Existence of Initial Dip for BCI: An Illusion or Reality

Abstract

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.

Reduction of Onset Delay in Functional Near-Infrared Spectroscopy: Prediction of HbO/HbR Signals

“…In the literature, this issue has been discussed considering several issues. One interesting report is that it is due to the use of caffeine before the experiment (Behzadi and Liu, 2006;Hong and Zafar, 2018). In addition, the detection time of the initial dip varies among trials and subjects (Hu et al, 2013).…”

“…Another means of addressing this delay is to utilize the initial dip for fast fNIRS-BCI applications. The initial dip is an early change in oxygenation prior to any subsequent increase in CBF, which is spatially more specific to the site of neuronal activity (Vanzetta and Grinvald, 2008;Hong and Zafar, 2018). However, there is also a time lag in detecting the initial dip .…”

Early Detection of Hemodynamic Responses Using EEG: A Hybrid EEG-fNIRS Study

“…An initial dip is the early decrease/increase in Δ HbO /Δ HbR as result of neurovascular coupling. Therefore, both the initial dip as well as the hemodynamic signal can be detected using the vector-phase diagram (Hong and Zafar, 2018 ). The original method uses eight phases that are created using a pair of Δ HbO and Δ HbR signals (or Δ HbT and Δ COE signals) (Yoshino and Kato, 2012 ; Sano et al, 2013 ; Yoshino et al, 2013 ; Oka et al, 2015 ).…”

Performance Improvement of Near-Infrared Spectroscopy-Based Brain-Computer Interface Using Regularized Linear Discriminant Analysis Ensemble Classifier Based on Bootstrap Aggregating

“…* Corrected-p < 0.05, * * corrected-p < 0.01, and * * * corrected-p < 0.001 (false discovery rate correction). to improving the bitrate of fNIRS-BCIs (Cui et al, 2010;Zafar and Hong, 2017;Hong and Zafar, 2018), it is difficult to bridge the performance gap between fNIRS-BCIs and EEG-BCIs. However, for such SSVEP-BCI which is a type of exogenous BCIs, the need for an external stimulus causing user fatigue easily could be problematic.…”