Functional connectivity of the visual brain and its alteration by external factors
Keywords: sevoflurane
visual system
isoflurane
V1 Lesion
functional connectivity
macaque

The brain’s ability to receive and interpret visual inputs is supported by an ensemble of brain areas. Damage to any of these areas severely impacts the brain’s ability to process visual information.
This study examines the case of two rhesus macaques (Macaca mullatta) with extensive, naturally occurring, bilateral lesions in their primary visual cortices (V1). V1 is essential to visual processing but these monkeys appear to have retained sight, suggesting their brains may have developed mechanisms compensating for these lesions. Abnormal functional networks, however, may indicate corresponding structural changes. The extent of these lesions strongly alluded to large disturbances to the lesion animals’ visual systems’ functional connectivity. Thus, the primary purpose of this study was to examine the functional connectivity of the visual system and how it is affected by different anaesthetics but also by the existence of lesions in the primary visual cortex.
Resting state connectivity is a measure of temporal correlation in spontaneous blood oxygen level–dependent (BOLD) signal, in at rest subjects. Temporally correlated areas are organized into resting state functional networks, which often have anatomic equivalents. Macaques’ resting-state data are usually acquired under anaesthesia. Anaesthetics impact functional connectivity strength and network sizes. It is unclear if all anaesthetics have the same impact. Thus, before examining how V1 lesions affect the visual system’s functional connectivity, we investigated the impact of two commonly used anaesthetics.
Independent component analysis was applied to data from 23 healthy animals (14 Isoflurane, 10 Sevoflurane) to examine differential anaesthetic effects at the whole brain level. Seed based correlation analysis (SBCA) was then used to examine the functional connectivity of the visual system. The results of these analyses indicated broadly equivalent effects from the two anaesthetics that are, however, characterised by small, localized, statistically significant differences over both the whole brain and the visual system specifically.
Our examination of compensatory visual processing mechanisms was limited to the 14 subjects anaesthetized using isoflurane –the anaesthetic applied to the lesion monkeys. We employed SBCA to compare the lesion monkeys’ V1 functional connectivity to that in the 14 healthy controls and to isolate variations in the functional connectivity of two thalamic pathways. We also examined the correlation between 12 regions of interest (ROI) in the lesion and control animals. Finally, we examined correlations between these ROI, the V1 calcarine, and V1 operculum in the 14 healthy subjects. Stronger connections may indicate structural connections –a baseline against which we were able to compare our lesion monkeys, seeking substantive insight into compensatory visual processing mechanisms.
The V1 lesions had surprisingly little impact on the aforementioned functional connectivity networks. There was increased connectivity strength between the lesion monkeys’ V1 remnants and several areas of the visual cortex. Examinations of the healthy animals returned limited-but-compelling evidence supporting the existence of stronger connections between V1 calcarine and two of these areas. One of the two lesion monkeys also presented stronger correlations between two areas belonging to one of the thalamic pathways, potentially indicative of new structural connections supporting otherwise perplexingly persistent sight.