858 nature neuroscience • volume 2 no 10 • october 1999

news and views

rable to that seen by Sugase and colleagues1.

However, the activity described by Sugaseand colleagues seems difficult to attributeto behavioral influences, because the ani-mals were not required to make any behav-ioral response but merely viewed thestimuli passively.

The above examples illustrate severalcoding strategies: multiple response mea-sures encoding the same stimulus, multipleresponse measures encoding multiple stim-ulus attributes, a single response measureencoding combinations of attributes, or asingle response encoding a stimulus and itsbehavioral significance. In contrast to all ofthese examples, Sugase and colleagues haveshown that a single response measure canevolve over time to encode new informa-tion, in the absence of any external cues.

How might this increase in informationcontent come about? Computational mod-els of visual processing often make use offeedback and lateral inhibition to enhanceweak or noisy signals10,11. These modelsreceive some experimental support fromrecordings in primary visual cortex12–14, butthey can only work in cases where theinformation for the feedback or inhibito-ry signals is already present in the respons-es. It seems unlikely that local feedback orlateral inhibition could explain the resultsof Sugase and colleagues, because theyfound very few neurons that contained anydetectable information about face identityor expression during the early phase of theresponse. Instead, this information mustbe introduced via top-down inputs fromother brain areas. Top-down processes aretypically used by modelers to allow senso-ry processing to be modulated by infor-mation not present in the initial responses,such as behavioral significance or seman-tic knowledge. However, they can also beused in models involving reciprocal inter-actions between different sensory areas, inwhich signals from one area are conveyedto another area where new informationfrom another source is added. The resul-tant signal containing the new information,which might reflect other sensory process-es, memory or behavioral significance, canthen be sent back to modulate activity inthe earlier area. In the present example,computations in the inferior temporal cor-tex would establish the presence of a face,and this information would be passed toother areas that would extract informationabout identity and facial expression. As theauthors discuss, the inferior temporal cor-tex has many connections to and from cor-tical and subcortical areas that areimplicated in social and emotional pro-cessing. The delay of some 50 ms between

the signal that a face was present and theappearance of information about identityor expression is also consistent with theidea that the initial signal from the tempo-ral cortex passes to other regions thatprocess it further and then relay the result-ing signals back to the temporal cortex.

As with any innovative and excitingstudy, the findings of Sugase and col-leagues raise more questions than theyanswer. Where does the ‘new’ informationabout identity and expression come from?Does the total amount of informationencoded by the inferior temporal neuronsincrease over time, or does the new infor-mation replace the old? Assuming that thenew information is introduced from else-where in the brain, how does it affect theprocessing that occurs within the inferiortemporal cortex itself? Ultimately, it willbe important to understand the relation-ship between this neural activity andbehavior. For instance, does the discrimi-natory behavior of these neurons matchthe ability of the monkey to make behav-ioral discriminations? Can the monkey’sbehavior be altered by disrupting this top-down processing, for instance using selec-tive lesions or microstimulation? If it turnsout that the changes in information con-tent over time have functional significance,

Switching the hippocampus off and onMemory consists of interdependent but dissociable processes, such as encoding,storage, consolidation and retrieval. Lesion studies have shown that the hippocampus isnecessary for declarative memory, but pinning down its exact contribution to theseindividual stages has proved tricky. Because brain lesions are permanent, they cannotunambiguously dissociate the stages of memory.

Richard Morris and his colleagues (University of Edinburgh Medical School) have nowdeveloped a technique that could overcome this problem. On page 898, they reportreversible, temporary inactivation of the hippocampus with LY326325, a selective watersoluble antagonist of AMPA/kainate glutamate receptors. The authors infused the dorsalhippocampus (inclusive of areas CA1-CA3 and the dentate gyrus) of rats with this drugduring training and/or retention on a version of the water maze test. The authors verifiedphysiologically that they couldselectively 'switch off' the dorsalhippocampus for varying periodsand then switch it on again andhave it work normally afterward.This technique revealed that thehippocampus is critical for bothencoding and retrieval of spatialmemory. This approach representsa potentially powerful way to testwhether the functional integrity ofvarious brain areas is necessary forspecific memory processes.

Kalyani Narasimhan

then our understanding of how brain areasprocess information, both locally and incombination with other areas, will havebeen substantially advanced.

1. Sugase, Y., Yamane, S., Ueno, S. & Kawano, K.Nature 401 (in press).

2. Optican, L. & Richmond, B. J. J. Neurophysiol.57, 162–178 (1987).

3. Gawne, T. J., Kjaer, T. W. & Richmond, B. J. J. Neurophysiol. 76, 1356–1360 (1996).

4. Worgotter, F., Opara, R., Funke, K. & Eysel, U.Neuroreport 7, 741–744 (1996).

5. Tanaka, K., Saito, H.-A., Fukada, Y. & Moriya,M. J. Neurophysiol. 66, 170–189 (1991).

6. Oram, M. W. & Perrett, D. I. J. Neurophysiol.76, 109–129 (1996).

7. Oram, M. W. & Perrett, D. I. J. Neurophysiol.68, 70–84 (1992).

8. Desimone, R. & Duncan, J. Annu. Rev.Neurosci. 18, 193–222 (1995).

9. Chelazzi, L., Miller, E. K., Duncan, J. &Desimone, R. Nature 363, 345–347 (1993).

10. Grossberg, S. Cognit. Sci. 11, 23–63 (1987).

11. McClelland, J. L. & Rumelhart, D. E.Explorations in Parallel Distributed Processing(MIT Press, Cambridge, Massachusetts, 1988).

12. Sillito, A. M. J. Physiol. (Lond.) 250, 305–329(1975).

13. Berman, N. J., Douglas, R. J., Martin, K. A. C.& Whitteridge, D. J. Physiol. (Lond.) 440,697–722 (1991).

14. Frith, C. & Dolan, R. J. Phil. Trans. R. Soc.Lond. B 352, 1221–1230 (1997).

© 1999 Nature America Inc. • http://neurosci.nature.com©

199

9 N

atu

re A

mer

ica

Inc.

• h

ttp

://n

euro

sci.n

atu

re.c

om