Online research in philosophy

Rafael Malach is currently a professor in the department of Neurobiology at the Weizmann Institute in Israel. His current research is aimed at understanding how the neuronal circuitry in the human brain translates a stream of sensory stimuli into meaningful perception. Rafael Malach received his PhD in physiological optics from UC Berkeley and did his post-doctorate research at MIT. Originally doing research on the organization of neuronal connections in the primate brain, his focus has recently shifted to the study of the human cerebral cortex using fMRI. Professor Malach has begun this research at Massachusetts General Hospital, exploring a new object-related region called the lateral occipital complex. Since then he expanded this research, studying the human visual cortex using a variety of methods, including adaptation paradigms, backward masking, and more recently naturalistic stimuli--all aimed at deciphering the intriguing link between perceptual experience and brain activity.

The cortical visual mechanisms involved in processing spatial relationships remain subject to debate. According to one current view, the ''dorsal stream'' of visual areas, emanating from primary visual cortex and culminating in the posterior parietal cortex, mediates this aspect of visual processing. More recently, others have argued that while the dorsal stream provides egocentric coding of visual location for motor control, the separate ''ventral'' stream is needed for allocentric spatial coding. We have assessed the visual form agnosic patient DF, whose lesion mainly affects the ventral stream, on a prehension task requiring allocentric spatial coding. She was presented with transparent circular disks. Each disk had circular holes cut in it. DF was asked to reach out and grasp the disk by placing her fingers through the holes. The disks either had three holes (for forefinger, middle finger, and thumb) or two holes (for forefinger and thumb). The distance between the forefinger and thumb holes, and the orientation of the line formed by them, were independently varied. DF was quite unable to adjust her grip aperture or her hand orientation in the three-hole task. Although she was able to orient her hand appropriately for the two-hole disks, she still remained unable to adjust her grip aperture to the distance between the holes. These findings are consistent with the idea that allocentric processing of spatial information requires a functioning ventral stream, even when the information is being used to guide a motor response.

Awareness is a personal experience, which is only accessible to the rest of world through interpretation. We set out to identify a neural correlate of visual awareness, using brief subliminal and supraliminal verbal stimuli while measuring cerebral blood flow distribution with H215O PET. Awareness of visual verbal stimuli differentially activated medial parietal association cortex (precuneus), which is a polymodal sensory cortex, and dorsolateral prefrontal cortex, which is thought to be primarily executive. Our results suggest participation of these higher order perceptual and executive cortical structures in visual verbal awareness.

What is the relationship between a visual percept and the underlying neuronal activity in parts of the brain? This manifesto reviews the theoretical framework of Crick and Kochfor answering these questions based on the neuroanatomy and physiology of mammalian cortex and associated subcortical structures. This evidence suggests that primates are not directly aware of neural activity in primary visual cortex, although they may be aware of such activity in extrastriate cortical areas. Psychophysical evidence in humans supporting this hypothesis is discussed.

When words are read, the visual cortex is activated, independent of whether visual or motor associations are elicited. This word-evoked brain activity is significantly influenced by semantic meaning. Such effects occur very early after stimulus presentation (at latencies between 80 and 130 msec), indicating that semantic meaning activates different neuronal assemblies in the human visual cortex when words are processed.