Site and parameters of microstimulation: evidence for independent effects on the properties of saccades evoked from the primate superior colliculus

Abstract

1. Microstimulation is used to investigate how activity in the superior colliculus (SC) contributes to determining the properties of primate saccadic eye movements. The site of collicular stimulation, the duration of the stimulation train, and the frequency of the stimulation train are each varied to examine the relative contributions of the locus, duration, and level of collicular activity to determining saccade amplitude, direction, duration, and velocity. 2. For any given site of stimulation, a relationship between movement amplitude and train duration can be demonstrated. Movement amplitude is a monotonically increasing, but saturating, function of increasing train duration. The size of the largest movement is dictated by the site of stimulation. Within the range over which amplitude can be modulated, movement offset is linked to the offset of the stimulation train. As a result, each decrement or increment in train duration produces a corresponding decrement or increment in movement duration. 3. The peak velocity of an evoked movement is influenced by the frequency of stimulation; a higher frequency of stimulation produces a movement of higher velocity. 4. The effects of train duration and frequency can be traded to produce movements that have comparable amplitudes but different dynamic characteristics; high-velocity movements of short duration and low-velocity movements of long duration can be produced by stimulating with high-frequency, short-duration, and low-frequency, long-duration trains, respectively. Across stimulation frequencies, the amplitude of an evoked movement is best related to the total number of pulses in the stimulation train. 5. Because it is possible to compensate for reduced velocity by increasing the duration of the stimulation train, the same site-specific maximum amplitude can be attained with different frequencies of stimulation. 6. Small, but significant, changes in movement direction occur as a result of varying train duration or train frequency. 7. The latency to movement onset (i.e., interval from stimulation onset to movement onset) depends upon the frequency of stimulation. A higher frequency of stimulation produces a movement of shorter latency. 8. These data demonstrate that both the site of stimulation and the parameters of stimulation contribute to determining the properties of a movement evoked from the primate SC. In doing so, they contradict the results of early microstimulation studies that suggest that the properties of eye movements evoked from the primate SC are determined solely by the site of stimulation. The findings conflict with the traditional view of collicular function that suggests that the collicular motor representation is purely anatomic. Rather, these data support a revised view whereby the locus, duration, and level of collicular activity contribute to determining the properties of a primate saccadic eye movement. According to this view, independent information relating to desired displacement and saccade velocity are extracted from the spatiotemporal p...

A neural model of the saccade generator in the reticular formation

“…Below this level, saccades of smaller amplitude are produced. Stanford et al (1996) also found that the function relating saccade amplitude to stimulation frequency saturated before the function relating saccade velocity to stimulation frequency (i.e. velocity continued to increase after amplitude had peaked).…”

“…Still, it may be the case that having a distribution of EBN on-directions assists in producing straight oblique saccades, as suggested by Quaia and Optican (1997). Stanford et al (1996) varied the frequency of electrical stimulation to the SC and found that the amplitude of a saccade evoked from a particular point on the SC is not only a function of stimulation location. Rather, the stereotypical saccade amplitude for a particular site is obtained only with sufficiently high stimulation frequency.…”

“…For any given region of the brain, current strength is a primary determinant of the size of the population of cells that is activated directly by microstimulation (Stanford et al, 1996). Since current was not varied in the Stanford et al (1996) study, it seems reasonable that for a given stimulation site, the effective current spread remained relatively constant.…”

“…Since current was not varied in the Stanford et al (1996) study, it seems reasonable that for a given stimulation site, the effective current spread remained relatively constant. The frequency of stimulation, however, was varied.…”

“…Y stands for 'Yes, simulations show that the model can explain the data', M stands for 'Might be able to explain the data, but this has not been simulated', and finally, N stands for 'No, unable to explain the data'. achieve straight oblique saccades, saturation of saccade velocity after saturation of saccade amplitude in response to high stimulation frequencies, tradeoffs between saccade velocity and duration to generate constant saccade amplitude, conservation of saccade amplitude in response to sufficiently brief stimulation of omnipause neurons, and high velocity smooth eye movements evoked by high levels of electrical stimulation of the superior colliculus (Schiller and Stryker, 1972;Evinger et al, 1981;Breznen et al, 1996;Missal et al, 1996;Stanford et al, 1996).…”