Timothy Ledgeway

University of Nottingham, Nottigham, England, United Kingdom

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Publications (58)153.57 Total impact

  • Timothy Ledgeway · David Heslip · Paul McGraw ·
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    ABSTRACT: Transcranial magnetic stimulation (TMS) has become a popular method for studying the functional properties, connectivity and chronometry of brain regions associated with visual encoding. However comparatively little is known about the precise mechanisms by which TMS influences on-going visual processing, though studies suggest it may suppress the processing of the signals associated with a task and/or induce increased levels of internal noise. To investigate this issue single-pulse TMS was applied over left-hemisphere V1 in eight observers during a forced-choice, orientation-identification task (horizontal vs. vertical) using a Gabor target (2 c/deg, centred 6 deg in the right visual field). Stimulus contrast was set to each observer's threshold, corresponding to 79% correct performance, measured in the absence of TMS. When TMS was applied over V1 performance decreased in all observers (~ 10% on average) compared to accuracy levels obtained during stimulation over a control site (Cz). Unexpectedly we found accuracy levels improved during V1 stimulation across a block of 200 trials in most (5/8) subjects, but remained stable during control site stimulation. Furthermore, no recovery was found when a brief, external, visual noise mask was used instead of a TMS pulse. These results show that the magnitude of TMS disruption can dissipate with repeated stimulation. To explore the potential mechanism underlying this recovery phenomenon we also measured the critical flicker fusion threshold (CFFT), using an LED driven by a square-wave temporal waveform of variable frequency, both prior to and following the same TMS protocol. For observers that previously exhibited TMS recovery, occipital simulation extended temporal integration periods by an average of 12% (by 3-8 ms). This suggests that the visual system can dynamically adapt to increased internal noise levels, by increasing the temporal interval over which visual stimuli are integrated, thus minimising the deleterious effects of TMS-induced cortical activity on sensory judgments. Meeting abstract presented at VSS 2015.
    Journal of Vision 09/2015; 15(12):809. DOI:10.1167/15.12.809 · 2.39 Impact Factor
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    Vision research 06/2015; 114. DOI:10.1016/j.visres.2015.06.002 · 1.82 Impact Factor
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    ABSTRACT: To study prelexical processes involved in visual word recognition a task is needed that only operates at the level of abstract letter identities. The masked priming same-different task has been purported to do this, as the same pattern of priming is shown for words and nonwords. However, studies using this task have consistently found a processing advantage for words over nonwords, indicating a lexicality effect. We investigated the locus of this word advantage. Experiment 1 used conventional visually-presented reference stimuli to test previous accounts of the lexicality effect. Results rule out the use of different strategies, or strength of representations, for words and nonwords. No interaction was shown between prime type and word type, but a consistent word advantage was found. Experiment 2 used novel auditorally-presented reference stimuli to restrict nonword matching to the sublexical level. This abolished scrambled priming for nonwords, but not words. Overall this suggests the processing advantage for words over nonwords results from activation of whole-word, lexical representations. Furthermore, the number of shared open-bigrams between primes and targets could account for scrambled priming effects. These results have important implications for models of orthographic processing and studies that have used this task to investigate prelexical processes.
    PLoS ONE 09/2013; 8(9):e72888. DOI:10.1371/journal.pone.0072888 · 3.23 Impact Factor
  • T. Ledgeway · P. McGraw · B. Thompson ·

    Journal of Vision 07/2013; 13(9):541-541. DOI:10.1167/13.9.541 · 2.39 Impact Factor
  • D. Heslip · T. Ledgeway · P. McGraw ·

    Journal of Vision 07/2013; 13(9):376-376. DOI:10.1167/13.9.376 · 2.39 Impact Factor
  • D Heslip · T Ledgeway · P McGraw ·
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    ABSTRACT: Background / Purpose: Local orientation and motion cues can be used to reduce the ambiguity of motion direction using a phenomenon known as “motion streaks”. We systematically quantify the orientation tuning of motion streak masking. Main conclusion: The orientation tuning of motion streak masking is non-monotonic, especially at “slow” speeds. “Off-orientation looking” may explain these results.
    Vision Sciences Society Annual Meeting 2013; 06/2013
  • P.J. Knox · T Ledgeway · A.J. Simmers ·
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    ABSTRACT: The presence of a general global motion processing deficit in amblyopia is now well established, although its severity may depend on image speed and amblyopia type, but its underlying cause(s) is still largely indeterminate. To address this issue and to characterize further the nature of the global motion perception deficit in human amblyopia, the effects of varying spatial offset (jump size - Δs) and temporal offset (delay between positional updates - Δt) in discriminating global motion for a range of speeds (1.5, 3 and 9 °/s) in both amblyopic and normal vision were evaluated. For normal adult observers (NE) and the non-amblyopic eye (FE) motion coherence thresholds measured when Δt was varied were significantly higher than those when Δs was varied. Furthermore when Δt was varied, thresholds rose significantly as the speed of image motion decreased for both NEs and FEs. AE thresholds were higher overall than the other eyes and appeared independent of both the method used to create movement and speed. These results suggest that the spatial and temporal limits underlying the perception of global motion are different. In addition degrading the smoothness of motion has comparatively little effect on the motion mechanisms driven by the AE, suggesting that the internal noise associated with encoding motion direction is relatively high.
    Vision research 04/2013; 86. DOI:10.1016/j.visres.2013.04.003 · 1.82 Impact Factor
  • Craig Aaen-Stockdale · Timothy Ledgeway · Paul McGraw · Robert F Hess ·
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    ABSTRACT: The intention of this series of experiments was to determine the extent to which the pathways sensitive to first-order and second-order motion are independent of one another at, and above, the level of global motion integration. We used translational, radial and rotational motion stimuli containing luminance-modulated dots, contrast-modulated dots, or a mixture of both. Our results show that the two classes of motion stimuli interact perceptually in a global motion coherence task, and the extent of this interaction is governed by whether the two varieties of local motion signal produce an equivalent response in the pathways that encode each type of motion. This provides strong psychophysical evidence that global motion and optic flow processing are cue-invariant. The fidelity of the first-order motion signal was moderated by either reducing the luminance of the dots or by increasing the displacement of the dots on each positional update. The experiments were carried out with two different types of second-order elements (contrast-modulated dots and flicker-modulated dots) and the results were comparable, suggesting that these findings are generalisable to a variety of second-order stimuli. In addition, the interaction between the two different types of second-order stimuli was investigated and we found that the relative modulation depth was also crucial to whether the two populations interacted. We conclude that the relative output of local motion sensors sensitive to either first-order or second-order motion dictates their weight in subsequent cue-invariant global motion computations.
    Vision research 09/2012; 68:28-39. DOI:10.1016/j.visres.2012.07.004 · 1.82 Impact Factor
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    ABSTRACT: We agree with many of the principles proposed by Frost but highlight crucial caveats and report research findings that challenge several assertions made in the target article. We discuss the roles that visual processing, development, and bilingualism play in visual word recognition and reading. These are overlooked in all current models, but are fundamental to any universal model of reading.
    Behavioral and Brain Sciences 08/2012; 35(5):300-1. DOI:10.1017/S0140525X1200012X · 20.77 Impact Factor
  • M. Long · C. Hutchinson · H. A. Allen · T. Ledgeway · A. Arena ·

    Perception 01/2012; 41(10):1268-1268. · 0.91 Impact Factor
  • A. Arena · C. V. Hutchinson · T. Ledgeway · H. A. Allen · M. D. Long ·

    Perception 01/2012; 41(10):1268-1268. · 0.91 Impact Factor
  • F. Rocchi · T. Ledgeway · B. S. Webb ·

    Journal of Vision 09/2011; 11(11):707-707. DOI:10.1167/11.11.707 · 2.39 Impact Factor
  • T. Ledgeway · P.V. McGraw · A.J. Simmers ·

    Journal of Vision 09/2011; 11(11):704-704. DOI:10.1167/11.11.704 · 2.39 Impact Factor
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    A.J. Simmers · T Ledgeway · C V Hutchinson · P J Knox ·
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    ABSTRACT: It is well established that amblyopes exhibit deficits in processing first-order (luminance-defined) patterns. This is readily manifest by measuring spatiotemporal sensitivity (i.e. the "window of visibility") to moving luminance gratings. However the window of visibility to moving second-order (texture-defined) patterns has not been systematically studied in amblyopia. To address this issue monocular modulation sensitivity (1/threshold) to first-order motion and four different varieties of second-order motion (modulations of either the contrast, flicker, size or orientation of visual noise) was measured over a five-octave range of spatial and temporal frequencies. Compared to normals amblyopes are not only impaired in the processing of first-order motion, but overall they exhibit both higher thresholds and a much narrower window of visibility to second-order images. However amblyopia can differentially impair the perception of some types of second-order motion much more than others and crucially the precise pattern of deficits varies markedly between individuals (even for those with the same conventional visual acuity measures). For the most severely impaired amblyopes certain second-order (texture) cues to movement in the environment are effectively invisible. These results place important constraints on the possible architecture of models of second-order motion perception in human vision.
    Vision research 08/2011; 51(18):2008-20. DOI:10.1016/j.visres.2011.07.012 · 1.82 Impact Factor
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    Ben S Webb · Timothy Ledgeway · Francesca Rocchi ·
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    ABSTRACT: The brain estimates visual motion by decoding the responses of populations of neurons. Extracting unbiased motion estimates from early visual cortical neurons is challenging because each neuron contributes an ambiguous (local) representation of the visual environment and inherently variable neural response. To mitigate these sources of noise, the brain can pool across large populations of neurons, pool the response of each neuron over time, or a combination of the two. Recent psychophysical and physiological work points to a flexible motion pooling system that arrives at different computational solutions over time and for different stimuli. Here we ask whether a single, likelihood-based computation can accommodate the flexible nature of spatiotemporal motion pooling in humans. We examined the contribution of different computations to human observers' performance on two global visual motion discriminations tasks, one requiring the combination of motion directions over time and another requiring their combination in different relative proportions over space and time. Observers' perceived direction of global motion was accurately predicted by a vector average readout of direction signals accumulated over time and a maximum likelihood readout of direction signals combined over space, consistent with the notion of a flexible motion pooling system that uses different computations over space and time. Additional simulations of observers' performance with a population decoding model revealed a more parsimonious solution: flexible spatiotemporal pooling could be accommodated by a single computation that optimally pools motion signals across a population of neurons that accumulate local motion signals on their receptive fields at a fixed rate over time.
    The Journal of Neuroscience : The Official Journal of the Society for Neuroscience 03/2011; 31(13):4917-25. DOI:10.1523/JNEUROSCI.6185-10.2011 · 6.34 Impact Factor
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    D Ellemberg · T L Lewis · D Maurer · B Lee · T Ledgeway · J P Guilemot · F Lepore ·
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    ABSTRACT: We compared the development of sensitivity to first- versus second-order global motion in 5-year-olds (n=24) and adults (n=24) tested at three displacements (0.1, 0.5 and 1.0 degrees). Sensitivity was measured with Random-Gabor Kinematograms (RGKs) formed with luminance-modulated (first-order) or contrast-modulated (second-order) concentric Gabor patterns. Five-year-olds were less sensitive than adults to the direction of both first- and second-order global motion at every displacement tested. In addition, the immaturity was smallest at the smallest displacement, which required the least spatial integration, and smaller for first-order than for second-order global motion at the middle displacement. The findings suggest that the development of sensitivity to global motion is limited by the development of spatial integration and by different rates of development of sensitivity to first- versus second-order signals.
    Seeing and perceiving 11/2010; 23(5-6):517-32. DOI:10.1163/187847510X532702 · 1.32 Impact Factor
  • R. F. Hess · T. Ledgeway ·

    Journal of Vision 11/2010; 2(7):121-121. DOI:10.1167/2.7.121 · 2.39 Impact Factor
  • R. F Hess · T. Ledgeway ·

    Journal of Vision 10/2010; 3(9):787-787. DOI:10.1167/3.9.787 · 2.39 Impact Factor
  • H. A Allen · R. F Hess · T. Ledgeway ·

    Journal of Vision 10/2010; 3(9):532-532. DOI:10.1167/3.9.532 · 2.39 Impact Factor
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    Claire V Hutchinson · Timothy Ledgeway ·
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    ABSTRACT: This study assessed spatial summation of first-order (luminance-defined) and second-order (contrast-defined) motion. Thresholds were measured for identifying the drift direction of 1c/deg., luminance-modulated and contrast-modulated dynamic noise drifting at temporal frequencies of 0.5, 2 and 8Hz. Image size varied from 0.125 degrees to 16 degrees . The effects of increasing image size on thresholds for luminance-modulated noise were also compared to those for luminance-defined gratings. In all cases, performance improved as image size increased. The rate at which performance improved with increasing image size was similar for all stimuli employed although the slopes corresponding to the initial improvement were steeper for first-order compared to second-order motion. The image sizes at which performance for first-order motion asymptote were larger than for second-order motion. In addition, findings showed that the minimum image size required to support reliable identification of the direction of moving stimuli is greater for second-order than first-order motion. Thus, although first-order and second-order motion processing have a number of properties in common, the visual system's sensitivity to each type of motion as a function of image size is quite different.
    Vision research 08/2010; 50(17):1766-74. DOI:10.1016/j.visres.2010.05.032 · 1.82 Impact Factor

Publication Stats

1k Citations
153.57 Total Impact Points


  • 2002-2013
    • University of Nottingham
      • School of Psychology
      Nottigham, England, United Kingdom
  • 1999-2005
    • McGill University
      • Division of Ophthalmology
      Montréal, Quebec, Canada
  • 1997-1999
    • University of Oxford
      • Department of Experimental Psychology
      Oxford, England, United Kingdom
  • 1994-1998
    • Royal Holloway, University of London
      • Department of Psychology
      Эгхем, England, United Kingdom
    • Cardiff University
      Cardiff, Wales, United Kingdom
  • 1994-1995
    • University of London
      Londinium, England, United Kingdom