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ScienceWeek
NEUROSCIENCE: ON SPATIAL MEMORY
The following points are made by David K. Bilkey (Science 2004 305:1245):
1) Spatial memory underlies our ability to move purposefully through the environment. The search for the neural foundations of this ability center on the hippocampus, a brain structure involved in the type of spatial memory that we use to navigate to a location hidden from view, such as a parking space.
2) When a rat is foraging freely in an open arena, the firing rate of many hippocampal neurons will be modulated by the animal's position in space. These neurons are known as "place cells", and the region in the animal's environment in which a particular cell fires is known as its "place field". Each place cell has its own place field, which usually covers about 10 to 20% of the current environment, and the cell is silent when the animal is outside this location (3). A network of place cells could thus represent the current environment, and the ensemble activity of the network could potentially encode the animal's own position in relation to distinctive features and events within that environment. Rats have good spatial memory, and we assume that similar basic mechanisms underlie this ability in many different species, including humans (4).
3) Fyhn et al(1) ask where in the brain place fields are built. Are they constructed within the hippocampus, or is the process completed in other "upstream" brain regions? Previous electrode recordings from upstream regions such as the entorhinal cortex have revealed that neural firing is only weakly modulated by the position of the animal. This implies that the hippocampus is able to construct a high-resolution representation of spatial location. Fyhn et al(1), however, have obtained data that cast doubt on this conclusion. They recorded specifically from the dorsocaudal region of the rat entorhinal cortex, which contains cells that project to the portion of the hippocampus where spatial firing is most prominent. They found that cells in this region encode almost the same amount of information about the animal's location as hippocampal place cells. In contrast, cells in other regions of the entorhinal cortex have poorer spatial specificity, explaining the conclusions of earlier studies.
4) Fyhn et al. then showed that hippocampal lesions had minimal effects on the spatial firing of entorhinal neurons. Thus, the entorhinal cortex does not merely receive spatial information projected back from the hippocampus. Fyhn et al(1) also confirmed that cells in the cortical regions that provide major inputs to the entorhinal cortex responded minimally to the animal's location, supporting the notion that spatial representation is built in the entorhinal cortex.(2,5)
References (abridged):
1. M. Fyhn et al., Science 305, 1258 (2004)
2. S. Leutgeb et al., Science 305, 1295 (2004)
3. J. O'Keefe, J. Dostrovsky, Brain Res. 34, 171 (1971)
4. E. A. Maguire et al., Science 280, [921] (1998)
5. I. Lee, R. P. Kesner, Hippocampus 14, 301 (2004)
Science http://www.sciencemag.org
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NEUROBIOLOGY: ON THE BIOLOGICAL BASIS OF MEMORY
Notes by ScienceWeek
Exactly 100 years ago, two psychologists, G.E. Mueller and A. Pilzecker, proposed what came to be called the perseveration-consolidation hypothesis of memory. In studies with human subjects, Mueller and Pilzecker found that memory of newly learned information was disrupted by the learning of other information shortly after the original learning, and they suggested that processes underlying new memories initially persist in a fragile state and then consolidate over time. This consolidation hypothesis still guides research, particularly research in neurobiology on the time-dependent involvement of neural systems and cellular processes enabling lasting memory.
At the present time, the concept of "synaptic plasticity" underlies nearly all theories of memories, the term referring to changes in the behavior of the junction (synapse) between two nerve cells resulting from past history.
Two prominent aspects of synaptic plasticity considered to be related to memory are "facilitation" and "potentiation". The term "facilitation" refers to a progressive increase in the amount of *neurotransmitter substance released at a synapse by successive nerve impulses (action potentials), the increase occurring during an input barrage consisting of repetitive stimulation (stimulus train). The term "potentiation" refers to an increase in neurotransmitter substance released by an action potential following repetitive stimulation of a synapse.
Both facilitation and potentiation can be long-lasting, and "long-term potentiation" has been a focus of much research on the cellular basis of memory, particularly in the hippocampus, a brain cortex structure in the medial part of the temporal lobe. In humans, among other functions, the hippocampus is apparently involved in short-term memory, and analysis of the neurological correlates of learning behavior in the rat indicates that the hippocampus of the rat is also involved in memory.
The following points are made by James L. McGaugh (Science 2000 287:248):
1) The author points out that the idea that synaptic mechanisms of long-term potentiation and long-term facilitation underlie memory remains a hypothesis.
2) The author points out that although studies of long-term potentiation and memory have focused on the involvement of the hippocampus, much evidence indicates that the hippocampus has only a time-limited role in the consolidation and/or stabilization of lasting memory.
3) The author points out that there are forms of memory that apparently do not involve the hippocampus and that may not use any known mechanisms of synaptic plasticity.
4) The author points out that despite theoretical conjectures, little is known about system and cellular processes mediating consolidation that continues for several hours or longer after learning, consolidation that creates lifelong memories.
Concerning the above caveats, the author concludes: "These issues remain to be addressed in this new century of research on memory consolidation."
Science http://www.sciencemag.org
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Notes by ScienceWeek:
neurotransmitter substance: Neurotransmitters are chemical substances released at the terminals of nerve axons in response to the propagation of an impulse to the end of that axon. The neurotransmitter substance diffuses into the synapse, the junction between the presynaptic nerve ending and the postsynaptic neuron, and at the membrane of the postsynaptic neuron the transmitter substance interacts with a receptor. Depending on the type of receptor, the result may be an excitatory or an inhibitory effect on the postsynaptic nerve cell.
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NEUROBIOLOGY: VISUAL EXPERIENCE AND SPATIAL PERCEPTION
The following points are made by Martin Eimer (Current Biology 2004 14:R115):
1) It is commonly believed that vision, hearing and touch are entirely separate "perceptual modules", each operating independently to provide us with unique information about the external world. Recent studies, however, have revealed that our perceptual experience is in fact shaped by a multitude of complex interactions between sensory modalities. A number of powerful multisensory illusions demonstrate that the senses are inextricably linked, and that our perception of visual, auditory or tactile events can be altered dramatically by information from other senses.
2) When a sound is accompanied by a visual stimulus at another location, people tend to perceive this sound incorrectly at the same position as the visual stimulus -- the ventriloquism effect [1]. When two objects are lifted that differ visibly in size, but are equal in weight, the larger object is felt to be heavier --the size weight illusion [2]. When people see a life-sized rubber model of their hand being touched at the same time as their own hand, which is hidden from view, they experience the touch on the rubber hand, and often report that the rubber hand feels as if it was their own [3].
3) In these cases, auditory and tactile perception are substantially altered by simultaneously available visual information. As a general rule, our sensations tend to be dominated by the modality that provides the most detailed and reliable information about the external world. Because vision provides highly accurate and detailed spatial information about three-dimensional properties of external objects, it is used to guide spatial judgements in other modalities as well, and can therefore influence (and sometimes distort) our spatial perception of auditory and tactile events.
4) Recent research has begun to uncover the neural basis of such interactions between sensory modalities in spatial perception. Neurons responding to multimodal stimuli have been found in numerous brain areas, including the superior colliculus and parietal areas [4]. These multisensory neurons typically have spatially overlapping receptive fields, which means that they are activated maximally in response to simultaneous visual, auditory and tactile events at the same external location.(5)
References (abridged):
1 Bertelson, P. and Aschersleben, G. (1998). Automatic visual bias of perceived auditory location. Psych. Bull. Rev. 5, 482-489
2 Flanagan, J.R. and Beltzner, M.A. (2000). Independence of perceptual and sensory motor predictions in the size-weight illusion. Nat. Neurosci. 3, 737-741
3 Botvinick, M. and Cohen, J. (1998). Rubber hands "feel" touch that eyes see. Nature 391, 756
4 Stein, B.E. and Meredith, M.A. (1993). The merging of the senses. (Cambridge, Mass.: MIT Press)
5 Roeder, B., Ruesler, F., and Spence, C. (2004). Early vision impairs tactile perception in the blind. Curr. Biol. 14:121
Current Biology http://www.current-biology.com
ScienceWeek http://scienceweek.com
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