One neuron, two views

ResearchBlogging.org The receptive field of a visual neuron is the region in space that the neuron responds to. It is the tiny window through which the neuron “sees” the world. You shine a spot of light inside the receptive field, you hear the neuron fires away with gusto. You move the spot of light away from the receptive field just a tiny bit, silence. I love doing receptive field mapping demos for students looking for lab projects because it really is one of the most rewarding things you can do as an educator. Without fail, the students widen their eyes and smile (some of them screamed) as the small light spot moves across the invisible boundary in darkness, even if they are familiar with the concept and know what’s coming. It’s one of those magic moments in science. I became so fascinated by receptive fields that I decided to become a visual electrophysiologist.

The scientific research of receptive fields has been focused on finding out what stimulus you can use to get a neuron to fire. Recently I became interested in something more obscure but more basic. I became interested in the geometry of receptive fields. Most vision scientists envision receptive fields to be small circular or rectangular patches. Can they take on more complicated shapes? Are there receptive fields with holes in the middle, or with one side missing? In a recent paper I documented an inverted L-shaped receptive field in area MT. It was recorded in an animal with a partial lesion placed in its primary visual cortex, which created a scotoma in one of its visual fields. The scotoma is indicated by a dotted red line in the figure. As you can see, the scotoma seemed to have torn a  hole in the receptive field.

That certainly was an unusual receptive field, but it was found in an animal whose natural development  had been artificially altered. Indeed, the neural plasticity literature has shown many examples of unusual receptive field shapes induced by physical lesions or chemical/genetic manipulations. What about normal receptive fields? Are they always the same old circular patches?

In primates, the answer seems to be yes … but not quite. I am glad that I found a paper by Ivan Pigarev and his coworkers published in 2001. Recording from area V4 of behaving macaques, they found a population of neurons with unusually large receptive fields. Some of them are really weird: they seem to respond to stimuli presented in two disjointed regions in the visual field. In other words, they are neurons with two windows.  More interestingly, the two regions are always at symmetrical locations across the vertical meridian, one looking at the ipsi-hemifield, the other looking at the contra-hemifield. What are they for? We don’t really know. Stereo vision? Symmetry detection? There are many possibilities. We need to do more research to find out.

An interesting thing about this finding is that this kind of receptive field is actually predicted by neuroanatomy. Many neuroscientists have the impression that the corpus callosum carries information about receptive fields around the vertical meridian. That is not entirely correct. Clusters of callosal fibers have been found to terminate in loci that are far away from cortical representation of the vertical meridian. It would be very interesting if we can work out the neural circuits that underlie these odd receptive fields.

Pigarev, I., Nothdurft, H., & Kastner, S. (2001). Neurons with large bilateral receptive fields in monkey prelunate gyrus Experimental Brain Research, 136 (1), 108-113 DOI: 10.1007/s002210000566

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~ by hhyu on January 15, 2011.

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