We examined a visual search task, in which observers responded to the high-acuityaspect of a popout target (shape of an odd-colored diamond or vernier offset of an odd spatial-frequency patch). Repetition of the attention-driving feature (color or spatial frequency) in this task primes the popout; repetition of the high-acuity aspect (shape, vernier offset) does not. Priming of pop-out is due to a decaying memory trace of the attention-focusing feature laid down with each trial. The trace exerts a diminishing effect over the following five to eight trials (-30 sec), and its influence over this time is cumulative. Observers cannot willfully overcome the priming, which suggests that it is passive and autonomous. Both target facilitation and distractor inhibition are evident; the former has a greater effect. The phenomenon shows complete binocular transfer.In a typical visual search experiment, the observer's task is to detect the presence of the odd target within a field of distractors. Two well-known effects have been observed. Serial search is the term attributed to the finding that an increasing number ofdistractors increases the time necessary to find the target, yielding positive slope functions (Sagi & Julesz, 1985;Treisman & Gelade, 1980). Parallel search is the term attributed to the finding that the target is detected equally quickly regardless of the number of distractors, yielding flat reaction time versus distractor number functions. In this situation, the target also "pops out"; that is, attention is automatically drawn to the odd item. The flat functions and the presence of"pop-out" were generally assumed to be causally related. Bravo and Nakayama (1992), however, dissociated these two aspects by adding an additional requirement to the task, which tapped into its attention-focusing aspect. They asked observers to respond to the shape of the odd-colored target, not its presence or absence. It was their claim that responding to the shape necessitates the spatial focusing of attention to the odd target. As such, this task examines the characteristics of attention focusing in pop-out. In contrast, the previously used presence versus absence task does not require the focusing of attention and is not informative regarding the deployment of focal attention.Drawing on theoretical accounts of attentional guidance, Bravo and Nakayama (1992) made a number of experimental predictions. To start, they reviewed two general processes that would be useful for focusing atThis work was supported by AFOSR Grant F49620-92-J-0016 to K.N. We thank Shinsuke Shimojo for his comments on an earlier version of this manuscript. V.Maljkovic is now in the Department of Brain and Cognitive Sciences, Massachusetts Institute of Technology. Correspondence should be addressed to K. Nakayama,

In an earlier paper (Maljkovic & Nakayama, 1994)we showed that repetition of an attention-driving feature primes the deployment of attention to the same feature on subsequent trials. Here we show that repetition of the target position also primes subsequent trials. Position priming shows a characteristic spatial pattern. Facilitation occurs when the target position is repeated on subsequent trials, and inhibition occurs when the target falls on a position previously occupied by a distractor. Facilitation and inhibition also exist, though somewhat diminished, for positions adjacent to those of the target and distractors. Assessing the effect of a single trial over time, we show that the characteristic memory trace exerts its strongest influence on immediately following trials and decays gradually over the succeeding, approximately five to eight, trials. Throughout this period, target-position facilitation is always stronger than distractor-position inhibition. The characteristics of position priming are also seen under conditions in which the attention-driving feature either stays the same or differs from the previous trial, suggesting that feature and position priming operate independently. In a separate experiment, using the fact that position priming is cumulative over trials, we show that position priming is largely object-or landmark-centered.In an earlier companion paper, we reported that repeating odd target colors in a pop-out task speeds subsequent attentional deployment to same-color stimuli (Maljkovic & Nakayama, 1994). The finding was revealed in a visual search task initially developed by Bravo and Nakayama (1992). In that paradigm, the observer must direct his or her focal attention to the odd target and respond to the target's subtle shape differences. The pop-out feature and the response feature are dissociated from each other and the priming is observed only when the feature, but not the response, is repeated. In the present paper we continue to explore the characteristics of the priming of pop-out, focusing now on the effect of stimulus position.We first summarize the findings offeature priming reported in Maljkovic and Nakayama (1994). We conceive ofthe visual search task as a two-step process (see , with a controlling stimulus property associated with each step. The first step, the attentiondriving feature-say, color-determines where attention is to be deployed. The second step requires focusing attention on a subtle aspect of the shape-which side of the target diamond is chopped off-which in turn determines the manual response, pressing the right or the left button. In the first step, the observer is required to find the odd "pop-out" target and, in the second step, to identify a subtIe aspect in the shape of the target.A memory trace of the attention-driving feature is laid down with each trial, and this trace facilitates deployment of attention to the same feature on the subsequent five to

Cerebral blood flow was measured using positron emission tomography (PET) in three experiments while subjects performed mental imagery or analogous perceptual tasks. In Experiment 1, the subjects either visualized letters in grids and decided whether an X mark would have fallen on each letter if it were actually in the grid, or they saw letters in grids and decided whether an X mark fell on each letter. A region identified as part of area 17 by the Talairach and Tournoux (1988) atlas, in addition to other areas involved in vision, was activated more in the mental imagery task than in the perception task. In Experiment 2, the identical stimuli were presented in imagery and baseline conditions, but subjects were asked to form images only in the imagery condition; the portion of area 17 that was more active in the imagery condition of Experiment 1 was also more activated in imagery than in the baseline condition, as was part of area 18. Subjects also were tested with degraded perceptual stimuli, which caused visual cortex to be activated to the same degree in imagery and perception. In

We performed two sets of experiments in which observers were instructed to make saccades to an odd colored target embedded in an array of distractors. First, we found that when the colors of the target and distractors switched unpredictably from trial to trial (the mixed condition), saccadic latencies decreased with increasing numbers of distractors. In contrast, saccadic latencies were independent of the number of distractors when the color of the target and distractors remained the same on each trial (the blocked condition). This pattern of results mirrors visual search tasks in which focal rather than distributed attention is required (Bravo, M.J., Nakayama, K. (1992). The role of attention in different visual search tasks. Perception and Psychophysics, 51, 465-472.). Second, we found that saccades to an odd target were made more quickly and accurately when the target was the same color as on previous trials than when it changed color. This priming of the target color accumulates across five to seven trials over a period of approximately 30 s. A similar priming effect has been previously shown for the deployment of focal attention (Maljkovic, V., Nakayama, K. (1994). Priming of popout: III. Role of features. Memory and Cognition, 22(6), 657-672.). Thus, we show a close congruence between the pattern of saccadic eye movement latencies and the deployment of focal attention. This supports the view that (1) the execution of saccades requires focal as opposed to distributed attention and that (2) this focal attention is guided by a short term memory system which facilitates the rapid refixation of gaze to recently foveated targets.

The priming of popout (PoP) (Maljkovic & Nakayama, 1994, 1996 increases the speed of attentional deployment to subsequent targets having the same feature characteristic and relative position, it lasts for approximately 5-8 trials, and is cumulative. Here we establish PoP as an example of short-term implicit memory by showing that it is qualitatively different from explicit memory. Using a postcued recall procedure embedded in the stream of search task trials, we show that explicit memory is not selective as is PoP and is of much shorter duration. As such we argue that explicit memory is unlikely to account for the properties of PoP. In examining the decay of PoP, we find that: PoP is not evident after a 90sec delay, it does not show passive decay over much shorter intervals (1-3sec), and it gets decremented by attentional deployments to visually dissimilar stimuli, the size of the decrement being related to task difficulty. The results, taken together, suggest that PoP reflects a functionally beneficial memory system, specialized for the rapid and automatic selection of items for focal attention and saccadic eye movements.One of the most exciting developments in memory research came about through the study of amnesic patients, when it was discovered that learning can occur without conscious awareness. For example, the famous amnesic patient HM first studied by Scoville and Milner (1957) is able to improve greatly in motor skills, yet has no recollection of ever practising them. The memory preserved in amnesic patients is not constrained to motor skills but extends to Correspondence should be addressed to V. Maljkovic,