Research Group
Visual Perception
The visual system receives an optical image as input and produces visual perception and visually-guided action as output. Current understanding is that this process involves both serial processing at different stages of complexity in different brain areas, and parallel processing of different sources of information such as colour and motion, also in different areas.
Our aim is to produce computational models of the different modules (colour, motion, stereo, shape) and to understand their interactions.
Techniques
a) Psychophysics
physical measurement of the discrimination capacities and biases of normal observers
b) Functional Magnetic Resonance Imaging (fMRI)
c) Eye movement and pupillary responses
The use of a recorder with 2000 Hz sampling rate allows us to process and alter the visual image in real time during saccadic eye movements.
d) Comparative measurements
Testing of abnormalities of visual perception in dyslexia, synaesthesia and following brain injury.
Examples of current research projects
- Poggendorf Illusion
Perception and Action
We have been testing the idea that the same cortical mechanisms are involved during visuo-motor extrapolation with eye movements, and perceptual extrapolation using the Poggendorff illusory figure.
In this illusion, tilted lines are apparently partly occluded by a figure. In the figure on the right, the upper line on the left is perceived as being aligned with the right hand line; however, the lower line on the left is in fact physically continuous with the right side line.
Attention and visual sensitivity
Does conscious attention select which inputs control behaviour, or can it in addition enhance the accuracy of low-level processes such as motion detection?
- On-off signal
On- and off-pathways in visual cortex
Selective adaptation to brightness increases/decreases indicates separate on- and off channels such as those found in retina and lateral geniculate nucleus. fMRI will be used to look for evidence of functional segregation of these pathways at a cortical level.
Signal detection theory
Normal psychophysical methods measure the observer’s signal/noise ratios as dimensionless quantities and do not determine whether an improvement in performance is due to signal enhancement or noise reduction. We have developed a method of distinguishing between these alternatives.
Basic visual geometry
Equal physical displacement of an object on the retina can be visible in some contexts and invisible under others. By comparing vernier and bisection thresholds we aim to understand how the visual system computes the relative positions of objects.
Texture processing
The visual system can distinguish textures by their statistics, such as the mean and variance of local orientation. We are measuring the statistical efficiency of these computations, in order to model their underlying physiology.
Illustrative publications:
Tibber, M., Grant, S. & Morgan, M.J. (2009) Occulomotor responses and visuospatial perpeptual judgments compete for common limited resources.?Journal of Vision, 9, 1-13, 3217-24.
Tibber, M.S., Anderson, E.J., Melmoth, D.R., Rees, G. & Morgan, M.J. (2009). Common Cortical Loci Are Activated during Visuospatial Interpolation and Orientation Discrimination Judgements. PLoS ONE, 4(2) e4585.
Spang, K. & Morgan, M.J. (2008). Cortical correlates of stereoscopic depth produced by temporal delay. Journal of Vision, 8, 1-12.
Gheri, C., Chopping, S. & Morgan, M.J. (2008). Synaesthetic colours do not camouflage form in visual search. Proc. Roy. Soc. B, 275(2) 841-246.
Parkes, L., Lund, J., Angelucci, A., Solomon, J. A. & Morgan, M. J. (2001). Compulsory averaging of crowded orientation signals in human vision. Nature Neuroscience, 4, 739-744.