I'm just coming to the end of a busy week where I had two conferences to attend. The first was the Vision Sciences Society conference in St. Pete Beach, Florida. I was presenting some research I have done recently with other members of my lab, where we applied the types of summation ideas I worked on in my PhD to a new area (more details here). Although the publication of papers is the real engine of how the field progresses, conferences seem to be a very important accelerant. Without them there would be a much greater dependence on waiting for journal articles to spread new ideas, and any progress would be glacially slow.

Immediately after VSS we had the 25th anniversary reunion for McGill Vision Research. I joined MVR two years ago, though I had visited a couple of times before that. We had a very interesting day of talks (I much prefer talks in this sort of smaller conference setting, as was the case at the AVA meetings I used to attend in the UK). Hopefully there is some way that the reunion can be made into a recurring event.

Why do psychophysics?

Much of science is founded on taking measurements of one sort or another. The task of the scientist can be thought to consist of two parts: firstly ensuring that the correct measurements are made (i.e. "Am I measuring what I am trying to measure?"), and secondly coming up with explanations for what those measurements show. Frequently the validity of a scientific theory is assessed according to how accurately it predicts the outcome of measurements made in the future.

Measure what is measurable, and make measurable what is not so.
— Galileo Galilei

Although other fields suffer their own complications when it comes to measuring the world (e.g. How does one measure the size of the Earth? Or how much of the amyloid protein in a solution has clumped to form a deposit?), psychologists have the particular problem of wanting to measure things that many people believe are unmeasurable. You cannot directly take a ruler to somebody's thoughts, so a more subtle solution is required.

Psychophysics is a set of theoretical tools which make sensations measurable. The internal states of the subject are modelled by a set of variables and processes, based on assumptions about how perceptual decisions are made. Experiments can be designed to probe these internal mechanisms by relating them to the behaviours they would predict under a carefully chosen set of conditions. The models typically involve some sort of variability (e.g. noisy neuronal responses) so rather than predicting a specific response to a stimulus they instead make a probabilistic prediction of how often each of a set of responses should be expected.

The basic measurements which psychophysicists make are therefore the probabilities with which subjects respond in a particular way to a stimulus under some experimental paradigm. A simple example would be showing a subject a faint (low contrast) striped pattern and asking them whether they see it or not. The observer will make both "yes" and "no" responses on different trials (even with an identical stimulus). The probability with which the observer says "yes" can then be used to infer the magnitude of some internal response variable. The way in which that can be done will be the subject of  future blog post.


Placeholder first blog post here. I am hoping to use this space to put up some helpful explanations of psychophysical techniques, and approaches to modelling vision.

We are so familiar with seeing, that it takes a leap of imagination to realize that there are problems to be solved. But consider it. We are given tiny distorted upside-down images in the eyes, and we see separate solid objects in surrounding space. From the patterns of stimulation on the retina we perceive the world of objects and this is nothing short of a miracle.
— Richard Gregory