Fig. 31 An example of the use of fMRI to study brain substrates of learning. Human subjects were subjected to fMRI scans while being presented with three types of visual stimuli (a: from left to right): word, nameable object, and face. The instructions were to remember each item for a later memory test. The word-encoding task was expected to depend on verbal processing, the object on both verbal and nonverbal processing, and the face on nonverbal processing. Coronal sections (b) show dorsal frontal cortex activation. Peak activation was observed in the left frontal cortex for word encoding, the left and right dorsal frontal for object encoding, and the right dorsal frontal cortex for face encoding. (c) The per cent signal change in the left (L) and right (R) hemispheres. There is a clear hemispheric asymmetry in the verbal and nonverbal encoding situations. Adapted from Kelley et al. (1998; in the original publication, the fMRI images are presented in pseudocolours).
Optical imaging of intrinsic signals relies on activity-dependent changes in light reflected from the imaged tissue. The intrinsic signals stem from activity dependent alterations in local blood volume, oxygenation of haemoglobin, and light scattering caused by the local movement of water, ions, and released "neurotransmitters) (Grinvald et al. 1986; Malonek et al. 1997; Vanzetta and Grinvald 1999). Optical imaging of intrinsic signals in its current state of the art is invasive, as it requires opening the skull. The optical changes can be visualized through intact dura or thinned bone, hence its invasiveness in experimental animals can be minimized. For the imaged tissue itself the process is nondestructive. The spatial resolution is high (|lm range), whereas the temporal resolution depends on the rise time of the haemodynamic and metabolic changes. Intrinsic imaging has been extensively employed to analyse the functional architecture of the mammalian brain, especially the visual system. The measurements can be repeatedly performed on the same subject over long periods (in fact, many weeks). The latter property is particularly advantageous in the study of the use and reuse of memory ("phase).
The use of voltage sensitive days involves application of dyes to the brain surface. This is invasive, and restricts the approach to animal studies. The advantage is a high temporal and spatial resolution (Shoham et al. 1999). Among the optical imaging methods, this is the one that follows neuronal activity most faithfully, as it monitors real-time alterations in membrane potential. An example of another optical imaging method that relies on extrinsic chromophores is "calcium imaging, i.e. using compounds that change their optical properties as a function of calcium concentration (e.g. Faber et al. 1999).
The contribution of optical imaging methods to the neurobiology of memory has so far been limited. These methods, none the less, have a remarkable potential to contribute to the analysis of brain mechanisms of learning and memory in experimental animals. The use of optical imaging in the analysis of the olfactory perception and olfactory memory is only one example. Calcium imaging has been used to detect experience-dependent alterations in odour "maps in the olfactory brain of the "classically conditioned "honeybee (Faber et al. 1999). Intrinsic imaging has been used to map the topography of odorant representation in the rat olfactory bulb as well (Rubin and Katz 1999).
All in all, functional neuroimaging is exciting, useful, and popular. Its potential for being even more so in the future is substantial. It has already provided us with remarkable data on the involvement of distinct brain areas and circuits in various phases and types of memory. But it is definitely not a magic bullet. The proper exploitation of its potential depends on proper adaptation of the specific technique to the right question and system, and in combining multiple synergistic methodologies in the experimental protocol. Selected examples include the use of molecular biology to image the dynamics of gene expression in the brain of small laboratory animals (Service 1999), hence creating the opportunity to monitor correlates of "consolidation in real time; the use of behavioural "assays to image brain substrates of memory illusions (Schacter 1996c, Cabeza et al. 2001, "false memory); and the differential contribution of brain circuits to mathematical thinking (Dehaene et al. 1999). Functional neuroimaging will be judged in the history of memory research not by the dazzling feats of the technology or by the impressive aesthetics of pseudocoloured images, but rather by the ability to solve questions (e.g. Kosslyn 1999, "enigma)
that cannot be unravelled without it. Judged by these "criteria, some authors think that so far, the contribution of functional neuroimaging to memory research is a bit overrated, whereas others think that this set of methodologies has already passed the test of memory research in flying colours.
Selected associations: Dimension, Engram, Homo sapiens, Phrenology
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