Greater Expectations

LTP (“depotentiation”) is a separate entity. NSF–GluR2and Shadlen (1999) record from two prefrontal areas and find that neurons in only one of them fire more when mediated AMPA receptor trafficking, however, plays an the monkey expects three squirts of water to drink than important role in maintaining synaptic strength and is a when it expects one. potential target for LTD-inducing stimuli. The experiment uses a clever and carefully balanced design. The monkey fixates on a red dot in the center Dimitri M. Kullmann of a computer screen. Then, the color changes to green University Department of Clinical Neurology or white, signaling, respectively, that either one or three Institute of Neurology squirts of water will be rewarded if the animal completes University College London the trial successfully. A red light briefly flashes someQueen Square where else on the computer screen, in the target locaLondon WC1N 3BG tion. The monkey waits for 2–4 s until the fixation light United Kingdom is extinguished; during this crucial delay period, only the colored fixation-cum-reward signal is on the screen. Selected Reading Then, finally, the monkey shifts its gaze, almost always to the remembered location. In a variation on this protoDerkach, V., Barria, A., and Soderling, T.R. (1999). Proc. Natl. Acad. col, the sequence of reward and target cues is reversed Sci. USA 96, 3269–3274. on half of the trials. Engert, F., and Bonhoeffer, T. (1999). Nature 399, 66–70. At the same time, the experimenters record action Jia, Z., Agopyan, N., Miu, P., Xiong, Z., Henderson, J., Gerlai, R., potentials from what they believe is a single neuron in Taverna, F.A., Velumian, A., MacDonald, J., Carlen, P., et al. (1996). Neuron 17, 945–956. either area 46 or the frontal eye field (FEF) that responds selectively to a light in a limited region of the screen (a Lledo, P.M., Zhang, X., Südhof, T.C., Malenka, R.C., and Nicoll, R.A. (1998). Science 279, 399–403. receptive field) during the delay period. In fact, the target