Wednesday, April 23, 2008
Monkey Lip Reading
First it was Broca's area in the chimp, now there is a new study examining audiovisual integration in the perception of monkey vocalizations.
The study by Ghazanfar, Chandrasekaran, & Logothetis (J. Neurosci. 2008, 28:4457-69) recorded single units as well as local field potentials in the STS and in auditory cortex of macaque monkeys. They report that responses in auditory cortex (lateral belt regions) are influenced by visual inputs from the STS. (Abstract below)
This looks like a pretty nice study that provides direct evidence for multisensory integration in belt areas of auditory cortex. This may not be the only source of input to these multisensory cells in the lateral belt region. In humans, lip reading activates a large network that include frontal regions. Feedback projections from motor-speech areas may also influence responses in auditory cortex (at least in humans) as mentioned previously.
Interactions between the Superior Temporal Sulcus and Auditory Cortex Mediate Dynamic Face/Voice Integration in Rhesus Monkeys
Asif A. Ghazanfar,1,2 Chandramouli Chandrasekaran,1 and Nikos K. Logothetis2
1Neuroscience Institute and Department of Psychology, Princeton University, Princeton, New Jersey 08540, and 2Max Planck Institute for Biological Cybernetics, 72076 Tuebingen, Germany
Correspondence should be addressed to Asif A. Ghazanfar, Neuroscience Institute and Department of Psychology, Green Hall, Princeton University, Princeton, NJ 08540. Email: email@example.com
The existence of multiple nodes in the cortical network that integrate faces and voices suggests that they may be interacting and influencing each other during communication. To test the hypothesis that multisensory responses in auditory cortex are influenced by visual inputs from the superior temporal sulcus (STS), an association area, we recorded local field potentials and single neurons from both structures concurrently in monkeys. The functional interactions between the auditory cortex and the STS, as measured by spectral analyses, increased in strength during presentations of dynamic faces and voices relative to either communication signal alone. These interactions were not solely modulations of response strength, because the phase relationships were significantly less variable in the multisensory condition as well. A similar analysis of functional interactions within the auditory cortex revealed no similar interactions as a function of stimulus condition, nor did a control condition in which the dynamic face was replaced with a dynamic disk mimicking mouth movements. Single neuron data revealed that these intercortical interactions were reflected in the spiking output of auditory cortex and that such spiking output was coordinated with oscillations in the STS. The vast majority of single neurons that were responsive to voices showed integrative responses when faces, but not control stimuli, were presented in conjunction. Our data suggest that the integration of faces and voices is mediated at least in part by neuronal cooperation between auditory cortex and the STS and that interactions between these structures are a fast and efficient way of dealing with the multisensory communication signals.