Ng et al., observed that there was a positive and significant association between land use mix diversity, walkability and repeatable Battery for the Assessment of Neuropsychological Status (RBANS) z score, immediate and delayed memory, recall, visuospatial and constructional ability but not attention.¹⁸ This is in contrast to the attention restoration theory of Kaplan. Allen et al., showed better cognitive scores in people having 1 or 2 green building days in their workspace.²⁷ Ward et al., found no association between greenspace exposure and cognition as measured by computerized CNS vital signs.²² Besser et al., found association between denser urban environments and worse cognition.36Dadvand et al., 2015 had shown better cognitive development in children in green spaces.³²

de Keijzer et al., showed slower cognitive decline in people exposed to greater residential surrounding greenness over 10 years; especially in the reasoning and fluency tests and not in short term memory.¹⁶ Mediation analysis of three potential mediators (physical activity, social support, air pollution) for this association suggested no mediation. Provision of public parks in childhood and early adulthood is associated with lesser cognitive decline according to Cherrie and colleagues.³¹ Wu et al., (2017) had shown that living in areas with higher land use mix is associated with 30% decreased odds of cognitive impairment.¹⁷ No significant association has been found between PACC and exposure to urban environments according to Crousbou and colleagues.³³

Besser found that the association between natural environment characteristics and cognitive decline is associated with APOE2 and not with APOE3 or 4.³⁶ According to Dadvand et al., there is an association between lifetime greenspace exposure and larger brain volumes, especially the right and left prefrontal cortex.20Crousbou et al., have shown greenspace to be associated with higher cortical thickness in the brain and air pollution the converse.³³

All studies on cognition have been cross sectional studies or studies taking samples from larger cohorts wherein there is a possibility for bias. The results of studies on greenspace and cognition are not comparable as there is heterogeneity in testing of cognition. Studies on cognitive decline have no matching for possible physical and psychiatric comorbidities that might increase the risk of cognitive impairment. Overall, the studies lack investigation on long term memory, executive functioning, social cognition or perceptual motor functioning. The very intrusive nature of an objective ‘cognitive function’ testing after brief exposure to any environment is not devoid of the confounder of performance anxiety and bias.

There might be a complex interaction of multiple factors in the association between green spaces and cognition. Oliver Mytton et al., have found a positive correlation between green space and physical activity and physical activity.³⁸ This mediates better cognition and neuroplasticity through epigenetic mechanisms.³⁹ Thus, physical activity may be an important mediator in the effects of green space on cognition. Various studies have proven green spaces to reduce state anxiety and cause stress recovery. It is also known well that stress and anxiety have a dual role on cognition,⁴⁰ but higher the anxiety, lower the cognitive performance.⁴¹ Probable mediators for attention restoration in greenspace like anxiety in green environment versus other built spaces and their effect on EEG high beta band and P 300 latency offer insights into mechanisms behind such restorative benefits as expressed by the above studies.28

A study by Lin et al., observes 6 neutral emotional parameters. Out of the 6, only valence and meditation show consistent results.¹³ The “valence” & “meditation” parameters are higher in participants who were walking (M=0.71 & 0.58) in green space, when compared to participants who were sitting (M=0.63 & 0.33) in green space. Whereas, the “focus” parameter showed a small contrary result of (M=0.55) in the sitting group which is higher than (M=0.42) in the walking group. Gao et al in their study subjected the participants to 6 different environments through VR. It was observed that there was no significant impact of the exposed environments on restoration of attention fatigue or positive mood. “open-green space” environment showed restorative effects of negative mood.

Both of the studies have been carried out in a short time frame. A better understanding of the same can be achieved by conducting long term studies. In the study by Gao et al., VR technology was used to expose the subjects to green spaces. Adopting technology to study the effect of green environment may not give authentic results when compared to studies that use natural environments.²⁸

Wood et al., used the attention restoration theory (ART) questionnaire in their cross-sectional study. It was found out that, out of all factors (such as site facilities), biodiversity played an important role in attention restoration by 43%.¹² In another study, Amicone et al., divided the children into 2 groups. The first group that was given recess break in the natural environment, showed improvement in “sustained and selective- attention”. The students’ score in Bell’s test was higher after the recess (M=32.61) than when compared to before the recess (M=31.85). The other group of students, who were given recess breaks in the built environment, showed no significant results.³⁰

The above 2 studies were carried out with a local residential population. Therefore, it cannot be referred to show any ethnic, social or cultural differences. To get more holistic results, a much more random population can be taken and tools that meet standard criteria can be used.

A cross sectional studyshowed that children’s behavioural problems were associated with their access to green space.²³ Using the Strengths & difficulties questionnaire, the behavioural problems were assessed. Children who had more behavioural problems such as mainly: hyperactivity or inattention were not exposed much to green space. It was also found that there is no association between exposure to forest area and behaviour. Donovan et al., found out in their longitudinal study that, children who lived in rural areas, after 2 years of age were less likely to develop ADHD. Their odds ratio was found out to be 0.607. Exposure to rural living or NDVI only for the first 2 years of life did not decrease the risk of developing ADHD.²⁵ Amoly et al., in their cross- sectional study observed that there was a significant inverse relationship between exposure to green spaces and the scores in SDQ & ADHD questionnaire (DSM IV).²⁹

In the 3 studies above, Amoly et al., & Tiesler et al., use the Strengths & Difficulties Questionnaire to study the behaviour and hence the findings can be compared.^{23, 29} Amoly et al., & Donovan et al., used normalized difference vegetation index (NDVI) to measure greenness which gives more uniform findings.^{25, 29} The drawbacks of Amoly et al., & Tiesler et al., are that they are cross-sectional studies. They need more evidence to back up their results.^{23, 29} These 2 studies can be supported using Donovan et al. Their study is a longitudinal study. This way the findings of each study can be contrasted and compared to establish a link and further give it more value.

Among 190 children in a cross-sectional study, 96 were diagnosed with ADHD/inattention. 47% of these children showed improvement of symptoms with age.²⁶ This study also shows that children in neighbourhoods that are rich and affiuent showed less clinical symptoms of ADHD.

This cohort-study has studied the influence of neighbourhood on children with ADHD. While doing so, it is known that neighbourhood factors are vast and cannot be controlled. Sharp et al have considered only a few baseline factors and hence the results do not give us a better understanding of the study. No standard tools were used to measure these factors. This study has to be associated with others that study familial and neighbourhood factors to give its results more evidence and value.

Kodama et al. in their study showed that, in patients with TBI, P300 latency was significantly shorter in the green environment than in darkness (p < 0.01).³⁷ Rehabilitation intended to improve selective attention or information processing speed might be more effective if performed in a low anxiety environment (i.e., a green environment).

Markevych et al., in their study observed that a 0.1 increase in NDVI decreased the risk of ADHD by a factor of 0.82. They also saw that an increase in PM10 & NO2 raised the risk of ADHD by a factor of 1.97.21 In the study, no proper information on SES and psychopathology of the family to assess risk of ADHD were found out. Understanding information of these factors will help in achieving a more holistic view of the results.

In another study, Bodin et al., observed that most participants were annoyed due to the noise and pollution from the traffic.24 Those who had bedrooms facing a green space had less concentration problems as they had proper sleep. This study lacks socio- demographic information. Further on, they use self- report questionnaires to assess the level of pollution. This can bring the possibility of bias.