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These stages are not the same as the [[trimesters]] of a woman's pregnancy.
The germinal stage least resembles a grown human. It begins when a [[sperm]] penetrates an [[egg]] in [[The Act|the act ]] of [[conception]] (normally the result of [[sexual intercourse]] between a [[man]] and a [[woman]]). At this point a [[zygote]] is formed. Through the process of [[mitosis]] the cells [[divide]] and [[double]].
The embryonic stage occurs once the zygote has firmly implanted itself into the uterine wall. It is in this stage that the [[vital organs]] are formed, and while the external [[body]] is still extremely dissimilar from an [[adult]] human, some features such as eyes and arms, and eventually ears and feet become recognizable.
Infants respond to stimuli differently when in these different states. [[Habituation]] is frequently used in testing [[psychological phenomenon]]. Both infants and adults look less and less as a result of consistent exposure to a particular stimulus. The amount of time spent [[looking]] to a presented alternate stimulus (after habituation to the initial stimulus) is indicative of the strength of the remembered percept of the previous stimulus, or dishabituation.
Habituation is used to discover the [[resolution]] of perceptual systems, for example, by habituating a [[subject]] to one stimulus, and then observing responses to similar ones, one can detect the smallest degree of [[difference]] that is detectable by [[The Subject|the subject]].
Infants have a wide variety of reflexes, some of which are permanent (blinking, gagging), and others transient in [[nature]]. Some with obvious purposes, some are clearly vestigal and some do not have obvious purposes. [[Primitive]] reflexes reappear in adults under certain conditions. Namely, neurological conditions like [[dementia]], [[traumatic]] lesions, etc. A [[partial]] [[list]] of [[infantile reflexes]] includes:
When studying infants, the habituation methodology is an example of a method often used to assess their performance. This method allows researchers to obtain information about what types of [[stimuli]] an infant is able to discriminate. In this [[paradigm]], infants are habituated to a particular stimulus and are then tested using different stimuli to evaluate discrimination. The critical measure in habituation is the infants' level of interest. Typically, infants prefer stimuli that are novel relative to those they have encountered previously. Several methods are used to measure infants' preference. These include the high-amplitude sucking procedure, in which infants suck on a pacifier more or less depending on their level of interest, the conditioned foot-kick procedure, in which infants move their legs to indicate preference, and the head-turn preference procedure, in which infants level of interest is measured by the amount of time spent looking in a particular direction. A key feature of all these methods is that, in each [[situation]], the infant controls the stimuli [[being]] presented. This gives researchers a means of measuring discrimination. If an infant is able to discriminate between the habituated stimulus and a novel stimulus, they will show a preference for the novel stimulus. If, however, the infant cannot discriminate between the two stimuli, they will not show a preference for one over the other.
[[Object permanence]] is an important stage of cognitive development for infants. Numerous tests regarding it have been done, usually involving a toy, and a crude [[barrier]] which is placed in front of the toy, and then removed, repeatedly. In sersorimotor stages 1 and 2, the infant is completely unable to comprehend object permanence. [[Jean Piaget]] conducted experiments with infants which led him to conclude that this [[awareness]] was typically achieved at eight to nine months of age. Infants before this age are too young to [[understand]] object permanence, which explains why infants at this age do not cry when their mothers are gone. "Out of sight, out of mind." A [[Lack of object|lack of Object ]] Permanence can lead to [[A-not-B error]]s, where children reach for a [[thing]] at a place where it should not be.
===Toddlerhood===
