Brain and Cognition Some New Technologies (1989) / Chapter Skim
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4. Four New Technologies: Critical Problems
4. Four New Technologies: Critical Problems
Pages 43-59
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From page 43... ...
Positron emission tomography presents such problems as the precise location of physiological processes, spatial and temporal discriminations, appropriate statistical analysis of the data, and risks to participants producecl by the imaging technique. And studies of the effects of brain damage have been limited by problems of extrapolating from animal to human work.
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From page 44... ...
Even if the dependent psychophysiological variables are measured with great precision, a careless definition of the independent task variables renders the experiment inadequate. An investigator may, for example, employ a PET scan and pub fish results of examining the metabolism in different parts of the brain when people are asked to "think" compared with the metabolism when they are asked not to ~think." The results appear exciting, especiaITy as they are presented in color pictures that have anatomical reference.
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From page 45... ...
There is at this tune no consensual terminology and no systematic attempt to standardize measurement procedures to ensure that components are equivalently defined in all paradigms by ad subjects (Fabian) et al., in press)
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From page 46... ...
Difficulties With Clinical Studies Malting sense of the psychophysiological literature is a formidable task. The number of papers published annually is very large.
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From page 47... ...
This is entirely adequate, even desirable, when conducting a diagnostic test, yet for many research projects it is critical to sort the data prior to averaging (e.g. to compare the ERP elicited when subjects erred or responded correctly, or to compare ERPs elicited when subjects were fast with those elicited when they were sIow)
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From page 48... ...
the scalp is due to intracellular current flow and ~ not affected significantly by extracellular volume currents. Although it is possible to detect the magnetic field associated with action potentials In isolated axons and nerve bundles (Swinney and Wikswo, 1980)
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From page 49... ...
Nevertheless, it IS important to precisely determine the magnitudes of contributions of secondary sources since, if they are significant but unaccounted for, they may bias computations of source locations and strengths. Magnetically Silent Sources As mentioned earlier, sources tangential to the surface of the skull are associated with observable fields, while So-called radial sources do not contribute to the external field.
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From page 50... ...
These will exhibit opposed patterns of current flow, and their electrical and magnetic fields will tend to cancel each other, as do current flows in the dendrites of closed field neurons. Hence, both EP and EF experiments in which visual stimuli are presented to both hemi-retinas produce responses that represent the residual effect of the anatomical asymmetry of the hemispheres.
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From page 51... ...
For example, high resolution and accurately scaled MR! scans together with reviews of histological data on aD areas of the human brain may furnish computational theorists with information useful in estimating the proportion of closed field neurons in the brain and of areas in which there is significant mirror symmetry.
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From page 52... ...
Straightforward measurements of the somatosensory evoked field produce data in excellent agreement with this physiologically established ordering, thus providing face validity for the inverse solutions that have been presented. Similarly, MRI scans of human subjects who had served in evoked fields experiments can show that the computed source locations lie at positions in various suIci known to contain neurons that respond to the particular sensory stunuli.
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From page 53... ...
They should influence decisions about the way in which neuromagnetism and other technologies are developed and used in experimental work. IMAGING T1:CENIQUES I: POSITRON EMISSION TOMOG1lAPlIY Positron emission tomography is a complex technology requiring adequate human and physical resources in the areas of instrumentation, radiochemistry, tracer kinetics, computer programming, and neurobiology as well as easy access to patients and normal control subjects.
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From page 54... ...
The resolution of a PET system is defined as the width of this distribution of radioactivity at one half of the magnum counting rate, the so-called full width at half maximum, usually abbreviated FWHM. One very important consequence of this definition of resolution is that when two point sources of radioactivity occur simultaneously in the field of view of a PET device, they cannot be distinguished as two separate sources if they are closer than a distance of one FWHM.
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From page 55... ...
By functional studies is meant studies in which some type of activation paradigm Is employed to produce a change in local blood flow or metabolism from a resting or control state. Under such circumstances PET data, obtained from the subtractions of a control state image from an image obtained during some type of functional activity, can be used to determine whether areas separated by significantly less than one FWHM are differentially activated by specific alterations in stimulus conditions (Fox et al., 1985~.
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From page 56... ...
Times of this magnitude obviously make difficult a second study in the same individual on the same day. As a result, most functional studies of the human brain with PET and fluorodeoxyglucose involve a single measurement in each subject studied with control measurements from a separate group of subjects.
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From page 57... ...
It is always viewed as a necessary goal of all studies involving PET not only to keep radiation exposure below the FDA limits but also to keep exposure to the lowest possible amount consistent with obtaining adequate data. Gains can be anticipated in reducing radiation by improving the efficiency of PET imaging devices dedicated to functional brain unaging in normal human subjects.
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From page 58... ...
It is still the belief of many students of linguistics that discrete brain lesions may disrupt particular grammatical rules or at least disrupt lexical modules versus phrasal modules. Furthermore, seeing how other processes such as attention or spatial mechanisms contribute to language processing is currently of interest.
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From page 59... ...
Yet, as Francis Crick has said, the lesion approach, whether one likes it or not, is one of the few approaches available to the brain scientist.
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