Our senses tether the supercomputer inside our heads (our brains) to the outside world. Through them, we experience physical phenomenon (sounds, sights, smells, tastes, textures and pain) that shape the experiences we have into memories over time.

Our lab is interested in how our brains encode sound. The auditory system is always ‘on’, detecting changes in pressure over multiple orders of magnitude. Some sounds alarm us of danger, and others fill us with joy. Depending on the context, some sounds do both (compare fireworks with explosions for example). We react to sounds in ~150ms, about as fast as a blink of the eye. We discriminate between frequencies of sine waves with extreme precision; adjacent keys on a piano, for example, represent a change in frequency at a ratio of 1.059.

Our organ of hearing, the cochlea, is a biological marvel of engineering. Exquisitely organized to receive complex pressure waves and extract necessary features – frequency, loudness and timing – with precision and fidelity.  The cochlea is not a passive detector however, it is an active machine kitted with a battery (the stria vascularis), and amplifier (outer hair cells) to support activation of the sensory receptors (inner hair cells). 

Chemicals known as neurotransmitters released from the hair cells activate the first neurons of the auditory pathway (spiral ganglion cells), which together form the auditory nerve.  From here, the central auditory system takes over.  Afferent (meaning ‘towards’ the brain) pathways use specialized anatomical, cellular and physiological mechanisms at brainstem, midbrain and cortical processing stations – breaking down and re-building acoustic signals to convey meaning. 

Auditory perception is not simply hierarchical ‘bottom-up’ processing.  Complex ‘top-down’ feedback loops and pathways connect the cortex directly to the cochlea via efferent (meaning ‘away from’ the brain) pathways.  Efferent neurons enable the brain to directly attenuate sound coming into the cochlea, modulating both the gain control mechanisms afforded by the mechanosensory outer hair cells and directly altering the excitability of spiral ganglion neurons.

Because these efferent neurons effectively apply the brakes to the cochlea’s in-built amplification system, they possess the ability to curtail acoustic information received by the brain. Since we can’t perceive what we don’t detect, it is important to understand the conditions under which these neurons are activated, and how they may be modulated to protect our ears from loud sounds, or provide contextual gain to salient stimuli.

The Descending Auditory Circuits (DAC) Laboratory is interested in understanding:
(1) how descending efferent circuits contribute to the encoding of sound information in the brain
(2) how descending activity is shaped by neuromodulatory molecules such as serotonin, noradrenalin and dopamine (which play important roles in regulating mood)
(3) how such neuromodulatory activity in turn contributes to functional auditory perception