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Biomedical subjects

Vincent P Ferrera

Publications and source records attributed to Vincent P Ferrera.

14 recordsLinked to original sources
PubMed2006

Modulation of visual responses in macaque frontal eye field during covert tracking of invisible targets.

The purpose of this study was to investigate the interaction between internal representations of invisible moving targets and visual responses of neurons in frontal eye fields (FEFs). Monkeys were trained to make saccades to the extrapolated position of a target that was temporarily rendered invisible for variable durations as if it had passed behind an occluder. Flashed, task-irrelevant visual probe stimuli were used to study the visual responsiveness of FEF neurons during this task. Probes were flashed at various times and locations during the occlusion interval. Net changes in neuronal activity were obtained by comparing the activity on trials with probes with randomly interleaved trials without any probe. Most neurons showed an increase in firing rate in response to the probe, but some showed a decrease. Both types of responses were enhanced when the invisible target moved toward the receptive field (RF) as compared with trials on which the target moved away from the RF. Some neurons showed a spatial shift in the visual response during the occlusion interval. For cells that were excited by the probe, the shift tended to be correlated with the direction of motion of the target, whereas for cells that were inhibited the shift tended to be in the opposite direction. These results suggest that the role of FEF in predicting invisible target motion includes a sensory/perceptual component.

Adaptation, Physiological↗
PubMed2006

A neural representation of categorization uncertainty in the human brain.

The ability to classify visual objects into discrete categories ("friend" versus "foe"; "edible" versus "poisonous") is essential for survival and is a fundamental cognitive function. The cortical substrates that mediate this function, however, have not been identified in humans. To identify brain regions involved in stimulus categorization, we developed a task in which subjects classified stimuli according to a variable categorical boundary. Psychophysical functions were used to define a decision variable, categorization uncertainty, which was systematically manipulated. Using event-related functional MRI, we discovered that activity in a fronto-striatal-thalamic network, consisting of the medial frontal gyrus, anterior insula, ventral striatum, and dorsomedial thalamus, was modulated by categorization uncertainty. We found this network to be distinct from the frontoparietal attention network, consisting of the frontal and parietal eye fields, where activity was not correlated with categorization uncertainty.

Adolescent↗
PubMed2006

A flashing line can warp your mind.

Keeping pace with a constantly changing world requires the ability to make predictions about the future on a variety of timescales. A very basic example of this is the ability to predict the future location of a moving object in the brief time that it takes to perceive and respond to that object. In this issue of Neuron, experiments by Sundberg, Fallah, and Reynolds reveal a potential neural substrate for making short-range predictions about motion in visual area V4.

Animals↗
PubMed2005

Effects of electrical microstimulation in monkey frontal eye field on saccades to remembered targets.

Spatially selective delay activity in the frontal eye field (FEF) is hypothesized to be part of a mechanism for the transformation of visual signals into instructions for voluntary movements. To understand the linkage between FEF activity and eye movement planning, we recorded neuronal responses of FEF neurons while monkeys performed a memory-saccade task. We then electrically stimulated the same sites during the memory-delay epoch of the task. The stimulation currents used were subthreshold for evoking saccades during a gap-fixation task. Microstimulation resulted in changes in the spatial and temporal components of saccade parameters: an increase in latency, and a shift in amplitude and direction. We performed a vector analysis to determine the relative influence of the visual cue and electrical stimulus on the memory-saccade. In general, the memory-saccade was strongly weighted toward the visual cue direction, yet the electrical stimulus introduced a consistent bias away from the receptive/movement field of the stimulation site. The effects of sub-threshold stimulation were consistent with a combination of vector subtraction and averaging, but not with vector summation. Vector subtraction may play a role in spatial updating of movement plans for memory-guided saccades when eye position changes during the memory period.

Animals↗
PubMed2005

Microstimulation of the dorsolateral prefrontal cortex biases saccade target selection.

A long-standing issue concerning the executive function of the primate dorsolateral prefrontal cortex is how the activity of prefrontal neurons is linked to behavioral response selection. To establish a functional relationship between prefrontal memory fields and saccade target selection, we trained three macaque monkeys to make saccades to the remembered location of a visual cue in a delayed spatial match-to-sample saccade task. We electrically stimulated sites in the prefrontal cortex with subthreshold currents during the delay epoch while monkeys performed this task. Our results show that the artificially injected signal interacts with the neural activity responsible for target selection, biasing saccade choices either towards the receptive/movement field (RF/MF) or away from the RF/MF, depending on the stimulation site. These findings might reflect a functional link between prefrontal signals responsible for the selection bias by modulating the balance between excitation and inhibition in the competitive interactions underlying behavioral selection.

Animals↗
PubMed2004

Modification of saccades evoked by stimulation of frontal eye field during invisible target tracking.

We investigated the internal representation of invisible moving targets using electrical microstimulation in the prefrontal cortex. Monkeys were trained to make saccades to the extrapolated position of a small moving target that was rendered invisible during part of its trajectory. Although the target was invisible, involuntary saccades were evoked by electrical microstimulation of the frontal eye field. Stimulation was applied at different times relative to the disappearance of the target while the monkey fixated. When stimulation was applied immediately after target disappearance, electrically evoked saccades were biased toward the starting point of the target trajectory. When stimulation was applied later in the trial, evoked saccades were biased toward the end of the trajectory. The bias in evoked saccade direction changed continuously over time. The magnitude and statistical significance of the electrically evoked saccade deviation depended on the accuracy of the monkeys' voluntary saccades relative to the invisible target. The results suggest that covert tracking is accompanied by a continuously shifting saccade plan that moves along the target path.

Animals↗
PubMed2003

Effects of spontaneous eye movements on spatial memory in macaque periarcuate cortex.

Persistent activity in prefrontal cortex during delayed response tasks is a putative neural correlate of spatial working memory. We tested whether this activity was sensitive to eye movements made during the memory interval by recording from prefrontal neurons while monkeys performed a delayed spatial matching saccade task in which they were allowed to make eye movements freely. We found that eye movements degraded the spatial tuning of persistent activity even as there was an improvement in behavioral performance. Although the strength of the memory signal decreased, delay activity continued to signal the location of cue. The results suggest that free eye movements reduce neuronal gain rather than add variability. The saccades performed during the delay suggest the existence of a rehearsal mechanism that could contribute to working memory maintenance. The results do not provide support for a segregation of storage and executive functions in the periarcuate cortex.

Animals↗
PubMed2003

Effects of gaze shifts on maintenance of spatial memory in macaque frontal eye field.

The activity of 91 neurons in the frontal eye fields (FEFs) of two macaque monkeys was recorded while the animals performed a delayed spatial match-to-sample task. During the delay, the animals were required to shift their gaze to one of four eccentric locations. Neuronal activity during the delay was analyzed for sensitivity to cue location and eye position. One-third of the neurons showed significant delay activity selective for cue location, whereas slightly more than one-half of the neurons showed significant modulation of delay activity when the gaze was shifted to an eccentric location. Despite this modulation, the neurons continued to signal their preferred cue location during most of the delay. However, after recentering saccades, the memory signal was temporarily abolished and then reemerged over a period of few hundred milliseconds. This is consistent with the idea that spatial working memory is buffered outside of the FEF. For most neurons, delay activity tended to increase when the gaze was shifted away from the preferred location and to decrease when the gaze was shifted toward the preferred location. This pattern of modulation is consistent with a vector subtraction mechanism that allows for the superposition of multiple saccade plans.

Animals↗
PubMed2003

Comparison of performance on memory-guided saccade and delayed spatial match-to-sample tasks in monkeys.

To investigate the sources of spatial error in memory-guided saccades (MGS), we have trained monkeys on two different tasks: a MGS task and a delayed spatial match-to-sample (MTS) task. We first tested the effect of introducing a post-saccadic visual feedback on the accuracy of MGS. We found that visual feedback had a pronounced effect on the systematic saccade error, but less of an effect on the variable error. Visual feedback can improve the accuracy of saccadic eye movements over several days, while feedback removal can decrease accuracy in a reversible way. These effects also depend both on target eccentricity and the duration of the memory delay. To test whether saccade error is due to the accuracy of spatial memory storage or arises downstream from that memory, we measured behavioral performance on a spatial MTS task both before and after training with visual feedback. The results showed no significant difference in performance of the MTS task before and after feedback training despite significant changes in MGS accuracy. The results suggest that the accuracy of spatial memory is not the source of the systematic errors that accompany MGS.

Analysis of Variance↗
PubMed2003

Estimating invisible target speed from neuronal activity in monkey frontal eye field.

Working memory involves transient storage of information and the ability to manipulate that information for short-range planning and prediction. The computational aspect of working memory can be probed using dynamic sensorimotor behavior requiring complex stimulus-response mappings. Such a transformation occurs when extrapolating the future location of a moving target that is rendered temporarily invisible. Estimating the trajectory of an invisible moving target requires encoding and storing several target features, including the direction and speed of motion. We trained monkeys to make saccades to the estimated position of invisible targets moving at various speeds. The activity of neurons in the frontal eye field (FEF) was consistently modulated according to the speed of target motion. A reconstruction algorithm showed that estimates of target speed based on FEF activity were similar to behavioral speed estimates. FEF may therefore be involved in updating an internal representation of target trajectory for predictive saccades.

Action Potentials↗
PubMedDate not supplied

Radial motion bias in macaque frontal eye field.

The visual responsiveness and spatial tuning of frontal eye field (FEF) neurons were determined using a delayed memory saccade task. Neurons with visual responses were then tested for direction selectivity using moving random dot patterns centered in the visual receptive field. The preferred axis of motion showed a significant tendency to be aligned with the receptive-field location so as to favor motion toward or away from the center of gaze. Centrifugal (outward) motion was preferred over centripetal motion. Motion-sensitive neurons in FEF thus appear to have a direction bias at the population level. This bias may facilitate the detection or discrimination of expanding optic flow patterns. The direction bias is similar to that seen in visual area MT and in posterior parietal cortex, from which FEF receives afferent projections. The outward motion bias may explain asymmetries in saccades made to moving targets. A representation of optic flow in FEF might be useful for planning eye movements during navigation.

Action Potentials↗