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Neuronal selectivityNeuronal selectivitySparse but not 'Grandmother-cell' coding in the medial temporal lobe Quiroga, Q., Kreiman, G., Koch, C., & Fried, I. Trends in Cognitive Science, vol. 12, No. 3, (2008) Keywords: neuron-selectivity, consciousness P. Studies over the last decade have shown that some neurons in the medial temporal lobe respond selectively to complex visual stimuli. The studies suggest a hierarchical organisation along the ventral visual pathway. Neurons in V1 code for basic visual features, whereas at the stage of the inferior temporal cortex neurons can code selectively for complex shapes or even faces. There is a tension at present between evidence relating subjective perception to the activity of large neuronal assemblies linked by the global gamma synchrony and other studies relating it to the activity of much smaller numbers of neurons. Since the correlations with consciousness appear strong in both cases, it seems likely that consciousness will be found to involve both types of process. With theories related to small numbers of neurons, it is shown that neurons are selective for particular images or categories of image, and that most neurons will be inactive in relation to most objects. The inferior temporal cortex projects to the medial temporal cortex where neurons are found to be selectively responsive to categories such as animals, faces and houses, as well as the degree of novelty of images. Activity in the medial temporal lobe is thought to be linked to creating memories rather than actual recognition, which seems to be more closely linked to the inferior temporal lobe. In a study by the authors, a hippocampal neuron fired in response to the image of a particular actor. Recording of the activity of a handful of neurons could be used to predict which of a number of images a subject was viewing at an accuracy far above chance. About 40% of medial temporal lobe neurons were found to be selective in this way, although some could fire selectively in response to more than one image. However, when this was case the images were often connected, such as two actresses in the same soap opera, or two famous towers in Europe. In fact it is estimated that selectively responding cells would respond to between 50 and 150 images. P. The authors are not trying to revive the idea of the 'grandmother cell' where one and only one neuron could respond to a particular image, for instance the image of the subject's grandmother. Rather than that the author's have estimated that out of one billion cells in the medial temporal lobe, two million could be responsive to specific percepts. These cells respond to percepts that are built up in the ventral pathway rather than detailed information falling on the retina. |
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