ANY-maze Help > The ANY-maze reference > The Protocol page > The elements of a protocol > Behaviour > Immobility detection > Forced swim test (FST) and Tail suspension test immobility detection

Forced swim test (FST) and Tail suspension test immobility detection

Introduction

ANY-maze can automatically detect periods when the animal is immobile in the Forced swim test (FST) (which is sometimes called the Porsolt Forced Swim Test) and in the Tail suspension test.

In the FST immobility excludes periods when the animal is drifting and small movements necessary to keep the head above water, while in the TST it excludes swinging.

This type of immobility detection is different to the immobility detection used when the animal is being tracked (which is described in detail here) and is only available when the Forced swim test and Tail suspension test protocol mode is being used.

Specifying the type of test

There are small differences to how immobility is determined between the Forced swim test and Tail suspension test, so you must specify which type of test you are performing.

Adjusting immobility sensitivity

You can alter how immobile the animal needs to be in order for ANY-maze to consider that it is immobile. Usually the Standard sensitivity is about right, but you can increase the sensitivity if you want the animal to be moving less before being scored as immobile. Alternatively you can decrease the sensitivity if you want to be more permissive (i.e. let the animal move a bit more while still considering it to be immobile).

You can change the immobility sensitivity at any time, including after you have performed tests. ANY-maze will simply reanalyse the data it recorded during the tests and update the immobility results accordingly.

How it works

Immobility detection in the Forced swim and Tail suspension tests works by analysing the frequency of movements in the images. 'Struggle' which is a feature of both tests, is characterised by higher frequency movements, whereas things like drifting and swinging are lower frequency movements. Thus ANY-maze actually detects 'struggle' and waits for struggle to stop for a short period of time after which is reports the animal as immobile.

Note that this is very different to how immobility is detected when the animal is being tracked.

Why ANY-maze reports that the video display is 'delayed'

In order to determine the frequency of movement in the images (see previous section), ANY-maze needs to analyse a sequence of images. To build this sequence ANY-maze stores approximately one second's worth of images, removing the oldest image in the sequence whenever a new one is added - it then analyses the movements in this sequence. This means that it detects the animal's immobility state around half a second behind real-time. Also, as mentioned in the previous section, ANY-maze doesn't update the immobility state immediately when it changes, rather it waits a short time to see how long the change will last - for example, if an animal was mobile, became immobile for one tenth of a second and then became mobile again, you probably wouldn't score it as being immobile - ANY-maze does the same. However, if after waiting a short time the animal's immobility state is still the same then the change is deemed to have occurred at the time that the state changed (i.e. before the wait).

The effect of all of this is that when, in real-time, ANY-maze detects a change in immobility, the change will actually have occurred around 1s earlier. As ANY-maze can only report the change when it detects it, it means that you would see ANY-maze saying 'Immobile' on the screen ~1s after the animal stops struggling and then removing 'Immobile' ~1s after the animal starts struggling again. This can make it very hard to assess ANY-maze's scoring. To address this ANY-maze can delay the video display by ~1s, so then the immobility reported relates to exactly what is happening on screen. It's important to understand that this delay just affects the video that is displayed, it makes no difference to the timing of the test or anything that happens in it.

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ANY-maze help topic T1393