Multisensory Facilitation of Behavior in Monkeys: Effects of Stimulus Intensity

Abstract

Multisensory stimuli can improve performance, facilitating RTs on sensorimotor tasks. This benefit is referred to as the redundant signals effect (RSE) and can exceed predictions on the basis of probability summation, indicative of integrative processes. Although an RSE exceeding probability summation has been repeatedly observed in humans and nonprimate animals, there are scant and inconsistent data from nonhuman primates performing similar protocols. Rather, existing paradigms have instead focused on saccadic eye movements. Moreover, the extant results in monkeys leave unresolved how stimulus synchronicity and intensity impact performance. Two trained monkeys performed a simple detection task involving arm movements to auditory, visual, or synchronous auditory-visual multisensory pairs. RSEs in excess of predictions on the basis of probability summation were observed and thus forcibly follow from neural response interactions. Parametric variation of auditory stimulus intensity revealed that in both animals, RT facilitation was limited to situations where the auditory stimulus intensity was below or up to 20 dB above perceptual threshold, despite the visual stimulus always being suprathreshold. No RT facilitation or even behavioral costs were obtained with auditory intensities 30-40 dB above threshold. The present study demonstrates the feasibility and the suitability of behaving monkeys for investigating links between psychophysical and neurophysiologic instantiations of multisensory interactions.

Multisensory interactions in early evoked brain activity follow the principle of inverse effectiveness

“…This observation fits with previous studies, which demonstrated an inverse relationship between the magnitude of the RTE for bisensory inputs and stimulus intensity (Corneil et al, 2002;Diederich and Colonius, 2004;Rach et al 2011). Interestingly, an inverse association between the magnitude of the RTE and the intensity of multisensory stimuli has been recently found also in monkeys (Cappe et al, 2010a). This observation, in addition to the present data, suggests that stimulus intensity influences integrative multisensory processing such that the RT facilitation effect to bisensory redundant targets is strongest when stimuli are low in intensity.…”

Multisensory Integration in Non-Human Primates during a Sensory-Motor Task

“…This detection task was comparable to previous studies conducted in human subjects (Giard and Peronnet, 1999; Molholm et al, 2002; Gori et al, 2011) and in macaques (Cappe et al, 2010). However, for the first time stimuli levels used with monkeys were based on individually determined perceptual thresholds in order to precisely determine the auditory versus visual stimuli intensities required to modify the multisensory gain in motor response latencies.…”

“…Mk-LI was 9 years old and Mk-JZ was 7 years old at the time of the onset of electrophysiological recordings. The monkeys Mk-LI and Mk-JZ are distinct from the two monkeys used in a previous study from this laboratory based on an earlier, less elaborate version of the psychophysical paradigm (Cappe et al, 2010), with some important differences as outlined in the discussion. The monkeys’ weight was monitored daily and both weighed around 8 kg.…”

“…This training was based on a positive reinforcement protocol described previously (Durif et al, 2003; Cappe et al, 2010) and modified from Smith and Olszyk (1997) (Figure 1). Briefly all the recording sessions were performed in a sound-attenuated chamber and the trials were initiated by the animal when pressing a lever with the left hand.…”

Auditory–Visual Multisensory Interactions in Humans: Timing, Topography, Directionality, and Sources

“…The importance of resolving this issue is highlighted by the mounting evidence in animal models documenting the prevalence of subadditive interactions that result in enhanced information content of responses (Bizley et al, 2007;Angelaki et al, 2009;). The topic of superadditive and subadditive interactions is likewise often intertwined with discussion of the applicability of the principle of inverse effectiveness, which would stipulate that unisensory stimulus conditions presented alone that are more effective in generating a neural response would more likely result in additive or even subadditive interactions when presented simultaneously as a multisensory pair Holmes, 2009;Cappe et al, 2010). Given that most ERP and fMRI studies of AV interactions involving rudimentary stimuli (i.e., tones/ noises and shapes/flashes) in humans have presented loud and highcontrast stimuli (i.e., suprathreshold stimuli), subadditive interactions would be expected to have been occurring (though as mentioned above were not statistically assessed) and are anticipated in the present study.…”