Variability of the hemodynamic response in infants: Influence of experimental design and stimulus complexity

Abstract

Measuring brain activity in developmental populations remains a major challenge despite great technological advances. Among the numerous available methods, functional near-infrared spectroscopy (fNIRS), an imaging modality that probes the hemodynamic response, is a powerful tool for recording brain activity in a great variety of situations and populations. Neurocognitive studies with infants have often reported inverted hemodynamic responses, i.e. a decrease instead of an increase in regional blood oxygenation, but the exact physiological explanation and cognitive interpretation of this response remain unclear. Here, we first provide an overview of the basic principles of NIRS and its use in cognitive developmental neuroscience. We then review the infant fNIRS literature to show that the hemodynamic response is modulated by experimental design and stimulus complexity, sometimes leading to hemodynamic responses with non-canonical shapes. We also argue that this effect is further modulated by the age of participants, the cortical regions involved, and the developmental stage of the tested cognitive process. We argue that this variability needs to be taken into account when designing and interpreting developmental studies measuring the hemodynamic response.

Keywords: Development; Experimental complexity; Infants; Inverted Hemodynamic Response; fNIRS.

Fig. 1

A typical Hemodynamic Response Function as observed in a newborn participant in our laboratory. Red: HbO, blue: HbR, rectangle: stimulation.

Fig. 2

Pictures of different fNIRS headgears and their respective approximative channel locations. A: The UCL system (adapted from Lloyd-Fox et al., 2018). B: The Hitachi ETG-4000 system (adapted from May et al., 2011). C: The NIRx NIRScout system as used in our laboratory.

Fig. 3

Canonical (A) and inverted (B) responses as observed in newborn infants in our laboratory. Red: HbO, blue: HbR.

Fig. 4

Hemodyamic response in the temporo-parietal junction as a function of age. Adapted from Lloyd-Fox et al. (2018).

Fig. 5

Alternating/non-alternating design with numerous variable stimuli within conditions. Adapted from Gervain et al. (2012).