powerful factor in perception, action, and mem- 

 ory — in the unity of conscious experience. At 

 any given moment, animals are inundated with a 

 vast number of sensory stimuli — far more than 

 the brain can process. Attention acts as a filter, 

 selecting some objects for further processing. As 

 the American psychologist William James noted 

 in his seminal book, The Principles of Psychology, 

 in 1890: 



Millions of items ... are present to my senses which never 

 properly enter into my experience. Why? Because they 

 have no interest for me. My experience is what I agree to 

 attend to. . . . Every one knows what attention is. It is the 

 taking possession by the mind, in clear and vivid form, of 

 one out of what seem several simultaneously possible ob- 

 jects or trains of thought. 



baseline level of attention, which we called ambient 

 attention. A mouse was given some time to run 

 around in the enclosure, without any distracting 

 stimuli, and the mouse's position and the firing of 

 the cells were recorded simultaneously. In another 

 condition, we engineered things so that as the mouse 

 walked around in its enclosure, bright lights and loud 

 sounds, which the mouse hates, came on periodi- 

 cally and at random. The only way the mouse could 

 turn them oif was to run to a small "goal region" on 

 the floor of the enclosure and sit there for a moment. 

 The region itself was unmarked, but the mouse 

 could find it by paying attention to the available vi- 

 sual cues. Mice learn this task very well. 



Kentros and I determined that even with ambi- 

 ent attention, the mouse forms a spatial map that 



Serotonin 

 Serotonin receptor 

 Hypothesized intracellular process 

 Recessive prionlike protein 

 |^i Dominant prionlike protein 

 /\/\/\ Dormant mRNA 

 /Y/VA Active mRNA 

 / Peptide 



Structural protein 



Novel biochemical pathway, postulated by the author and his colleagues, would maintain a new 

 synaptic connection on the tip of a nerve-cell axon. Such new growth strengthens the synaptic 

 communication with another nerve cell and is the basis for the formation of long-term memory. 

 The process begins (a) as a neurotransmitter, in this case serotonin, binds with a receptor mole- 

 cule on the surface of the axon, both stimulating the new growth and, through a hypothesized 

 intracellular process, triggering prionlike proteins within the axon to convert from a recessive to 

 a dominant shape (b). Dominant prions convert additional recessives to the dominant form (c), 

 which gives rise to a self-perpetuating chain reaction. When paired, the dominant prionlike pro- 

 teins help regulate the local synthesis of proteins by binding to and activating messenger RNA 

 (mRNA) present in the synaptic region (d). By maintaining local protein synthesis, prionlike pro- 

 teins maintain the new synapses and prevent them from regressing (e). Because the dominant 

 prionlike proteins are constantly being renewed, the mechanism ensures that the new connec- 

 tion, built as a result of some vital event, is maintained even if it is called upon only infrequently. 



Is selective attention required to form and retain a 

 spatial map? Clifford G. Kentros, a postdoctoral 

 fellow, and I exposed mice to experimental condi- 

 tions that required increasing degrees of spatial atten- 

 tion [see illustration on preceding page]. We implanted a 

 probe in the brain of a mouse that could measure the 

 individual firing of as many as four place cells as we 

 tracked the animals position in a test enclosure. The 

 enclosure was circular, with enough visual cues on its 

 walls for the animal to orient itself and, perhaps, to 

 form a spatial map. 



One condition was designed simply to establish a 



remains stable for an hour or two. Such a map, 

 however, becomes unstable after three to six hours. 

 When a mouse is forced to pay a lot of attention to 

 a new environment, by having to use visual land- 

 marks to learn a spatial task, the spatial map remains 

 stable for days. 



So what is the attentional mechanism in the 

 brain? How does it contribute to the strong en- 

 coding of information about space and the ready re- 

 call of that information after long intervening peri- 

 ods? Michael E. Goldberg and Robert H. Wurtz, 



natural insioRY March 2006 



