John Schlag

Determination of antidromic excitation by the collision test: problems of interpretation.

Extracellular unit recordings were made from pontine reticular neurons in the cat and cells of the motor cortex in monkeys. In all cases, the characteristics of responses to electrical stimulation were studied using the tests of invariance of latency, high frequency following, and collision for determining the orthodromic or antidromic nature of the responses. The results of these tests show that their conclusions are not always consistent. A systematic error was found between the experimental and predicted values of the collision interval. It is argued that this error is due to differences in the application of measured parameters in calculating the collision interval. The collision test can be considerably improved by repeating the test with stimuli of progressively greater strengths.

Antisaccade performance predicted by neuronal activity in the supplementary eye field

The voluntary control of gaze implies the ability to make saccadic eye movements specified by abstract instructions, as well as the ability to repress unwanted orientating to sudden stimuli. Both of these abilities are challenged in the antisaccade task, because it requires subjects to look at an unmarked location opposite to a flashed stimulus, without glancing at it,. Performance on this task depends on the frontal/prefrontal cortex and related structures, but the neuronal operations underlying antisaccades are not understood. It is not known, for example, how excited visual neurons that normally trigger a saccade to a target (a prosaccade) can activate oculomotor neurons directing gaze in the opposite direction. Visual neurons might, perhaps, alter their receptive fields depending on whether they receive a pro- or antisaccade instruction. If the receptive field is not altered, the antisaccade goal must be computed and imposed from the top down to the appropriate oculomotor neurons. Here we show, using recordings from the supplementary eye field (a frontal cortex oculomotor centre) in monkeys, that visual and movement neurons retain the same spatial selectivity across randomly mixed pro- and antisaccade trials. However, these neurons consistently fire more before antisaccades than prosaccades with the same trajectories, suggesting a mechanism through which voluntary antisaccade commands can override reflexive glances.

Oculomotor localization relies on a damped representation of saccadic eye displacement in human and nonhuman primates

The oculomotor system has long been thought to rely on an accurate representation of eye displacement or position in a successful attempt to reconcile a stationary targets retinal instability (caused by motion of the eyes) with its corresponding spatial invariance. This is in stark contrast to perceptual localization, which has been shown to rely on a sluggish representation of eye displacement, achieving only partial compensation for the retinal displacement caused by saccadic eye movements. Recent studies, however, have begun to case doubt on the belief that the oculomotor system possess a signal of eye displacement superior to that of the perceptual system. To verify this, five humans and one monkey (Macaca nemestrina) served as subjects in this study of oculomotor localization abilities. Subjects were instructed to make eye movements, as accurately as possible, to the locations of three successive visual stimuli. Presentation of the third stimulus (2-ms duration) was timed so that it fell before, during, or after the subjects saccade from the first stimulus to the second. Localization errors in each subject (human and nonhuman) were consistent with the hypothesis that the oculomotor system has access to only a damped representation of eye displacement--a representation similar to that found in perceptual localization studies.

Induction of oculomotor responses by electrical stimulation of the prefrontal cortex in the cat

Abstract The prefrontal cortex of the cat (ence´phale isole´preparation) was explored using minimal electrical stimulation in order to locate as exactly as possible the critical regions from which rapid ocular movements could be induced. Two such regions have been found: one on the mesial face of the hemisphere under the cruciate sulcus, the other within the presylvian sulcus. The extent of these zone and their functional characteristics are described. The problem of the organization of a frontal eye field in the cat is discussed with reference t the monkey.

Through the eye, slowly: delays and localization errors in the visual system.

Reviews on the visual system generally praise its amazing performance. Here we deal with its biggest weakness: sluggishness. Inherent delays lead to mislocalization when things move or, more generally, when things change. Errors in time translate into spatial errors when we pursue a moving object, when we try to localize a target that appears just before a gaze shift, or when we compare the position of a flashed target with the instantaneous position of a continuously moving one (or one that appears to be moving even though no change occurs in the retinal image). Studying such diverse errors might rekindle our thinking about how the brain copes with real-time changes in the world.

Visual responses of thalamic neurons depending on the direction of gaze and the position of targets in space

SummaryVisual receptive field properties of neurons in the region of the thalamic internal medullary lamina were studied in alert cats while they fixated in various directions. In slightly more than 50% of the cells, the responsiveness of the cells was found to depend on the location of the stimulus with respect to the head-body axis (stimulus absolute position). A cell could ignore a stimulus outside its absolute field even if it was well placed within its receptive field.Three types of neurons were distinguished. Neurons with small central receptive fields were tonically activated when the animal fixated the stimulus in one half of the screen (usually contralateral). The firing rate of these cells was related to the stimulus absolute position measured along a preferred axis. Similarly, neurons with large receptive fields fired as a function of stimulus absolute position but stimulus fixation was not required. Neurons with eccentric fields responded to stimuli located in a target area defined in head-body coordinates. Such cells gave presaccadic bursts with eye movements terminating in the target area.The conclusion proposed is that neurons exist which code visual spatial information in a non-retinal frame of reference. This coding takes place at the time of stimulus presentation. Its role may be seen in the initiation of visually guided movements.

Unit activity related to spontaneous saccades in frontal dorsomedial cortex of monkey

SummarySingle unit activity was studied in the dorsomedial edge of the frontal lobe, above the superior arcuate sulcus in three trained monkeys (Macaca nemestrina). Gaze and head movements were recorded with two magnetic search coils. Discharges preceding spontaneous eye movements in a preferred direction were consistently observed in light and in dark, in a limited cortical territory at the anterior border of the supplementary motor area. Microstimulation at these sites elicited saccades in the unit preferred direction. Five presaccadic units were studied head fixed and head free and showed the same saccade-related activity under both conditions. Preliminary data suggest that the area studied may be a supplementary eye field distinct from the arcuate frontal eye field.

Electrophysiological properties of units of the thalamic reticular complex

Abstract Unit activity was recorded in and close to the thalamic reticular complex in encephale isole cats. A group of units was characterized by their long high-frequency burst responses to 10/sec thalamic and cortical stimulations, their sustained high-frequency firing induced by 40–250/sec thalamic and cortical stimulations, the depression of their firing by high-frequency stimulations of the midbrain reticular formation and caudate nucleus, and their tendency to be more active during EEG spindles. These units were located along tracks histologically found crossing the thalamic reticular complex. They differed from cells collected along tracks not crossing the reticular complex, on all four electro-physiological properties mentioned above. These properties are shown to be intrinsic for a class of neurons, presumably of the reticular complex. The timing of firing of reticularis and nonreticularis neurons is compared and it is suggested that reticularis neurons are at the origin of inhibition of dorsal thalamic cells. It is also postulated that they play a role in the control of thalamic spindle waves.

Vestibular-oculomotor interaction in cat eye-head movements

When cats make slow scanning head movements, intersaccadic counterrotary eye movements are driven by the vestibulo-ocular reflex, but reset or forward saccades are not directly affected by vestibular afference. When the movements are rapid (approximately 200 deg./sec), large (greater than 40 degrees), and executed in single step shifts gaze (whether in the dark or during visual fixation shifts between known targets), there is no longer any clear vestibular effect on any eye movements during the gaze shift. The vestibulo-ocular reflex is active only at the beginning of head rotation and again at its termination as the gaze reaches its goal, even in the absence of vision. It is postulated that head-in-space and eye-in-orbit movements are perfectly monitored to adjust the amplitude of gaze shifts, despite the lack of overt vestibular effects on eye movements.

Illusory localization of stimuli flashed in the dark before saccades.

A photic stimulus flashed just before a saccade in the dark tends to be mislocalized in the direction of the saccade. This mislocalization is not only perceptual; it is also expressed by errors of ocular targeting. A particular situation arises if the point of light is flashed twice at the same place, the second time, just before a saccade. The point of light may appear at two different places even though neither the site of its retinal image nor the direction of gaze change between the flashes. Experiments were run on five human subjects, head fixed in the dark, with flashes repeated at the site of the saccade goal or at the initial point of fixation. In both cases, the test stimulus was mislocalized. However, its apparent displacement never produced the perception of a streak. Streaks were reported only when there was an actual stimulus movement on the retina (e.g. by flashing the stimulus during the saccade). Mislocalization did not occur if the two flashes were not separated by a dark interval. This implies that, as long as a steady stimulus remains continually visible, there is no updating of the internal representation of eye position assumed to be used for stimulus localization.