Professor Tom Freeman

BSc PhD Birm

Publication

• Date
• Type

Research

Research topics and related papers

1. Combining prior expectations and uncertainty to explain motion illusions during pursuit.

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).

Freeman, T.C.A., Culling, J.F., Akeroyd, M.A. & Brimijoin, W.O. Auditory compensation for head rotation is incomplete. Journal of Experimental Psychology: Human Perception and Performance 43, 371-380, (2017).

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

Recent Funding

Leverhulme Trust (2019) £239,204
T Freeman, J Culling “Active audiovisual perception: Listening and looking while moving”.

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”

Research group

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)

Research collaborators

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

Marc Ernst / Jan Souman (Max Planck Institute, Tubingen): Motion perception during walking

Jon Erichsen (infantile nystagmus, Huntington’s disease)

Tom Margrain (age, eye movement and motion sensitivity)

Biography

Undergraduate education

1984-1987 BSc Hons (2i), School of Psychology, University of Birmingham

Postgraduate education

1987-1990 Doctoral degree supervised by Dr M.G. Harris, School of Psychology,   University of Birmingham

Employment

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

Honours and awards

Awards/external committees

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).

Editor, Perception

Supervisions

Postgraduate research interests

I am happy to discuss PhD projects in any area of psychophysics and/or motor control, especially those that relate vision, hearing and self-motion. Current projects in the lab include: auditory and visual compensation for head and eye movement; audiovisual integration; Bayesian models of motion perception (with or without self-motion!).

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.

Students

Current

Joshua Haynes

Pengyuan Wang

Recent Alumni

Joshua Stevenson-Hoare

Lee McIlreavy

James Brawn