The neural basis of response time variability

Lead Research Organisation: University of Bristol
Department Name: Experimental Psychology


Response times are highly variable. For example, when driving the onset of a red traffic light can lead to large variability in the time to press the brake even when all other factors such as alertness, vigilance, visibility etc. are constant. Such response variability is still present in highly controlled laboratory conditions. The origins of this variability remains a mystery, but understanding it is a central question for scientists with an interest in understanding the relation between brain and behaviour, and - as the example above illustrates - is relevant in a number of applied contexts. A comprehensive account of response time variability should explain at a functional level where the variability originates and determine the neural processes and anatomical structures involved. In my research group we focus on the study of the eye movement response. Eye movements are interesting for a number of reasons. First, they are important for visual perception: we only see fine detailed information when the eyes point directly at a location. Second, eye movements are ubiquitous: we make more eye movements in a day than heart beats. Third, we have a detailed knowledge of the neurophysiology of eye movements control in non-human primates which allows us to link functional and neural explanations. For the last ten years the major focus of my research has been to identify functional explanations for eye movement responses using mathematical models and behavioural experiments. The next major step for this research is to investigate the brain processes that account for this variability. Functional Magnetic Radiation Imaging (fMRI) and Magnetoencephalography (MEG) are both methods for imaging the human brain while participants carry out a task. This allows the brain areas involved in the task to be identified. This Fellowship will allow me to introduce these methods to my research to study the neural basis of response time variability.

Technical Summary

The project will use the human saccadic eye movement response to investigate the more general problem of response time variability. Our previous work has established the importance of the visual stimulus and the nature of the response required in determining response time. Alongside this we have used mathematical models to explain changes in response time in terms of changes in threshold and accumulation rate of the decision variable to respond (c.f. Carpenter & Williams, 1995). The next step in this research is to use brain imaging (MEG and fMRI in this case) to investigate in more depth the neural basis of response time variability. A central first question for the development of models of choice and reaction time variability is understanding the origins of changes in response time when the stimulus and the response are held constant. The variability, which must be internally generated, cannot be investigated used standard behavioural methods which infer properties of the systems by changing either the stimulus or response. Equally the computational models in this area do not specify directly where this variability comes from and understanding the neural substrate of this variability will provide a significant constraint on these models. Pilot work, using MEG, collected in preparation for this application showed that the state of visual cortex in the pre-target period plays a major role in determining response latency. This leads to a set of questions that will be addressed in this application using MEG and fMRI: Why is there decreased Gamma activity for fast saccades in the pre target interval? Do the small changes in the visual response to target really reflect the differences between fast and slow saccades? Do the differences in visual response for fast saccades match responses to more visually salient targets?

Planned Impact

The work outlined in this application addresses a fundamental question: what is the basis of response time variability. In humans in particular, answering this question has extremely wide ranging implications within and outside the academic community. My research group has always engaged a range of non-academic stake holders. Three currently active research projects that apply the research developed in this fellowship illustrate this. Our work recording and modelling eye movement behaviour of CCTV operators has Manchester City Council as research partner. Our project on natural dynamic scenes and human vision is jointly funded by EPSRC and The Defence Science and Technology Laboratory (Dstl). The relationship with Dstl and the Vision Group at the University of Bristol is a long established one and provides a mechanism by which our research has a direct effect on government. We are developing a working model of human vision which includes movements of the eyes. The project is funded by QinetiQ Group PLC which ensures that our work will have an impact within the commercial sector. My group will continue to work in this manner with local government, central government and the commercial sector to ensure the impact of this new research. The new Clinical Research and Imaging Centre (CRIC) project in Bristol will bring a wide range of clinical staff from the NHS into contact with basic researchers to develop new solutions for the care of patients. My ongoing involvement in CRIC will provide a structured opportunity for this work to have a potential impact on patient care within the NHS. I am a member of the Applied Vision Association (AVA) and Bristol Vision Institute (BVI) which bring together researchers from Universities, Industry and Government for regular conferences to exchange ideas and new research. Alongside a wide range of public engagement work on a more ad-hoc basis, for a number of years I have been working with an artist and staging art exhibitions in which the art is inspired and often produced by eye movement data. These exhibitions are always accompanied by a number of joint talks in which I discuss the science in detail to members of the public. I have a strong track record and continuing commitment to ensuring that my work has an impact outside the academic community. The structures and relationships for this to continue are in place and I will, of course, add to this portfolio of contacts as this project progresses.


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Description In this grant we focused on understanding the neural basis of response time variability. This is a fundamental biological phenomenal that occurs across the whole animal kingdom: variability between individual behavioural response times, even in identical conditions is ubiquitous. In this grant we carried out two set of linked studies that used MEG and fMRI to investigate this core question.
In a first series we used magnetoencephalography (MEG) and a correlational source reconstruction approach to focus on the state of the perception-action brain network in the period before the response evoking stimulus to investigate the influence of this brain state on subsequent response time in humans. We used a saccadic eye movement response as these are a well characterised ballistic response for which we have a good knowledge of the underlying neural pathways that control the response. The MEG signal allows us to extract both the power and phase of the oscillating neural responses. We found a relationship between future response time and pre-stimulus power, but not phase, in occipital, parietal, posterior cingulate and superior frontal cortices, consistently across a wide range of frequencies. These correlations were not explained by deterministic sources of variance, such as experimental factors and trial history. Our results further suggest that occipital areas mainly reflect short-term (trial to trial) stochastic fluctuations, while the frontal contribution largely reflects longer-term effects such as fatigue or practice. Parietal areas reflect fluctuations at both time scales. We found no evidence of lateralization: these effects were indistinguishable in both hemispheres of the brain and for both saccade directions, and non-predictive of choice - a finding with fundamental consequences for models of action decision, where independent, not coupled, noise is normally assumed. These results were published in the leading journal Neuroimage.
In the second series we used Function Magnetic Resonance Imaging (fMRI) of the brain to ask how changes in the task demand affect response times. In these studies, we contrasted four response conditions in which human participants generated a response to: an isolated stimulus; the most salient stimulus in the presence of a less salient distractor; the less salient stimulus in the presence of a more salient distractor; and made a free choice between two stimuli. We developed a novel saccade-contingent behavioural paradigm to investigate the neural basis of these distinct response behaviours: selection, inhibition and choice. The novel paradigm allows for exceptionally well-matched contrasts across these conditions and we linked these processes to our previous computation modelling work with stochastic accumulation-to-threshold models. We replicated the core cortical eye-movement network for saccade generation (frontal eye fields, posterior parietal cortex and higher-level visual areas). However, in contrast to previously published tasks, saccadic selection and inhibition recruited only this core network. However, the core network was insufficient for the free choice response which recruited anterior brain regions including dorsolateral prefrontal cortex and anterior cingulate cortex. The results indicate that extra-saccadic activity observed for free choice, and in previously published tasks probing saccadic control, is probably due to increased load on higher-level cognitive processes, and not response selection per se, which, in this case, is achieved within the classical cortical eye movement network. These results are now published in the leading international journal, The Journal of Neuroscience.
Exploitation Route The finding will have implications for neurology and applied behavioural science. We will be taking forward the use of these methods to answer questions about response time variability which is a key feature of aging.
Sectors Creative Economy,Digital/Communication/Information Technologies (including Software),Education,Pharmaceuticals and Medical Biotechnology

Description I am involved as a consultant in an SME who's work is informed by these results in a general way.
First Year Of Impact 2015
Sector Creative Economy,Digital/Communication/Information Technologies (including Software)
Impact Types Economic

Description FSL Imaging Workshop in Bristol 2012 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Training of graduate students and post-doctoral researchers. The course covered both the theory and practice of functional and structural brain image analysis. Main purpose: To stimulate thinking
Year(s) Of Engagement Activity 2012
Description MRI time laps film 
Form Of Engagement Activity A magazine, newsletter or online publication
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Participants in your research and patient groups
Results and Impact As part of the fellowship I worked with Catherine Baker, who is an artist, to deliver the public engagement aspects of this project as outlined in the original proposal. She produced a time-laps film that can be used to calm participants during structural MRI scans.

30 copies of the film were distributed at the 2011 Bristol Magnetic Resonance Summer School and a number of centres internationally are using the video.
Year(s) Of Engagement Activity 2011
Description Talk at the European Conference on Eye Movements 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Gave a talk on the work from PhD which was invaluable - received insightful feedback from researchers in the field. Main purpose: to share information. Result description: The effect of reward and probability on responses. Impact: Feedback on research and communication of research to others.
Year(s) Of Engagement Activity 2015