When analysing probe trials in Morris water maze, two of the most commonly used measures of spatial memory retention are the time spent in the target quadrant (the quadrant where the platform was previously located), and the number of crossings of the exact platform location.

These have obvious limitations, for example the time spent in the target quadrant will show to what extent the subject spends time searching the general area where the platform was located (assuming it was in the center of the quadrant), but does not show the degree of accuracy – it gives no distinction between a subject with a vague knowledge of the target location and a subject who has learned it much more precisely.

Conversely the number of exact platform crossings gives a useful measure of whether the subject knows the target location very precisely, but gives no distinction between subjects who make a focused search of the immediate area, repeatedly coming very close to the target location though not quite crossing it, and those who perform no better than chance.

Consequently, significant behavior and findings may be overlooked, as non-exact but very close spatial memory retention is ignored, even when both these measures are used at once.

The close encounter measures in the HVS Image system overcome this issue.

All HVS Image users automatically get measures relating to a circular area around the center of the set platform location (without having to configure zones or set anything up). This circular area, known as the ‘counter’, is twice the diameter of the platform by default, meaning that for a 10 cm platform, any swimming within 5 cm of the set platform location will be recognised as close.

In fact these meausues are given for the current platform location, for the learned platform location when the platform is removed in a probe trial, and for any additional calibrated platform positions (see below).

Close encounter measures include:

• The number of passes over or close to the set platform position (passes through the counter).
• The percentage of the trial time spent at or close to the set platform position (within the counter).
• The time to the first close encounter with the set platform position, showing how quickly the subject almost reaches the set platform position (how quickly it reaches the counter), even if the exact location is just missed.
• The path to the first close encounter with the set platform position, showing the path length taken to almost reach the set position, even if the exact location is just missed.
• These give robust measures of spatial memory in the probe test.

Other advantages of the HVS Image close encounter measures vs quadrant and exact location measures:

• The degree of precision you want to measure can easily be adjusted if required, making the counter area smaller if you only want to include very precise spatial memory, or larger if you want to include less precise spatial memory, or focussed searches with a greater degree of accuracy than is given by the time or path in the target quadrant.
• You can re-analyse with different sized counter areas, to find significant results that do not show up in the more basic measures or with the default counter size.
• As well as being potentially crucial in probe trials, the close encounter measures are also given in regular trials, allowing more precise assessment of learning, and direct comparison between learning and probe trials.
• Close encounter measures are given for each calibrated platform position, allowing the same precision in assessing different types of search strategies, such as egocentric vs. allocentric learning, where the platform may be placed or moved relative to the start point, or for example in studies on reference vs working memory, where the platform may be moved every day.
• The close encounter measures work regardless of where in the pool the platform was placed, so give detailed insights across the whole range of studies carried out with the water maze including those where the platform is placed near the pool wall, near a fixed landmark or at random, not just when it’s in the center of a quadrant, as assumed by the time in target quad measure.