Early, low-level auditory-somatosensory multisensory interactions impact reaction time speed

Abstract

Several lines of research have documented early-latency non-linear response interactions between audition and touch in humans and non-human primates. That these effects have been obtained under anesthesia, passive stimulation, as well as speeded reaction time tasks would suggest that some multisensory effects are not directly influencing behavioral outcome. We investigated whether the initial non-linear neural response interactions have a direct bearing on the speed of reaction times. Electrical neuroimaging analyses were applied to event-related potentials in response to auditory, somatosensory, or simultaneous auditory-somatosensory multisensory stimulation that were in turn averaged according to trials leading to fast and slow reaction times (using a median split of individual subject data for each experimental condition). Responses to multisensory stimulus pairs were contrasted with each unisensory response as well as summed responses from the constituent unisensory conditions. Behavioral analyses indicated that neural response interactions were only implicated in the case of trials producing fast reaction times, as evidenced by facilitation in excess of probability summation. In agreement, supra-additive non-linear neural response interactions between multisensory and the sum of the constituent unisensory stimuli were evident over the 40-84 ms post-stimulus period only when reaction times were fast, whereas subsequent effects (86-128 ms) were observed independently of reaction time speed. Distributed source estimations further revealed that these earlier effects followed from supra-additive modulation of activity within posterior superior temporal cortices. These results indicate the behavioral relevance of early multisensory phenomena.

Keywords: auditory; crossmodal; event-related potential; multisensory; reaction time; redundant signals effect; somatosensory; tactile.

Figure 1

Behavioral results. (A) Mean reaction times (SEM shown) for auditory–somatosensory multisensory pairs, auditory, and somatosensory stimuli (white, light gray, and dark gray bars, respectively). The left and right panels show data from trials producing faster and slower RTs, respectively. (B) Results of applying Miller's (1982) race model inequality to the cumulative probability distributions of the reaction time data. The model is the arithmetic sum of the cumulative probabilities from auditory and somatosensory trials. The x-axis indicates the percentile of the reaction time distribution after median split of the data. As above, the left and right panels show data from trials producing faster and slower RTs, respectively.

Figure 2

Group-averaged voltage waveforms. Data are displayed at an exemplar left-lateralized central electrode site (C3) from each condition. Separate graphs depict ERPs from trials producing fast and slow RTs. Non-linear neural response interactions appear to start over the ∼40–80 ms post-stimulus onset for trials leading to fast but not to slow RTs.

Figure 3

Global field power (GFP) waveforms and analyses. (A) group-averaged GFP waveforms are displayed for each condition and are also separated according to later RT speed. The dotted insets indicate the 40–84 ms post-stimulus interval over which area measures were calculated. (B) Mean (SEM shown) GFP area measurements over the 40–84 ms post-stimulus interval. (C) Mean (SEM shown) GFP area measurements over the 86–128 ms post-stimulus interval.

Figure 4

Source estimations and statistical contrasts. The upper panels depict the mean source estimations over the 40–84 ms post-stimulus interval for each condition and subsequent RT speed at the sagittal slice of maximal amplitude (x = −53 mm using the Talairach and Tournoux (1988) coordinate system). The lower panels depict the mean (SEM shown) scalar values across nodes within the superior temporal cluster identified across conditions.