Cells in the foveal representation of V1 cortex of adult primates became visually responsive after normal sensory input was removed. Immediately after fovea were lesioned bilaterally, a region was found where no cells' activity could be modulated by visual stimulation. Recordings made in that deafferented zone at greater than 2.5 months after lesions revealed that activity of over half of the cells could be modulated by visual stimuli presented to intact peripheral retina. Although response characteristics made cells with recovered driving quite unlike normal cells, the result suggests a level of visual cortical reorganization previously observed only in immature animals.
A region of dorsomedial frontal cortex (DMFC) has been implicated in planning and executing saccadic eye movements; hence it has been referred to as a supplementary eye field (SEF). Recently, activity related to executing smooth-pursuit eye movements has been recorded from the DMFC, and microstimulation here has been shown to evoke smooth eye movements. This report documents neuronal activity present in smooth-pursuit tasks where the predictability of target motion was manipulated. The activity of many neurons in the DMFC reached a peak when a predictable change in target motion occurred. Furthermore, the peak activity of some cells was systematically shifted by manipulating the duration of the target event, indicating that the network these neurons were in could learn the temporal characteristics of new target motion. Finally, the activity of most neurons tested was greater when target motion was predictable than when it was unpredictable. The results suggest that the DMFC participates in planning smooth-pursuit eye movements based on past stimulus history.
Smooth pursuit eye movements are guided largely by retinal-image motion. To compensate for neural conduction delays, the brain employs a predictive mechanism to generate anticipatory pursuit that precedes target motion (E. Kowler, 1990). A critical question for interpreting neural signals recorded during pursuit concerns how this mechanism is interfaced with sensorimotor processing. It has been shown that the predictor is not simply turned-off during randomization because anticipatory eye velocity remains when target velocity is randomized (E. Kowler & S. McKee, 1987; G. W. Kao & M. J. Morrow, 1994). This study was completed to compare pursuit behavior during randomized motion-onset timing with that occurring during direction or speed randomization. We found that anticipatory eye velocity persisted despite motion-onset randomization, and that anticipation onset time was between that observed in the different constant-timing conditions. This centering strategy was similar to the bias of eye velocity magnitude away from extremes observed when direction or speed was randomized. Such a strategy is comparable to least-squares error minimization, and could be used to facilitate acquisition of a target when it begins to move. Centering was in some observers accounted for by a shift of eye velocity toward that generated in the preceding trial. The results make unlikely a model in which the predictor is disengaged by randomizing stimulus timing, and suggest that predictive signals always interact with those used in sensorimotor processing during smooth pursuit.
When viewing a moving object, details may appear blurred if the object's motion is not compensated for by the eyes. Smooth pursuit is a voluntary eye movement that is used to stabilize a moving object. Most studies of smooth pursuit have used small, foveal targets as stimuli (e.g. Lisberger SG and Westbrook LE. J Neurosci 1985;5:1662-1673.). However, in the laboratory, smooth pursuit is poorer when a small object is tracked across a background, presumably due to a conflict between the primitive optokinetic reflex and smooth pursuit. Functionally, this could occur if the motion signal arising from the target and its surroundings were averaged, resulting in a smaller net motion signal. We asked if the smooth pursuit system could spatially summate coherent motion, i.e. if its response would improve when motion in the peripheral retina was in the same direction as motion in the fovea. Observers tracked random-dot cinematograms (RDC) which were devoid of consistent position cues to isolate the motion response. Either the height or the density of the display was systematically varied. Eye speed at the end of the open-loop period was greater for cinematograms than for a single spot. In addition, eye acceleration increased and latency decreased as the size of the aperture increased. Changes in the density produced similar but smaller effects on both acceleration and latency. The improved pursuit for larger motion stimuli suggests that neuronal mechanisms subserving smooth pursuit spatially average motion information to obtain a stronger motion signal.
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