In my first post, I had mentioned that as babies my girls kept reminding me of rats, my former experimental subjects. Although I am currently a stay-at-home Mom and no longer actively practice science, I sometimes try observing my children as if they were rats, detecting their developmental milestones from an animal behaviorist’s point of view.
Perhaps I simply spent too much time with rats in my career. Since undergraduate years, I spent many many many hours with rats. So many that I have developed rat allergies. By the time I was a postdoc, I could not survive 5 minutes with a rat without a full protective gear including a respirator mask.
As a systems / behavioral neuroscientist, my interests lay in understanding how memory is formed. With that goal, I trained thousands of rats in various memory tasks over the duration of almost twenty years. Unlike with humans, who can verbalize what or whether they remember, we behavioral neuroscientists rely on detecting memory based on what animals do, and design tasks around it.
For the types of experiments my former lab conducted, in general we ordered rats at 250-275 grams, which is about 8 weeks old. When they arrive at the lab, they are pups, small and exuberant. Due to their sedentary lifestyle, er, I mean a living condition we put them in, within 3-4 weeks they become about 400-500 grams, monstrous, stout, and rigid. Because we do not want stress to confound their abilities to learn and form memory we “handle” rats for ~5 days before a start of behavioral training. We want rats to get used to being picked up and down, and to be comfortable enough to explore surroundings. For 5 minutes a day, we “play” with rats, picking them up from their cages, give them massages in the back of the neck and all over, and have them walk on our toweled, lab coat-covered lap. For any given experiment we work with 15 – 30 rats at a time, so we may sit with them 3-4 hours a day during the handling phase of an experiment (time to listen to audio books).
When rats are that young, they are way cuter than adult rats. Once they had gotten used to me, their tiny dainty fingers would wrap around and cling to my gloved finger. Their bright red eyes gaze at me with a curious look. They sniff everything and the top of their nose is a bit moist. They would often tug my lab coat as they try to climb onto my chest or play with the buttons of my lab coat. Their body is so soft and supple, with shiny fur. If given an opportunity, they would snuggle their noses in any warm and dark space, in this case under my hand or armpit.
Now just replace “rats” with “my babies” in the previous paragraph. My babies did pretty much do all that, except they have bright brown eyes and creamy smooth skin. Ok, their noses are not as moist, either.
Many memory tasks in the rodent model utilize motivation or reinforcers to get them to do something, and we score presence or absence of certain behavior and conclude it as memory. For example, rats can swim in water but do not like to — so we dump them in rather cold water (~72 F) to have them learn the location of a platform where they can stand. There are also tasks that utilize electrical shocks — rats learn to fear the shock, so they exhibit a freezing behavior (absence of all movement except for heart beating) when they are put in the context where they previously received the shock(s) (contextual fear conditioning) or avoid the place altogether where they received the shock (inhibitory avoidance). We measure the length of time that they freeze or avoid and use it as an index of memory.
As apparent, not all of animal or human memories are fearful or stressful. Tasks that do not involve aversive stimuli in the rodent model are more subtle. Some utilize rodents’ novelty-seeking behavior. For instance, in context-object discrimination task rats are placed in two different contexts with benign objects and learn the relationship between the context and object. The two contexts differ in shape, surface texture, lighting, and room. In each context, there are two objects specific to the context. Objects A (e.g. yellow balls) in context A, and objects B (wooden cube) in context B. On Day 1, rats explore the two contexts for 5 min each, separated by 25 min. On Day 2, the rats explore the same contexts in reverse order, the same amount of time. On Day 3, the testing day, we place one Object B next to one Object A in Context A, and we measure time a rat explores each object. If the rats have formed the memory of the relationship between the context and object, then the rats would detect the mismatch of the object B in Context A, and spend more time exploring object B. We have used this task to study the importance of hippocampal plasticity in formation of this type of memory .
I have, accidentally, conducted the exact task on my older baby L. In our old apartment where L spent first two years of her life, the living room was where we mainly stayed and L played. The changing table was in L’s nursery, ~20 steps from the living room in a separate room. We kept the same toys in the living room, and a separate set in her nursery. When L was around 6 months, she started to fuss during diaper change. When we gave her a toy during diaper changes, she was more cooperative. The same toy remained at the changing table. One day however, there was a toy switch. A “living room toy” with which she was saturated (i.e. she lost interest) appeared at the changing table. When this toy was given to her during diaper change, L spent significant time playing with the toy, much longer than the usual toy at the changing table or the amount of time she spent with the toy in the living room in recent days. A light bulb flashed in my head — “context-object discrimination!” This was my first time to vividly witness relevance of my laboratory rat experiments in humans. It was also a relief to know that my baby was developing and had cognitive capabilities (at least) equivalent to mature rats.
In order to perform this task, the hippocampus is required. I read somewhere then (and I now could not find the article for citation) that the human hippocampus fully matures by the age of 6 – 8 months. It made sense to me that she now could perform this task. I had wished that I routinely tested L in this task to determine at what age she could do this task for the first time.
When L was just shy of turning 2 years old, she undeniably exhibited a 10-hour retention of information. When we left our house one morning, we noticed a snail crawling on a leaf just outside our door. We stopped, crouched, and observed the snail for a while. In the evening after picking up L at her day care and coming home, without any provocation L crouched at the same spot and looked for the snail (grown-ups did not bother to look, since we knew the snail would have been gone by then). At that time I did not expect a 22 month-old to retain that type of explicit information for that long period of time, even after processing so much interfering information at day care. Her verbal skills were still immature – I was still relying on what she did to conclude her memory.
As L becomes older, and is more capable of verbalizing, I am always amazed to discover what she remembers. Just the other day as I drove in the car with her she saw a restaurant out of the window and blurted out that this was the restaurant where Mommy forgot her credit card and had to go back to retrieve it. It had to have been at least 6 weeks since that event, and we never talked about it in between. She has, what memory researches might categorize, a solid “episodic memory” of that event.
Sometimes I wish I could systemically test my daughters’ memory capabilities. Sometimes I wish I could just open up their brains and examine what is happening. For now, I’m just thankful that my girls seem to be getting smarter than rats.