Karthik Ganesan:

Title:
Silent lip reading generates speech signals in auditory cortex: Evidence from intracranially implanted electrodes in humans

Abstract:
Audiovisual integration plays a vital role in speech perception, especially during face-to-face communication. Crossmodal activation of auditory processes by visual stimuli is an important aspect of natural speech perception. It has been previously shown that lip reading activates areas areas in the primary auditory cortex (PAC) including the superior temporal gyrus (STG). Though visual stimuli have been shown to influence neural representations in auditory cortex, it has not been conclusively shown whether auditory and visual stimuli activate the same population of neurons in the PAC. Here, we examine the spatial distribution of silent lip reading signals in the PAC in a large cohort of patients to study if this is indeed the case. We recorded electrocorticographic (ECoG) activity from macroscopic depth electrodes implanted within the STG of 13 patients with epilepsy. On each trial, patients were presented with one of three types of stimuli: (1) single phonemes, (2) videos showing the lip movements articulating each phoneme (visemes), or (3) videos showing audio-visual speech movements. Group-level analyses using parametric statistics were performed to show that visual lip -reading generates neural responses broadly along the PAC, spatially overlapping with the distribution of phoneme responses. Furthermore, we also investigated whether the identity of these phonemes and visemes could be discriminated from neural responses in auditory areas. Several electrodes across patients reliably discriminated between specific instances of the phonemes or visemes. However, preliminary analyses indicate that auditory and visual speech information are encoded at distinct areas of the STG. These results demonstrate that observing silent visual speech crossmodally activates speech-processing areas in a content-specific manner in the PAC. It is also shown that maximum information for phoneme discrimination in the PAC is carried in the frequency band of 4-8 hz.

Dalia Khammash

Title:
Probing cortical inhibition in visual cortex with transcranial magnetic stimulation

Abstract:
Transcranial magnetic stimulation (TMS) is a non-invasive method to stimulate localized brain regions. Despite widespread use in motor cortex, TMS is seldom performed in sensory areas due to variable, qualitative metrics. Our objective was to assess the reliability and validity of tracing TMS-induced phosphenes (short-lived artificial percepts) to investigate the stimulation parameters necessary to elicit decreased visual cortex excitability with paired-pulse TMS at short inter-stimulus intervals.