Professor Tom Freeman - BSc PhD Birm
Images move mainly because we do. Simple actions like rotating the eye or head make stationary objects sweep across the retina, while in hearing, head movements cause dynamic changes in auditory cues to location. Our own movements therefore create some very basic problems for the perceptual brain – how do the vision and hearing know whether we are moving, objects are moving, or both? One solution the brain adopts is to use signals from the motor system to help interpret incoming sensory information during self-movement.
My work investigates the fundamental mechanisms that drive this process, using a variety of techniques. These include measuring how well we control eye and head movements, how successful we are at judging moving objects that are seen and heard, and which regions of the brain are active as we do so. The findings help test and develop key theories in perception, in particular how sensory evidence and prior knowledge are integrated. Some of the results are applied to various clinical conditions, such as schizophrenia, nystagmus and Huntington’s disease.
Levels 1 and 2:
I give a variety of tutorials as Levels 1 and 2 on perception, cognition, developmental and abnormal psychology (supporting PS1014, PS2003, PS2008, PS2009). At Level 2, I teach one half of the Perception module (PS2009), concentrating on ‘The Other senses’. I also run practicals in Perception.
I teach and coordinate the ‘Vision and Action’ module, where we examine our current understanding of visual perception during motor activity and how visual perception helps support self-motion. I supervise projects on a number of topics in perception.
Selected publications (2014 onwards)
Full list of publications
Research topics and related papers
1. Combining prior expectations and uncertainty to explain motion illusions during pursuit.Your browser does not support the video tag. As the pictures of Charlie above demonstrate, smooth eye pursuit adds motion to the retinal image – movement on the retina doesn’t ‘line-up’ with the motion of objects in the world. Hence, when we track Charlie (right picture), the cupboard moves in the image, and he is more or less stationary. One way the visual brain solves this problem is to add estimates of eye velocity to estimates of image motion, which gives an estimate of the ‘real’ motion. This process doesn’t always work accurately - observers often misperceive velocity when they move their eyes. Thus, pursued stimuli appear slower (Aubert-Fleishl phenomenon), stationary objects appear to move (Filehne illusion), the perceived direction of objects moving on a different trajectory to the eye is distorted and self-motion veers away from its true path (e.g. the slalom illusion). Each of these illusions demonstrate that eye speed is often underestimated with respect to image speed, a finding that many authors have taken as evidence of early sensory signals that differ in accuracy. In this project we tested an alternative, based on the idea that perceptual estimates are increasingly influenced by prior expectations when motion signals become more uncertain. Most objects are stationary or move slowly; hence the visual system’s prior expectation is a distribution that peaks at 0 (represented by the black line in the movie above). Motion signals are represented by the red line, with their uncertainty (= precision) captured by the width of the distribution shown. Perceived speed is based the combination of the two (they are multiplied together according to Bayes rule to yield the blue distribution). As motion signals become more uncertain, the blue distribution moves towards the prior. Hence perceived speed slows down.
Freeman, T. C. A., Champion, R. A. and Warren, P. A. (2010). A Bayesian model of perceived head-centered velocity during smooth pursuit eye movement. Current Biology, 20(8), 757-762. (10.1016/j.cub.2010.02.059)
2. Auditory compensation for head rotation
A fundamental yet almost entirely overlooked problem in hearing is how auditory motion cues that occur ‘at the ears’ are interpreted when the head moves. The motivation for this project is based on the idea that the world is largely stationary, containing only a few moving objects, while the head continually moves (just as the eyes do in vision – see above). At the ear, therefore, the auditory cues to motion are dominated by self-motion not object motion. A considerable amount is known about how retinal image motion is integrated with ‘extra-retinal signals’ that are based on motor and vestibular system activity and provide information about eye and head movement. Continuing the analogy, we are currently exploring whether the auditory system uses equivalent ‘extra-cochlear’ information to interpret dynamic changes in acoustic images.
To explore this issue, we use a technique based on linking moving sounds with real-time measurements of head rotation. The technique allows us to determine a simple, yet fundamental, assessment of auditory motion perception, namely the degree to which a sound must rotate around the listener in external space in order to appear stationary. Like vision, we discover that hearing is able to compensate for this type of self-movement quite well, but there is a persistent error that we provide evidence for over the course of two experiments. This perceptual error is the auditory analogue of the Filehne illusion in vision, first described almost 100 years ago, in which a static object appears to move against a smooth pursuit eye movement. We are currently investigating whether Bayesian models like that described above could account for the auditory Filehne illusion.
In collaboration with John Culling (Cardiff), Owen Brimijoin (MRC Institute of Hearing, Glasgow Section) and Michael Akeroyd (MRC Institute of Hearing, Nottingham).
3. Oculomotor control: precision and accuracy as a function of age
Click here for an interview with Tom Freeman on this and related ageing projects
We know that as we grow older, the accuracy of our tracking eye movements decreases. However, little is know about how the precision (variability) of eye movements in either young or older observers. In this project we developed a new analysis that allowed us to account for two types of precision – short-duration ‘shake’ and longer-duration ‘drift’. We found that older observers were less precise at faster eye speeds. We also found that the way the two precision measures depended on speed differed with the type of eye movement our observers made. When they made reflexive eye movements, shake was largely independent of speed and resembled the variability measured during fixation. When they made deliberate eye movements, both shake and drift increased with speed. This suggests that the two different types of eye movements do not share the same noise source.
Kolarik, A.J., Margrain, T.H., & Freeman, T.C.A. (2010). Precision and accuracy of ocular following: Influence of age and type of eye movement. Experimental Brain Research. 201, 271-282.
Funded by BBSRC/EPSRC ‘Strategic Promotion of Ageing Research Capacity’ (SPARC) initiative
Wellcome ISSF (2016), £38,601
K Singh, J Walters, T Freeman & J Zhang “Neurophysiologically- informed models and machine learning classification of task-driven and
resting state oscillatory dynamics in schizophrenia”.
Fight for Sight / Nystagmus network (2013), £14,350
J Erichsen & T Freeman “Understanding the basis for oscillopsia in nystagmus to provide a basis for treatment”
JE Williams Endowment studentship, £ 55,235
J Erichsen & T Freeman “Evaluating eye movements as biomarkers for monitoring the progression of Huntington’s Disease to facilitate early
intervention and clinical management.”
Wellcome ISSF (2013), £12,152
P Sumner, S.K. Rushton, T.C.A. Freeman “Improving rehabilitation of visual vertigo through understanding the visual triggers”
MRC (2013), £924,429
K Singh, T.C.A. Freeman, J Walters, L Wilkinson “Defining the disturbances in cortical glutamate and GABA function in psychosis, its origins and consequences”
Royal Society International Travel Grant (2010), £4000
T.C.A. Freeman “Auditory and audio-visual motion perception during eye movement and head rotation”
Wellcome Trust (2007), £135, 799
T.C.A. Freeman “Visual motion sensitivity during eye movement: Investigating the interaction between retinal and extra-retinal noise”
BBSRC/EPSRC SPARC initiative (2006), £37,230
T.C.A. Freeman & T.H. Margrain“Age, eye movement and motion perception”
John Culling (active hearing)
Krish Singh (schizophrenia, GABA/gamma and orientation perception; brain imaging and active motion perception)
Petroc Sumner (OKN, smooth pursuit and saccades; infantile nystagmus)
Owen Brimijoin (MRC Institute of Hearing, Scottish section), Michael Akeroyd (MRC. Institute of Hearing, Nottingham)
David Alais, Simon Carlile (University of Sydney): Auditory motion perception
Rebecca Champion / Paul Warren (University of Manchester): Bayesian models of head-centred motion perception
Jon Erichsen (infantile nystagmus, Huntington’s disease)
Tom Margrain (age, eye movement and motion sensitivity)
Postgraduate research interests
I am happy to discuss PhD projects in any area of psychophysics and/or motor control, especially those that relate visual performance with motor activity. Current PG projects include: age, eye movement and motion sensitivity; interaction between retinal and extra-retinal motion afterefffects; extra-retinal signatures in MEG and fMRI.
If you are interested in applying for a PhD, or for further information regarding my postgraduate research, please contact me directly (contact details available on the 'Overview' page), or submit a formal application here.
Laura Whitlow, Graduated 2015
James Harrison, Graduated 2014
Ben Dunkley, Graduated 2012
Emer O’Connor, Graduated 2011
Rhys Davies, Graduated 2011
Joni Karanka, Graduated 2009
Rhodri Woodhouse Graduated 2009
Jenny Naji, Graduated 2004
Alastair Barrowcliff, Graduated 2003
1984-1987 BSc Hons (2i), School of Psychology, University of Birmingham
1987-1990 Doctoral degree supervised by Dr M.G. Harris, School of Psychology, University of Birmingham
External panel member, Liverpool Hope University Psychology Dept. Review, 2014
Expert reviewer for Bournemouth University mock-REF, School of Psychology, 2013
EPSRC peer-review college (2006-present day).
2012 – present day Professor, School of Psychology, Cardiff University
2008-2012: Reader in Psychology, School of Psychology, Cardiff University
2003 - 2008: Senior Lecturer, School of Psychology, Cardiff University
1999 - 2003: Lecturer, School of Psychology, Cardiff University
1997 - 1999: Fixed-term lecturer, School of Psychology, Cardiff University
1995 - 1997: Research Fellow with Prof. M. S. Banks, School of Optometry, University of California, Berkeley
1992 - 1995: Research Fellow with Prof. M. A. Georgeson, Department of Vision Sciences, Aston University
1991 - 1992: Lecturer, School of Psychology, University of Birmingham
1990 - 1991: Research Associate with Prof. G.W. Humphreys, School of Psychology, University of Birmingham