Brain stimulation: feeling the buzz

Causal Evidence of Motion Signals in Macaque Middle Temporal Area Weighted-Pooled for Global Heading Perception

Accurate heading perception relies on visual information integrated across a wide field, that is, optic flow. Numerous computational studies have speculated how local visual information might be pooled by the brain to compute heading, but these hypotheses lack direct neurophysiological support. In the current study, we instructed human and monkey subjects to judge heading directions based on global optic flow. We showed that a local perturbation cue applied within only a small part of the visual field could bias the subjects' heading judgments, and shift the neuronal tuning in the macaque middle temporal (MT) area at the same time. Electrical microstimulation in MT significantly biased the animals' heading judgments predictable from the tuning of the stimulated neurons. Masking the visual stimuli within these neurons' receptive fields could not remove the stimulation effect, indicating a sufficient role of the MT signals pooled by downstream neurons for global heading estimation. Interestingly, this pooling is not homogeneous because stimulating neurons with excitatory surrounds produced relatively larger effects than stimulating neurons with inhibitory surrounds. Thus our data not only provide direct causal evidence, but also new insights into the neural mechanisms of pooling local motion information for global heading estimation.

Closing in on the constitution of consciousness

The science of consciousness is a nascent and thriving field of research that is founded on identifying the minimally sufficient neural correlates of consciousness. However, I have argued that it is the neural constitution of consciousness that science seeks to understand and that there are no evident strategies for distinguishing the correlates and constitution of (phenomenal) consciousness. Here I review this correlation/constitution distinction problem and challenge the existing foundations of consciousness science. I present the main analyses from a longer paper in press on this issue, focusing on recording, inhibition, stimulation, and combined inhibition/stimulation strategies, including proposal of the Jenga analogy to illustrate why identifying the minimally sufficient neural correlates of consciousness should not be considered the ultimate target of consciousness science. Thereafter I suggest that while combined inhibition and stimulation strategies might identify some constitutive neural activities—indeed minimally sufficient constitutive neural activities—such strategies fail to identify the whole neural constitution of consciousness and thus the correlation/constitution distinction problem is not fully solved. Various clarifications, potential objections and related scientific and philosophical issues are also discussed and I conclude by proposing new foundational claims for consciousness science.