Search PubMed⌕ Search

Biomedical subjects

Douglas P Munoz

Publications and source records attributed to Douglas P Munoz.

At least 19 recordsLinked to original sources
PubMed2006

Warning signals influence motor processing.

When observers initiate responses to visual targets, they do so sooner when a preceding stimulus indicates that the target will appear shortly. This consequence of a warning signal may change neural activity in one of four ways. On the sensory side, the warning signal may speed up the rate at which the target is registered by the brain or enhance the magnitude of its signal. On the motor end, the warning signal may lower the threshold required to initiate a response or speed up the rate at which activity accumulates to reach threshold. Here, we describe which explanation is better supported. To accomplish this end, monkeys performed different versions of a cue-target task while we monitored the activity of visuomotor and motor neurons in the superior colliculus. Although the cue target task was designed to measure the properties of reflexive spatial attention, there are two events in this task that produce nonspecific warning effects: a central reorienting event (brightening of central fixation marker) that is used to direct attention away from the cue, and the presentation of the cue itself. Monopolizing on these tendencies, we show that warning effects are associated with several changes in neural activity: the target-related response is enhanced, the threshold for initiating a saccade is lowered, and the rate at which activity accumulates toward threshold rises faster. Ultimately, the accumulation of activity toward threshold predicted behavior most closely. In the discussion, we describe the implications and limitations of these data for theories of warning effects and potential avenues for future research.

Animals↗
PubMed2006

Effector-specific fields for motor preparation in the human frontal cortex.

We investigated the neural correlates of advance motor preparation in two experiments that required a movement in response to a peripheral visual stimulus. In one experiment (the memory delay paradigm), subjects knew the target location during a preparatory 'memory delay' interval; in the other experiment they did not know the target location during a 'gap period' (the gap paradigm). In both experiments we further varied the effector that was instructed, either the eye or the forelimb. An area that codes motor preparation should exhibit increases during the memory delay and gap period and such increases should predict some attribute of performance (planning to use the eye or the forelimb). We first identified the frontoparietal visuomotor areas using standard fMRI block designs. Subjects were then scanned using event-related fMRI. With the exception of primary motor cortex (M1), all areas (putative lateral intraparietal area (putLIP), dorsal premotor cortex (PMd), frontal eye field (FEF), ventral frontal eye field (FEFv), supplementary motor area (SMA)) showed gap and memory delay activation for both saccades and pointing. Gap activity in the frontal areas was higher than in the parietal area(s) investigated. The observation that 'memory delay' activity was equivalent or less than gap activity in all areas suggests that what is commonly considered to be memory-related responses largely represents advance motor preparation. Certain areas showed increased activation during the gap or memory delay intervals for pointing (PMd, FEF, FEFv) or saccades (SMA, putLIP). These observations suggest an important role of the frontal cortex in advance motor preparation.

Adult↗
PubMed2006

Priming of head premotor circuits during oculomotor preparation.

Large, rapid gaze shifts necessitate intricate coordination of the eyes and head. Brief high-frequency bursts of activity within the intermediate and deeper layers of the superior colliculus (dSC) encode desired gaze shifts regardless of component movements of the eyes and head. However, it remains unclear whether low-frequency activity emitted by oculomotor neurons within the dSC and elsewhere has any role in eye-head gaze shifts. Here we test the hypothesis that such low-frequency activity contributes to eye-head coordination by selectively priming head premotor circuits. We exploited the capacity for short-duration (10 ms, 4 pulses) dSC stimulation to evoke neck muscle responses without compromising ocular stability, stimulating at various intervals of a "gap-saccade" task. Low-frequency neural activity in many oculomotor areas (including the dSC) is known to increase during the progression of the gap-saccade task. Stimulation was passed during either a fixation-interval while a central fixation point was illuminated, a 200-ms gap-interval between fixation point offset and target onset, or a movement-interval following target onset. In the two monkeys studied, the amplitude of evoked responses on multiple neck muscles tracked the known increases in low-frequency oculomotor activity during the gap-saccade task, being greater following stimulation passed at the end of the gap- versus the fixation-interval, and greater still when the location of stimulation during the movement interval coincided with the area of the dSC generating the ensuing saccade. In one of these monkeys, we obtained a more detailed timeline of how these results co-varied with low-frequency oculomotor activity by stimulating, across multiple trials, at different times within the fixation-, gap- and movement-intervals. Importantly, in both monkeys, baseline levels of neck EMG taken immediately prior to stimulation onset did not co-vary with the known pattern of low-frequency oculomotor activity up until the arrival of a transient burst associated with visual target onset. These baseline results demonstrate that any priming of the head premotor circuits occurs without affecting the output of neck muscle motoneurons, We conclude that low-frequency oculomotor activity primes head premotor circuits well in advance of gaze shift initiation, and in a manner distinct from its effects on the eye premotor circuits. Such distinctions presumably aid the temporal coordination of the eyes and head despite fundamentally different biomechanics.

Action Potentials↗
PubMed2006

Salience, relevance, and firing: a priority map for target selection.

The salience map is a crucial concept for many theories of visual attention. On this map, each object in the scene competes for selection - the more conspicuous the object, the greater its representation, and the more likely it will be chosen. In recent years, the firing patterns of single neurons have been interpreted using this framework. Here, we review evidence showing that the expression of salience is remarkably similar across structures, remarkably different across tasks, and modified in important ways when the salient object is consistent with the goals of the participant. These observations have important ramifications for theories of attention. We conclude that priority--the combined representation of salience and relevance--best describes the firing properties of neurons.

Action Potentials↗
PubMed2006

Discharge properties of monkey tectoreticular neurons.

The intermediate layers of the superior colliculus (SC) contain neurons that clearly play a major role in regulating the production of saccadic eye movements: a burst of activity from saccade neurons (SNs) is thought to provide a drive signal to set the eyes in motion, whereas the tonic activity of fixation neurons (FNs) is thought to suppress saccades during fixation. The exact contribution of these neurons to saccade control is, however, unclear because the nature of the signals sent by the SC to the brain stem saccade generation circuit has not been studied in detail. Here we tested the hypothesis that the SC output signal is sufficient to control saccades by examining whether antidromically identified tectoreticular neurons (TRNs: 33 SNs and 13 FNs) determine the end of saccades. First, TRNs had discharge properties similar to those of nonidentified SC neurons and a proportion of output SNs had visually evoked responses, which signify that the saccade generator must receive and process visual information. Second, only a minority of TRNs possessed the temporal patterns of activity sufficient to terminate saccades: Output SNs did not cease discharging at the time of saccade end, possibly continuing to drive the brain stem during postsaccadic fixations, and output FNs did not resume their activity before saccade end. These results argue against a role for SC in regulating the timing of saccade termination by a temporal code and suggest that other saccade centers act to thwart the extraneous SC drive signal, unless it controls saccade termination by a spatial code.

Action Potentials↗
PubMed2006

Stabilizing the visual world.

Sensory systems are thought to use an internal copy of motor commands to determine which sensations are caused by the self versus the world. A new paper proves that an identified pathway performs this function for eye movements.

Afferent Pathways↗
PubMed2005

Crossmodal integration in the primate superior colliculus underlying the preparation and initiation of saccadic eye movements.

Saccades to combined audiovisual stimuli often have reduced saccadic reaction times (SRTs) compared with those to unimodal stimuli. Neurons in the intermediate/deep layers of the superior colliculus (dSC) are capable of integrating converging sensory inputs to influence the time to saccade initiation. To identify how neural processing in the dSC contributes to reducing SRTs to audiovisual stimuli, we recorded activity from dSC neurons while monkeys generated saccades to visual or audiovisual stimuli. To evoke crossmodal interactions of varying strength, we used auditory and visual stimuli of different intensities, presented either in spatial alignment or to opposite hemifields. Spatially aligned audiovisual stimuli evoked the shortest SRTs. In the case of low-intensity stimuli, the response to the auditory component of the aligned audiovisual target increased the activity preceding the response to the visual component, accelerating the onset of the visual response and facilitating the generation of shorter-latency saccades. In the case of high-intensity stimuli, the auditory and visual responses occurred much closer together in time and so there was little opportunity for the auditory stimulus to influence previsual activity. Instead, the reduction in SRT for high-intensity, aligned audiovisual stimuli was correlated with increased premotor activity (activity after visual burst but preceding saccade-aligned burst). These data provide a link between changes in neural activity related to stimulus modality with changes in behavior. They further demonstrate how crossmodal interactions are not limited to the initial sensory activity but can also influence premotor activity in the SC.

Acoustic Stimulation↗
PubMed2005

Deficits in saccadic eye-movement control in Parkinson's disease.

In contrast to their slowed limb movements, individuals with Parkinson's disease (PD) produce rapid automatic eye movements to sensory stimuli and show an impaired ability to generate voluntary eye movements in cognitive tasks. Eighteen PD patients and 18 matched control volunteers were instructed to look either toward (pro-saccade) or away from (anti-saccade) a peripheral stimulus as soon as it appeared (immediate, gap and overlap conditions) or after a variable delay; or, they made sequential saccades to remembered targets after a variable delay. We found that PD patients made more express saccades (correct saccades in the latency range of 90-140 ms) in the immediate pro-saccade task, more direction errors (automatic pro-saccades) in the immediate anti-saccade task, and were less able to inhibit saccades during the delay period in all delay tasks. PD patients also made more directional and end-point errors in the memory-guided sequential task. Their inability to plan eye movements to remembered target locations suggests that PD patients have a deficit in spatial working memory which, along with their deficit in automatic saccade suppression, is consistent with a disorder of the prefrontal-basal ganglia circuit. Impairment of this pathway may release the automatic saccade system from top-down inhibition and produce deficits in volitional saccade control. Parallel findings across various motor, cognitive and oculomotor tasks suggest a common mechanism underlying a general deficit in automatic response suppression.

Adult↗
PubMed2005

Correlates of capture of attention and inhibition of return across stages of visual processing.

How do visual signals evolve from early to late stages in sensory processing? We explored this question by examining two neural correlates of spatial attention. The capture of attention and inhibition of return refer to the initial advantage and subsequent disadvantage to respond to a visual target that follows an irrelevant visual cue at the same location. In the intermediate layers of the superior colliculus (a region that receives input from late stages in visual processing), both behavioral effects link to changes in the neural representation of the target: strong target-related activity correlates with the capture of attention and weak target-related activity correlates with inhibition of return. Contrasting these correlates with those obtained in the superficial layers (a functionally distinct region that receives input from early stages in visual processing), we show that the target-related activity of neurons in the intermediate layers was the best predictor of orienting behavior, although dramatic changes in the target-related response were observed in both subregions. We describe the important consequences of these findings for understanding the neural basis of the capture of attention and inhibition of return and interpreting changes in neural activity more generally.

Action Potentials↗
PubMed2004

fMRI activation in the human frontal eye field is correlated with saccadic reaction time.

Variation in response latency to identical sensory stimuli has been attributed to variation in neural activity mediating preparatory set. Here we report evidence for a relationship between saccadic reaction time (SRT) and set-related brain activity measured with event-related functional magnetic resonance imaging. We measured hemodynamic activation time-courses during a preparatory "gap" period, during which no visual stimulus was present and no saccades were made. The subjects merely anticipated appearance of the target. Saccade direction and latency were recorded during scanning, and trials were sorted according to SRT. Both the frontal (FEF) and supplementary eye fields showed pre-target preparatory activity, but only in the FEF was this activity correlated with SRT. Activation in the intraparietal sulcus did not show any preparatory activity. These data provide evidence that the human FEF plays a central role in saccade initiation; pre-target activity in this region predicts both the type of eye movement (whether the subject will look toward or away from the target) and when a future saccade will occur.

Brain↗
PubMed2004

Sensory biases produce alternation advantage found in sequential saccadic eye movement tasks.

In two-choice reaction time tasks, participants respond faster when the correct decision switches across consecutive trials. This alternation advantage has been interpreted as the guessing strategies of participants. Because the participants expect that the correct decision will switch across consecutive trials, they respond faster when this expectation is confirmed and they respond more slowly when it is disconfirmed. In this study, we evaluated the veracity of this expectancy interpretation. After replicating a long-lasting alternation advantage in saccadic reaction times (Experiment 1), we show that reducing the participants' ability to guess with a challenging mental rotation task does not change the alternation advantage, which suggests that expectancy is not responsible for the effect (Experiment 2). Next, we used prosaccade and antisaccade responses to dissociate between the sensory and motor contributions of the alternation advantage (Experiment 3) and we found that the alternation advantage originates from sensory processing. The implications of these findings are discussed with regard to guessing strategies, sensory processing, and how these findings may relate to inhibition of return.

Analysis of Variance↗
PubMed2004

Visual responses on neck muscles reveal selective gating that prevents express saccades.

Express saccades promote the acquisition of visual targets at extremely short reaction times. Because of the head's considerable inertia, it is unknown whether express saccades are accompanied by a parallel command to the head. Here, by recording electromyographic (EMG) activity from monkey neck muscles, we demonstrate that visual target presentation elicits time-locked, lateralized recruitment of neck muscles at extremely short latencies (55-95 ms). Remarkably, such recruitment not only accompanies express saccades, but also precedes nonexpress saccades, occasionally by up to 150 ms. These results demonstrate selective gating of components of descending commands from the superior colliculus to prevent express saccades yet permit recruitment of a head orienting synergy. We conclude that such selective gating aids eye-head coordination by permitting force development at neck muscles while a decision to commit to a gaze shift is being made, optimizing the contribution of the more inertial head to the ensuing gaze shift.

Animals↗
PubMed2004

Neural correlates of the automatic and goal-driven biases in orienting spatial attention.

How do stimuli in the environment interact with the goals of observers? We addressed this question by showing that the relevance of an abruptly appearing visual object (cue) changes how observers orient attention toward a subsequent object (target) and how this target is represented in the activity of neurons in the superior colliculus. Initially after the appearance of the cue, attention is driven to its locus. This capture of attention is followed by a second bias in orienting attention, where observers preferentially orient to new locations in the visual scene-an effect called inhibition of return. In the superior colliculus, these two automatic biases in orienting attention were associated with changes in neural activity linked to the appearance of the target-relatively stronger activity linked to the capture of attention and weaker activity linked to inhibition of return. This behavioral pattern changes when the cue predicts the upcoming location of the target-the benefit associated with the capture of attention is enhanced and inhibition of return is reduced. These goal-driven changes in behavior were associated with an increase in pretarget- and target-related activity. Taken together, the goals of observers modify stimulus-driven changes in neural activity with both signals represented in the salience maps of the superior colliculi.

Action Potentials↗
PubMed2003

Using auditory and visual stimuli to investigate the behavioral and neuronal consequences of reflexive covert orienting.

Reflexively orienting toward a peripheral cue can influence subsequent responses to a target, depending on when and where the cue and target appear relative to each other. At short delays between the cue and target [cue-target onset asynchrony (CTOA)], subjects are faster to respond when they appear at the same location, an effect referred to as reflexive attentional capture. At longer CTOAs, subjects are slower to respond when the two appear at the same location, an effect referred to as inhibition of return (IOR). Recent evidence suggests that these phenomena originate from sensory interactions between the cue- and target-related responses. The capture of attention originates from a strong target-related response, derived from the overlap of the cue- and target-related activities, whereas IOR corresponds to a weaker target-aligned response. If such interactions are responsible, then modifying their nature should impact the neuronal and behavioral outcome. Monkeys performed a cue-target saccade task featuring visual and auditory cues while neural activity was recorded from the superior colliculus (SC). Compared with visual stimuli, auditory responses are weaker and occur earlier, thereby decreasing the likelihood of interactions between these signals. Similar to previous studies, visual stimuli evoked reflexive attentional capture at a short CTOA (60 ms) and IOR at longer CTOAs (160 and 610 ms) with corresponding changes in the target-aligned activity in the SC. Auditory cues used in this study failed to elicit either a behavioral effect or modification of SC activity at any CTOA, supporting the hypothesis that reflexive orienting is mediated by sensory interactions between the cue and target stimuli.

Acoustic Stimulation↗
PubMed2003

Altered control of visual fixation and saccadic eye movements in attention-deficit hyperactivity disorder.

Attention-deficit hyperactivity disorder (ADHD) is characterized by the overt symptoms of impulsiveness, hyperactivity, and inattention. A frontostriatal pathophysiology has been hypothesized to produce these symptoms and lead to reduced ability to inhibit unnecessary or inappropriate behavioral responses. Oculomotor tasks can be designed to probe the ability of subjects to generate or inhibit reflexive and voluntary responses. Because regions of the frontal cortex and basal ganglia have been identified in the control of voluntary responses and saccadic suppression, we hypothesized that children and adults diagnosed with ADHD may have specific difficulties in oculomotor tasks requiring the suppression of reflexive or unwanted saccadic eye movements. To test this hypothesis, we measured eye movement performance in pro- and anti-saccade tasks of 114 ADHD and 180 control participants ranging in age from 6 to 59 yr. In the pro-saccade task, participants were instructed to look from a central fixation point toward an eccentric visual target. In the anti-saccade task, stimulus presentation was identical, but participants were instructed to suppress the saccade to the stimulus and instead look from the central fixation point to the side opposite the target. The state of fixation was manipulated by presenting the target either when the central fixation point was illuminated (overlap condition) or at some time after it disappeared (gap condition). In the pro-saccade task, ADHD participants had longer reaction times, greater intra-subject variance, and their saccades had reduced peak velocities and increased durations. In the anti-saccade task, ADHD participants had greater difficulty suppressing reflexive pro-saccades toward the eccentric target, increased reaction times for correct anti-saccades, and greater intra-subject variance. In a third task requiring prolonged fixation, ADHD participants generated more intrusive saccades during periods when they were required to maintain steady fixation. The results suggest that ADHD participants have reduced ability to suppress unwanted saccades and control their fixation behavior voluntarily, a finding that is consistent with a fronto-striatal pathophysiology. The findings are discussed in the context of recent neurophysiological data from nonhuman primates that have identified important control signals for saccade suppression that emanate from frontostriatal circuits.

Adolescent↗
PubMed2002

Contribution of the primate superior colliculus to inhibition of return.

The phenomenon of inhibition of return (IOR) has generated considerable interest in cognitive neuroscience because of its putative functional role in visual search, that of placing inhibitory tags on objects that have been recently inspected so as to direct further search to novel items. Many behavioral parameters of this phenomenon have been clearly delineated, and based on indirect but converging evidence, the widely held consensus is that the midbrain superior colliculus (SC) is involved in the generation of IOR. We had previously trained monkeys on a saccadic IOR task and showed that they displayed IOR in a manner similar to that observed in humans. Here we recorded the activity of single neurons in the superficial and intermediate layers of the SC while the monkeys performed this IOR task. We found that when the target was presented at a previously cued location, the stimulus-related response was attenuated and the magnitude of this response was correlated with subsequent saccadic reaction times. Surprisingly, this observed attenuation of activity during IOR was not caused by active inhibition of these neurons because (a) they were, in fact, more active following the presentation of the cue in their response field, and (b) when we repeated the same experiment while using the saccadic response time induced by electrical micro-stimulation of the SC to judge the level of excitability of the SC circuitry during the IOR task, we found faster saccades were elicited from the cued location. Our findings demonstrate that the primate SC participates in the expression of IOR; however, the SC is not the site of the inhibition. Instead, the reduced activity in the SC reflects a signal reduction that has taken place upstream.

Action Potentials↗