Signal processing in the visual system


Research in my laboratory focuses on signal processing in the visual system. I use three methodologies - psychophysics, computational modelling, and the analysis of existing neural data. There are several ongoing projects.


Dr Alan Freeman

Research Location

Camperdown - School of Medical Sciences

Program Type



Neural mechanisms underlying motion sensitivity
It has been known for 50 years that many neurons in primary visual cortex are selective for motion direction. There is no consensus, however, on the physiological mechanisms underlying this selectivity. My aim is throw light on this question through three experimental approaches.
Analysis of electrophysiological data. There is a simple hypothesis for the mechanism, namely, that a cortical cell's preferred direction can be predicted from its receptive field. My students and I have been testing this hypothesis on responses collected from mammalian primary visual cortex in a colleague's laboratory.
Psychophysics. The psychophysical extension of the hypothesis described above is that human responses to moving patterns of light and dark will depend on which way they are moving. We are in the early stage of collecting data to test this idea.
Computational modelling. A third approach is computational modelling of signal processing in the subcortical pathways and primary visual cortex. Building on my previously published model (Hesam Shariati and Freeman, 2012), we will determine whether the model can match motion-selective responses recorded in the laboratory.

Machine vision
The computational model that we have built is potentially very general. It currently reproduces several important visual capabilities - such as orientation selectivity - and can potentially display motion direction selectivity. The ultimate goal here is to provide the front end of a machine-based object recognition system. The most powerful system resides in the human brain, and it therefore makes sense to copy the biological version as closely as possible.

Rhythmic modulation of sensation
There is increasing evidence that visual sensitivity varies periodically with a frequency close to 10 Hz. This variation, the alpha rhythm, will increase the noise in psychophysical measurements unless taken into account. My students and I are therefore measuring the rhythm psychophysically. The goal is to see whether there is a specific rhythm for each subject and, if so, to time psychophysical measurements so that variation resulting from the rhythm is minimised.

Additional Information


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brain, Neuroscience, Vision, sensation, perception, Hearing & vision problems, Neuroscience & psychology, The senses

Opportunity ID

The opportunity ID for this research opportunity is: 1167