What is a multisensory cortex? A laminar, connectional, and functional study of a ferret temporal cortical multisensory area

Deafness induces complete crossmodal plasticity in a belt region of dorsal auditory cortex

Many neural areas, where patterned activity is lost following deafness, have the capacity to become activated by the remaining sensory systems. This crossmodal plasticity can be measured at perceptual/behavioural as well as physiological levels. The dorsal zone (DZ) of auditory cortex of deaf cats is involved in supranormal visual motion detection, but its physiological level of crossmodal reorganisation is not well understood. The present study of early-deaf DZ (and hearing controls) used multiple single-channel recording methods to examine neuronal responses to visual, auditory, somatosensory and combined stimulation. In early-deaf DZ, no auditory activation was observed, but 100% of the neurons were responsive to visual cues of which 21% were also influenced by somatosensory stimulation. Visual and somatosensory responses were not anatomically organised as they are in hearing cats, and fewer multisensory neurons were present in the deaf condition. These crossmodal physiological results closely correspond with and support the perceptual/behavioural enhancements that occur following hearing loss.

Cross-modal and subliminal effects of smell and color

In the present study, we examined whether the cross-modal effect can be obtained between odors and colors, which has been confirmed under olfactory recognizable conditions and also occurs under unrecognizable conditions. We used two flavors of red fruits such as strawberries and tomatoes for this purpose. We also aimed to compare whether similar cross-modal effects could be achieved by setting the flavors at recognizable (liminal) and unrecognizable (subliminal) concentrations in the experiment. One flavor at a normal concentration (0.1%, Liminal condition) and one at a concentration below the subliminal threshold (0.015%, Subliminal condition), were presented, and the color that resembled the smell most closely from among the 10 colors, was selected by participants. Except for the subliminal tomato condition, each odor was significantly associated with at least one color (p < 0.01). Participants selected pink and red for liminal strawberry (0.1%) (p < 0.05), pink for subliminal strawberry (0.015%) (p < 0.05), and orange for liminal tomato (0.1%) (p < 0.05), but there was no color selected for subliminal tomato (0.015%) (p < 0.05). The results of this study suggest that the flavor of tomato produced a cross-modal effect in liminal conditions, but not in subliminal conditions. On the other hand, the results of the present study suggest that the flavor of strawberries produces a cross-modal effect even under subliminal conditions. This study showed that cross-modal effects might exist, even at unrecognizable levels of flavor.

A comparison of multisensory features of two auditory cortical areas: primary (A1) and higher-order dorsal zone (DZ)

From myriads of ongoing stimuli, the brain creates a fused percept of the environment. This process, which culminates in perceptual binding, is presumed to occur through the operations of multisensory neurons that occur throughout the brain. However, because different brain areas receive different inputs and have different cytoarchitechtonics, it would be expected that local multisensory features would also vary across regions. The present study investigated that hypothesis using multiple single-unit recordings from anesthetized cats in response to controlled, electronically-generated separate and combined auditory, visual, and somatosensory stimulation. These results were used to compare the multisensory features of neurons in cat primary auditory cortex (A1) with those identified in the nearby higher-order auditory region, the Dorsal Zone (DZ). Both regions exhibited the same forms of multisensory neurons, albeit in different proportions. Multisensory neurons exhibiting excitatory or inhibitory properties occurred in similar proportions in both areas. Also, multisensory neurons in both areas expressed similar levels of multisensory integration. Because responses to auditory cues alone were so similar to those that included non-auditory stimuli, it is proposed that this effect represents a mechanism by which multisensory neurons subserve the process of perceptual binding.

Multisensory integration in neurons of the medial pulvinar of macaque monkey

The pulvinar is a heterogeneous thalamic nucleus, which is well developed in primates. One of its subdivisions, the medial pulvinar, is connected to many cortical areas, including the visual, auditory, and somatosensory cortices, as well as with multisensory areas and premotor areas. However, except for the visual modality, little is known about its sensory functions. A hypothesis is that, as a region of convergence of information from different sensory modalities, the medial pulvinar plays a role in multisensory integration. To test this hypothesis, 2 macaque monkeys were trained to a fixation task and the responses of single-units to visual, auditory, and auditory-visual stimuli were examined. Analysis revealed auditory, visual, and multisensory neurons in the medial pulvinar. It also revealed multisensory integration in this structure, mainly suppressive (the audiovisual response is less than the strongest unisensory response) and subadditive (the audiovisual response is less than the sum of the auditory and the visual responses). These findings suggest that the medial pulvinar is involved in multisensory integration.

MULTISENSORY RESPONSES IN A BELT REGION OF THE DORSAL AUDITORY CORTICAL PATHWAY

A basic function of the cerebral cortex is to receive and integrate information from different sensory modalities into a comprehensive percept of the environment. Neurons that demonstrate multisensory convergence occur across the necortex, but are especially prevalent in higher-order, association areas. However, a recent study of a cat higher-order auditory area, the dorsal zone (DZ) of auditory cortex, did not observe any multisensory features. Therefore, the goal of the present investigation was to address this conflict using recording and testing methodologies that are established for exposing and studying multisensory neuronal processing. Among the 482 neurons studied, we found that 76.6% were influenced by non-auditory stimuli. Of these neurons, 99% were affected by visual stimulation, but only 11% by somatosensory. Furthermore, a large proportion of the multisensory neurons showed integrated responses to multisensory stimulation, constituted a majority of the excitatory and inhibitory neurons encountered (as identified by the duration of their waveshape), and exhibited a distinct spatial distribution within DZ. These findings demonstrate that the dorsal zone of auditory cortex robustly exhibits multisensory properties and that the proportions of multisensory neurons encountered are consistent with those identified in other higher-order cortices.