Research Profile

July 2009

Principal Investigator:Gene R. Stoner
Co-Investigators: Hulusi Kafaligonul, Ladan Shams

The Mechanisms Underlying Multisensory Interactions in the Brain

As we are all aware, objects and events encountered in our daily lives stimulate several of our senses at the same time. Generally, however, the processing within the brain of objects and events is segmented, with different brain areas responding to different sensory stimuli.

The temporal dynamics of different sensory modalities also differ. For example, the processing of sound within the ear is much faster than the processing of light within the eye. This means that the brain’s responses to a single auditory-visual event occur at different locations and in different timeframes. And learning just how these spatially- and temporally-distributed activities interact to create our unified multisensory experiences is one of the most fundamental problems in neuroscience.

Thus, the longterm goal of this team’s research is to understand the neuronal mechanisms underlying such cross-modal modulation. Their unusual experimental paradigm was inspired by an illusion developed just last year by U.K. scientists Elliot Freeman and Jon Driver. In the illusion, brief sounds systematically change the perceived direction of visual motion stimuli, although the sounds themselves provide no motion cues.

The illusion is believed to rely on the ability of sound to “capture” the timing of visual stimuli, a phenomenon known as temporal ventriloquism, and with use of their KIBM award the research team has successfully adapted it for use in certain neurophysiological experiments.

It seems that Freeman and Driver used “long-range” visual stimuli, involving large spatial and temporal displacements, and the neuronal basis of these is believed to involve higher-order processing than is associated with the “short-range” motion stimuli that activate area MT. Although it was believed unlikely that the F&D illusion would extend to short-range stimuli, the team happily discovered that it does. This has led to the hypothesis that area MT underlies this cross-modal illusion.

The team is now testing several other hypotheses about the way that auditory stimuli shift the timing of visual stimuli. And soon to start are neuronal recording experiments from macaque monkeys. This will be the first examination of auditory-visual perceptual interactions at the cellular level in area MT. By examining this neuronal data together with the human behavioral experiments, they envision being able to identify the mechanisms that underlie the amazing unified multisensory experience of the world that we all enjoy.

Isn’t it a wonder that in spite of nearly overwhelming quantities of information bombarding our sensory systems every minute of the day, we are able to reduce, condense, amalgamate it into a manageable flow and to focus on only what relates to whatever is our current task? And the big question is how do we selectively attend to one aspect of our environment and back-burner the rest?

For starters there’s mounting evidence from both primate and human studies suggesting:

• That cortical neurons are able to synchronize their activity, producing brain rhythms at different frequencies.

• That these brain rhythms, particularly in the gamma frequency range (30-80 Hz), are instrumental in our selective attention.

• That disruption of gamma rhythms is a key marker for such serious cognitive and attentional maladies as bipolar disorder, schizophrenia and attention deficit hyperactivity disorder (ADHD).

It’s true that basic mechanisms underlying gamma frequency synchronization have been found to be the same in many species. What has not been understood is the way that the synchronization of these neurons affects their targets. How do changes, such as attentional enhancement in gamma activity, interact with small circuits or single cells to control input processing? And just what is it that makes these changes happen?

Now, taking advantage of the similarities across different species, a team of Salk Institute researchers that includes electrophysiologists, engineers, systems biologists and virologists is utilizing their Kavli Foundation award to study the functional and physiological consequences of gamma frequency spike synchronization of neurons in transgenic mice.

Making use of innovative genetic, viral and computational techniques, the collaborators aim to better understand the biological underpinnings of selection and attention. Using a real-time neuron/computer hybrid system to introduce controllable conductances into neurons, as well as targeted recordings of specific neuronal types, they have determined the differing ways that different types of neurons respond to gamma frequency synchronized inputs.

These techniques have led the group to discover systematic differences in the way distinct neuron types respond to changes in gamma frequency oscillatory synchrony and how changes in this synchrony change the neurons’ processing of input. Further, they have identified specific electrical properties that account for the differences.

Interestingly, during cortical activity these properties can quickly change, suggesting that the cortex may indeed make moment-to-moment adjustments in filtering input to fit moment-to-moment changes in functional needs --- such as what input to process and what to ignore.

Currently the team is creating viral tools to change the way that cells integrate gamma frequency inputs and to affect the emergence of gamma oscillations. Through this work they hope to uncover the role of these oscillations in attentional selection, in the long term potentially leading to treatments for attentional disorders in humans.