Asymmetry of the latent periods of saccades in human depending on visual space complicity

Saccadic latencies of visually-guided saccades of 10 right-handed subjects with right-leading eyes were studied. Stimulation paradigm was spatially bidimentional, and stimuli were shown along horizontal and vertical meridians. Three traditional single step GAP - NO DELAY - OVERLAP temporal paradigms were used. In the first experiment, each paradigm was applied separately (simple visual space). In the second experiment, all the three paradigms were varied pseudo-random order and equiprobably, which complicated the time parameters of visual stimulation (complicated visual space). Asymmetry of visually-guided saccades along the vertical and horizontal meridians was revealed. The character of this asymmetry varied between subjects. MANOVA showed that the factor of visual space complicity (simple or complicated visual space) affected the latent period of saccades to a greater extent than the factor of stimulus lateralization (stimulus presentation in the left/right or upper/lower visual hemifields).     

Latent period of antisaccades in various conditions of visual stimulation in man

The latent periods (LP) of normal saccades and antisaccades were studied in 10 right-handed healthy subjects in two series of experiments. Peripheral visual stimuli were located at an angle of 10 degrees with respect to the central fixation stimulus in the left and right visual semifields. Two standard schemes of visual stimulation: 1) SS (single step), i.e., switching the peripheral stimulus on immediately after switching the central stimulus of; 2) GAP, i.e., the same with the interstimulus interval in 200 ms. It was shown that in the GAP stimulation condition, the LP of both saccades and antisaccades was 30-50 shorter than in the SS condition. The LP of antisaccades was longer than that of saccades by 145-300 ms. The LP of the leftward antisaccades was by 10-100 ms shorter than that of the rightward ones. Probably, this phenomenon reflects the dominance of the right hemisphere in spatial attention.     

Cortical slow negative potentials during eye fixation and preparing of visually triggered saccades in a man

In 10 right-handed healthy subjects EEGs preceding saccades with mean latent periods were selectively averaged. Two standard schemes of visual stimulation were used: with immediate presentation of a peripheral target stimuli after the central fixation stimulus (a single step paradigm) and with the interval between the stimuli in 200 ms (GAP paradigm). Two waves of slow premotor negativity (early PMN1 and late PMN2) that appeared 930 +/- 79 and 609 +/- 82 ms, respectively, prior to a saccade onset were observed. The PMN2 was followed by the negative potentials N-3, N-2, and N-1 (saccadic initiation potential). It was found that in GAP stimulation condition the PMN1 was less pronounces and N-1 was increased as compared to the single step. These findings suggest that disengage of attention from the central point during the GAP period clears the saccadic system for decision making and initiation of a saccade. Under such conditions, the expectation of a visual target does not require a high level of nonspecific activation and motor attention.     

Sound improves distinction of low intensities of light in the visual cortex of a rabbit

Electrodes were implanted into cranium above the primary visual cortex of four rabbits (Orictolagus cuniculus). At the first stage, visual evoked potentials (VEPs) were recorded in response to substitution of threshold visual stimuli (0.28 and 0.31 cd/m2). Then the sound (2000 Hz, 84 dB, duration 40 ms) was added simultaneously to every visual stimulus. Single sounds (without visual stimuli) did not produce a VEP-response. It was found that the amplitude ofVEP component N1 (85-110 ms) in response to complex stimuli (visual and sound) increased 1.6 times as compared to "simple" visual stimulation. At the second stage, paired substitutions of 8 different visual stimuli (range 0.38-20.2 cd/m2) by each other were performed. Sensory spaces of intensity were reconstructed on the basis of factor analysis. Sensory spaces of complexes were reconstructed in a similar way for simultaneous visual and sound stimulation. Comparison of vectors representing the stimuli in the spaces showed that the addition of a sound led to a 1.4-fold expansion of the space occupied by smaller intensities (0.28; 1.02; 3.05; 6.35 cd/m2). Also, the addition of the sound led to an arrangement of intensities in an ascending order. At the same time, the sound 1.33-times narrowed the space of larger intensities (8.48; 13.7; 16.8; 20.2 cd/m2). It is suggested that the addition of a sound improves a distinction of smaller intensities and impairs a dis- tinction of larger intensities. Sensory spaces revealed by complex stimuli were two-dimensional. This fact can be a consequence of integration of sound and light in a unified complex at simultaneous stimulation.     

Slow Cortical Potentials Preceeding Visually Guided Saccades in Schizophrenics

The parameters of saccades and presaccadic slow potentials were studied in right-handed men with a dominant right eye, including 19 schizophrenics and 12 healthy subjects. For visual stimulation, three light-emitting diodes were used, which were located in the center of the visual field (the central fixation stimulus) and 10° to the right and left of it (peripheral stimuli). Two stimulation protocols were used: with a simultaneous switching off of the central fixation stimulus and switching on of the peripheral stimuli (test 1) and with an interstimulus gap of 200 ms (test 2). According to the latency, saccades were divided into anticipatory, express, and regular. Slow EEG potentials preceding regular saccades were analyzed. It was found that the proportion of anticipatory saccades is considerably higher than the normal value in schizophrenia. The analysis of the presaccadic potentials demonstrated a significant decrease in the amplitude of negative potentials in the vertex region at early stages of presaccadic preparation and its increase in the occipital region at late stages. Test 2 in the patients demonstrated an increase in the positivity focus in the frontal region of the right hemisphere. It was assumed that the alterations found in schizophrenia result from the deficit of frontal cortical fields.     

Potential of the human cerebral cortex prior to antisaccades

Latencies and other parameters of presaccadic potentials preceding antisaccades and normal saccades to visual stimuli were studied in 10 right-handed healthy subjects. The EEG was recorded in F3, F4, Fz, C3, C4, Cz, P3, P4, O1 and O2 derivation. EEG records preceding saccades and antisaccades with mean latencies were selected and averaged. The latencies of the leftward antisaccades were shorter than of the rightward antisaccades. The slow presaccadic negativity (in the period of central eye fixations) and fast N -2 and P -1 potentials within the latent period were more prominent before antisaccades than normal saccades. Spatiotemporal analyses of presaccadic potentials showed that the right frontal cortex was activated to a greater extent before antisaccades than before saccades. These findings suggest that right-hemispheric dominance in the spatial attention and inhibition of automatic saccades to visual stimuli in the period of antisaccades preparation.     

Lateralization of saccadic latency during monocular presentation of stimuli to a leading and non-leading eye

Saccadic latencies were studied in ten healthy subjects. Peripheral targets were presented monocularly to a leading and nonleading eyes in the right and left hemifields. SS (single step) and OVERLAP (200 ms) schemes of visual stimulation were used. Under OVERLAP conditions, the saccadic latency was longer by 30-39 ms and the number of long-latency saccades was higher than under SS conditions, especially in subjects with mixed asymmetry profiles. In the majority of subjects with right asymmetry profile, the latencies of saccades during stimulation of the leading eye were by 12 ms shorter than during stimulation of the nonleading eye, and the latencies of right saccades were by 24 ms shorter than that of the left saccades independently of the stimulated eye. The obtained results explain some characteristic features of hemyspheric asymmetry in organization of saccadic movements.     

Cortical mechanisms for trans-saccadic memory and integration of multiple object features

Constructing an internal representation of the world from successive visual fixations, i.e. separated by saccadic eye movements, is known as trans-saccadic perception. Research on trans-saccadic perception (TSP) has been traditionally aimed at resolving the problems of memory capacity and visual integration across saccades. In this paper, we review this literature on TSP with a focus on research showing that egocentric measures of the saccadic eye movement can be used to integrate simple object features across saccades, and that the memory capacity for items retained across saccades, like visual working memory, is restricted to about three to four items. We also review recent transcranial magnetic stimulation experiments which suggest that the right parietal eye field and frontal eye fields play a key functional role in spatial updating of objects in TSP. We conclude by speculating on possible cortical mechanisms for governing egocentric spatial updating of multiple objects in TSP.     

Electrical stimulation of the primate lateral habenula suppresses saccadic eye movement through a learning mechanism

The lateral habenula (LHb) is a brain structure which represents negative motivational value. Neurons in the LHb are excited by unpleasant events such as reward omission and aversive stimuli, and transmit these signals to midbrain dopamine neurons which are involved in learning and motivation. However, it remains unclear whether these phasic changes in LHb neuronal activity actually influence animal behavior. To answer this question, we artificially activated the LHb by electrical stimulation while monkeys were performing a visually guided saccade task. In one block of trials, saccades to one fixed direction (e.g., right direction) were followed by electrical stimulation of the LHb while saccades to the other direction (e.g., left direction) were not. The direction-stimulation contingency was reversed in the next block. We found that the post-saccadic stimulation of the LHb increased the latencies of saccades in subsequent trials. Notably, the increase of the latency occurred gradually as the saccade was repeatedly followed by the stimulation, suggesting that the effect of the post-saccadic stimulation was accumulated across trials. LHb stimulation starting before saccades, on the other hand, had no effect on saccade latency. Together with previous studies showing LHb activation by reward omission and aversive stimuli, the present stimulation experiment suggests that LHb activity contributes to learning to suppress actions which lead to unpleasant events.     

Predictive adjustment of the perceived direction of gaze during saccadic eye movements

When we look at a stationary object, the perceived direction of gaze (where we are looking) is aligned with the physical direction of eyes (where our eyes are oriented) by which the object is foveated. However, this alignment may not hold in a dynamic situation. Our experiments assessed the perceived locations of two brief stimuli (1 ms) simultaneously displayed at two different physical locations during a saccade. The first stimulus was in the instantaneous location to which the eyes were oriented and the second one was always in the same location as the initial fixation point. When the timing of these stimuli was changed intra-saccadically, their perceived locations were dissociated. The first stimuli were consistently perceived near the target that will be foveated at saccade termination. The second stimuli once perceived near the target location, shifted in the direction opposite to that of saccades, as its latency from saccades increased. These results suggested an independent adjustment of gaze orientation from the physical orientation of eyes during saccades. The spatial dissociation of two stimuli may reflect sensorimotor control of gaze during saccades.     

Saccades differentially modulate human LGN and V1 responses in the presence and absence of visual stimulation

Saccades occur several times each second in normal human vision. The visual image moves across the retina at high velocity during a saccade, yet no blurring of the visual scene is perceived . Active suppression of visual input may account for this perceptual continuity, but the neural mechanisms underlying such saccadic suppression remain unclear. We used functional MRI to specifically examine responses in the lateral geniculate nucleus (LGN) and primary visual cortex (V1) during saccades. Activity in both V1 and LGN was strongly modulated by saccades. Furthermore, this modulation depended on whether visual stimulation was present or absent. In complete darkness, saccades led to reliable signal increases in V1 and LGN, whereas in the presence of visual stimulation, saccades led to suppression of visually evoked responses. These findings represent unequivocal evidence for saccadic suppression in human LGN and retinotopically defined V1 and are consistent with the earliest site of saccadic suppression lying at or before V1.     

Saccade generation by the frontal eye fields in rhesus monkeys is separable from visual detection and bottom-up attention shift

The frontal eye fields (FEF), originally identified as an oculomotor cortex, have also been implicated in perceptual functions, such as constructing a visual saliency map and shifting visual attention. Further dissecting the area's role in the transformation from visual input to oculomotor command has been difficult because of spatial confounding between stimuli and responses and consequently between intermediate cognitive processes, such as attention shift and saccade preparation. Here we developed two tasks in which the visual stimulus and the saccade response were dissociated in space (the extended memory-guided saccade task), and bottom-up attention shift and saccade target selection were independent (the four-alternative delayed saccade task). Reversible inactivation of the FEF in rhesus monkeys disrupted, as expected, contralateral memory-guided saccades, but visual detection was demonstrated to be intact at the same field. Moreover, saccade behavior was impaired when a bottom-up shift of attention was not a prerequisite for saccade target selection, indicating that the inactivation effect was independent of the previously reported dysfunctions in bottom-up attention control. These findings underscore the motor aspect of the area's functions, especially in situations where saccades are generated by internal cognitive processes, including visual short-term memory and long-term associative memory.     

Concept of automaticity of saccades

All-round researches of a human being's eye movements in norm and in pathology have been carried out (1967-2006). An analysis of generating of rapid eye movements, those are saccades, has been done. A concept of automaticity of saccades has been formulated. According to the concept a saccade is generated by rhythmo-genesis type, without any external and internal stimuli in their own rhythm. The role of automaticity of saccades in the process of visual perception, and data of impairments of automaticity of saccades in pathology and in uncomfortable visual environment were show.     

Enhanced tactile performance at the destination of an upcoming saccade

Previous work has demonstrated that upcoming saccades influence visual and auditory performance even for stimuli presented before the saccade is executed. These studies suggest a close relationship between saccade generation and visual/auditory attention. Furthermore, they provide support for Rizzolatti et al.'s premotor model of attention, which suggests that the same circuits involved in motor programming are also responsible for shifts in covert orienting (shifting attention without moving the eyes or changing posture). In a series of experiments, we demonstrate that saccade programming also affects tactile perception. Participants made speeded saccades to the left and right side as well as tactile discriminations of up versus down. The first experiment demonstrates that participants were reliably faster at responding to tactile stimuli near the location of upcoming saccades. In our second experiment, we had the subjects cross their hands and demonstrated that the effect occurs in visual space (rather than the early representations of touch). In our third experiment, the tactile events usually occurred on the opposite side of upcoming eye movement. We found that the benefit at the saccade target location vanished, suggesting that this shift is not obligatory but that it may be vetoed on the basis of expectation.     

Saccadic tracking of targets mediated by the anterior-lateral eyes of jumping spiders

The modular visual system of jumping spiders (Salticidae) divides characteristics such as high spatial acuity and wide-field motion detection between different pairs of eyes. A large pair of telescope-like anterior-median (AM) eyes is supported by 2-3 pairs of 'secondary' eyes, which provide almost 360 degrees of visual coverage at lower resolution. The AM retinae are moveable and can be pointed at stimuli within their range of motion, but salticids have to turn to bring targets into this frontal zone in the first place. We describe how the front-facing pair of secondary eyes (anterior lateral, AL) mediates this through a series of whole-body 'tracking saccades' in response to computer-generated stimuli. We investigated the 'response area' of the AL eyes and show a clear correspondence between the physical margins of the retina and stimulus position at the onset of the first saccade. Saccade frequency is maximal at the margin of AL and AM fields of view. Furthermore, spiders markedly increase the velocity with which higher magnitude tracking saccades are carried out. This has the effect that the time during which vision is impaired due to motion blur is kept at an almost constant low level, even during saccades of large magnitude.     

A model of the saccade-generating system that accounts for trajectory variations produced by competing visual stimuli

Variable saccade trajectories are produced in visual search paradigms in which multiple potential target stimuli are present. These variable trajectories provide a rich source of information that may lead to a deeper understanding of the basic control mechanisms of the saccadic system. We have used published behavioral observations and neural recordings in the superior colliculus (SC), gathered in monkeys performing visual search paradigms, to guide the construction of a new distributed model of the saccadic system. The new model can account for many of the variations in saccade trajectory produced by the appearance of multiple visual stimuli in a search paradigm. The model uses distributed feedback about current eye motion from the brainstem to the SC to reduce activity there at physiologically realistic rates during saccades. The long-range lateral inhibitory connections between SC cells used in previous models have been eliminated to match recent physiological evidence. The model features interactions between visually activated multiple populations of cells in the SC and distributed and topologically organized inhibitory input to the SC from the SNr to produce some of the types of variable saccadic trajectories, including slightly curved and averaging saccades, observed in visual search tasks. The distributed perisaccadic disinhibition of SC from the substantia nigra (SNr) is assumed to have broad spatial tuning. In order to produce the strongly curved saccades occasionally recorded in visual search, the existence of a parallel input to the saccadic burst generators in addition to that provided by the distributed input from the SC is required. The spatiotemporal form of this additional parallel input is computed based on the assumption that the input from the model SC is realistic. In accordance with other recent models, it is assumed that the parallel input comes from the cerebellum, but our model predicts that the parallel input is delayed during highly curved saccadic trajectories.     

Double Receptive Fields of Neurons of the Lateral Geniculate Body of the Cat

We studied spatial organization of the receptive fields (RF) of neurons of the lateral geniculate body (LGB) on unanesthetized cats (pretrigeminal brainstem section). After identification of localization and borders of the RF of the neuron under study, we scanned with a sufficient resolution the entire visual field and tried to find additional space zones, whose stimulation could influence the impulse activity of the neuron. These experiments demonstrated that 24 neurons of 167 examined units (14%) could react to the presentation of visual stimuli within the visual space outside the main RF, but we were unable to determine borders of these additional zones with a sufficient accuracy. In 12 neurons (7% of the group under study), localization, dimensions, and specific features of an additional RF (RF-2) could be clearly determined. As a rule, the center of the RF-2 was localized at a distance of 20-40^O; or even farther from the center of the main RF (RF-1). To activate the neuron from the RF-2, application of greater visual stimuli was necessary (as compared with stimulation of the RF-1). Thus, two RF of one and the same neuron had dissimilar spatial organizations and qualitatively differed from each other in their stationary and dynamic characteristics. Considering our findings, we hypothesize that the RF-2 of LGB neurons can play a certain role in perception of large objects within the visual field of the animal by promoting formation of the avoidance reaction.     

Computation of color and brightness differences by neurons in the rabbit visual cortex

Changes in activity of 54 neurons in the rabbit visual cortex evoked by the replacement of eight color and eight achromatic stimuli in pairs were analyzed. The diffused stimuli generated by color SVGA monitor were used in the experiments. The earliest response of phasic neurons (50-90 ms after the replacement) was strongly correlated with differences between stimuli in color or intensity. This response ("the signal of differences") was used as a basis of a matrix (8 x 8) constructed for each neuron. Such matrices included mean numbers of spikes per second in responses to changes of different stimuli pairs. All matrices were subjected to factor analysis, and the basic axes (the main factors) of sensory spaces were revealed. It was found that 16 neurons (30%) detected only achromatic differences between stimuli. Perceptual spaces of these neurons were two-dimensional with brightness and darkness orthogonal axes. The spaces of 12 neurons (22%) were four-dimensional with two chromatic and two achromatic axes. The structure of the perceptual space reconstructed from neuronal spikes was similar to the space calculated from the early VEP components recorded under similar conditions and to another space reconstructed on the basis of rabbit's instrumental learning. The fundamental coincidence of color spaces revealed by different methods may reflect the general principle of vector coding in the visual system and suggests the coexistence of two independent cortical mechanisms of the detection of chromatic and achromatic differences.     

Effects of binaural decorrelation on neural and behavioral processing of interaural level differences in the barn owl (Tyto alba)

The effect of binaural decorrelation on the processing of interaural level difference cues in the barn owl (Tyto alba) was examined behaviorally and electrophysiologically. The electrophysiology experiment measured the effect of variations in binaural correlation on the first stage of interaural level difference encoding in the central nervous system. The responses of single neurons in the posterior part of the ventral nucleus of the lateral lemniscus were recorded to stimulation with binaurally correlated and binaurally uncorrelated noise. No significant differences in interaural level difference sensitivity were found between conditions. Neurons in the posterior part of the ventral nucleus of the lateral lemniscus encode the interaural level difference of binaurally correlated and binaurally uncorrelated noise with equal accuracy and precision. This nucleus therefore supplies higher auditory centers with an undegraded interaural level difference signal for sound stimuli that lack a coherent interaural time difference. The behavioral experiment measured auditory saccades in response to interaural level differences presented in binaurally correlated and binaurally uncorrelated noise. The precision and accuracy of sound localization based on interaural level difference was reduced but not eliminated for binaurally uncorrelated signals. The observation that barn owls continue to vary auditory saccades with the interaural level difference of binaurally uncorrelated stimuli suggests that neurons that drive head saccades can be activated by incomplete auditory spatial information.     

Closing the loop between neurobiology and flight behavior in Drosophila

Fruit flies alter flight direction by generating rapid stereotyped turns called saccades. Using a combination of tethered and free-flight methods, both the aerodynamic mechanisms and the sensory triggers for saccades have been investigated. The results indicate that saccades are elicited by visual expansion, and are brought about by remarkably subtle changes in wing motion. Mechanosensory feedback from the fly's 'gyroscope' complements visual cues to terminate saccades, as well as to stabilize forward flight. Olfactory stimuli elicit tonic increases in wingbeat amplitude and frequency but do not alter the time course or magnitude of visual reflexes.