Alex S. Baldwin
Postdoctoral Research Fellow at McGill Vision Research
Areas of Specialisation
The human visual system is one of the great wonders of the natural world, hidden in plain sight. It performs operations beyond that capable by our most advanced technology in a way that is so unobtrusive that it can be difficult to apprehend the complexity of the task set for it. Our brains somehow transform the input from the retina – a distorted, constantly-shifting “spotlight” cast on the visual world – into a stable and detailed representation of the outside world. The visual stimulus can contain many different types of information: for example the simple contrast between light and dark regions in the stimulus, more complex texture patterns, objects with shapes and surfaces, motion both of objects in the world and of the observer moving through it, and depth information from stereoscopic disparity. Of these stimulus domains the processing of contrast is the most fundamental, however even in this case our understanding is still incomplete.
In order to investigate how the brain processes visual information I use psychophysical methods. I set my human observers carefully-designed tasks that require them to use information from visual stimuli to make decisions. By collecting a large amount of data on how humans behave I can establish a relationship between the properties of the stimulus and the response. The principle is that if we understand the visual system then we should be able to predict how humans will perform on these tasks. In order to explain the results from a specific experiment I draw on the background of what is known from psychophysics, neurophysiology (e.g. electrical recordings made from brain cells), and imaging methods such as fMRI, to develop mathematical and computational models of how the brain might perform the task. The human performance can then be tested against the simulations made by a variety of such models in order to find that which best accounts for the data. The combination of psychophysics and modelling is particularly powerful when the experiment is designed with the possible models already in mind, as one can design an experiment that is specifically focused on giving the data needed to distinguish between them.
I have experience designing and conducting psychophysical experiments using Matlab (with Psychtoolbox), PsychoPy, and bespoke software (writing my own routines in Delphi). I have also developed both analytic and stochastic (Monte Carlo) computational models in Matlab in order to interpret the data from those experiments.
My research to this date has involved investigating the combination and summation of signals over space in human vision. I initially developed a descriptive model of how luminance contrast sensitivity varies across the central visual field. I have since used this attenuation surface both as a stage in my models of the summation of contrast over area and to develop novel stimuli for use in my experiments. Recently I have applied techniques from my previous work on area summation in the contrast domain to the study of the integration of local orientation signals into coherent textures.
PhD in Neurosciences: 2009-2013
BSc in Music Technology & Neuroscience (first class honours): 2006-2009
- Alex S. Baldwin (2013). Pattern Integration in the Normal and Abnormal Human Visual System. Life and Health Sciences, Aston University.
- Alex S. Baldwin, Minnie Fu, Reza Farivar & Robert F. Hess (in prep). The equivalent internal orientation and position noise for contour integration.
- Daniel P. Spiegel, Alex S. Baldwin, & Robert F. Hess (in press). Ocular dominance plasticity; inhibitory interactions and contrast equivalence. Scientific Reports.
- Daniel P. Spiegel, Alex S. Baldwin, & Robert F. Hess (2016). The relationship between fusion, suppression and diplopia in normal and amblyopic vision. Investigative Ophthalmology and Visual Science, 57(13), pp. 5810-5817. [IOVS]
- Alex S. Baldwin, Gunnar Schmidtmann, Frederick A. A. Kingdom & Robert F. Hess (2016). Rejecting probability summation for radial frequency patterns, not so Quick! Vision Research, 122, pp. 124-134. [ScienceDirect]
- Alex S. Baldwin, Daniel H. Baker & Robert F. Hess (2016). What do contrast threshold equivalent noise experiments actually measure? Noise vs. nonlinearity in different masking paradigms. PLoS One, 11(3):e0150942, pp. 1-26. [PLoS]
- Alex S. Baldwin & Tim S. Meese (2015). Fourth-root summation of contrast over area: no end in sight when spatially inhomogeneous sensitivity is compensated by a witch's hat. Journal of Vision, 15(15):4, pp. 1-12. [JoV]
- Frederick A. A. Kingdom, Alex S. Baldwin & Gunnar Schmidtmann (2015). Modelling probability and additive summation for detection across multiple mechanisms under the assumptions of signal detection theory. Journal of Vision, 15(5):1, pp. 1-16. [JoV]
- Alex S. Baldwin, Jesse S. Husk, Lauren Edwards, Robert F. Hess (2015). The efficiency of second order orientation coherence detection. Vision Research, 109, pp. 45-51. [ScienceDirect]
- Alex S. Baldwin, Jesse S. Husk, Tim S. Meese & Robert F. Hess (2014). A two-stage model of orientation integration for Battenberg-modulated micropatterns. Journal of Vision, 14(1):30, pp. 1-21. [JoV]
- Alex S. Baldwin, Tim S. Meese & Daniel H. Baker (2012). The attenuation surface for contrast sensitivity has the form of a "witch's hat" within the central visual field. Journal of Vision, 12(11):23, pp. 1-17. [JoV]
- Emily House, Matthew Mold, Joanna Collingwood, Alex Baldwin, Steven Goodwin & Christopher Exley (2009). Copper Abolishes the β-Sheet Secondary Structure of Preformed Amyloid Fibrils of Amyloid-β42. Journal of Alzheimer's Disease. 18:811-817 [PubMed]
- Angela Zhang, Paul Khayat, Hassan Akhavein, Alex S. Baldwin, Robert F. Hess & Reza Farivar (in press). Monkeys see snakes like humans do: a comparative analysis of external noise and efficiency in contour integration. (Society for Neuroscience 2016, San Diego).
- Tatiana Ruiz, Daniel Spiegel, Alex S. Baldwin, Robert F. Hess & Reza Farivar. (in press). Mild traumatic brain injury patients recruit compensatory mechanisms to integrate visual information. (Society for Neuroscience 2016, San Diego).
- Alex S. Baldwin, Gunnar Schmidtmann, Frederick A. A. Kingdom & Robert. F. Hess (2015). Rejecting probability summation for RF patterns, not so Quick! Journal of Vision, 15(12), pp. 1031-1031. (VSS 2015, St. Pete Beach). [Poster]
- Frederick A. A. Kingdom, Alex S. Baldwin & Gunnar Schmidtmann (2015). Modelling probability summation for the detection of multiple stimuli under signal detection theory. Journal of Vision, 15(12), pp. 473-473. (VSS 2015, St. Pete Beach).
- Yi Gao, Alex S. Baldwin & Robert. F. Hess (2015). Dipper functions for second-order modulation of contrast, orientation, and motion. Journal of Vision, 15(12), pp. 469-469. (VSS 2015, St. Pete Beach).
- Daniel P. Spiegel, Alex S. Baldwin, Mark A. Georgeson, Reza P. Farivar & Robert. F. Hess (2015). The relationship between fusion, suppression, and diplopia in amblyopia. Investigative Ophthalmology & Visual Science, 56(7), pp. 2196-2196 (ARVO 2015, Denver).
- Alex S. Baldwin & Robert F. Hess (2014). Investigating the shape of the contrast sensitivity function using white, bandpass, and contrast jitter noise. Journal of Vision, 14:10, 1421. (VSS 2014, St. Pete Beach).
- Alex S. Baldwin, Jesse S. Husk, Tim S. Meese & Robert F. Hess (2012). Pooling strategies for the integration of orientation signals depend on their spatial configuration. Perception, 41(12), pp. 1512-1513. (AVA Christmas Meeting 2012, UCL).
- Alex S. Baldwin, Tim S. Meese & Daniel H. Baker (2012). A reevaluation of area summation of contrast with compensation for retinal inhomogeneity. Perception, 41 (ECVP Abstract Supplement): 223. (ECVP 2012, Sardinia).
- Tim S. Meese, Rob Summers, Alex S. Baldwin (2012). Theory and data for area summation of contrast with and without uncertainty: Evidence for a noisy integrator model. Journal of Vision, 12:9, 601. (VSS 2012, Naples, Florida).
- Alex S. Baldwin, Tim S. Meese & Daniel H. Baker (2012). Extensive physiological summation of contrast signals over area revealed by Witch’s Hat compensation for retinal inhomogeneity. Perception 41(3): 366. (AVA Spring Meeting 2011, Cardiff University)
- Alex S. Baldwin, Tim S. Meese & Daniel H. Baker (2011). Retinal inhomogeneity and the witch's hat: Contrast sensitivity declines as a bi-linear function of eccentricity in each direction. Perception, 40: 112. (AVA Christmas Meeting 2010, Paris)
- Alex S. Baldwin & Tim S. Meese (2010). A new approach to investigating the contrast sensitivity to contour integration in the fovea. Perception, 39 (ECVP Abstract Supplement): 26. (ECVP 2010, Switzerland).
- Alex S. Baldwin, Tim S. Meese & Daniel H. Baker (2010). Loss of contrast sensitivity at 4 c/deg depends on eccentricity and meridian but not grating orientation for the central 9 deg of the visual field. Perception, 39(8): 1151. (AVA Spring Meeting 2010, Liverpool)
Teaching assistant for Vision Science and Research Methods: 2009-2013
Teaching assistant for Human Physiology and Pathology: 2008 - 2009
Teaching assistant for Introduction to Neuroscience: 2008 - 2009
Awards and Memberships
Awarded the Keele University Dean's Scholarship: 2006 - 2009
Member of the Applied Vision Association: 2009 - Present
Member of the Vision Sciences Society: 2013 - Present