The Gallagher proximity measures, which analyse how close the subject was to the platform or target location throughout the trial, were introduced by HVS Image in the 1990s for Dr. Michela Gallagher.

There are three measures, as follows:

Gallagher Global

The Gallagher global (mean) measure was initially used in probe trials. Its value is the average distance the subject was from the learned location over the whole trial or slice being analysed.

As noted by Gallagher et al, 1993, conventional probe trial acquisition criteria (e.g. more than 35% of trial time spent in the target quadrant and at least two platform crossings being made) do not distinguish between either:

• subjects that spend much of the trial swimming close to the target vs those that spend the same amount of time in the target quadrant but without focusing on the target itself,
  or
• subjects that spend much of the trial swimming close to the target but without making exact platform crossings vs those that not only don’t make platform crossings but also don’t show any knowledge of the target location.

In contrast the Gallager global measure, the average distance the subject was from the target location, shows a clear difference and reflects actual performance:

• When taken over the entire trial duration, the Gallager global measure reflects the entire distribution of the search during the trial.
• When use with time slices, it allows more fine-grained analysis of behavior, for example to assess extinction.

The HVS Image system gives you both the raw value and the value corrected for start point so as not to be biased by different start points being different distances from the target location.

Gallagher Cumulative

The measure was also adapted for use with training trials, to show the degree of search error or deviation from an ideal path, as in training there may be no difference in the conventional measures of latency and path length for subjects searching close to the platform (without actually finding it) compared to those not searching the right area.

The Gallagher cumulative measure gives the total distance from the platform summed over time (the averages for each second of the trial all added together). While latency and path length may be similar, the sum of the distances from the target will be much greater for a widely distributed path with little focused search than for a path that is focused in proximity to the platform.

As with the global measure, the HVS Image system gives both the raw value and the value corrected for start point.

Gallagher by Segment

In the HVS Image system the global and cumulative measures are given by default (as with all analyses you can omit them from your results exports if not wanted). At the check of a box the system also provides you with the average distance for each one second of each trial (or slice), known as Gallager by segment.

While the mean and cumulative proximity give summary statistics, the 1-second time series allows you to:

• See how behavior evolved during the trial, for example, was the subject near the platform early on and then moved away, did proximity improve as the trial progressed?
• Detect early search strategies, for example, in probe trials, what did the subject do in the first few seconds, when memory recall is most apparent?
• In reversal learning or strategy shift experiments, pinpoint when the subject abandoned the old location and began to search elsewhere.
• In extinction trials, pinpoint when the subject abandoned the search of the learned location.
• Map proximity changes against known events in the trial (e.g., cue onset or trial midpoint).
• Plot proximity over time, generating insights into search consistency, hesitation, or response to spatial uncertainty.