Biology subjects

Sharifpour, R.

Publications and source records attributed to Sharifpour, R..

6 recordsLinked to original sources

bioRxiv2023

Regional response to light illuminance across the human hypothalamus

Light exerts multiple non-image-forming biological effects on physiology including the stimulation of alertness and cognition. However, the subcortical circuitry underlying the stimulating impact of light is not established in humans. We used 7 Tesla functional magnetic resonance imaging to assess the impact of variations in light illuminance on the regional activity of the hypothalamus while healthy young adults (N=26; 16 women; 24.3 {+/-} 2.9y) were completing two auditory cognitive tasks. We find that, during both the executive and emotional tasks, higher illuminance triggered an activity increase over the posterior part of the hypothalamus, which includes part of the tuberomamillary nucleus and the posterior part of the lateral hypothalamus. In contrast, increasing illuminance evoked a decrease in activity over the anterior and ventral parts of the hypothalamus, encompassing notably the suprachiasmatic nucleus and another part of the tuberomammillary nucleus. Critically, the performance of the executive task was improved under higher illuminance and was negatively correlated with the activity of the posterior hypothalamus area. These findings reveal the distinct local dynamics of different hypothalamus regions that underlie the impact of light on cognition.

neuroscience↗

bioRxiv2023

Light modulates task-dependent thalamo-cortical connectivity during an auditory attentional task

Exposure to blue wavelength light stimulates alertness and performance by modulating a widespread set of task-dependent cortical and subcortical areas. How light affects the crosstalk between brain areas to trigger this stimulating effect is not established. We recorded the brain activity of 19 healthy young participants (24.05{+/-}2.63; 12 women) while they completed an auditory attentional task in darkness or under an active (blue-enriched) or a control (orange) light, in an ultra-high-field 7 Tesla MRI scanner. We tested if light modulated the effective connectivity between an area of the posterior associative thalamus, encompassing the pulvinar, and the intraparietal sulcus (IPS), key areas in the regulation of attention. We found that only the blue-enriched light strengthened the connection from the posterior thalamus to the IPS. Our results provide the first empirical data supporting that blue wavelength light affects ongoing non-visual cognitive activity by modulating task-dependent information flow from subcortical to cortical areas.

neuroscience↗

bioRxiv2023

Impact of light on task-evoked pupil responses during cognitive tasks.

Light has many non-image-forming functions including modulation of pupil size and stimulation of alertness and cognition. Part of these non-image-forming effects may be mediated by the brainstem locus coeruleus. The processing of sensory inputs can be associated with a transient pupil dilation that is likely driven in part by the phasic activity of the locus coeruleus. Here, we aimed to characterise the task-evoked pupil response associated with auditory inputs under different light levels and across two cognitive tasks. We continuously monitored the pupil of 20 young healthy participants (24.05y {+/-}4.0; 14 women) while they completed an attentional and an emotional auditory task whilst exposed to repeated 30-to-40s-blocks of light interleaved with darkness periods. Blocks could either consist of monochromatic orange light [0.16 melanopic Equivalent Daylight Illuminance (EDI) lux] or blue-enriched white light of three different levels [37, 92, 190 melanopic EDI lux; 6500K]. For the analysis 15 and then 14 participants were included in the attentional and emotional tasks respectively. Generalized Linear Mixed Models showed a significant main effect of light level on the task-evoked pupil responses triggered by the attentional and emotional tasks (p[≤].0001). The impact of light was different for the target vs. non-target stimulus of the attentional task but was not different for the emotional and neutral stimulus of the emotional task. Despite a smaller sustained pupil size during brighter light blocks, a higher light level triggers a stronger task-evoked pupil response to auditory stimulation, presumably through the recruitment of the locus coeruleus.

neuroscience↗

bioRxiv2023

Impact of repeated short light exposures on sustained pupil responses in an fMRI environment

Light triggers numerous non-image forming (NIF), or non-visual, biological effects. The brain correlates of these NIF effects have been investigated, notably using Magnetic Resonance Imaging (MRI) and short light exposures varying in irradiance and spectral quality. However, it is not clear whether having light in subsequent blocks may induce carry over effects of one light block onto the next, thus biasing the study. We reasoned that pupil light reflex (PLR) was an easy readout of one of the NIF effects of light that could be used to address this issue. We characterized the sustained PLR in 13 to 16 healthy young individuals under short light exposures during three distinct cognitive processes (executive, emotional and attentional). Light conditions pseudo-randomly alternated between monochromatic orange light [0.16 melanopic Equivalent Daylight Illuminance (mel EDI) lux] and polychromatic blue-enriched white light of three different levels [37, 92, 190 mel EDI lux]. As expected, higher melanopic irradiance was associated with larger sustained PLR in each cognitive domain. This result was stable over the light block sequence under higher melanopic irradiance levels as compared to lower ones. Exploratory frequency-domain analyses further revealed that PLR was more variable within a light block under lower melanopic irradiance levels. Importantly, PLR varied across tasks independently of the light condition pointing to a potential impact of the light history and/or cognitive context on PLR. Together, our results emphasize that the distinct contribution and adaptation of the different retinal photoreceptors influence the NIF effects of light and therefore potentially their brain correlates.

neuroscience↗

bioRxiv2023

In vivo Locus Coeruleus activity while awake is associated with REM sleep quality in healthy older individuals

The locus coeruleus (LC) is the primary source of norepinephrine (NE) in the brain, and the LC-NE system is involved in regulating arousal and sleep. It plays key roles in the transition between sleep and wakefulness, and between slow wave sleep (SWS) and rapid eye movement sleep (REMS). However, it is not clear whether the LC activity during the day predicts sleep quality and sleep properties during the night, and how this varies as a function of age. Here, we used 7 Tesla functional Magnetic Resonance Imaging (7T fMRI), sleep electroencephalography (EEG) and a sleep questionnaire to test whether the LC activity during wakefulness was associated with sleep quality in 52 healthy younger (N=33; ~22y; 28 women) and older (N=19; ~61y; 14 women) individuals. We find that, in older, but not in younger participants, higher LC activity, as probed during an auditory mismatch negativity task, is associated with worse subjective sleep quality and with lower power over the EEG theta band during REMS (4-8Hz), which are two sleep parameters significantly correlated in our sample of older individuals. The results remain robust even when accounting for the age-related changes in the integrity of the LC. These findings suggest that the activity of the LC may contribute to the perception of the sleep quality and to an essential oscillatory mode of REMS, and that the LC may be an important target in the treatment of sleep disorders and age-related diseases.

neuroscience↗

bioRxiv2023

MRI-assessed locus coeruleus contrast and functional response are not associated in young and late middle-aged individuals

The brainstem locus coeruleus (LC) influences a broad range of brain processes, including cognition. The so-called LC contrast is an accepted marker of the integrity of the LC that consists of a local hyperintensity on specific Magnetic Resonance Imaging (MRI) structural images. The small size of the LC has, however, rendered its functional characterization difficult in humans, including in aging. A full characterization of the structural and functional characteristics of the LC in healthy young and late middle-aged individuals is needed to determine to potential roles of the LC in different medical conditions. Here, we wanted to determine whether the activation of the LC in a mismatch negativity task changes in aging and whether the LC functional response was associated to the LC contrast. We used Ultra-High Field (UHF) 7-Tesla functional MRI (fMRI) to record brain response during an auditory oddball task in 53 healthy volunteers, including 34 younger (age: 22.15y {+/-} 3.27; 29 women) and 19 late middle-aged (age: 61.05y {+/-} 5.3; 14 women) individuals. Whole-brain analyses confirmed brain responses in the typical cortical and subcortical regions previously associated with mismatch negativity. When focusing on the brainstem, we found a significant response in the rostral part of the LC probability mask generated based on individual LC images. Although bilateral, the activation was more extensive in the left LC. Individual LC activity was not significantly different between young and late middle-aged individuals. Critically, while the LC contrast was higher in older individuals, the functional response of the LC was not associated with its contrast. These findings show that the age-related alterations of the LC structural integrity may not necessarily be related to changes in its functional response. The results further indicate that LC responses could remain stable in healthy individuals aged 20 to 70.

neuroscience↗