What is the difference between sensitive period and critical period

How long do sensitive periods last? During their first six years of life, children move through five main categories of sensitive periods, including: order, language, sensory skills, movement, and social skills. Each sensitive period lasts for as long as it is necessary for a child to complete a particular stage in their development. What is an example of a sensitive period? An example of a sensitive period occurs in vision development. Infants are born with the basic ability to see unless their vision is impaired by prenatal damage or genetic defects , but a newborn's vision is not as good as the vision of an 8-month-old.

What is sensitive period in psychology? Sensitive periods are periods of psychological development in the child. This period is a time of limited duration. During the sensitive periods, the child has very powerful capacities. She based the majority of her method on these sensitive periods. What was the critical period? More specifically, the "Critical Period" refers to the period of time following the end of the American Revolutionary War in to the inauguration of George Washington as President in During this time, the newly independent former colonies were beset with a wide array of foreign and domestic problems.

This is why it's so important for parents and caregivers to understand how their children are growing in all ways and channels and to know what stimuli, or stuff, they need to give their children to help them thrive.

From time to time children without any cognitive or physical problems at birth may not be able to develop certain milestones during the stage or time period they are most receptive. There may be an injury, illness, caregiver neglect or abuse, or a shortage of needs such as food or medical care, that make it difficult for a child to absorb all the basic building blocks and stimulation they need to gain certain abilities at certain times in life.

When this occurs, affected children will generally have a harder time gaining those abilities even if they later get special attention and resources designed to help them compensate. It's like children have a window of opportunity when they are ready to grow in certain ways if they have the right stuff and tools in their environment.

When that window closes, it will never be as easy to grow in those ways again. Theorists disagree about how important it is for children to have that special stimuli at each growing stage in order to reach their milestones. Some theorists call these times critical periods, but other theorists call them sensitive periods. The difference between critical periods and sensitive periods is subtle. Scientists have found that when cochlear implants are installed to bypass the non-functional inner ears in these children before age 3.

When applied to language learning, the Critical Period Hypothesis states that there is a critical time during which individuals are more capable of acquiring new languages with native-like proficiency. The original hypothesis was first popularized by Eric Lenneberg, a linguist and neurologist, in a landmark book Biological Foundations of Language in According to this theory, the process of learning a new language is constrained by a critical period. There is a distinct discontinuity in outcomes between learning within the critical period and learning outside of it.

It may feel overwhelming that there are so many different critical periods in the brain development journey. Critical period is a controversial science concept because it implies there is a hard cutoff. If the skill is not developed during that time, the opportunity to develop this function will be gone forever. But some of those skills are actually experience-expectant rather than experience-dependent , meaning the stimuli required for development are expected.

The expected experiences are practically guaranteed to be available in everyday life, e. Parents rarely have to make an effort to introduce those common experiences.

Abilities that depend on the presence of specific experiences are experience-dependent. Parents need to provide the appropriate early life experiences for these skills to develop.

Some examples are emotional regulation, a second language, and the absolute pitch. But the good news is many experience-dependent traits have sensitive periods rather than critical periods. Even when the particular life experiences are missing during the optimal time, the skills can still develop.

It might just be harder or take longer. So the most important thing for parents to do is to provide a nurturing environment for your child and help your child build resilience.

The first neuroimaging studies used positron emission tomography PET to study the glucose metabolism of the occipital cortex at rest in both EB—individuals that become blind during the first few years of life see Box 1 —and sighted individuals Wanet-Defalque et al.

It was shown that the glucose metabolism observed in occipital cortex of blind individuals was greater than that observed in blindfolded sighted subjects, but comparable to what was observed when the blindfold was removed. These initial observations obviously raised important questions on the functionality of the EB's visual cortex. Subsequently, Uhl et al. Despite the impressive nature of the observed crossmodal activations in the occipital cortex, important questions still remained regarding their exact significance.

Are they truly task-related or simply an epiphenomenon associated with the absence of visual input? Several findings suggest that the occipital cortex does indeed play a functional role in processing non-visual information following early blindness. The first line of evidence stems from research demonstrating strong correlations between brain activity in occipital cortex of EB and behavioral performance on a variety of tasks including verbal memory Amedi et al.

This is perhaps not so surprising given the wealth of evidence documenting the development of heightened compensatory perceptual and cognitive abilities in EB see Voss et al. Auditory spatial abilities in particular have been heavily investigated in light of substantial questions concerning a blind person's ability to form adequate spatial representations in the absence of vision; consequently, an abundance of compelling evidence linking occipital functioning and sound localization in early blindness has been brought to light see Figure 1 ; see also Collignon et al.

Figure 1. Functional relevance of crossmodal plasticity. Illustrated here are demonstrations of the functional role played by the occipital cortex in spatial hearing tasks in early blind individuals. The top row panel A depicts the finding that occipital activity in early blind individuals black dots was predictive of their performance in a sound localization task Gougoux et al.

The bottom row panel B illustrates the effect that TMS has when applied to the occipital cortex black bars when both blind and sighted subjects were asked to localize sounds Collignon et al. Compared to Sham-TMS white bars , TMS applied over occipital cortex reduced the performance of early blind subjects only, which is indicative that this region is functionally relevant for spatial processing in the early blind.

Adapted with permission from Gougoux et al. Additional evidence supporting the functional relevance of the crossmodal recruitment of occipital cortices in early blindness comes from the use of trans-magnetic stimulation TMS which enables inferences on causality via the temporary disruption of cortical functioning within very specific brain areas.

Indeed, the application of TMS to occipital areas significantly hampers the performance of EB in tasks assessing sound localization Collignon et al. Perhaps the most striking form of evidence comes from a blind expert Braille reader, who completely lost the ability to read Braille following an ischemic stroke causing bilateral lesions to her occipital cortex Hamilton et al.

Similarly, a middle-aged blind individual was reported as having transient difficulties in reading Braille while he experienced temporary visual hallucinations Maeda et al. The fact that his ability returned to normal following the hallucinations suggests a causal relationship between occipital functioning and Braille reading in this blind individual.

Taken together, these findings suggest that occipital cortex might still serve some functional purpose following blindness. What is not clear at this point, however, is how these crossmodal plastic adaptations come to be? Properly understanding how non-visual sensory inputs are processed within occipital cortex is a challenging task and is discussed in the following section.

As highlighted earlier, many neural processes and connections are the result of competitive interactions between different neurons and sensory inputs, and as previously suggested by Pascual-Leone and Hamilton , visual inputs might actually gain access to occipital regions by means of such competitive processes with the other senses during early development. One popular hypothesis is that occipital cortex might be by design best suited to carry out predetermined specialized functions for which the visual system provides the most adequate sensory input.

For instance, regions specializing in the spatial processing of sounds in blind individuals appear to map onto areas of the dorsal visual stream known for similar processing of visual stimuli Collignon et al. Similarly, the visual word form area, which, as its name indicates, responds well to the visual presentation of words, has been shown to be highly responsive to tactually presented Braille words in EB subjects Reich et al.

Furthermore, Pietrini et al. Similarly, distinct regions within the ventral visual pathway of blind individuals show neural specialization for non-living and living stimuli in the auditory modality, suggesting that the conceptual domain organization in the ventral visual pathway does not require visual experience to develop Mahon et al.

Several studies have shown that this region in blind individuals becomes responsive to both tactile motion on the fingers Ricciardi et al. These findings, taken together, provide compelling evidence that the functional specialization of occipital regions is preserved in early blindness, and that the operations subserved by each region need not depend on visual input to be solicited by a given task.

Although many higher tier visual areas seem to have preserved there functional specialization following blindness, it is still undetermined how the non-visual input reaches occipital cortex.

Two obvious possibilities are either via already existing connections or through the establishment of new connections not present in sighted individuals. The latter, however, appears unlikely for at least two reasons. The first, as discussed later on, stems from a growing body of evidence demonstrating that crossmodal recruitment of occipital cortex is possible in normal sighted individuals after brief transient periods of visual deprivation, which suggests that already existing intermodal connections are at play [see reviews on potential multisensory pathways by Schroeder et al.

The second, results from animal work investigating the developmental synaptic pruning period in early infancy. It has been shown that corticocortical projections from auditory to visual cortex are present in infant kittens only to be soon after pruned away due to competitive processes Innocenti and Clarke, ; Innocenti et al.

However, in kittens deprived of vision at birth, these extrinsic connections to the occipital cortex seem to remain Berman, ; Yaka et al. These findings rather suggest that it is the strengthening of normally transient intermodal connections, and not the formation of new connections following blindness, that is likely to provide the substrate for the crossmodal innervation of occipital cortex following early blindness.

Research with animal models of blindness has illustrated several such pathways that could potentially mediate the crossmodal processing of sound in blindness. For instance, studies with blind rodents have shown the existence of connections between the inferior colliculus an important auditory relay and the lateral geniculate nucleus LGN—an important visual relay Doron and Wollberg, ; Izraeli et al. Alternatively, auditory information could be fed via direct connections between the medial geniculate nucleus MGN—an important auditory relay and the occipital cortex Laemle et al.

Furthermore, Karlen et al. Evidence in humans is a little sparser, but several recent findings also support corticocortical pathways between auditory and visual areas as a likely source for streaming auditory input into the occipital cortex. For instance, a recent diffusion tensor imaging DTI tractography study in normal seeing humans has revealed the existence of connections between Heschl's gyrus and the calcarine sulcus Beer et al.

Whether this pathway is different in blind individuals has yet to be established, although it perhaps need not be to subserve the crossmodal recruitment of visual areas by sound. Moreover, a pair of recent studies used dynamic causal modeling DCM to investigate the effective connectivity between regions underlying auditory activations in the primary visual cortex of EB individuals.

DCM is a powerful hypothesis-driven tool that allows for inferences on the causality between the activity observed in different brain areas and, analogously, to study how information flows in the brain Friston et al. It was found that auditory-driven activity in V1 is best explained by direct connections with A1 Collignon et al.

A final argument in favor of corticocortical pathways underlying auditory recruitment of occipital areas stems from neuroanatomical investigations showing the optic radiations geniculocortical tracts of EB humans to be severely atrophied Noppeney et al.

So far only research findings relating to early or congenital blindness have been covered see Box 1 , more or less ignoring the notion of critical periods. This is partly due to the fact that most research has primarily focused on the effects of early blindness, and also because, there is little consensus on the effects of late-onset visual deprivation.

The following sections attempt to disentangle the different findings relating to late blindness and to contrast them with those relating to early blindness. One of the first neuroimaging studies to investigate the occipital brain metabolism in EB individuals Veraart et al. It was shown that occipital functioning in LB was different from that of EB: while EB were found to have higher occipital glucose metabolism relative to sighted individuals, LB showed a reduction.

This finding obviously served as an early indication that the age of blindness onset was potentially a determining factor in the changes that occur in occipital cortex following visual deprivation. Indeed, a pair of early investigations of task-related activations showed that while crossmodal recruitment was observed in EB, no such observation was made in LB Cohen et al.

This finding suggested the existence of a strict critical period for the development of crossmodal plasticity within the occipital cortex 14 years of age: Cohen et al.

However, findings from a large number of other studies have since challenged this view. Kujala et al. This was later followed by a series of studies by Burton et al. Similarly, several auditory spatial tasks elicited occipital activations in late-onset blind individuals Voss et al.

However, these crossmodal changes were not accompanied by behavioral enhancements, as is the case in EB individuals, raising questions concerning the functional relevance of the observed crossmodal plasticity in LB. Despite some exceptions, there thus appears to be some agreement that crossmodal recruitment of deafferented visual areas is not exclusive to EB and can be observed in cases of late-onset blindness as well.

While this is the case, the crossmodal recruitment in LB appears to be nonetheless generally reduced both in terms of intensity and spatial extent relative to EB, suggesting that while the development of crossmodal plastic processes might not be bound by a critical period, it is definitely modulated by a sensitive period in early development during which reorganization is likely to be more pronounced.

Additional evidence supporting the existence of adult crossmodal plasticity stems from research investigating the effects of temporary visual deprivation in normal sighted individuals. One of the first studies to document such effects revealed that short-term light deprivation enhances the excitability of visual cortex.

Indeed, a brief period of visual deprivation was shown to not only induce a reduction in the TMS thresholds required for eliciting phosphenes but also lead to an increase in visual cortex activation by photic stimulation Boroojerdi et al. These findings were soon followed by research inspired by a school for the blind in Spain, which required that its instructors experience daily life without sight for an entire week during training Pascual-Leone and Hamilton, The instructors reported having heightened awareness for sounds, being able to better distinguish different speakers and to better orient themselves in response to incoming sounds.

To follow up on these reports, Pascual-Leone and Hamilton developed a protocol in which sighted volunteers would be blindfolded for 5 days. Preliminary findings revealed an increase in BOLD signal within the occipital cortex in response to tactile stimulation after 5 days of complete visual deprivation, and that this increase was no longer present the day following blindfold removal.

These findings indicated that rapid crossmodal changes can occur in the occipital cortex of adults when temporarily deprived of vision, and were further documented in Merabet et al. Remarkably, such crossmodal deprivation-related effects were limited to the blindfolding period and were rapidly reversible. Subsequent work has impressively shown that very short time periods of visual deprivation are sufficient to induce marked crossmodal changes in occipital cortex.

For instance, Weisser et al. In a recent study, we used a novel technique to determine whether occipital cortex processes auditory input in a similar manner to auditory cortex Lazzouni et al. We developed a blindfolding protocol to assess the effects of short-term visual deprivation on the auditory steady state response ASSR.

The ASSR can be defined as an electrophysiological response to rapidly changing auditory stimuli, where neuronal populations respond at the same frequency as the modulation rate of an amplitude-modulated AM tone and, importantly, for which the sources of the activity can be extracted using dipole analyses.

The ASSR therefore constitutes a powerful tool as it evokes a response that is intrinsically linked to the stimulus and can be tracked within the brain. The results showed that the two spectral peaks associated with the modulation rates of two dichotically presented stimuli 39 and 41 Hz were observed only within auditory cortex prior to blindfolding.

Following 6 h of visual deprivation, however, two peaks were also observed in occipital cortex see Figure 2 , thus shedding light on the timeline associated with short-term crossmodal recruitment of input-deprived sensory cortices.

This finding also demonstrates that visual cortex can display auditory cortex-like functioning in response to auditory input during periods of deprivation. Figure 2. Crossmodal plasticity in temporarily deprived sighted individuals.

This figure portrays a recent MEG finding that testifies to the impressive speed at which the visual cortex can display auditory cortex-like functioning following a short period of visual deprivation. The left graph shows that prior to blindfolding the two spectral peaks left temporal in red; right temporal in green associated with modulation rate of the auditory stimuli presented to both ears 39 and 41 Hz are clearly restricted to the temporal electrodes auditory cortex.

However, as shown in the right graph, the same peaks can now be found in visual cortex purple peaks following a 6 h visual deprivation period. Adapted with permission from Lazzouni et al. The previous sections documented multiple demonstrations of the crossmodal processing that occurs in the mature occipital cortex. However, an important question to ask concerns whether the plasticity observed in the adult brain is similar to what is observed in the visually deprived immature brain.

Indeed these findings point not only to quantitative differences i. Figure 3. How early and late blind differ. Illustrated here are two examples of how the crossmodal plasticity observed in early and late blind individuals differs. The top row panel A illustrates the differential effect TMS has when applied over the occipital cortex black bars of LB first bar graph and EB second bar graph on their performance in a Braille task, where only the early blind showed an increase in error rate Cohen et al.

The bottom row panel B consists in a schematic representation of how auditory information flows toward V1 in the congenitally blind and late blind, illustrating the DCM findings of Collignon et al. Adapted with permission from Cohen et al. As highlighted above, there is an abundance of evidence demonstrating the functional relevance of the crossmodal recruitment of occipital areas in EB.

Several studies have showed strong correlations between behavioral performance and occipital activity Amedi et al. Interestingly, there is little to no evidence of this in LB. This is likely in part due to the limited evidence of enhanced perceptual abilities in LB, as they are often found to be indistinguishable from sighted individuals in terms of performance.

The observed crossmodal recruitment in LB therefore seemingly doesn't lead to any behavioral gain as it does in the EB. This assumption is supported by data provided by Cohen et al.

While there are a few exceptions where LB have demonstrated heightened perceptual abilities compared to sighted individuals e. Indeed, several other factors could explain increased performance e. One previously proposed hypothesis to explain occipital activations observed in the late-blind stated that they might be the result of mental imagery processes. While such visual imagery processes have been shown to activate components of the visual system in normal sighted individuals Kosslyn et al.

Moreover, the visual imagery hypothesis loses traction when considering that occipital recruitment is seldom observed in the sighted when performing non-visual tasks that are also performed by the blind. This would imply that the unlikely scenario where LB resort to visual imagery and not sighted individuals takes place.

In fact, it is often reported that when sighted individuals perform non-visual tasks, cross-modal inhibitory mechanisms are engaged e. One exception that has linked superior performance in LB to brain changes has done so using an auditory spatial change-detection task and ERP measurements Fieger et al.

As such, it appears that CB persons possess a more sharply tuned early attentional filtering, manifested in the N1 component, while LB show superiority at deploying late attentional processes of target discrimination and recognition, indexed by the P3 component. These findings therefore strongly suggest that even when both CB and LB individuals show a behavioral advantage over sighted subjects on a given task, these enhancements are potentially mediated by different underlying cerebral mechanisms.

The potential role played by corticocortical connections in mediating the crossmodal recruitment of occipital cortex was specifically underlined in previous sections. For instance, a DTI tractography analysis has shown the existence of direct connections between primary auditory and visual areas in normal seeing individuals Beer et al.

To addresses the possible differences between EB and LB individuals, we have recently shown that the flow of auditory information into the occipital cortex might be mediated by a different pathway in LB using DCM analyses Collignon et al.