Students at Normandy University (Normandie Université) in Rouen, France were curious to test whether the concept of ‘sensory substitution’ exists within rats. ‘Sensory substitution’ is when an agent temporarily stops a specific sensory system from functioning properly, only to have a different sensory system to make up for the sensory system that was lost by using the other sensory system to function in its place. These students set out an experiment that would test whether the auditory system can be used in replace of the visual system to help rats navigate through a particular path. There have been numerous studies throughout the world that have been performed to test similar concepts, which led to this study being performed. In 1987, Strelow et al have demonstrated that visual-auditory sensory substitution was evident in three macaque monkeys when they were unable to see.
Through the help of their auditory system, the macaque monkeys were able to perceive the environment that they were in although they were essentially blind. The monkeys learned to avoid any challenges that stood in their way, to grab hold of any object that stood in their path or to differentiate between various textures through this experiment. Moreover, in 2013, Thomson, Carra and Nicolelis have shown that rats had the ability to use infra-red light emissions to navigate to a reward. This was due to the electrical stimulations that resulted within their vibrissal cortex from the infra-red light emissions. With that being said, these studies have led to an interest in further discovering sensory substitution within certain animals and what causes them. It is clear that certain sensory signals, that are termed reafferent signals and exafferent signals, have allowed animals such as the macaque monkeys or the rats to navigate and interact with their environment despite the lack of functionality within a particular sensory system.
Reafferent signals are signals that are carried from within the body such as from the vestibular system or receptors in the muscles and tendons, whereas exafferent signals are created from the environment and activate a variety of senses in the body. However, there has not been any demonstration of the reafferent signals that show the impact of sound on rodents, which has led to the rise of this particular study. With that being said, the students at Normandy University hypothesized that the rats will be successful in achieving a visual-auditory sensory substitution in order to navigate through a spatial path by following the sound. Moreover, the students believe that there will be a correlation between the effects of sound and the rats’ activity, such as the rats stopping when the sound is lost or the rats gaining speed as sound is evident. To perform this experiment, nine male rats that were all aged 50 give or take 5 days, were placed into animal cages filled with sawdust and straw. There was plenty of food available for the rats, whereas there was a limited supply of water provided to them, since that would be used as their reward.
The experimental room was completely dark, which was used to prevent the rat’s visual system from functioning. This study was performed in three stages: stage 1 was conducted in a three closed arms maze with no time constraint, stage 2 was conducted in a three open arms maze with a time constraint of 45 seconds, and stage 3 was conducted in a three open arms maze, where the left and right arms were moved upward to form an oblique structure, and there was a time constraint of 45 seconds. Each stage consisted of 40 trials each day, where stage 1 lasted eight days, stage 2 lasted seven days and stage 3 lasted four days. On the last of the four days for stage 3, there was no sound being played at all, whereas the days prior to that, there was sound being played to direct the path of the rats. In each trial of each stage, there was a rat placed in the center of the apparatus and sound would be played at one of the extremities of the goal positions. At the end of each of these goal positions, there was a reward of water that was distributed to the rats at ground level once they went into the extremities of the correct goal positions.
The rats’ activities were monitored, which allowed parameters such as the number of errors per trial, the number and percentage of successful trials and the rats’ average speed to be recorded. Thereafter, various quantitative and qualitative analyses were performed such as the Wilcoxon signed-rank test, the Holm method and Chi square tests to generalize the findings of the experiment. From this experiment, each stage had its own set of results that unraveled intriguing information about the rats’ activity in complete darkness when exposed to sound. In the first stage of the experiment, the number of errors, which was highlighted by the number of entries into a goal position that was not exposed to sound, has decreased significantly over the eight days that the trials were performed in. The first day had the greatest number of errors, while the sixth, seventh and eighth days had the least number of errors.
Moreover, the average amount of time it took to complete a trial decreased significantly from approximately 95 seconds on the first day to approximately 15 seconds on the last three days. During the second stage, it is clear that the percentage of successful trials increased from 55% to 94% from days 9 to 15. On day 9 of the experiment, it is clear that trials on the left and right arms of the maze are the most successful, as opposed to the central arm. In addition, the percentage of successful trials increases significantly when the reward is in the central arm, but this was no longer the case when day 15 neared since the difference between successful trials whether the reward was in the left or right arms compared to the center arm no longer made a difference. There was an increased presence of the scanning strategy used by the rats as the experimental days progressed from day 9 to day 15. During the third stage, the percentage of the number of successful trials went from 45.55% on day 16 to 74.45% on day 17 to 89.45% on day 18 due to the exposure of sound. It is also clear that on the first day of the third stage, the percentage of successful trials was greater on the central arm, rather than the left or right arms that were moved upwards into an oblique position. This difference between the three paths was minimized by the eighteenth day of the experiment. However, on the 19th day, when no sound was played, the number of successful trials declined significantly to approximately the same amount of successful trials on day 16.
The number of successful trials due to the rats’ scanning strategy declines significantly to that of day 16, which is a significant difference from number of successful trials on day 18. With that being said, there are numerous clear patterns that have resulted from performing this experiment. As mentioned previously, there are numerous clear patterns that have resulted from this experiment, which shows that the active sensory substitution turned out to be successful. The numerous trials for each stage show that with each trial, the success rate of the rate getting to the extremity of the correct arm that the sound is coming from increases.
Moreover, it is clear that the rat is navigating to the correct arm in less time each trial. This exemplifies that the rat is adapting and learning how to function using the auditory system in replace of the visual system. Similarly, when the sound is not playing, it is clear that the rats are lost when attempting to reach the correct arm that the reward of water is in. This signifies that the rats developed a reliance on the auditory system for navigation when the visual system is no longer evident. The lack of sound leaves them in an unsure state of where to go or what to do. In addition, it is clear that the rats learned to associate the sound with the reward of water, which can be seen from the rats’ scanning strategy and use of Uturns when they are in an arm.
Overall, the findings of this experiment support the hypothesis and shows that there is a possible functional sensory substitution in rodents, such as the use of the auditory system when the visual system is not functional. The results from this experiment highlights the efficiency of the learning process that occurs from this sensory substitution, which makes it clear that sensory substitution is functional and evident in rodents. This signifies the heavy reliance that rodents, as do humans, have on sensory systems in order to carry out daily functions and to interact with the environment. However, the study was only limited to rats and it should have been tested on other types of rodents since it would provide a greater understanding of sensory substitution in the class of rodents as a whole.