Biomedical subjects
Publications and source records attributed to Lawrence H Snyder.
Response inhibition, or impulse control, is critical for normal cognitive function. In this issue of Neuron, Hasegawa and colleagues use a spatial nonmatch-to-sample task to reveal neurons in and around the frontal eye fields that encode where an animal should not look.
The primate posterior parietal cortex (PPC), part of the dorsal visual pathway, is best known for its role in encoding salient spatial information. Yet there are indications that neural activity in the PPC can also be modulated by nonspatial task-related information. In this study, we tested whether neurons in the PPC encode signals related to cognitive set, that is, the preparation to perform a particular task. Cognitive set has previously been associated with the frontal cortex but not the PPC. In this study, monkeys performed a cognitive set shifting paradigm in which they were cued in advance to apply one of two different task rules to the subsequent stimulus on every trial. Here we show that a subset of neurons in the PPC, concentrated in the lateral bank of the intraparietal sulcus and on the angular gyrus, responds selectively to cues for different task rules.
Neurons in many cortical areas involved in visuospatial processing represent remembered spatial information in retinotopic coordinates. During a gaze shift, the retinotopic representation of a target location that is fixed in the world (world-fixed reference frame) must be updated, whereas the representation of a target fixed relative to the center of gaze (gaze-fixed) must remain constant. To investigate how such computations might be performed, we trained a 3-layer recurrent neural network to store and update a spatial location based on a gaze perturbation signal, and to do so flexibly based on a contextual cue. The network produced an accurate readout of target position when cued to either reference frame, but was less precise when updating was performed. This output mimics the pattern of behavior seen in animals performing a similar task. We tested whether updating would preferentially use gaze position or gaze velocity signals, and found that the network strongly preferred velocity for updating world-fixed targets. Furthermore, we found that gaze position gain fields were not present when velocity signals were available for updating. These results have implications for how updating is performed in the brain.
We investigated whether macaque monkeys possess the ability to prepare abstract tasks in advance. We trained two monkeys to use different stimulus-response (S-R) mappings. On each trial, monkeys were first informed with a visual cue which of two S-R mapping to use. Following a delay, a visual target was presented to which they would respond with a left or right button-press. We manipulated delay time between cue and target and found that performance was faster and more accurate with longer delays, suggesting that monkeys used the delay time to prepare each task in advance.
Executive control involves concentrating on one task without losing the ability to switch to a second task at will. We studied this ability in monkeys (Macaca mulatta) performing arbitrary stimulus-response mappings in a task-switching paradigm. We found relatively low switch costs but high task interference costs. This is the reverse of the typical human pattern of relatively large switch costs and small interference costs. This difference in the behavior of the two species may reflect anatomical differences in the sizes of the prefrontal and parietal cortices. These results indicate that monkeys are an excellent model for some but not all aspects of human task-switching.
Sensory-motor transformations are often studied using memory-guided movements to small numbers of targets. Whether target locations are directly converted into motor plans on every trial, or subjects use targets to select one of a small number of previously memorized trajectories is unknown. Well-trained monkeys made memory-guided saccades to familiar or nearby novel targets. Performance was superficially similar under the two conditions. However, saccades to novel targets close to the vertical meridian were repulsed away from the nearest familiar target. These findings suggest that sensory-to-motor transformations are performed on every trial, but that previous experience may bias the transformation.
A vertical asymmetry in memory-guided saccadic eye movements has been previously demonstrated in humans and in rhesus monkeys. In the upright orientation, saccades generally land several degrees above the target. The origin of this asymmetry has remained unknown. In this study, we investigated whether the asymmetry in memory saccades is dependent on body orientation in space. Thus animals performed memory saccades in four different body orientations: upright, left-side-down (LSD), right-side-down (RSD), and supine. Data in all three rhesus monkeys confirm previous observations regarding a significant upward vertical asymmetry. Saccade errors made from LSD and RSD postures were partitioned into components made along the axis of gravity and along the vertical body axis. Up/down asymmetry persisted only in body coordinates but not in gravity coordinates. However, this asymmetry was generally reduced in tilted positions. Therefore the upward bias seen in memory saccades is egocentric although orientation in space might play a modulatory role.
During a shift of gaze, an object can move along with gaze or stay fixed in the world. To examine the effect of an object's reference frame on spatial working memory, we trained monkeys to memorize locations of visual stimuli as either fixed in the world or fixed to gaze. Each trial consisted of an initial reference frame instruction, followed by a peripheral visual flash, a memory-period gaze shift, and finally a memory-guided saccade to the location consistent with the instructed reference frame. The memory-period gaze shift was either rapid (a saccade) or slow (smooth pursuit or whole body rotation). This design allowed a comparison of memory-guided saccade performance under various conditions. Our data indicate that after a rotation or smooth-pursuit eye movement, saccades to memorized world-fixed targets are more variable than saccades to memorized gaze-fixed targets. In contrast, memory-guided saccades to world- and gaze-fixed targets are equally variable following a visually guided saccade. Across all conditions, accuracy, latency, and main sequence characteristics of memory-guided saccades are not influenced by the target's reference frame. Memory-guided saccades are, however, more accurate after fast compared with slow gaze shifts. These results are most consistent with an eye-centered representational system for storing the spatial locations of memorized objects but suggest that the visual system may engage different mechanisms to update the stored signal depending on how gaze is shifted.
A localized cluster of neurons in macaque posterior parietal cortex, termed the parietal reach region (PRR), is activated when a reach is planned to a visible or remembered target. To explore the role of PRR in sensorimotor transformations, we tested whether cells would be activated when a reach is planned to an as-yet unspecified goal. Over one-third of PRR cells increased their firing after an instruction to prepare a reach, but not after an instruction to prepare a saccade, when the target of the movement remained unknown. A partially overlapping population (two-thirds of cells) was activated when the monkey was informed of the target location but not the type of movement to be made. Thus a subset of PRR neurons separately code spatial and effector-specific information, consistent with a role in specifying potential motor responses to particular targets.
When primates reach for an object, they very often direct an eye movement toward the object as well. This pattern of directing both eye and limb movements to the same object appears to be fundamental to eye-hand coordination. We investigated interactions between saccades and reaching movements in a rhesus monkey model system. The amplitude and peak velocity of isolated eye movements are positively correlated with one another. This relationship is called the main sequence. We now report that the main sequence relationship for saccades is changed during coordinated eye and arm movements. In particular, peak eye velocity is approximately 4% faster for the same size saccade when the saccade is accompanied by a coordinated arm movement. Saccade duration is reduced by an equivalent amount. The main sequence relationship is unperturbed when the arm moves simultaneously but in the opposite direction as the eyes, suggesting that eye and arm movements must be tightly coordinated to produce the effect. Candidate areas mediating this interaction include the posterior parietal cortex and the superior colliculus.