Multisensory processing and oscillatory activity: analyzing non-linear electrophysiological measures in humans and simians

Abstract

Stimulus-related oscillations are known to be closely linked to integrative processing in the brain. One research domain within which there has been tremendous interest in oscillatory mechanisms is in the integration of inputs across the widely separated sensory systems. Under the standard approach of assessing multisensory interactions in electrophysiological datasets, the event-related response to a multisensory stimulus is directly compared with the sum of the responses to its unisensory constituents when presented alone. When using methods like wavelet transformation or fast Fourier transformation to derive induced oscillatory signals, however, such linear operations are not appropriate. Here we introduce a simple bootstrapping procedure wherein the linear summation of single unisensory trials forms a distribution against which multisensory trials may be statistically compared, an approach that circumvents the issue of non-linearity when combining unisensory oscillatory responses. To test this approach we applied it to datasets from intracranial recordings in non-human primates and human scalp-recorded EEG, both derived from a simple audio-visual integration paradigm. Significant multisensory interactions were revealed in oscillatory activity centered at 15 and 20 Hz (the so-called beta band). Simulations of different levels of background noise further validated the results obtained by this method. By demonstrating super- and sub-additive effects, our analyses showed that this approach is a valuable metric for studying multisensory interactions reflected in induced oscillatory responses.

Appendix: Materials and paradigm

Intracranial field potentials in macaques

To evaluate the application of this technique to intracranial field potential data, we used data recorded during the course of a multisensory investigation in auditory cortex of an awake behaving macaque monkey (see Lakatos et al. 2005a). All procedures were approved in advance by the Animal Care and Use Committee of the Nathan Kline Institute. The subject, a Rhesus monkey (Maccaca mulatta) was adapted to a custom fitted primate chair and to the recording chamber and then surgically prepared for chronic awake electrophysiological recording. Surgery was performed under anesthesia (1–2% isoflourane), using standard aseptic surgical methods (see Schroeder et al. 2001). To allow electrode access to the brain, and to promote an orderly pattern of sampling across the surface of the auditory cortices, matrices of 18 gauge stainless steel guide tubes were placed over auditory cortex. These matrices were angled so that the electrode track would be perpendicular to the plane of auditory cortex, as determined by pre-implant MRI (Schroeder et al. 1998). They were placed within small, appropriately shaped craniotomies, to rest against the intact dura. The matrices, along with a titanium head post (permitting painless head restraint), were embedded in dental acrylic and secured to the skull with titanium orthopaedic screws.

Recordings were made in an electrically shielded, sound-attenuated chamber with SONEX ProSPEC Composite™ sound absorbing foam. Laminar profiles of field potentials (EEG) were recorded using a linear array multi-contact electrode (24 contacts, 100 μm intercontact spacing) positioned to sample from all the layers simultaneously (Lakatos et al. 2005b). Signals were impedance matched with a pre-amplifier (10× gain, bandpass dc-10 kHz) situated on the electrode, and after further amplification (500×) the signal was band pass filtered by analogue filtering (0.1–500 Hz) to extract the field potential sampled at 2 kHz/16 bit precision.

Eye position was monitored using an ISCAN ETL-200 eye tracking system, and stimuli were presented only when the monkey’s gaze was held within a 10° degree window surrounding the fixation point in the middle of the monitor. Auditory stimuli consisted of broadband Gaussian noise bursts (16 ms duration; 70 dB SPL, 1 ms rise/fall times). The visual stimuli consisted of a bright white monitor flash (16 ms duration). Unisensory-auditory, unisensory-visual, and multisensory audiovisual stimuli were presented in separate blocks (SOA = 767 ms), each consisting of 100 stimuli. Trial blocks were separated by brief breaks in which the monkey was checked and fed dried fruits and other preferred treats.

In accordance with the time–frequency representations (Fig. 2), for the analysis of multisensory interactions a 15 Hz WT was performed. The wavelet had a duration (2σ _t ) of 67 ms and a spectral bandwidth (2σ _f ) of 4.8 Hz. As a measure of 15 Hz activity, the maximum value of the modulus of the complex transform coefficient was computed in a time interval between 10 and 110 ms for each epoch.

Human EEG recordings

Induced oscillatory beta responses for one subject (female, 27 years, right handed) from one right-frontal channel were analyzed (Fig. 5). This channel was selected because a right-frontal maximum was found at this electrode site. The nearest neighboring site within the 10–20 system of this electrode is the F4 electrode. The EEG was recorded in an electrically shielded chamber from 2 EOG and 128 scalp electrodes (impedances < 5 kΩ), referenced to the nose at a sample rate of 500 Hz. Epochs for EEG beta activity lasted from 500 ms before to 1000 ms after stimulus onset. Baselines were computed from −300 to −100 ms pre-stimulus. For artifact suppression, trials were automatically excluded from averaging if the standard deviation within a moving 200 ms time interval exceeded 30 μV in any one of the EEG channels and 40 μV at the EOG channels in a time interval between −300 and 500 ms.

Auditory stimuli in the experiment consisted of a 1,000 Hz tone (60 ms duration; 75 dB SPL) presented from a single speaker located atop the monitor on which the visual stimuli were presented. The visual stimulus consisted of a red circular disk subtending 1.2° in diameter presented on a black background. Visual stimuli were presented 1.56° lateral left above a central fixation cross for 60 ms. Unisensory-auditory (A), unisensory-visual (V), and multisensory audiovisual (AV) were presented at inter-stimulus-intervals ranging between 750 and 3,000 ms. The subject was instructed to maintain central fixation at all times and to make a speeded button response with their right index finger when a stimulus in either modality was detected. A total number of 533 auditory, 547 auditory and 530 audiovisual trials were submitted to the analysis.

For the analysis of beta responses, the maximum value of the modulus of the complex 20 Hz transformed coefficient was computed in a time interval between 80 and 170 ms for each epoch. The duration (2σ _t ) of the wavelet was 100 ms with a spectral bandwidth (2σ _f ) of 6.4 Hz. The subject gave written informed consent to participate in the study.