Biomedical subjects

R M McPeek

Publications and source records attributed to R M McPeek.

6 recordsLinked to original sources

PubMed2001

Short-term priming, concurrent processing, and saccade curvature during a target selection task in the monkey.

In human subjects, two mechanisms for improving the efficiency of saccades in visual search have recently been described: color priming and concurrent processing of two saccades. Since the monkey provides an important model for understanding the neural underpinnings of target selection in visual search, we sought to explore the degree to which the saccadic system of monkeys uses these same mechanisms. Therefore, we recorded the eye movements of rhesus monkeys performing a simple color-oddity pop-out search task, similar to that used previously with human subjects. The monkeys were rewarded for making a saccade to the odd-colored target, which was presented with an array of three distractors. The target and distractors were randomly chosen to be red or green in each trial. Similar to what was previously observed for humans, we found that monkeys show the influence of a cumulative, short-term priming mechanism which facilitates saccades when the color of the search target happens to repeat from trial to trial. Furthermore, we found that like humans, when monkeys make an erroneous initial saccade to a distractor, they are capable of executing a second saccade to the target after a very brief inter-saccadic interval, suggesting that the two saccades have been programmed concurrently (i.e. in parallel). These results demonstrate a close similarity between human and monkey performance. We also made a new observation: we found that when monkeys make such two-saccade responses, the trajectory of the initial saccade tends to curve toward the goal of the subsequent saccade. This provides evidence that the two saccade goals are simultaneously represented on a common motor map, supporting the idea that the movements are processed concurrently. It also indicates that concurrent processing is not limited to brain areas involved in higher-level planning; rather, such parallel programming apparently occurs at a low enough level in the saccadic system that it can affect saccade trajectory.

Animals↗

PubMed2000

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.

Humans↗

PubMed2000

Evidence against direct connections to PPRF EBNs from SC in the monkey.

Direct projections from the superior colliculus (SC) to the paramedian pontine reticular formation (PPRF) have been demonstrated anatomically. The PPRF contains cells called excitatory burst neurons (EBNs) that execute the final premotoneuronal processing for saccadic eye movements, as well as other burst cells called long-lead burst neurons (LLBNs). Previous electrophysiological tests in monkey have failed to find evidence for monosynaptic connections from the SC to EBNs, but have shown that direct projections to LLBNs exist. The validity of these results has been questioned because EBNs are known to be inhibited during periods of fixation by cells called omnipause neurons (OPNs). In later experiments in cat, the stimulus in the SC was triggered during saccades (when OPNs are off) and direct connections to EBNs were found. The present experiments were conducted to determine whether direct connections from the SC to EBNs could be demonstrated in monkey. LLBNs located near EBNs were also recorded. Single-pulse stimuli were delivered at sites in the SC at current levels well above those required to evoke saccades with pulse train stimuli. The stimuli were triggered shortly after the onset of ipsilateral or contralateral saccades and also slightly after the end of saccades. A sample of 21 EBNs was recorded and none were activated by postsaccadic stimulation or during contralateral saccades. The high spontaneous discharge rates of EBNs during ipsilateral saccades made activation of these cells more difficult to detect; however, when the results were quantified by peristimulus time histograms aligned on stimulus onset, only 1/21 EBNs showed evidence of activation in the monosynaptic range of latencies (<1.6 ms), 13 EBNs were activated at di- or polysynaptic latencies, and 7 were not activated. In contrast, 15/21 LLBNs were activated with latencies in the monosynaptic range. Further evidence supporting the absence of direct connections to EBNs was obtained by realigning the peristimulus time histograms for a subset of EBNs with similar firing rates on the time of occurrence of the last spike before stimulus onset. A subset of EBNs was also studied during drowsy ipsilaterally directed eye drifts, during which these cells were firing at low spontaneous rates and OPNs were off. No evidence for direct connections to EBNs was found in this behavioral state. The variance in results obtained for cat and monkey may be due to a species difference that reflects the more complex signal processing required in the monkey's saccadic system.

Action Potentials↗

PubMed1999

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.

Attention↗

PubMed1994

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.

Animals↗

PubMedDate not supplied

Using locations to store shape: an indirect effect of a lesion.

This study had three general points. First, it examined possible visual consequences of frontal lesions. A patient with focal damage to the subcortical regions of the left frontal lobe, and a small amount of damage near Broca's area, was predicted to have impaired brain function in posterior regions that are anatomically connected to the damaged site. Second, it showed the utility of using positron emission tomography (PET) in conjunction with magnetic resonance imaging to characterize "functional lesions." PET revealed reduced metabolism in posterior cortical loci that are innervated by fibers from the damaged regions. Some of the affected areas are hypothesized to be involved in visual functions, specifically the encoding of lines and edges. Third, a series of tests was designed to document that the patient had difficulty encoding visual stimuli, and then to distinguish among alternative possible causes of this deficit. The results suggested that the patient encoded shapes as sets of filled locations if possible, which allowed him to use intact processes subserved by brain areas that were not affected by the damage. The data were best explained if the lesion slowed processing in the ventral system (which encodes object properties), allowing the dorsal system (which encodes spatial properties) to produce a response more quickly than the ventral system.

Adult↗