747 Coordinate transformations for eye and arm movements in the brain Lawrence H Snyder Recent work on the coding of spatial information in the brain has significantly advanced our knowledge of sensory to motor transformations on several fronts The encoding of information referenced to the retina eye centered but modulated by eye position called a gain field representation has proved to be very common throughout parietal and occipital cortex The use of an eye centered representation as a working memory of spatial location is problematic if the eyes move during the memory period Details regarding the manner in which the brain solves this problem are beginning to emerge Finally the discovery of eye centered representations of ongoing or intended arm movements has changed the way we think about the order of operations in the sensory to motor coordinate transformation Addresses McDonnell Center for Higher Brain Function Department of Anatomy Neurobiology Box 8108 Washington University School of Medicine 660 South Euclid Avenue St Louis MO 63110 USA e mail larry eye hand wustl edu Current Opinion in Neurobiology 2000 10 747 754 0959 4388 00 see front matter 2000 Elsevier Science Ltd All rights reserved Abbreviations FEF frontal eye fields LIP lateral intraparietal area PM premotor cortex PMd dorsal premotor cortex PMv ventral premotor cortex PPC posterior parietal cortex PRR parietal reach region SC superior colliculus SEF supplementary eye fields V1 visual area 1 Introduction A major brain function is the manipulation of spatial information Much of what we do involves extracting spatial information from sensory input and then using that spatial information to direct a motor response A simple example is reaching for something we have seen This act requires that spatial information be transformed from a retinal coordinate frame to the coordinate frame of the muscles or joints involved in moving the arm A major endeavor of systems neuroscience has been to discover the algorithms and the sites at which such transformations are performed 1 4 5 6 This review will focus on several recent issues related to this endeavor It will consider only transformations of visual information used to drive eye and arm movements in primates and will focus on the involvement of the parietal cortex It has long been known that the lateral intraparietal area LIP in parietal cortex is involved in coding targets for saccadic eye movements Recently it has become clear that cortical regions medial to LIP are involved in coding arm movements The details of how spatial information is represented and manipulated in these regions is of critical importance to our understanding of sensory to motor transformations Eye centered coding and gain fields Many occipital and parietal areas involved in early processing of visual information do so in an eye centered frame of reference 7 In many areas eye centered spatial signals are modulated by postural information Cells in these areas have receptive fields whose locations are fixed with respect to the retina For example the receptive field of a particular cell may always lie 10 above the fovea regardless of the positions of the eyes head and body At the same time however the magnitude of the activity evoked by any given stimulus depends on eye head arm or body position The original description of this modulation called a gain field was a monotonic influence of eye position on visually evoked and saccade related activity in areas 7a and LIP 8 Since then gain fields have been described in numerous other brain areas as well Effects of eye position have been described in visual area 1 V1 the cortical area closest to the retinal input and therefore the most likely to be purely eye centered or retinotopic 9 10 11 12 However these observations are controversial 13 14 Elsewhere the data for eye position gain fields are well established Spatial responses in areas V3a MT middle temporal area MST medial superior temporal area VIP ventral intraparietal area V6 V6a 7m PMv ventral premotor cortex PM PMd dorsal PM and SEF supplementary eye fields have all been shown to be modulated by the position of the eyes in the head 15 22 23 24 Head position gain fields have been reported in both areas LIP and 7a 25 In LIP the gain fields are referenced to the body whereas in 7a they are referenced to the inertial frame that is to the fixed world around you Recently eye position effects have been identified in V4 12 26 demonstrating that eye position gain fields occur even in the ventral what visual pathway Several roles for gain fields in coordinate frame transformations have been proposed Zipser and Andersen 27 first showed that retinotopic signals modulated by eye position could be used as an intermediate stage in computing head centered location of visual targets As very few neurons in parietal cortex code in explicit head centered coordinates 28 29 an alternative proposal is that gain field representations themselves constitute a distributed representation of head centered spatial information Consistent with this idea Bremmer et al 30 recently showed that a network using actual recorded responses could be used to generate a reliable and precise estimate of the head centered location of a visual stimulus 748 Motor systems Remarkably despite 15 years of experimental and theoretical work on gain fields only circumstantial evidence exists that gain field information is in fact used in a spatial computation gain fields are ubiquitous and the distributed representation contains spatial information that is rarely or never explicitly represented elsewhere At one time an argument could be made that gain field information is not used by the brain This argument was based on the idea that LIP one of the first areas in which gain fields were well characterized serves to identify the location of visual targets for saccadic eye movements and to transfer that spatial information to the superior colliculus SC As such LIP would appear to have no use for the head centered position information contained in the eye position gain field However recent studies using animals whose heads are not fixed in place now link SC to the control of head as well as eye movements 31 32 for this purpose head centered target location is in fact highly relevant Thus the argument that head centered information in LIP is superfluous is no longer valid More generally the investigation of animals that are free to move their heads may well
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