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M J Nichols

Publications and source records attributed to M J Nichols.

7 recordsLinked to original sources
PubMed1996

Component stretching during oblique stimulation-evoked saccades: the role of the superior colliculus.

1. During oblique visually guided saccades, the peak velocity of each component is reduced from what it would be for a purely horizontal or vertical saccade of the same amplitude, and the durations of the components are prolonged. We tested predictions of two competing accounts of the neural basis of this "component stretching" phenomenon. Using a recent experimental approach, we electrically stimulated sites in the monkey superior colliculus (SC) immediately after either vertical or horizontal visually guided saccades. Under these conditions, the amplitude of one component (corresponding to the direction of the preceding movement, horizontal or vertical) of the stimulation-evoked saccade varies systematically, while the amplitude of the other component ("constant-amplitude component") remains essentially constant. These changes in saccade metrics occur even though both the locus and parameters of collicular stimulation are fixed. 2. As with visually guided saccades, the peak velocity of the constant-amplitude component in stimulation-evoked saccades decreased with increasing amplitude of the orthogonal component, while the duration of the constant-amplitude component increased with orthogonal amplitude. The component stretching effect in these stimulation-evoked saccades was qualitatively indistinguishable from component stretching in metrically matched, visually guided movements. Yet, unlike visually guided saccades, the locus of stimulation-induced activity was fixed from one stimulation-evoked saccade to the next. 3. Therefore component stretching in stimulation-evoked oblique saccades does not depend on the locus of activity in the collicular motor map. These results are inconsistent with the predictions of common source models of component stretching, but are consistent with cross-coupling models. We discuss the present results in the context of a new interpretation of collicular interaction with downstream saccadic controllers.

Animals↗
PubMed1996

Independent feedback control of horizontal and vertical amplitude during oblique saccades evoked by electrical stimulation of the superior colliculus.

1. In early local feedback models for controlling horizontal saccade amplitude, a feedback signal of instantaneous eye position is continuously subtracted from a reference signal of desired eye position at a comparator. The output of the comparator is dynamic motor error, the remaining distance the eyes must rotate to reach the saccadic goal. When feedback reduces dynamic motor error to zero, the saccade stops on target. Two classes of local feedback model have been proposed for controlling oblique saccades (i.e., saccades with both horizontal and vertical components). In "independent comparator" models, separate horizontal and vertical comparators maintain independent representations of horizontal and vertical dynamic motor error. Thus, once an oblique desired displacement signal is established, the horizontal and vertical amplitudes of oblique saccades are under independent feedback control. In "vectorial comparator" models, output cells in the motor map of the superior colliculus act as site-specific vectorial comparators. For a given oblique desired displacement, a single comparator controls the amplitudes of both components. Because vectorial comparator models do not maintain separate representations of horizontal and vertical dynamic motor error, they cannot exert independent control over the component amplitudes of oblique saccades. 2. We tested differential predictions of these two types of models by electrically stimulating sites in the superior colliculus of rhesus monkey immediately after either vertical or horizontal visually guided saccades. We have shown previously that, despite the fixed site of collicular stimulation, the amplitude of the visually guided saccades systematically alters the amplitude of the corresponding component (horizontal or vertical) of stimulation-evoked saccades. However, in the present study, we examined the effect of the visually guided saccades on the amplitude of the orthogonal component of stimulation-evoked saccades. 3. For a fixed site of collicular stimulation, vectorial comparator models predict that the initial visually guided saccade will influence both components of the ensuing stimulation-evoked saccade via the single feedback comparator. By contrast, independent comparator models permit the independent manipulation of the horizontal and vertical amplitudes of these oblique stimulation-evoked saccades. 4. In total, we collected data from 15 collicular stimulation sites. Immediately after either horizontal or vertical visually guided saccades of different amplitudes, we measured the horizontal and vertical amplitudes of saccades evoked by stimulation of the intermediate or deep layers of the superior colliculus. For each site, the duration, frequency, and current of the stimulation train were held constant. 5. Under these conditions, stimulation-evoked saccades followed visually guided saccades with short latency (18.1 +/- 6.7 ms, mean +/- SD). For every stimulation site tested, although the amplitude of the component of stimulation-evoked saccades corresponding to the direction of the preceding saccade (horizontal or vertical) varied systematically, the amplitude of the orthogonal component was roughly constant. 6. Thus the horizontal and vertical amplitudes of oblique stimulation-evoked saccades can be manipulated independently. Moreover, the peak velocity-amplitude relationships, the instantaneous velocity profiles, and the ratio of horizontal and vertical velocities and durations were very similar to those of visually guided saccades. 7. Independent comparator models can readily account for the ability to manipulate the amplitude of one component of oblique saccades without affecting the other. However, two-dimensional local feedback models that cannot exert independent control over the horizontal and vertical amplitudes of oblique saccades should be carefully reevaluated.

Animals↗
PubMed1995

Nonstationary properties of the saccadic system: new constraints on models of saccadic control.

1. We tested the predictions of two models of the saccadic burst generator by electrically stimulating sites in primate superior colliculus (SC) immediately following visually guided movements. 2. The amplitude and direction of stimulated saccades depend systematically on the amplitude and direction of preceding visually guided saccades, and that effect decays exponentially with a time constant of approximately 45 ms. The saccadic system, then, displays an amplitude-dependent non-stationarity that follows an exponential time course during the intersaccadic interval (ISI). 3. These results are consistent with a variant of the eye displacement model proposed by Jurgens et al. but not with Robinson's classic model of the burst generator. Moreover, since all models of saccadic control must predict either stationary or nonstationary behavior during the ISI, these results provide a powerful new constraint on those models. 4. Finally, the success of the displacement model in accounting for our data suggests a new explanation for the results of colliding saccade experiments.

Animals↗
PubMed1985

The assessment of two methods for removing eye movement artefact from the EEG.

A quantitative assessment of both computerised correlation and analogue techniques for the removal of eye movement artefact from the electroencephalogram was undertaken. In both methods a fraction of the measured EOGs was subtracted from the measured EEG to leave the corrected EEG. In the correlation method the correction factors were computed from the cross-correlations of the EOGs and measured EEGs. In the analogue method the fraction subtracted was derived using a potential divider arrangement. The relative effectiveness of the two methods was determined by comparing the autocorrelation functions of the corrected EEGs at a lag of 2 sec. It was found that the computerised correlation technique was superior. A 3-channel EOG correlation correction procedure was found adequate in which two horizontal and one vertical EOGs were used. The analogue technique was very time consuming and possibly erroneous.

Electroencephalography↗