Existence of Initial Dip for BCI: An Illusion or Reality

A tight coupling between the neuronal activity and the cerebral blood flow (CBF) is the motivation of many hemodynamic response (HR)-based neuroimaging modalities. The increase in neuronal activity causes the increase in CBF that is indirectly measured by HR modalities. Upon functional stimulation, the HR is mainly categorized in three durations: (i) initial dip, (ii) conventional HR (i.e., positive increase in HR caused by an increase in the CBF), and (iii) undershoot. The initial dip is a change in oxygenation prior to any subsequent increase in CBF and spatially more specific to the site of neuronal activity. Despite additional evidence from various HR modalities on the presence of initial dip in human and animal species (i.e., cat, rat, and monkey); the existence/occurrence of an initial dip in HR is still under debate. This article reviews the existence and elusive nature of the initial dip duration of HR in intrinsic signal optical imaging (ISOI), functional magnetic resonance imaging (fMRI), and functional near-infrared spectroscopy (fNIRS). The advent of initial dip and its elusiveness factors in ISOI and fMRI studies are briefly discussed. Furthermore, the detection of initial dip and its role in brain-computer interface using fNIRS is examined in detail. The best possible application for the initial dip utilization and its future implications using fNIRS are provided

A multimodal investigation of retrosplenial function

The retrosplenial cortex (RSC) has attracted much attention due to its proposed role in learning and memory. It forms a part of the Papez circuit and is connected with the anterior thalamic nuclei, the hippocampal formation and sensory areas including the visual cortex. Damage to the RSC impairs episodic and spatial memory. Furthermore, dementias such as Alzheimer's Disease have been shown to involve retrosplenial pathology, highlighting the need to better understand the role of this region. The current work explores the contributions of the RSC to visual and spatial processing as well as its vulnerability in a model of amnesia. It is demonstrated here that visual stimulation of anaesthetised mice elicits intrinsic signal responses in the RSC, similar to those seen in the primary visual cortex. Further, it is shown that training on a spatial memory task is paralleled by the gradual formation of a context-specific retrosplenial memory engram, which re-activates upon re-exposure to the task weeks from initial acquisition. Moreover, the overall level of retrosplenial activity and the stability of the engram show a link to the successful expression of spatial memory upon re-exposure to the task. Finally, it is revealed that the disconnection of the mammillary bodies from the anterior thalamus, which is a common feature of diencephalic amnesia, leads to the reduction of the metabolic marker, cytochrome oxidase, in the RSC as well as to widespread microstructural changes revealed by diffusion tensor imaging. Taken together, it is demonstrated here that the RSC is an important integratory hub contributing to the formation of episodic memory and aspects of visual processing and that it displays high sensitivity to the loss of its inputs, which may explain its involvement in a variety of conditions