Localization of Motion Perception the Cortex

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                Localization of Motion Perception the Cortex

Are there any principles of cortical organization common to all the sensory areas of the cortex?  What is the significance of topographic and nontopographic maps? What functions are represented in cortex and are there basic structural and functional uniformities in the organization of the cortex?  Gregory (1974) discusses the difficulties in taking over engineering methods into biology and some implications of this approach for the study of the central nervous system and most importantly localization of function.  Modern methods attempt to analyze the whole in such a manner that its activity can be completely described by the causal relations between the parts.  When a feature of behaviour is said to be localized in a part of the brain, the intended meaning is that some necessary, though not sufficient, condition for this behaviour is localized in a specific region of the brain Gregory, R.L. (1974).

In this essay the notion of localisation in the cortex is considered by examining the current experimental findings in relation to motion perception in the human cortex.  Firstly the ‘where’ pathway for motion related information in the brain, the parietal stream will be explained and broken down.  Continuing on to look at its individual parts MT, MST, FST and VIP.  Following this evidence relating to the discovery of these specialised centres including lesion studies as well as imaging studies like fMRI (functional magnetic resolution imaging) and PET (positron emission topography).  Converging evidence strongly implicates area MT (V5) as being important in motion perception. In recent years we have also come to appreciate that MT provides substantial input to other ‘satellite’ areas, particularly the medial superior temporal (MST) area where local motion signals are combined by neurons that respond to optic flow (Badcock & Khuu 2001). These areas also have an important role in motion perception.

The parietal pathway

The study of the visual striate cortex and extra- striate cortex led to the notion of specialization.  Different parts of the brain seem to be involved in processing the visual image for different attributes (Snowden, R.J. 1994).  Two streams exist in the monkey cortex that transmit signals from the striate cortex into the extra-striate cortex.  These are the parietal and temporal streams, these two streams have different functions.  The temporal pathway is important for identifying objects and has therefore been called the ‘what’ pathway’. The human visual system also contains a functional sub-system that is specialized to extract image motion (Badcock D.R Khuu, S. R. 2000). An area specifically concerned with motion processing seems to be one node in the hierarchical flow of visual information.  The parietal pathway is crucial for locating objects and has been termed the ‘where’ pathway (Sekular, R. & Blake, R. 1994). Detection of motion of the image across the retina is performed locally in parallel for all points in the image.  All subsequent computations (object motion, three-dimensional trajectories, self-motion are based on these subsequent measurements.  Exact boundaries of the parietal and temporal streams in humans are hard to specify.  However a human parietal stream would presumably include area MT (V5), MST and possibly VIP and LIP.

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Area MT contains a complete representation of the contralateral visual field. Receptive field size increases linearly with eccentricity and is about ten times larger than in striate cortex, suggesting a significant convergence of inputs.  The area is organized in a columnar fashion with blocks of cortical tissue containing similar directional selectivity.  Projections from V1 originate from layer4B and 6, while projections from V2 originate from the cytochrome oxidase thick strips, that in turn receive a direct projection from 4B.  Hence whether directly or indirectly through V2, the input to MT seems to be dominated by the M pathway.

Strong physiological ...

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