BACKGROUND: Increasing evidence suggests that metabolism affects brain physiology. Here, we examine the effect of GLP-1 on simple visual-evoked functional Magnetic Resonance Imaging (fMRI) responses in cortical areas. METHODS: Lean (n = 10) and nondiabetic obese (n = 10) subjects received exenatide (a GLP-1 agonist) or saline infusion, and fMRI responses to visual stimuli (food and nonfood images) were recorded. We analysed the effect of exenatide on fMRI signals across the cortical surface with special reference to the visual areas. We evaluated the effects of exenatide on the raw fMRI signal and on the fMRI signal change during visual stimulation (vs rest). RESULTS: In line with previous studies, we find that exenatide eliminates the preference for food (over nonfood) images present under saline infusion in high-level visual cortex (temporal pole). In addition, we find that exenatide (vs saline) also modulates the response of early visual areas, enhancing responses to both food and nonfood images in several extrastriate occipital areas, similarly in obese and lean participants. Unexpectedly, exenatide increased fMRI raw signals (signal intensity during rest periods without stimulation) in a large occipital region, which were negatively correlated to BMI. CONCLUSIONS: In both lean and obese individuals, exenatide affects neural processing in visual cortex, both in early visual areas and in higher order areas. This effect may contribute to the known effect of GLP1 analogues on food-related behaviour.
Posterior brain lesions selectively alter alpha oscillatory activity and predict visual performance in hemianopic patients. Cortex, 121, 347-361.
Alpha oscillatory frequency and amplitude have been linked to visual processing and to the excitability of the visual cortex at rest. Therefore, posterior brain lesions, which damage the neural circuits of the visual system might induce alterations in the alpha oscillatory activity. To investigate this hypothesis, EEG activity was recorded during eyes-closed resting state in patients with hemianopia with posterior brain lesions, patients without hemianopia with anterior brain lesions and age-matched healthy controls. Patients with posterior lesions revealed a selective slowdown of individual alpha frequency in both the intact and the lesioned hemisphere and a reduction of alpha amplitude in the lesioned hemisphere, resulting in an interhemispheric imbalanced oscillatory alpha activity, while no significant alterations in the alpha range were found in patients with anterior lesions. This suggests a crucial role of posterior cortices in coordinating alpha oscillations in the visual system. Moreover, right posterior lesions had a more severe reduction of individual alpha frequency and altering of the interhemispheric distribution of the alpha amplitude, in line with the notion of the prominence of the right posterior cortices in balancing the interhemispheric functioning. Crucially, the duration of the in individual alpha frequency and the interhemispheric imbalance in alpha amplitude were directly linked to visuo-spatial performance across all participants and to impaired visual detection abilities in hemianopics, therefore supporting a functional role of alpha oscillations in visual processing and suggesting that activity in this frequency range at rest represents a neurophysiological marker reliably reflecting the integrity and the functionality of the visual system in humans.