Close passes (or close encounters) in Morris water maze analysis refer to measures based on entries into a defined, circular area (“counter”) surrounding the platform location, allowing quantification of near-miss spatial accuracy (adding to platform finding in acquisition trials and to exact platform crossings in probe trials).

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 important limitations. Time spent in the target quadrant reflects general search location (assuming the target was in the center of the quadrant), but does not show the degree of accuracy. A subject with only a vague representation of the platform location may produce similar quadrant-level measures to one with precise spatial knowledge.

Conversely the number of exact platform crossings identifies highly precise memory, but fails to capture near-miss behavior. Subjects that repeatedly search very close to the platform location without crossing it are not distinguished from those performing at chance level.

Consequently, significant behavior and effects may be overlooked, as non-exact but spatially accurate search is not captured by these conventional measures.

The close encounter measures in the HVS Image system address this limitation.

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, being within 5 cm of the set platform location will be recognised as close.

These measures are calculated not only for the current platform location, but also for the learned platform location in probe trials, and for any additional calibrated platform positions – for example new and old locations in reversal learning trials, and marked positions in dual strategy trials.

Close encounter measures include:

• The number of passes over or close to the set platform position (entries into the counter).
• The percentage of the trial time spent at or close to the set platform position (within the counter).
• The latency 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 provide sensitive and robust measures of spatial memory, particularly in probe trials.

Other advantages of the HVS Image close encounter measures:

• Adjustable precision: 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.
• Re-analysis flexibility: 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.
• Applicability across trial types: 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.
• Multi-location analysis: 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.
• Independence from platform placement: The close encounter measures work regardless of where in the pool the platform was placed.

This allows detailed assessment of spatial learning and search strategies across a wide range of experimental designs, including those where the platform is not located at the centre of a quadrant.

Technical note: Close pass measures are based on entries into circular regions (“counters”) centred on calibrated platform locations. These measures are calculated for all defined platform positions, whether or not the platform is present. The default counter diameter is twice that of the platform, but can be adjusted depending on the level of spatial precision required.

See technical notes, citation and background.

View the full Morris water maze analysis overview.