Acoustics: Stimulus Presentation in the rat

There are a number of things to consider when presenting auditory stimuli to laboratory animals. One thing that we must not forget is that auditory stimuli are perceived differently by rodents than they are by humans. Thus, if we present a relatively low frequency tone to an animal, it is likely that their perception is different than the perception of the experimenter setting its intensity. As you can see from the collection of audiograms below (Engle & Barnes, 2012), stimuli below 1 kHz quickly fall outside of rats' hearing range but are still near the optimal range for humans.

Knowing this, imagine if you were to associate a 500 Hz tone with food delivery and a 2 kHz tone with sucrose delivery in a Pavlovian paradigm. Can you guarantee that the increased rate of approach for sucrose is in fact related to the increased incentive salience for that reward? Hopefully you would have learned about counterbalancing long before this, but if not, you can see how you might have a problem on your hands.

Another aspect of stimulus presentation that is often ignored involves the method by which we measure stimulus intensity.  Standard decibel (dB) meters that one might buy at your local electronics store or hardware store are known to measure the intensity of sound.  However, most individuals are unaware that the decibel measurements output by these meters are adjusted depending on their frequency.  We can see this in the plot below:

The weightings applied to commercial dB meters are designed to comply with OSHA standards so that acoustic hazards in work environments can be detected. Unfortunately, this means that the commonly available detectors are optimized/weighted for human hearing.  Thus, is is best to use a C or Z weighting on a decibel meter in order to obtain a more absolute measure of dBs and then to manually correct the volume to ensure that it is well above the auditory threshold for the rodent at that frequency.

All in all, being aware of these factors allows you to choose proper stimuli, assess stimuli effectively, or make adjustments to stimuli when necessary to ensure experimental reliability.  Similar concerns arise when considering any other sensory stimulus.  So, without a doubt, you need to be aware of the limitations of the species you are working with.

D.J. Barker