Walker Group

We detect sounds with our ears, but it is the brain that transforms these pressure waves into the rich and meaningful perceptual world that we experience. The Walker Group investigates how neural activity in the auditory cortex enables humans and other animals to identify and locate sounds in their environment. Our research has provided new insights into how patterns of action potentials across time and across neural populations contribute to this complex process.

CURRENT RESEARCH QUESTIONS

How do neurons in the auditory cortex represent pitch? 
Pitch is our perception of the tonal quality of sounds. It allows us to recognise a melody across different instruments, and to interpret a person’s “tone of voice”. Pitch is far more complex than hearing the frequency of a pure tone, as it allows us to experience a sound composed of many frequency components as a single note. Interestingly, the brain seems to derive this unitary pitch percept through two parallel mechanisms – one that computes the sound’s harmonic structure, and another its temporal periodicity. Our behavioural studies compare how people and animals hear pitch, while our microelectrode studies are uncovering the neural mechanisms for pitch extraction.

How does the brain attend to one sound in a crowded acoustical environment? 
To have a conversation with a friend at a busy coffee shop, your brain must disentangle soundwaves entering your ear from multiple sources (e.g. music, other people, coffee grinders) and selectively attend to your friend’s voice. This challenging task becomes increasingly difficult with ageing and even mild hearing loss, and current treatments such as hearing aids and cochlear implants are often ineffective. We are investigating how pitch cues support this selective listening process in humans across the lifespan using psychophysical tasks and EEG. Parallel animal studies will examine how pitch guides selective attention at the level of individual neurons.

How do our expectations about sounds affect what we hear? 
Our brains are constantly monitoring which sounds are most likely to occur in our current environment, and these statistics affect how we experience sound. In collaboration with colleagues in labs across the UK and USA, we are investigating how expectations influence the detection of quiet sounds in noisy environments. This international project combines psychophysics (Holt, U. Texas), EEG (Lalor, U Rochester), fMRI (Dick, UCL), and intracranial EEG (Abel, U Pittsburgh) in human participants with our studies of ferret behaviour and electrophysiology.

How do we hear in reverberant environments? 
When a person speaks, their emitted sound waves reflect off objects, walls and floors, so that our ear receives multiple delayed and attenuated copies of their voice. This reverberation helps us recognize our surroundings and judge distance, but can also make speech more difficult to understand. Our previous work has shown that neurons in auditory cortex adapt to the reverberation of new environments within seconds, improving representation of the original sound source. We are further investigating the neural mechanisms of reverberation adaptation, and how these processes change with ageing.

EXPERIMENTAL APPROACHES

Our lab employs a range of complementary techniques to address these questions. The precise spiking responses of individual neurons and small groups of neurons are recorded in both anaesthetised and awake, behaving animals using Neuropixels microelectrodes. The activity of genetically defined populations of neurons are measured using in vivo two-photon imaging, providing a dynamic view of neural circuit function. Computational models are used to refine our predictions about brain function, and to better interpret our experimental results. Finally, psychophysical experiments measure the listening performance of humans and animals, linking neural activity to perception.

Graduate Student Opportunities

The Walker Group welcomes talented and motivated graduate students who are interested in understanding how the brain processes sound. Prospective DPhil students are required to secure their own tuition funding, either through University of Oxford scholarships or external sources. Master’s students from the MSc Neuroscience, MBiomedSci, and related programmes at Oxford are also very welcome to join the group for research projects.

Undergraduate Student Opportunities 

Dr Kerry Walker has supervised >40 FHS undergraduate dissertation students at the University of Oxford, including those studying Biomedical Sciences, Pre-Clinical Medicine, and visiting scholars from other universities. New students are encouraged to apply to join the group for their dissertation projects.