Modeling Intelligence in Rodents

Nicola Tesla, Marie Curie, Albert Einstein, and Stephan Hawking are names associated with high intelligence. People want to understand this fascinating trait so that they can learn how to enhance their own intelligence, recreate the great intelligent minds of history, and even create systems that have excellent intelligence of their own.

Although there have been a lot of studies into understanding intelligence, it has also resulted in controversy and disagreement between researchers.

To best research intelligence, a general definition of it should be considered. There is much contention on the definition of intelligence, as well as exactly which components constitute this trait, resulting in a variety of definitions.

What can be agreed upon is that intelligence is the capability to learn from experience by not only acquiring knowledge, but also retaining and retrieving this knowledge in order to use it when required to identify and solve problems.^[1] The speed with which this knowledge is accessed and applied is also of importance.

There are several theories used to describe the concept of intelligence, with the major theories being the general intelligence theory, theory of primary mental abilities, theory of multiple intelligences, and triarchic theory of intelligence.^[1]

General Intelligence Theory

English psychologist Charles Spearman introduced the concept of a general mental ability with his two-factor theory of intelligence. Spearman came to this discovery after he observed that people who performed satisfactorily well in one area of intelligence, would normally also perform well in other areas.^[2]

An example of this is the correlation between mathematical ability and being able to distinguish the pitch of music. He linked this relationship to the central factor of a general intelligence.^[2] He further deduced that a person’s general intelligence over several capabilities can be depicted by a single g-factor and the s-factor is the specific ability the person has for a certain area of intelligence.^[2]

In recent years, psychology has described two types of intelligence that are incorporated into Spearman’s g factor, namely fluid and crystallized.  Fluid intelligence refers to the potential to tackle any problem without using previous knowledge, but rather abstract thinking and logic.^[3]

Crystallized intelligence is the use of knowledge that has been obtained earlier to solve problems.^[2] Crystallized intelligence can very easily stem from fluid intelligence when formed into long-term memory.^[2]

Primary Mental Abilities Theory

Louis Leon Thurstone disagreed with Spearman’s idea of a g-factor but did not reject it. He rather came to the conclusion from his own research that intelligence is made up of multiple primary mental abilities in addition to the individual’s general ability.^[2]

He discovered seven of these primary mental abilities; namely verbal fluency, verbal comprehension, spatial visualization, memory, number facility, inductive reasoning, and perceptual speed.

Multiple Intelligences Theory

Howard Gardner built on the work of Thurstone and suggested that there are multiple independent types of intelligences, not just one, and each type has its own particular talents and skills.^[2] The eight types of intelligences that he identified are logical-mathematical, linguistic, musical, spatial, intrapersonal, interpersonal, naturalist and bodily-kinaesthetic.

Gardner also suggests that many activities will include a mixture of the different intelligences. This theory can assist in explaining concepts beyond intelligence, but it does have a downfall in that it has not been tested widely and does not consider other types of intelligences except those mentioned by Gardner.^[2]

Triarchic Theory of Intelligence

The final theory of intelligence mentioned in this article was proposed by Robert Sternberg.^[2] He constructed this theory based on the idea that intelligence is the ability to achieve success based on an individual’s sociocultural context and their personal standards.^[2]

Using this definition, he introduced three aspects of intelligence; creative, analytical, and practical intelligence. In 1997, Sternberg built on this idea and defined intelligence as a mental activity aimed at intentional adaptation to and the formation of real-world environments that are appropriate to an individual’s life.^[3]

In other words, how effectively an individual is able to handle and change within their environment during their life.

One of the first studies of intelligence was undertaken by Charles Darwin’s cousin, Sir Francis Galton, when he investigated his hypothesis that intelligence is a general mental ability and a result of biological evolution.^[2]

He designed a test site to evaluate the reaction times of people.^[2] By doing this he turned the abstract concept of intelligence into something concrete, such as reaction time, that can be measured and therefore studied in empirical terms. This set the ball rolling for future studies into intelligence.

Alfred Binet, a French psychologist, developed the first intelligence test, the Binet-Simon Intelligence Scale, with Theodore Simon. This was after the French government assigned him with the task to create something that can determine which school children required additional academic assistance.^[1],[4]

The scale was further adapted by Lewis Terman when he standardized it with subjects from an American sample and its name was changed to the Stanford – Binet Intelligence Scale. This scale of intelligence is still widely used today. Binet also presented the idea of mental age, which is a psychological measure used to indicate the mental capabilities of an individual in terms of the number of years it would take an average child to reach the same stage.^[5]

The Intelligence Quotient (IQ) was introduced by William Stern, a German psychologist.^[1] An individual’s IQ is a measure of how well they are able to take the information they have gained as well as logic and use it to answer questions, solve problems and make predictions.^[6] Tests used to determine an individual’s IQ first evaluate their Short- and Long-Term Memory followed by their ability with how swiftly they can recall information and solve problems.^[6]

Rats afford researchers the opportunity to test out some aspects of intelligence that they may not be able to in humans or may not have even considered.^[7] When given an adequate environment, these rodents have proved repeatedly that they have exemplary intelligence, much to the astonishment of many researchers that work closely with them.

Earlier cognitive studies with rats involved them being subjected to very poor conditions, such as the Skinner box which inhibits stimulation from the environment and lessens competitive behavior.^[7] This caused researchers to underestimate the abilities of the rats as it put a great restriction on what they were really capable of.

Another cause for shock at their potential is the fact that many people still don’t believe that animals, especially those on the lower spectrum of evolution, can be capable of such complex mental tasks. On the contrary, both mice and rats have proven themselves to be of great intelligence and therefore provide researchers with brilliant models to study this trait.^[7]

Rats are more regularly used than mice due to the belief that they are more intelligent, something that recent studies are proving to be false. Most paradigms and experiments have therefore been designed specifically for rats.

The following are a variety of mazes that researchers commonly use in the study of intelligence. These mazes mostly test spatial learning and memory.

Radial Arm Maze

The radial arm maze was designed in 1976 by Olton and Samuelson to evaluate the memory and spatial organization abilities of rodents and has provided researchers with much of the vital information pertaining to these capabilities in rats. The results obtained when using this maze indicate how exceptional rodents are at remembering spatial information, a skill that is required in the wild for foraging and navigation.^[8]

Olton et al^[8] conducted a study using their newly designed radial arm maze to evaluate the rat’s ability to remember, differentiate and process details that they have obtained from place learning while looking for food. Place learning refers to the type of learning that occurs when associating certain stimuli to a specific location. When this can be used to resolve a problem in their environment, rats are able to learn very quickly.^[8]

Rats were able to distinguish and remember which of the eight arms they had already walked through and taken a pellet from. It was not determined what cues the rats used to store the information into their long-term memory, but they did not choose adjacent arms, arms in a particular sequence nor did they make any markings on arms as they walked through them.^[8]

It was also discovered that rats were inclined to choose on average, six different arms from the eight choices provided, even when it was not reinforced. The researchers noted that the rats did not always achieve a perfect execution of the test, but their identification and memory of which arms they had traveled in was above chance.^[8]

They also observed that the rats experienced a decrease in their performance when increasing the number of choices presented to the rats, thereby interfering with previous choices they had already made.^[8]

Hebb Williams Maze

Hebb and Williams introduced their maze in 1946 to measure intelligence in rodents.^[9] It provides researchers with a tool to evaluate a variety of learning problems by giving the test subjects different choices. They designed this maze in such a way that other survival instincts, such as the dedication for food retrieval, will be minimized so that a rodent’s intelligence and general learning can be specifically measured without the other factors influencing it.^[9]

The maze uses a constant goal and setting, which is very on par with human intelligence tests and provides an opportunity to directly compare the results of rodents and humans in tasks that are behaviorally similar.^[10]

Morris Water Maze

The Morris Water Maze was designed by Richard Morris in 1984 and is utilized to evaluate spatial memory and learning.^[11] It consists of a platform, normally hidden, situated in a pool of opaque water. The test subject is placed in the water and must locate the platform.

There is usually a training period before the test in which the rodent is allowed to become familiar with the location of the platform, normally with the use of visual cues encouraging the rodents to find it.^[12]

The rodent’s performance is assessed on the amount of time it takes for the platform to be found.

Y-Maze (Spontaneous and Forced Alternation Task)

The Y-maze is used to test the exploratory behavior and working memory of rodents.^[12] In the forced alternation version of this maze, test subjects are given a sample trial in which they are only allowed access to two of the arms.

After this period, they are placed back in the maze, which now has the third arm open, and allowed to explore. All sorts of odor cues are normally removed so that the rodent’s working memory can assist in locating the new, undiscovered arm.

Results from this test are normally calculated by the percentage of the time spent in the novel arm.

There is a variation, known as spontaneous alternation, in which the position of the Y-maze is moved a bit. This changes the visual cues of the rodent and evaluates their spatial working memory and habituation skills.^[12]

T-Maze (Spontaneous and Forced Alternation Task)

The T-maze is very similar to the Y-maze and tests the same abilities in rodents. It gives rodents the choice between two arms, left and right. The rodent’s spatial memory is tested and they should be more inclined to choose the less discovered arm of the two after the trial period.^[13] The number of turns made in the novel arm is recorded over the course of the experiment.

There is also an opportunity to use the maze for forced and spontaneous alternation.

Researchers have devised a number of mental tasks for rodents in an attempt to better test and understand their intelligence. The following tasks described below are a few of these tests which highlight the interesting cognitive abilities of rodents.

Digging Task

Davis and Parriag conducted an experiment to test the response of rats to proportional distance cues.^[7] They accomplished this by hiding food underneath the enclosed area that a rat was situated in and then teaching the rat to dig and retrieve the hidden food.^[7]

These rats were able to swiftly comprehend where the food was located and dug for it in no time when placed in the enclosure. It was also revealed in subsequent studies using controls that no olfactory cues were used by the rats when searching for their food.

Number Task

Rodents have been known to display behavior towards number-related stimuli.^[14] Davis and Memmott demonstrated that rats are able to “count” to three when presented with a stimulus (e.g., a shock) that they could relate to numerical information. The researchers did this by subjecting the rats to three shocks in a thirty-minute time period, followed by an absence of any shock.

The researchers then presented the rats with a lever to press in order to evaluate their level of calmness based on the frequency with which it was pressed (i.e., the calmer the rat is, the more it will press the lever). It was observed that the rats learned to anticipate each upcoming shock until the third one was conveyed, after which they displayed an obvious calm behavior by increasing the rate with which they pressed the lever.^[14]

The conclusion made was that rats were able to interpret the third shock as the final one and knew they were going into a period of safety from any painful stimuli, therefore increasing their activity of whatever task was provided, in this case, lever-pressing. It was proven through the use of probe tests that the rats didn’t apply a timeframe to determine when the safe period would be, but rather “counted” the number of shocks.^[14]  The term “count” is used loosely when referring to this action because it has not yet been determined if rats utilize the same complex processes as humans to enumerate items.^[15]

Davis and Memmott also determined that rats behaved in relatively the same manner whether the three shocks were sent out early, later or spaced out during the experiment. Factors that diminished the observed behavior were stimuli such as sounds or odors added just before or with the provided shock.^[14]

This is thought to come about because the rats do not need to “count” the shocks anymore since they are provided with other signs that warn of approaching danger. It was also discovered in a subsequent study by Imada et al^[16],[17] that an optimal session time frame is required to replicate these results since a five-minute test session did not provide the same behavior from the rats as the longer, thirty-minute session did.

Although “counting” in rats is an unnatural behavior that occurs in extreme situations, it provides researchers with a better insight into the hidden abilities of this species and more opportunities to test intelligence.

Varied Number Tasks

The numerical abilities of rats have been demonstrated in multiple research studies following Davis and Memmott’s initial one. These included training rats to only consume a set number of food pieces,^[18] to distinguish how many times they have been petted^[19], and to travel through the correct numerically positioned tunnel from a choice of six different ones.^[20]

Davis et al^[19] were even able to train rats to determine which particular arm of a Y-maze they must travel through based on the number of tactile cues provided. For example, they entered one arm when two or four vibrissal deflections occurred and the other arm when only presented with three deflections.

When rats were tested on the ordinal position task twelve, and then again eighteen months after the initial study, the results were consistent with that of the original test. Therefore, indicating that they also have a good numerical memory and that they could retain, in many situations, what they have learned for most of their lifespan.^[7]

Logic Task

Davis et al^[7] conducted a logic test on rats using the methodology described by Gillian^[21] in which they used chimpanzees. Whereas Gillian trained the chimpanzees to compare colors, Davis trained the rats to compare olfactory stimuli. Davis first presented the rats with only two scents, training them to choose the one over the other (e.g., A > B), once this was correctly established by the rats, they introduced the next scent in the same manner (e.g., B > C) until all five scents were introduced (A > B > C > D > E).

To increase the validity of the experimental procedure, Davis ensured that each subject was given a different preferential sequence. For example, rat 1 may have been trained to have a preference for the smell of chocolate over banana, while rat 2 was trained to rather have a preference for the smell of banana over chocolate. This was to limit any species-specific preference for certain scents.^[7]

Once all the subjects had been adequately trained to identify their four premise pairs (A > B, B > C, C > D and D > E), they received a novel test in which they had to choose between two scents, B and D, that had not been directly compared. The correct response would be to choose scent B over scent D, which is exactly what was observed by the researchers.

Without the need for reinforcement or rewards, the rats responded just as well to the logic test as Gillian’s chimpanzees did and even gave researchers more revealing data to work with.^[7],[21] When an inconsistent scent was added, e.g., scent F that had not been previously introduced, the rats showed a decline in their performance of the logic task.

Davis was not able to identify the specific processes used by the rats to complete this logic task, but it is also not exactly known how human subjects come to logical decisions either.^[7]

Human Recognition Task

Davis and Norris designed a test to determine if rats are able to recognize a familiar human.^[22] The rats were given a period of time to bond with their particular human in which they were fed and petted. Once each rat was properly bonded to their human, they were given a test where they had to choose between two humans, one being familiar and the other being a stranger.

Significantly more rats correctly chose their familiar human over the stranger, indicating a skill for human recognition.^[22] These results were even observed when the time spent bonding was decreased and feeding was not conducted by the bonding human.^[7]

The reason for the rat’s ability to recognize the correct human stems from their great olfactory skills. They made use of olfactory cues when selecting between the humans.

Knowing which areas of the brain are most activated when certain intellectual activities are occurring, greatly assists when researching the properties of intelligence. Studies using brain imaging technology provide an opportunity to see inside brains and try to correlate which areas are responsible for intelligence.

The earliest anatomical studies in humans discovered that nearly all brain areas were associated with intelligence. Researchers made use of voxel-based morphometry (VBM) in an attempt to better distinguish which brain areas were most responsible. They found a relationship between cortical thickness and intelligence in several association areas of the temporal and frontal lobes.^[23] It has also been discovered that certain brain areas are associated with certain types of intelligence.

Human and rat brains are structurally and functionally similar enough to compare research between the two.^[24] The smaller and less complex brains of rats provide researchers with an easier opportunity to study systems that are too complicated in humans.

It has been discovered that the hippocampal and intrahippocampal regions in rats’ brains are responsible for working and spatial memory.^[25] These are the brain regions that will be focused on in this article. When viewing the hippocampus’ cytoarchitecture, it can be split up into a number of subregions, the three mentioned in this article will be the dentate gyrus, CA1 and CA3.^[25] Projections connect these areas and establish excitatory circuits, two of which are the temporoammonic and trisynaptic pathways.

Complete Hippocampal Lesions

Morris et al^[26] compared rats that had their entire hippocampal regions removed with rats that kept this area intact (normal). The rats were subjected to the Morris water maze and had to locate a fixed platform, either hidden or visible. The hippocampal-lesion rats showed a higher deficit when compared to the normal rats in locating the hidden platform, taking much longer to locate it.

When tested with a visible platform, the hippocampal-lesion group did notably better than their performance when it was hidden, and more in line with the normal group.^[26] The results from this study show the importance of the hippocampus in place-navigation and how removal of this area can cause detrimental consequences on rats’ ability to learn and remember navigation.

Hippocampal Subregion Lesions

With the importance of the hippocampus in mind, Okada and Okaichi set out to determine exactly which subregions of the hippocampus are responsible for spatial memory and if they are dependant on one another.^[25] They induced lesions in different areas of the hippocampus in rats using a neurotoxin injection and then tested the rats in a trial-unique platform position task (SM-DMPT) and the Morris water maze task (M-WMT), used to test spatial working memory and spatial reference memory respectively.

After comparison of the various lesioned hippocampal areas, the researchers discovered that the dentate gyrus is highly involved in spatial memory tasks after the rats who had lesions in this area performed poorly in both the SM-DMPT and M-WMT.

Further analysis of the CA1 and CA3 areas indicated the importance of the connection between the two in processing spatial information. This was proven by rats with a CA1 lesion performing poorly in both the SM-DMPT and M-WMT; whereas those with a CA3 lesion performed normally in the M-WMT but not the SM-DMPT.^[25] It was also observed that those who had CA1 and CA3 ipsilateral lesions performed much better than the rats with CA1 and CA3 contralateral lesions.

Ventral and Dorsal Hippocampal Lesions

Hock et al^[27] conducted a study on rats who received a neurotoxin-induced lesion either in their ventral hippocampal region (VH) or their dorsal hippocampal region (DH). The researchers observed that the rats with a DH lesion had a decline in their performance with the delayed reinforced alternation of the T-maze which was not seen in the VH group.^[27]

This result is consistent with previous studies done on ventral and dorsal hippocampal lesions and suggests that there are a greater number of place cells in the DH and spatial data is mainly conveyed to this area of the hippocampus in rats. Therefore, indicating that the DH is responsible for spatial memory.

When tested with an internal state-conditional task, which evaluates conditional learning, both the VH and DH groups showed impairment in their performances. These results are thought to be due to the rats not being able to create associations with their internal state since the VH contains hypothalamic projections and information will be inhibited from connecting with other sensory information.^[27]

The reason for the deficit in the DH group is thought to either be due to a connection between the VH and DH in processing the internal state or that this task requires the processing of both external and internal information, and therefore, it’s necessary for both hippocampal regions to be intact.

There is an age-old debate about how dependent an individual’s intelligence is on their genetic make-up. Although it can be agreed that environmental factors also have a strong influence on someone’s development of intelligence, this is one of the most inheritable traits and recent studies have discovered 1,041 genes related to cognitive abilities in humans.^[23] Therefore consideration of the genetic foundation of intelligence is of importance.

The two genes covered in this article will be the PDE4B and the CRBN genes.

Mutation of the PDE4B Gene

The phosphodiesterase 4B (PDE4B) gene is involved in the formation of the hippocampus and modifications of this gene have been linked to bipolar and schizophrenia due to its ability to bind DISC1.^[28],[29] PDE4B plays a vital role in signal transduction as it controls intraneuronal concentrations of cyclic adenosine monophosphate (cAMP) by hydrolyzing this messenger.

PDE4B was genetically engineered in mice so that cAMP hydrolyzation was decreased and CREB phosphorylation was increased.^[28] The mutant mice were observed to perform much better cognitively compared to normal mice due to the higher levels of cAMP causing improved long-term potentiation which assists in learning and memory. This enhancement in cognitive abilities extended to their capabilities in solving problems, remembering locations in mazes and previous mice they had met before.

Another effect of the PDE4B mutation was the decline in the mice’s anxiety, encouraging them to explore their surroundings more. Fear-memory was also observed to be completely lost within a week, suggesting the benefit this mutation may have on inhibiting PTSD.^[28]

An additional, and exciting, effect of the PDE4B mutation was evidence of neurogenesis occurring and the increased number of neural connections. This is not only advantageous in improving cognitive function but could also prevent neurodegeneration and brain cell death. A result that can lower the risk of neurodegenerative diseases such as dementia and Alzheimer’s.^[28]

CRBN Knockout Mice

The Cereblon (CRBN) gene is expressed in all circuits of the mouse hippocampus and is believed to be linked to brain development in rodents.^[30] Normal activity of CRBN results in the subsequent decrease of AMP-activated protein kinase (AMPK) activity. Increased activity of AMPK, observed in CRBN mutations, has been shown to cause a deficit in memory, learning, and synaptic plasticity.^[31]

A nonsense mutation of the CRBN gene situated on chromosome 3p26.2 is associated with intellectual disability in humans. The fact that CRBN is the main target of the teratogenic effects of thalidomide, connected to the development of autism, further highlights the importance of this gene being studied.^[31]

Bavley et al^[31] presented a CRBN knock-out (KO) mouse model that can be used to study isolated intellectual disabilities. The KO mice were created by first crossing CRBN-floxed mice on the C57BL/6J background with Cag^cre/+ deleter mice, resulting in progeny that had the CRBN gene deleted in all tissues by the Cre recombinase.^[31]

The KO mice were observed to have inadequate memory and learning that was dependent on the hippocampus compared to wild-type mice, as well as hyperphosphorylated levels of AMPK. This was revealed by impairments in expressions of synaptic proteins and long-term potentiation.^[31] These results were reinstated when the CRBN KO occurred in the dorsal hippocampal region.

KO mice did not present with any repetitive behaviors, abnormal social behaviors, or irregular working memories; a significance that encourages the use of this model to study isolated intellectual disabilities, as well as the neurobiological processes related to memory and learning.

There is extensive clinical significance in the study of intelligence. The results obtained in research can assist with a better understanding of the foundational aspects of intelligence. This can be applied to designing improved intellectual tests for identifying children who may need academic help or in the screening process of potential job applicants.^[1]

Studies of intelligence can also be used to create pharmaceutical agents that may enhance an individual’s cognitive ability, something that is desired by many people especially with life expectancy increasing.

Another advantage of this research is its use in the design of artificial intelligence (AI). These systems are created based on the knowledge garnered by neuroscience discoveries since artificial neural networks imitate known brain connections.^[32],[33] The more studies that can be done to better understand which pathways interact to form intelligence, and how they work together, the better the AI systems generated will be.

This article has mentioned models used to study intelligence in rodents. In order to cover the large variety of factors affecting intelligence, there are many different types of models available to study this intriguing trait.

When studying intelligence, it is important to remember that a number of different factors influence its development, occurrence, and potential. All these must be considered and therefore, a blend of the above-mentioned models is recommended. Studying factors that are comparable between rodents and humans will also be of benefit.

Knowing exactly which genes, brain regions, and cell types are involved with intelligence will advance the field of neuroscience and therefore a focus on this area of research is advisable.

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23. Goriounova, N.A. and Mansvelder, H.D. Genes, Cells and Brain Areas of Intelligence. Front. Hum. Neurosci. (2019): https://doi.org/10.3389/fnhum.2019.00044
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