Robert M. McPeek

Saccades require focal attention and are facilitated by a short-term memory system

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.

Concurrent processing of saccades in visual search

We provide evidence that the saccadic system can simultaneously program two saccades to different goals. We presented subjects with simple visual search displays in which they were required to make a saccade to an odd-colored target embedded in an array of distractors. When there was strong competition between target and distractor stimuli (due to color priming from previous trials), subjects were more likely to make a saccade to a distractor. Such error saccades were often followed, after a very short inter-saccadic interval ( approximately 10-100 ms), by a second saccade to the target. The brevity of these inter-saccadic intervals suggests that the programming of the two saccades (one to a distractor and one to the target) overlapped in time. Using a saccade-contingent change in the search display, we show that new visual information presented during the initial saccade does not change the goal of the second saccade. This supports the idea that, by the end of the first saccade, programming of the second saccade is already well underway. We also elicited two-saccade responses (similar to those seen in search) using a double-step task, with the first saccade directed to the initial target step and the second saccade directed to the second target step. If the two saccades are programmed in parallel and programming of each saccade is triggered by one of the two target steps, the second saccade should occur at a relatively fixed time after the onset of the second target step, regardless of the timing of the initial saccade. This prediction was confirmed, supporting the idea that the two saccades are programmed in parallel. Finally, we observed that the shortest inter-saccadic intervals typically followed hypometric initial saccades, suggesting that the initial saccade may have been interrupted by the impending second saccade. Using predictions from physiological studies of interrupted saccades, we tested this hypothesis and found that the hypometric initial saccades did not appear to be interrupted in mid-flight. We discuss the significance of our findings for models of the saccadic system.

The effects of visual scene composition on the latency of saccadic eye movements of the rhesus monkey

This study examined how variations in the visual scene affect the generation of bimodal saccadic latency distributions, the first mode of which is called the population of express saccades. The surface media used to make stimuli visible and the composition of the background were varied to determine the conditions under which express saccades can be generated in rhesus monkeys. The results show that express saccades to singly presented targets can readily be elicited when the stimuli are made visible by virtue of either luminance contrast, color contrast or motion cues. Express saccades are rarely obtained when stimuli are made visible by virtue of only stereoscopic depth or texture cues. Express saccades can, however, be elicited using random-dot stereograms or textures when luminance or chrominance information is added to the target. When single target stimuli are presented simultaneously with a set of non-target stimuli, express saccades are for the most part prevented unless either the non-target stimuli are near threshold or their numerosity is very high, in which case they form a texture-like array. However, when the non-target stimuli are continuously present in the display, express saccades reemerge. These findings suggest that express saccades are not unique to experimental situations in which only a single stimulus appears on an otherwise homogeneous surface; they can readily be generated as long as the target stimulus is made visible by virtue of luminance, chrominance, motion or a combination of more than one surface medium and as long as the target does not appear concurrently with a salient group of other non-target stimuli.

Concurrent processing of saccades

100 words): We summarize several experiments indicating that the saccadic system is capable of simultaneously programming two movements toward different goals. This concurrent processing of saccades can lead to the execution of two saccades separated by an extremely short inter-saccadic interval. This supports the idea of target competition proposed by the target article, but suggests a greater degree of parallel processing. In fact, we provide evidence that concurrent processing of two saccades is not limited to higher-level planning subsystems, but rather, also involves brain regions close enough to the motor output that it can systematically affect saccade trajectory. Please address correspondence to: Robert M. McPeek Smith-Kettlewell Eye Research Institute 2232 Webster St. San Francisco, CA 94115 USA phone: 415-561-1639 fax: 415-561-1610 email: rmm@ski.org Concurrent processing of saccades McPeek, Keller, and Nakayama 2 We find much to agree with in the target articles proposed model of the saccadic system, particularly with regard to the idea of saccade target selection in a distributed, coarse-coded network with competition among active sites. However, in our work on saccades in visual search, we have made several findings which suggest that the saccadic systems ability to simultaneously program multiple movements may be more extensive than suggested by the model. We examined saccade target selection using a pop-out visual search task in which subjects make a saccade to an odd-colored target. When the color of the target changes from trial to trial, performance is worse than when it remains the same. This is due to a phenomenon called the priming of pop-out which affects saccades in humans (McPeek et al. 1998) and rhesus monkeys (McPeek & Keller 1998), as well as the deployment of focal attention in the absence of eye movements (Maljkovic & Nakayama 1994). We have exploited this priming effect to provoke a strong competition between target and distractor stimuli in our search task. We find that when subjects make an incorrect saccade to a distractor, such error saccades may be followed by a second saccade to the correct target after only a very short intersaccadic interval (~10-100 msec; see Fig. 1 for an example). Initially found in human subjects (McPeek et al. 1996), this finding has been replicated in the rhesus monkey (McPeek & Keller 1998). The brief inter-saccadic intervals, often shorter than the latency of express saccades (Fischer & Weber 1993), suggest that the system has processed the two saccades in parallel. According to this view, subjects begin programming a saccade to a distractor, but soon after, become aware of the correct location of the target. As a result, the subject begins Concurrent processing of saccades McPeek, Keller, and Nakayama 3 programming a second saccade to the correct target, which is processed in parallel with the initial incorrect saccade. The two saccades are effectively pipelined by the system, such that their preparation overlaps in time. The saccade which was programmed first is executed first, and is quickly followed by the second saccade as soon as its programming is complete. This view is supported by results from the double-step paradigm. Becker and Jürgens (1979) observed that when two sequential, but temporally closely spaced, target steps are presented on opposite sides of fixation, an initial saccade directed toward the first target position is often followed after a very brief fixation by a second saccade to the second target position. The presumption is that programming of the initial saccade is triggered by the first target step, and programming of the second saccade is triggered by the second target step. The short inter-saccadic interval results from the fact that the programming of the two saccades overlaps in time. If this is true, the second saccade should always occur one normal saccadic reaction time after the presentation of the second target step, regardless of when the initial saccade occurs. Becker & Jürgens (1979) confirmed this prediction for horizontal target steps. In our own doublestep experiments with target steps in two dimensions, we have also found that the second saccade consistently occurs approximately 200-250 msec after the presentation of the second target step, regardless of the timing of the initial saccade, and across a wide range of inter-saccadic intervals (McPeek 1997). This clearly supports the idea that these two-saccade responses reflect the parallel or pipelined processing of two movements, each triggered by the presentation of a target step. Concurrent processing is not limited to the higher-level planning stages of the saccadic system. In both our search experiments and in our double-step experiments, we observed that when two saccades to different goals are Concurrent processing of saccades McPeek, Keller, and Nakayama 4 executed in rapid succession, the first movement of the pair may be hypometric, falling short of the stimulus that it is directed toward. When the fixation interval between the first and second saccades is very brief, this reduction in amplitude of the initial saccade is most prominent (McPeek et al. 1996; McPeek & Keller 1998). This points to an effect of the impending readiness of the second saccade on the execution of the initial saccade. We have found further evidence for an effect of concurrent processing of a second saccade on the execution of an initial saccade. In the rhesus monkey, we have observed that for two-saccade responses in search, the initial incorrect saccade tends to show a relatively large amount of curvature, compared to saccades to single targets, or even to correct saccades in search (McPeek & Keller 1998). Furthermore, we have shown that these incorrect initial saccades are systematically curved toward the goal of the second saccade (see Fig. 1). This effect of the subsequent saccade goal on the execution of the initial saccade not only supports the idea that the two saccades are processed concurrently, it also provides additional evidence that this parallel processing is not limited to higherlevel saccade planning centers. Apparently, the processing of the second saccade involves brain regions close enough to the motor output that it can result in systematic changes in the trajectory of the initial saccade. To summarize, we argue that two saccades to different targets can be processed concurrently and executed in rapid succession. Furthermore, this parallel processing engages even the lower levels of the saccadic system, as evidenced by the systematic effects of the processing of the second saccade on the amplitude and the curvature of the initially executed saccade. Several recent studies of search indicate that subjects often program an initial saccade before visual analysis at the current site is complete (Hooge & Erkelens 1996; Hooge & Erkelens 1998; Zelinsky 1996). The ability of the system to program two Concurrent processing of saccades McPeek, Keller, and Nakayama 5 saccades concurrently makes this seemingly sub-optimal search strategy more understandable: since concurrent processing reduces the penalty for an initial incorrect goal selection, it encourages an early saccade based on the probable target location rather than a slower, more conservative search strategy. Supported by NIH EY06881 to RMM, EY08060 to ELK, and AFOSR F49620-92-J0016 to KN. Concurrent processing of saccades McPeek, Keller, and Nakayama

Smooth pursuit to a movement flow and associated perceptual judgments

Smooth pursuit is typically regarded as foveal tracking of simple targets and is therefore usually examined with stimuli such as a small moving dot. In real world situations, however, it is rare to encounter such perceptually minimal stimuli. Can smooth pursuit follow a complex object, such as a movement flow that contains ambiguous local but unambiguous global pattern of motion? Does this type of eye tracking always correspond to the motion perception of the same complex moving target? Our results show that one can smoothly pursue a movement flow even when the detection of this driving signal requires significant integration of information across space and time. Additionally, the generation of smooth pursuit appears to require sensory signals slightly stronger than that for perceiving coherent motion.