies allow us to monitor and manipulate thousands of neurons simultaneously in awake behaving animals,” Dr. Williamson said, as he shared some experiments his lab is working on that focus on identifying neural circuit mechanisms that underlie sensory-guided behaviors.
Many of the neural circuits that govern complex behavior also underlie perceptual disorders such as the perception of sounds that do not exist (tinnitus or schizophrenia) or the irrepressible awareness of unwanted or distracting sounds (attention deficit hyperactivity disorder)
Basic science asks how neural circuits in the brain are organized and how they function. “We need to understand how neural circuits govern perception in order to understand what happens when they break,” Dr. Williamson said. This kind of research will lead to new insights into which neural circuits could be targeted to maximize therapeutic benefit and to guide therapeutic plasticity by reinforcing healthy patterns of activity to repair function of damaged cells.
“We only hear what the brain tells us to hear,” he added.