Category Archives: Human Development

Mental Retardation and Dendritic Spines


Dendrite is Greek for “tree-like” and to explain what they do in the simplest terms possible, they receive electrochemical signals from other neurons and then pass these signal down to the soma or neural cell body. Dendrites play a critical role in determining the frequency of the action potential, which drives the electrical signal down axons of the body of the neuron towards the axon terminals. Dendrites are so essential that their architecture is a great indicator of the complexity of our neural connections. In fact, our brain function depends on strong synaptic connections, connections which are cultivated during infancy and early childhood.

Unfortunately, as with all things complex, sometimes something goes wrong in the developing process. Mental retardation occurs when there is a disruption in this early refinement of dendrites that results in cognitive impairment severe enough to disrupt adaptive behaviour. There is a wide array of genetic disorders and poor environmental conditions that can result in mental retardation. For example, Down Syndrome and PKU (both genetic disorders), accidents during pregnancy and childbirth, maternal infections with rubella, Fetal Alcohol Syndrome and environmental impoverishment. Poor environmental conditions in young children such as poor nutrition, isolation and neglect can even result in brain damage severe enough to cause damage to these sensitive dendrites.

spine 2

Healthy dendrites have spines that look like small balloons that hang of the dendrite. In cases of mental retardation dendritic spines are very thin and long, resembling the dendritic spines of a fetus. This is clearly seen in the top most image, a) and c) are healthy dendrites. This clear difference reflects the failure of normal circuits in the brain’s development. Studies by Marin-Padilla and Purpura have discovered a correlation between extent of dendritic spine damage and degree of mental retardation.


Bear, Mark F., Barry W. Connors, and Michael Paradiso. “Neurons and Glia.”Neuroscience: Exploring the Brain. Baltimore, MD: Lippincott Williams & Wilkins, 2006. 43. Print.

Images courtesy of google images.


The Sleep Cycle

Sleep is a confusing topic of study because despite every human needing it, no one really knows with 100% certainty why we do it. There is no single theory that is accepted by the entire scientific community; however, two main hypotheses seem most fitting. First, the restoration hypothesis proposes that we sleep to rest and recover, a preparation for when we wake. What is being restored remains unclear. The second hypothesis proposes that sleep is an adaptive process that keeps us out of trouble for a large chunk of the day: hiding us from predators and conserving our energy reserves. The only thing we do know about sleep is that we cannot survive without it.

The Stages

The majority of our sleep (75%) is spent in non-REM (rapid-eye movement) sleep and occurs in four stages. An entire sleep cycle takes approximately 90 minutes and is classified as a form of ultradian rhythm (faster than circadian which is a 24 hour cycle).

Stage 1: The Transitional Stage

Stage 1 is known as the transitional stage as it is when our brain waves become less regular and begin to wane. If you were to look at an EEG we would pretty much look wide awake. You have probably felt the sensation of shifting between the two stages if you have ever fallen asleep in class or in front of the TV. That sense of falling is due to the slowing of our brain waves. Activity levels begin to change from alpha to theta waves; they are high amplitude but very low frequency. Due the small difference, however, stage 1 is the lightest stage of sleep and we are easily woken. Our eyes are making slow, rolling movements and the whole stage only lasts a few minutes. All in all though, it only occurs at the beginning of sleep and only lasts 5-10 minutes.

Stage 2: The First NREM Stage 

Stage 2 marks the true beginning of non-REM sleep. During stage 2, our brain waves become slightly deeper and occasional variations in wave movement (oscillations) occur between 8-14 Hz occurs. These oscillations are called sleep spindles and are produced by thalamic pacemaker cells.  It is proposed that sleep spindles occur because the brain is trying to inhibit processing to ease the transitional into sleep. Another characteristic stage 2 indicator is the K-complex. K-complexes are a high-amplitude, sharp wave and the largest brain event during sleep. Scientists believe that K-complexes help suppress arousal and aid in memory consolidation. Lastly, another indicator of transition from stage 1 to stage 2 sleep is that eye movements cease.

Stage 3 and 4: The Delta Rhythm Stages 

During stage 3, an EEG would show large-amplitude, low rhythm delta waves. Eve movements as well as body movements will be usually be absent. Stage 3 can also be seen as a transitional stage but between light and deep sleep. As we enter into deep sleep our body and brain become increasingly less sensitive to stimuli and less susceptible to arousal. Common childhood sleep issues such as bed wetting, night terrors and sleep walking tend to occur towards the end of stage 3. The main difference between stage 3 and 4 is the amount of delta waves. When less than 50% of deep sleep is delta, we are in stage 3. When more than 50% of our brain activity is delta waves, we are in stage 4 of sleep. As delta waves correspond with very deep sleep, a person in stage 4 of sleep is the hardest to wake. This can be extremely scary if a person is sleep walking or a child is having night terror. They may seem awake but they are completely unresponsive to external stimuli. Stage 4 of sleep only happens during the first cycle as such, our sleep becomes lighter throughout the night. This is extremely helpful as it prepares our body for waking. From an evolutionary stand point this makes sense. Our ancestors certainly could not set an alarm clock, the noise of other animals and the rising sun needed to be sufficient to wake us.

REM Sleep 

REM sleep is probably the most exciting and important of all the stages of sleep. REM stands for rapid eye movement and is suitable name for this stage as an EOG would show rapid eye movement under our eyelids. During this phase we also experience dreaming. Our brain activity mimics waking, showing a myriad of different brain waves: alpha, beta and dysynchronous waves. Despite our brain activity showing an incredible change in activity, our muscles are actually paralysed. A scary albeit common phenomenon known as sleep paralysis is when we wake-up during REM sleep and our muscles remain effectively paralysed.  During sleep however, this “paralysis” is known as sleep atonia. It is a beneficial process as it prevent us from acting out our dreams and putting ourselves in harms way. Certain neurons in our brain stem (specifically the tegmentum) known as REM sleep-on cells release monoamines inhibit motor neuron activity. Another curious attribute of REM sleep is the incredible recall of a person woken from it. Our dream world becomes as real and vivid as the real world. As REM mimics wakefulness, waking someone during REM means they will feel very alert. The exact reason for REM sleep is as elusive as sleep itself; however, scientists do know that like sleep, it is vital. When a person is repeatedly disturbed during REM sleep or does not get enough sleep in general, we go through a process called REM rebound. In other words, we spend the majority of our following sleep in REM. Lastly, scientists have also discovered that newborns and foetuses spend the majority of their sleep in REM. All in all, these findings suggest REM is vital to proper human functioning and development.

In Conclusion 

As we progress through the night we spend increasingly less time in stages 3 and 4. After the first cycle stage 1 of sleep is replaced by REM sleep and the amount of time spent in REM sleep increases. Even though the reason why we sleep is unclear, the change in brain activity and the determent of not sleeping is enough to say with certainty that sleep is necessary for normal human function.


Bear, M., & Connors, B. (2007). Neuroscience: Exploring the brain (3rd ed.). Philadelphia, PA: Lippincott Williams & Wilkins.

Dement, W.C. (1978). Some must watch while some must sleep. New York: W.W. Norton.

Hall, R. (1998, January 1). Stages of Sleep. Retrieved September 14, 2014.

K-complex. (2014, August 24). Retrieved September 14, 2014, from

Pinel, J.P.J. (1992). Biopsychology. Needham Heights, MA: Allyn & Bacon.

Sleep spindle. (2014, August 24). Retrieved September 14, 2014, from

The Development of Speech and Conversation (Age 0-5)


Communication at any age requires a skill set including the obvious such as a proper vocabulary and grammatical accuracy. For a child, a conversation is far more difficult because vocabulary and grammar are not the only things that are still developing. The five years leading up to this communicative breakthrough also depends on the phonological and social development of the child.

Component 1: Phonological Development 

Firstly, the quintessential ingredient to verbal speech of any kind is phonological development. When infants are born they all have the ability to perceive the sounds and tones used in all languages. Experience with their own language over the first year of their life, slowly allows them to tune into the phonemic contrasts between their own language over others (Smith et al. 2011). For example, when a Japanese baby is 8 months old, they can distinguish /r/ and /l/ sounds; however, by the time they are one, they no longer can (Purves, 2001).

This may not seem like a useful skill; however, it is necessary for an infant to become an expert at their own mothertongue. Adults play a quintessential role by building on the biological rhythm of their babies developing speech. By 10 months, the canonical babbling comes to reflect the prosody of their surrounding language of their parents (De Boysson-Bardies, 1993). Clearly when infants are this young, it is essential for close proximity between caregiver and infant. Characteristics of contingent talk such as exaggerated facial expressions, repetition and eye-contact ensure optimal attention. Without this attention, the infant would not gain the benefits of contingent talk and their vocabulary acquisition would suffer.

Component 2: Acquisition of Vocabulary 

Hand in hand with phonological development is the acquisition of vocabulary. The first step of any child’s vocabulary acquisition is contingent talk. Contingent talk is when a caregivers ‘scaffolds’ learning by talking about what the infants is already attending to (Carpenter et al. 1998). Various forms exist including child-direct speech, following in, expansion and clarification and all of these forms help co-regulate intentions (Fogel, 1993). In other words, both the child and the caregiver work together to improve the language of the child. As the name suggests, scaffolding language allows the caregiver to just teach their child new words, but also to improve the quality of their word choice and the coherence between words.

Furthermore, contingent talk is instrumental in the development of theory of mind (Astington & Baird, 2005). In relation, understanding the intention of a caregiver also helps build in a child’s vocabulary. As a paradigm, when an experimenter tells an 18-month-old that they are looking for a ‘toma’ and then proceeds to look for it, rejecting objects until settling, with satisfaction for a final object, the majority of 18-month-olds will infer that this final object is the ‘toma’ (Tomaselo, 2003). Lastly, what all of these skills lead up to is the 100 word transition phase. After a child learns his or her first words, the next couple of months consistent of holophrases such as “nomore” until they have learnt about 100 words. Beyond this point, at around 18 months, children switch from holophrases to telegraphic speech (Bates et al. 1995). Three to four word phrases then begin around 24-27 months, the stage when grammar acquisition becomes important.

Component 3:Acquisition of Grammar 

Grammar becomes increasingly more important around 24 months because now word order actually begins to change the meaning of speech. The two major components of grammar are syntax and morphology. Syntax is the organisation of words into larger structure like sentence; who did what to whom. Understanding knowing who did what to whom or the agent-patient relationships is vital to communication. Around the age of three, children start to show understanding on this relationship. Tomasello et al. (1998; 1999) taught children made-up verbs and by age three, the children were able to act out appropriate actions and modify the agent-patient relationships to fit new scenes.

Once this skill has been attained, children can also start using synaptic bootstrapping; using grammatical information to infer the meaning of unfamiliar words (Gleitman, 1990). This is particularly useful when a child is having a conversation with anyone with a broader vocabulary, allowing them to carry on speaking without actually knowing every word. To continue, understanding morphology or word structure like plurals, possession, tense etc. allows children to use words productively in conversation. The wug test (Berko, 1958), suggests that understanding morphemes develops between the age of four and five. Understanding both syntax and morphology helps a five year old get around the barriers of having a conversation with someone more versed than them.

Component 4: Developing Pragmatics 

Finally, the acquisition of pragmatics is also a necessary tool for holding a conversation, regardless of age. The first pragmatic skills occurs even in infants, maintaining eye-contact with the people speaking to them. Maintaining eye-contact and eventually responding with smiles and sounds lets the other speaker know that the child is paying attention. Eventually, around 11 months this skill helps develop joint attention whereby the child can actually direct their caregivers attention (Smith et al. 2011). By a child is five, expressing intent is necessary to get their point across, setting the topic and for taking turns in speaking (Bates, 1976; Strivers et al. 2009). Finally, developing an understanding for implicature and referencing is also a necessary skill for conversation. Studies show that by age five children have learnt that they can use pronouns to refer to people, persons and things which are clearly seen or recently spoken off (Matthews et al. 2006). Until this age, to refer to things children will point to remain ambiguous. Understanding implicature like “I ate some cake” meaning “some” and not “all” also develops by around five years of age. Both of these skills are extremely helpful tools for a five year especially for clear communication and reduces ambiguity and confusion. Fortunately, by this age, should ambiguity remain, a five-year old will actively search the scene for clues that will reduce confusion. In other words, by age five, children have learnt the pragmatic skills to steer them through must conversational confusion.


Astington, J. W., & Baird, J. A. (2005). Why language matters for theory
of mind. Oxford, England: Oxford University Press.

Bates, E. (1976). Language and context: Studies in the acquisition of pragmatics. New York: Academic Press.

Berko, J. (1958). The child’s learning of English morphology (Doctoral dissertation, Radcliffe College).

Carpenter, M., Akhtar, N., & Tomasello, M. (1998). Fourteen-through 18-month-old infants differentially imitate intentional and accidental actions. Infant Behavior and Development, 21(2), 315-330.

Caselli, M. C., Bates, E., Casadio, P., Fenson, J., Fenson, L., Sanderl, L., & Weir, J. (1995). A cross-linguistic study of early lexical development. Cognitive Development, 10(2), 159-199.

De Boysson-Bardies, B. (1993). Ontogeny of language-specific syllabic productions (pp. 353-363). Springer Netherlands.

Fogel, A. (1993). Developing through relationships. Chicago: University of Chicago Press.

Gleitman, L. (1990). Structural sources of verb learning. Language Acquisition, 1, 1-63.

Matthews, D., Lieven, E., Theakston, A., & Tomasello, M. (2006). The effect of perceptual availability and prior discourse on young children’s use of referring expressions. Applied Psycholinguistics, 27(03), 403-422.

Purves, D., Augustine, G. J., Fitzpatrick, D., Katz, L. C., LaMantia, A. S., McNamara, J. O., & Williams, S. M. (2001). The Development of Language: A Critical Period in Humans.

Smith, P., & Cowie, H. (2011). Understanding children’s development (5th ed.). Chichester, West Sussex: Wiley.

Stivers, T., Enfield, N. J., Brown, P., Englert, C., Hayashi, M., Heinemann, T., Levinson, S. (2009). Universals and cultural variation in turn-taking in conversation. Proceedings of the National Academy of Sciences, 106(26)

Tomasello M. 1998. Reference: intending that others jointly attend. Pragmat. Cogn. 6:219–34

Tomasello M. 1999. Perceiving intentions and learning words in the second year of life. See Bowerman & Levinson 1999. In press

Tomasello, M. (Ed.). (2003). The new psychology of language: Cognitive and functional approaches to language structure (Vol. 2). Psychology Press.