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Single-neurons and subjective visionSingle neurons and subjective visionSingle-neuron correlates of subjective vision in the human medial temporal lobe Gabriel Kreiman, Itzhak Fried & Christof Koch P. PNAS, June 11 2002, vol. 99, no. 12, pp. 8378-83 Keywords: single-neuron, subjectivity, correlates of consciousness INTRODUCTION: This and similar studies should be seen as laying to rest two favourite ideas of twentieth century consciousness studies. The first was the idea that consciousness was non-physical. This approach is not really coherent within a scientific paradigm in any case, but experiments now demonstrate a correlation between subjective perceptions and physical levels of activity in individual neurons. Similarly, the mind-brain identity concept seemed to propose that in some mysterious way consciousness was identical to the whole operation of the brain, whereas this and other experiments clearly relate consciousness to the activity of individual neurons and specific neuronal assemblies, albeit both the neurons and assemblies involved are constantly changing. The same environmental input to the retina can give rise to two quite different conscious visual percepts. In this study, the responses of individual neurons were recorded. Two-thirds of the visually selective medial temporal lobe neurons recorded showed changes in activity that correlated with the shifts in what was subjectively perceived, rather than the retinal input. P. Flash suppression is an experimental technique by which an image is sent to one eye and then a different image to the other eye. The newer image will suppress the first input. Neurons that select for the initial input and not the input to the second eye will be inactive when the first input is suppressed in this way, although the first image is still physically present on the retina. In visual illusions such as the Necker cube, the same retinal input can produce two different subjective perceptions. There is a distinction here between what happens in the primary visual cortex and in the later visual areas. Activity in the primary areas correlates to the retinal input, rather than any subjective perception. In this study performed in the US in 2002, a neuron in a subject's amygdala responded selectively to the image of President Clinton, while failing to respond to 49 other test images presented. In the case of Clinton's image, the neuron's firing rate jumped from a baseline of 2.8 spikes a second to 15.1 spikes per second. However, the neuron did not react when the initial image of Clinton was suppressed by an image for which it was not selective. Another amygdala neuron increased its firing in response to some faces, but was inactive when an image it didn't select for was flashed to the other eye. A neuron in the medial temporal lobe increased its firing in response to pictures of spatial lay outs and not to other stimuli. Here again the activity did not occur when a different image was flashed to the other eye. In all these cases, the physical input to the first eye was continuing, but was not getting into conscious perception.. Out of 428 neurons studied in the medial temporal lobe, 44 responded selectively to particular categories and 32 to specific images. None of these neurons were active when the images or categories they were selective for were part of the input on their retina, but were suppressed from subjective experience by a second image to the other eye. However, they could be active when both images were present, but the image they selected for was dominant. In the experimental subjects two out of three medial temporal lobe neurons changed their firing in line with subjective perceptions, but activity did not change if an input was present on the retina but not subjectively experiences because of retinal input to the other eye. |
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