Multisensory integration in the superior colliculus: a neural network model

Neurons in the superior colliculus (SC) are known to integrate stimuli of different modalities (e.g., visual and auditory) following specific properties. In this work, we present a mathematical model of the integrative response of SC neurons, in order to suggest a possible physiological mechanism underlying multisensory integration in SC. The model includes three distinct neural areas: two unimodal areas (auditory and visual) are devoted to a topological representation of external stimuli, and communicate via synaptic connections with a third downstream area (in the SC) responsible for multisensory integration. The present simulations show that the model, with a single set of parameters, can mimic various responses to different combinations of external stimuli including the inverse effectiveness, both in terms of multisensory enhancement and contrast, the existence of within- and cross-modality suppression between spatially disparate stimuli, a reduction of network settling time in response to cross-modal stimuli compared with individual stimuli. The model suggests that non-linearities in neural responses and synaptic (excitatory and inhibitory) connections can explain several aspects of multisensory integration.     

Fear conditioning induces guinea pig auditory cortex activation by foot shock alone

The present study used an optical imaging paradigm to investigate plastic changes in the auditory cortex induced by fear conditioning, in which a sound (conditioned stimulus, CS) was paired with an electric foot-shock (unconditioned stimulus, US). We report that, after conditioning, auditory information could be retrieved on the basis of an electric foot-shock alone. Before conditioning, the auditory cortex showed no response to a foot-shock presented in the absence of sound. In contrast, after conditioning, the mere presentation of a foot-shock without any sound succeeded in eliciting activity in the auditory cortex. Additionally, the magnitude of the optical response in the auditory cortex correlated with variation in the electrocardiogram (correlation coefficient: −0.68). The area activated in the auditory cortex, in response to the electric foot-shock, statistically significantly had a larger cross-correlation value for tone response to the CS sound (12 kHz) compared to the non-CS sounds in normal conditioning group. These results suggest that integration of different sensory modalities in the auditory cortex was established by fear conditioning.     

Japanese macaques form a cross-modal representation of their own species in their first year of life

We tested whether infant Japanese macaques (Macaca fuscata) have a cross-modal representation of their own species. We presented monkeys with a photograph of either a monkey or a human face on an LCD monitor after playing back a vocalization of one of those two species. The subjects looked at the monitor longer when a human face was presented after the monkey vocalization than when the same face was presented after human vocalization. This suggests that monkeys recall and expect a monkey’s face upon hearing a monkey’s voice.     

Cross-modal representation of human caretakers in squirrel monkeys

We tested whether squirrel monkeys have cross-modal representations of their human caretakers with a 0-delay symbolic matching-to-sample procedure. We first trained the monkeys to match photographs of two of their caretakers. After reaching criterion, they were exposed to two test sessions. In these sessions 32 all-reinforced test trials were interspersed among the training trials. In the test trials, a voice, either matching (congruent condition) or mismatching (incongruent condition) with the sample photographs, was played back after the sample stimulus disappeared. The monkeys' matching accuracies in the incongruent condition were lower than in the match condition. Post hoc analyses revealed that the presentation of the primary caretaker's voice interfered with performance in test trials where the secondary caretaker's face was presented (incongruent condition). This suggests that our subjects recalled their primary caretaker's representation upon hearing the appropriate voice.     

Vocal imitation in blindfolded zebra finches (<Emphasis Type="Italic">Taeniopygia guttata</Emphasis>) is facilitated in the presence of a non-singing conspecific female

Song acquisition in young male zebra finches, Taeniopygia guttata, is viewed as a learning process by imitation from a visible conspecific male acting as a song tutor. The role played by non-singing conspecific females in this process has been poorly documented. The influence of social contact on the song learning of blindfolded young males was investigated in both female-raised and pair-raised birds. Pupils with both eyes occluded from about 35 to 65 days post-hatch, the sensitive phase for song learning, copied significantly more from an adult male sharing their cage when tutored in the presence of a female sibling than in her absence. Interestingly, the effect was more pronounced in female-raised birds compared to pair-raised birds. I conclude that physical contact in the absence of visual cues is sufficient for song imitation to occur. The results demonstrate an instance of social stimulation by non-singing females in the song acquisition of male songbirds.     

EEG gamma-band activity during audiovisual speech comprehension in different noise environments

The presence of cross-modal stochastic resonance in different noise environments has been proved in previous behavioral and event-related potential studies, while it was still unclear whether the gamma-band oscillation study was another evidence of cross-modal stochastic resonance. The multisensory gain of gamma-band activity between the audiovisual (AV) and auditory-only conditions in different noise environments was analyzed. Videos of face motion articulating words concordant with different levels of pink noise were used as stimuli. Signal-to-noise ratios (SNRs) of 0, −4, −8, −12 and −16 dB were selected to measure the speech recognition accuracy and EEG activity for 20 healthy subjects. The power and phase of EEG gamma-band oscillations increased in a time window of 50–90 ms. The multisensory gains of evoked and total activity, as well as phase-locking factor, were greatest at the −12 dB SNR, which were consistent with the behavioral result. The multisensory gain of gamma-band activity showed an inverted U-shaped curve as a function of SNR. This finding confirmed the presence of cross-modal stochastic resonance. In addition, there was a significant correlation between evoked activity and phase-locking factor of gamma-band at five different SNRs. Gamma-band oscillation was believed to play a role in the rapid processing and information linkage strengthening of AV modalities in the early stage of cognitive processes.