R. Boch

Enhanced activation of neurons in prelunate cortex before visually guided saccades of trained rhesus monkeys

SummarySingle unit recording from trained rhesus monkeys demonstrate that the activity of the prelunate cortex is enhanced when a visual stimulus becomes a target of saccadic eye movement. As a rule, the enhancement is spatially selective: it does not occur if the animal makes an eye movement away from, rather than towards the stimulus. The results show that the prelunate cortex has access to an extraretinal signal which is activated in association with events preceding visually guided eye movements. Whether the signal reflects the initiation of eye movement or the animals interest in the stimulus, which he usually selects to initiate an eye movement, remains uncertain.

Express-saccades of the monkey: Effect of daily training on probability of occurrence and reaction time

SummaryTwo monkeys learned to make saccadic eye movements from a central fixation point to a peripheral target, when there was a temporal gap between fixation point offset and target onset. Under these conditions the animals made saccades after extremely short reaction times (< 100 ms), so called express-saccades. With ongoing training the rate of occurrence increased (10 to 100%) and the reaction time of the express-saccades decreased (95 to 75 ms). The training effects were mediated by the amount of previously executed express-saccades and they were also spatially selective for express-saccades to that target position that had been used during training. The training effects on the express-saccades can be saturated after less than 7 days of daily training and are reversible after another 7 days of no training. The results indicate the existence of a fast-operating visuo-to-oculomotor pathway which can be quickly and reversibly modified by daily exercise.

Express-saccades of the monkey: reaction times versus intensity, size, duration, and eccentricity of their targets.

SummaryMonkeys were trained to fixate a small spot of light (fixation spot) and to saccade to a peripheral target if and only if the fixation spot was turned off. If the offset of the fixation spot preceded the onset of the peripheral target by a temporal gap of more than 140 ms the animals could change their direction of gaze after saccadic reaction times of no more than 70–80 ms (express-saccades). The reaction times of the express-saccades depend on the luminance and the size of the target and decrease from about 120 ms for near threshold targets by about 50 ms in a range of 2,5 log units above threshold (gap duration 200 ms). The minimum reaction time and the target size for which the minimum is reached are functions of the retinal eccentricity of the target. Comparison with response latencies of afferent visual neurons suggests that the dependence of the reaction times of express-as well as regular-saccades on the physical parameters of the target is mostly determined by retinal factors. The short reaction times of the express-saccades are discussed in relation to the reaction times of other visually-guided goal-directed movements.

Further observations on the occurrence of express-saccades in the monkey

SummaryExpress-saccades, i.e. goal directed eye movements with extremely short saccadic reaction times (SRT) have recently been observed in rhesus monkey (70–80 ms) and human subjects (around 100 ms). In the gap task which has been used so far, a central fixation point (Fp) was turned off a short time before a new target (Tg) in the near periphery was presented. Therefore, express-saccades occurred when the goal of fixation was no longer visible. To determine whether or not the absence of the Fp is a necessary condition for the execution of an express-saccade, we used an overlap task in which the monkeys had to change the direction of gaze in the presence of the Fp. The results for this overlap task were compared to those found in the gap task. Three major observations have emerged from the present study. (a) Even though the Fp remained visible, a suddenly appearing peripheral target could be reached by an express-saccade. (b) Express-saccades persisted if the location as well as the time of the appearance of the target was randomized. It appears that for an express-saccade to occur, the process of interruption of previous active fixation must be completed at the time when a new target becomes visible. (c) The spectrum of the monkeys saccadic reaction times contains at least three different peaks: express-saccades with reaction times below 100 ms, fast regular saccades with reaction times around 130 ms, and slow regular saccades with reaction times around 180 ms.

Selection of visual targets activates prelunate cortical cells in trained rhesus monkey.

SummaryCells in monkey prelunate association cortex display an enhanced visual activity after the onset of a stimulus in the receptive field, when the stimulus is simultaneously selected as a target for a saccadic eye movement. In the present study we observed a separate activation which is independent of the passive visual on-response and occurs in a given cell when the animal saccades to a steady stimulus in its receptive field. The activation begins when the stimulus is selected for foveation before the eye actually moves, but is not necessarily predictive for an eye movement.

Saccadic reaction times and activation of the prelunate cortex: Parallel observations in trained rhesus monkeys

SummaryRhesus monkeys were trained to fixate a small spot and saccade to a second stimulus in the near periphery if the fixation spot went off. In different tests the target stimulus could occur at various delay times before or after the offset of the fixation spot. During periods of single unit recording from the prelunate cortex neural events were measured together with saccadic reaction times (SRT): If the stimulus was visible for a period of time (1 or 0.5 s) before the fixation spot disappeared (positive “delayed saccade” task) the SRT reached values of more than 300 ms. The SRTs were shorter when the target stimulus occurred simultaneously with the offset of the fixation spot (“saccade” task). SRT were shortest (∼ 150 ms) if the target stimulus appeared 100–250 ms after the offset of the fixation spot (negative “delayed saccade” task). Moreover, they decreased with the time of daily training.The different behavioural conditions resulted in different types of cortical activity with different latencies: In “saccade” and negative “delayed saccade” tasks the neurons on-responses could be enhanced in comparison to the passive visual onresponses during stationary fixation. The latencies of the on-response and the enhanced on-response were equal with approximately 80 ms. In striking contrast the latencies of the presaccadic activation (PSA) in the positive “delayed saccade” tasks were more than twice as long with about 200 ms. Daily training influences both the SRTs and the PSA: The SRTs become shorter by more than 150 ms in positive “delayed saccade” tasks (delay: 300–500 ms) and the percentage of PSA-neurons decrease from more than 70% to less than ∼ 20% after 3 weeks of daily training and recording. The temporal aspects of events preceding visually guided eye movements are important to understand the serial and parallel processing in cortical and subcortical structures that are involved in the learning, initiation, and execution of goal directed movements.

Behavioral modulation of neuronal activity in monkey striate cortex: excitation in the absence of active central fixation.

SummaryTwo rhesus monkeys were trained to fixate a small fixation point for randomly varying periods of time using the dimming paradigm. During single unit recording in the striate cortex the fixation point was turned off for a few hundred milliseconds in the presence of a peripheral stimulus. During the disappearance of the fixation point the peripheral stimulus could dim and because of that, in some trials the monkeys saccade to the stimulus right after the offset of the fixation point. In other trials of the same task the monkeys suppressed the execution of a saccade without missing the dimming of the stimulus. During the monkeys performance of this task, striate cortex neurons display an increase of activity after fixation point offset in the presence of a receptive field stimulus, independent of the monkeys decision to look at it or not. Its occurrence can be interpreted as a sign of the monkey having interrupted active fixation of the fixation point and/or having shifted its visual attention towards the peripheral target without necessarily executing a saccade to it.

Express-Saccades of the Monkey: A New Type of Visually Guided Rapid Eye Movements After Extremely Short Reaction Times

Monkeys are trained to fixate a stationary small central spot and to saccade to a peripheral target. The saccadic reaction times depend on the temporal sequence of fixation point off set and target on set and form a spectrum between 60 and 400 ms containing at least 4 peaks. If the fixation point is turned off before the peripheral target appears (gap duration in the order of 200 ms) a rather narrow (± 3 ms) distribution of reaction times is obtained with a peak at 75 ms (express-saccades). The exact mean value of the reaction times depends on the amount of daily training (and on the physical parameters of the target). Reaction times of the regular saccades may be shortened by as much as 200 ms by daily exercise those of the express-saccades by about 20 ms.

Chapter 4 Volitional Eye Movements and their Relationship to Visual Attention

Publisher Summary This chapter describes the classification of eye movements based on the physical parameters of the movements and on the physical conditions under which these movements occur. It illustrates those types and aspects of eye movements that are under volitional control and discuss their relationship to visual attention. Eye movements are classified in terms of their velocity, as being either fast, slow, or close to zero (stationary). Complex combinations of eye movements occur reflexively when the head moves. These reflexes, distinguished on the basis of the receptor detecting the motion, are of two types—namely, vestibular and optokinetic. Both slow and fast eye movements and the state of “no movement,” can be achieved by volition. The volitional control of eye movements is restricted to certain types of movement and in certain cases to the initiation or to the suppression of the movement. Among the various types of fast or saccadic eye movements, self-paced saccades are “volitional” and spontaneous saccades are “not volitional”.