Influence of eye movements on geniculo-striate excitability in the cat

A dynamic sequence of visual processing initiated by gaze shifts

Animals move their head and eyes as they explore the visual scene. Neural correlates of these movements have been found in rodent primary visual cortex (V1), but their sources and computational roles are unclear. We addressed this by combining head and eye movement measurements with neural recordings in freely moving mice. V1 neurons responded primarily to gaze shifts, where head movements are accompanied by saccadic eye movements, rather than to head movements where compensatory eye movements stabilize gaze. A variety of activity patterns followed gaze shifts and together these formed a temporal sequence that was absent in darkness. Gaze-shift responses resembled those evoked by sequentially flashed stimuli, suggesting a large component corresponds to onset of new visual input. Notably, neurons responded in a sequence that matches their spatial frequency bias, consistent with coarse-to-fine processing. Recordings in freely gazing marmosets revealed a similar sequence following saccades, also aligned to spatial frequency preference. Our results demonstrate that active vision in both mice and marmosets consists of a dynamic temporal sequence of neural activity associated with visual sampling.

Dependence of perceptual saccadic suppression on peri-saccadic image flow properties and luminance contrast polarity

Across saccades, perceptual detectability of brief visual stimuli is strongly diminished. We recently observed that this perceptual suppression phenomenon is jumpstarted in the retina, suggesting that the phenomenon might be significantly more visual in nature than normally acknowledged. Here, we explicitly compared saccadic suppression strength when saccades were made across a uniform image of constant luminance versus when saccades were made across image patches of different luminance, width, and trans-saccadic luminance polarity. We measured perceptual contrast thresholds of human subjects for brief peri-saccadic flashes of positive (luminance increments) or negative (luminance decrements) polarity. Thresholds were >6–7 times higher when saccades translated a luminance stripe or edge across the retina than when saccades were made over a completely uniform image patch. Critically, both background luminance and flash luminance polarity strongly modulated peri-saccadic contrast thresholds. In addition, all of these very same visual dependencies also occurred in the absence of any saccades, but with qualitatively similar rapid translations of image patches across the retina. This similarity of visual dependencies with and without saccades supports the notion that perceptual saccadic suppression may be fundamentally a visual phenomenon, which strongly motivates neurophysiological and theoretical investigations on the role of saccadic eye movement commands in modulating its properties.

Active vision at the foveal scale in the primate superior colliculus

The primate superior colliculus (SC) has recently been shown to possess both a large foveal representation as well as a varied visual processing repertoire. This structure is also known to contribute to eye movement generation. Here, we describe our current understanding of how SC visual and movement-related signals interact within the realm of small eye movements associated with the foveal scale of visuomotor behavior. Within the SC's foveal representation, there is a full spectrum of visual, visual-motor, and motor-related discharge for fixational eye movements. Moreover, a substantial number of neurons only emit movement-related discharge when microsaccades are visually guided, but not when similar movements are generated toward a blank. This represents a particularly striking example of integrating vision and action at the foveal scale. Beyond that, SC visual responses themselves are strongly modulated, and in multiple ways, by the occurrence of small eye movements. Intriguingly, this impact can extend to eccentricities well beyond the fovea, causing both sensitivity enhancement and suppression in the periphery. Because of large foveal magnification of neural tissue, such long-range eccentricity effects are neurally warped into smaller differences in anatomical space, providing a structural means for linking peripheral and foveal visual modulations around fixational eye movements. Finally, even the retinal-image visual flows associated with tiny fixational eye movements are signaled fairly faithfully by peripheral SC neurons with relatively large receptive fields. These results demonstrate how studying active vision at the foveal scale represents an opportunity for understanding primate vision during natural behaviors involving ever-present foveating eye movements.NEW & NOTEWORTHY The primate superior colliculus (SC) is ideally suited for active vision at the foveal scale: it enables detailed foveal visual analysis by accurately driving small eye movements, and it also possesses a visual processing machinery that is sensitive to active eye movement behavior. Studying active vision at the foveal scale in the primate SC is informative for broader aspects of active perception, including the overt and covert processing of peripheral extra-foveal visual scene locations.

Do cross-modal projections always result in multisensory integration?

Convergence of afferents from different sensory modalities has generally been thought to produce bimodal (and trimodal) neurons (i.e., exhibit suprathreshold excitation to more than 1 sensory modality). Consequently, studies identifying cross-modal connections assume that such convergence results in bimodal (or trimodal) neurons that produce familiar forms of multisensory integration: response enhancement or depression. The present study questioned that assumption by anatomically identifying a projection from ferret auditory to visual cortex Area 21. However, electrophysiological recording within Area 21 not only failed to identify a single bimodal neuron but also familiar forms of multisensory integration were not observed either. Instead, a small proportion of neurons (9%; 27/296) showed subthreshold multisensory integration, in which visual responses were significantly modulated by auditory inputs. Such subthreshold multisensory effects were enhanced by gamma-aminobutyric acid antagonism, whereby a majority of neurons (87%; 20/23) now participated in a significant, multisensory population effect. Thus, multisensory convergence does not de facto result in bimodal (or trimodal) neurons or the traditional forms of multisensory integration. However, the fact that unimodal neurons exhibited a subthreshold form of multisensory integration not only affirms the relationship between convergence and integration but also expands our understanding of the functional repertoire of multisensory processing itself.

Somatosensory evoked potentials during the ideation and execution of individual finger movements

Scalp somatosensory evoked potentials (SEPs) were recorded in 10 volunteers after median nerve stimulation, in four experimental conditions of hand movements performance/ideation, and compared with the baseline condition of full relaxation. The experimental conditions were (a) self-improvised hand-finger sequential movements; (b) the same movements according to a read sequence of numbers; (c) mental ideation of finger movements; and (d) passive displacement of fingers in complete relaxation. Latencies and amplitudes of the parietal (N20, P25, N33, and P45) and frontal peaks (P20-22, N30, and P40) were analyzed. Latencies did not vary in any of the paradigms. Among the parietal complexes, only the P25-N33 amplitude was significantly reduced in (a), (b), (c), and (d) and the N20-P25 was reduced in (a) and (d); among frontal waves, N30 and P40 were significantly reduced (20-75%) in (a) and (b). Coronal electrodes showed amplitude decrements maximal at the frontal-rolandic positions contralateral to the stimulated side.

A neuromagnetic study of movement-related somatosensory gating in the human brain

Neuromagnetic fields from the left cerebral hemisphere of five healthy, right-handed subjects were investigated under three different experimental conditions: (1) electrical stimulation of the right index finger (task S); (2) voluntary movement of the same finger (M); (3) M+S condition, consisting of voluntary movements of the right index finger triggering the electrical stimulus at the very beginning of the electromyogram. The three conditions were administered in random order every 5-8 s. In addition, the task somatosensory evoked fields (task SEFs) gathered during condition (1) were compared with control SEFs recorded at the beginning of the experiment during rest. In all subjects the overlay of somatosensory stimulation on movement provoked a decrement in brain responsiveness (gating) as determined by the amplitude of gated SEFs. The latter was found as the difference between the neuromagnetic fields during M+S condition (overlaying of movement and sensory stimulation) minus neuromagnetic fields under M condition (M only). The gating effect was found to begin approximately 30 ms after movement onset, and to last for the whole period of the ongoing movement. The theoretical locus of gating was estimated by dipole localisation of the difference between task SEFs and gated SEFS using a moving dipole model. The site of the "early" gating effect (< 40 ms) was found to be more anteriorly located than the "later" (> 40 ms) gating effect. The task SEFs were found to be larger (significant after 30 ms) than the control SEFs elicited under the basal condition. The results are discussed with respect to timing, mechanism (centrifugal and centripetal), locus and selectivity of gating. In addition, the results are discussed with regard to clinical application (measuring attentional deficits in patients with impairments of higher mental functions and measuring gating deficits in patients with disturbed sensorimotor integration.

Spectral characteristics of blink suppression in normal observers

Previous studies of the characteristics of suppression occurring under various visual conditions show similarities and differences which may indicative of the mechanism of suppression. The primary purpose of this study was to determine if the suppression that occurs in response to an eyelid blink (blink suppression) is similar to that which occurs during a saccade (saccadic suppression). In addition, the characteristics of blink suppression and other forms of suppression (i.e. permanent and binocular rivalry suppression) are compared. A test probe paradigm was utilized to determine the effect of blink suppression on the spectral sensitivity function in three normal observers. Employing a two alternative forced choice technique, thresholds were determined for wavelengths from 420 to 680 nm in 20 nm steps. At each wavelength, the threshold was determined at 0 and 400 msec after the onset of a voluntary blink. The magnitude of suppression was taken as the difference between the 0 and 400 msec thresholds. Similar to saccadic suppression, the magnitude of blink suppression increased as the stimuli biased detection towards the luminance channel. These results suggest that blink suppression and saccadic suppression are the result of a single mechanism. Similarities between blink suppression and other forms of visual suppression are also considered.

Suppression of contrast sensitivity during eyelid blinks

Each blink of the eyelids is associated with a concurrent suppression of vision that lasts as long as 200 msec. Saccadic eye movements are also associated with a concurrent suppression of vision. Previous studies suggested that blink and saccadic suppression may be the result of a single mechanism. Volkmann, Riggs, White and Moore [(1978) Vision Research, 18, 1193-1199] demonstrated that saccadic suppression is most evident for low spatial frequency stimuli. However, the effect of stimulus spatial frequency on blink suppression has not been evaluated. If blink suppression and saccadic suppression result from a single mechanism, then blink suppression should also exhibit its greatest effect at low spatial frequencies. The purpose of this study was to determine the effect of stimulus spatial frequency on blink suppression. The stimulus was a sine-wave grating presented at different times after the blink. Psychometric functions were produced from the data for each post-blink, stimulus onset time and a Weibull function was fit to the data to determine threshold. The magnitude and duration of blink induced contrast sensitivity suppression was found to depend on the spatial frequency of the stimulus employed (similar to saccadic suppression). This is further evidence that a single mechanism may produce both blink induced visual suppression and saccadic suppression.

Facilitation of somatosensory evoked potentials by exploratory finger movements

Modification of somatosensory processing depending on the behavioral setting was studied. Active alternating movements of the fingers, passive tactile stimuli to the hand, and active exploration of objects were performed during recording of somatosensory evoked potentials (SEPs). SEPs were elicited by compound electrical median nerve stimulation and electrical stimulation at detection threshold of cutaneous median nerve fascicles identified by microneurography. Electrical stimulation was not time-locked to the studied condition. In comparison with SEPs at rest there was attenuation of early cortical potentials up to 25 ms post-trigger in all nonresting conditions. In stimulation of the compound median nerve as well as of isolated cutaneous fascicles of a hand actively exploring an object there was an additional increased negativity, peaking at 28 ms. This facilitory effect was independent of attentional focusing and was absent during exploration using the ipsilateral, non-electrically stimulated hand. In patients with parietal lesions the facilitatory effect was diminished on the affected side. Spline interpolated brain maps at this latency based on 32-channel recordings in healthy volunteers showed a shift of local contralateral positive maximum from frontal to parietal during exploration, indicating enhancement of a tangential dipole. It is suggested that in conditions involving close sensorimotor interaction such as exploratory hand movements there is preactivation of a cortical area which is located in the central sulcus and receives cutaneous somatosensory inputs.

Saccade-related responses of centrifugal neurons projecting to the chicken retina

Centrifugal projections to several sensory systems modulate the afferent activity during active behaviors. To see whether such modulation occurred in the visual system, we recorded the activity of isthmo-optic neurons in awake chickens during eye movements. We find that the discharge of all isthmo-optic neurons tends to stop during saccades, although every neuron does not pause for every saccade. The pause begins at approximately the same time as the saccade, and pause duration is correlated with saccade duration. Pausing during saccades occurs in both dark and light suggesting that it is motoric rather that visual in origin. In addition, we find that the spontaneous activity of isthmo-optic neurons increases in darkness. We discuss the significance of the saccadic modulation of isthmo-optic activity in terms of possible functions of the centrifugal projection in modulation of ganglion cell activity.

How does the brain control its own activity? A new function for the basal ganglia

It has long been a problem in neuroscience to known how the brain controls its own activity, how it is able to control the level of CNS excitability and how it is able to select and act on some information as opposed to some other information. In this paper I propose a new theory in which the basal ganglia play a role in selecting information ("selective attention") and in controlling the general level of excitability of the CNS ("state control"), the two processes being to some extent interdependent. The basal ganglia achieve these functions by actions on the thalamic-frontal cortical axis and on the brainstem mesencephalic reticular formation.

Selectivity of attenuation (i.e., gating) of somatosensory potentials during voluntary movement in humans

Attenuation of somatosensory evoked potentials (SEPS) recorded from the scalp during voluntary movement occurs for specific combinations of the finger moved and the peripheral nerve stimulated. The cerebral potential component occurring at a latency of 27 msec (P27) evoked either by stimulation of median nerve at the wrist or by stimulation of 1st and 2nd digit nerves in the fingers were selectively attenuated during movement of 1st digit but were not altered during movement of 5th digit. By contrast, the cerebral P27 component evoked by stimulation of ulnar nerve at the wrist or by stimulation of 5th digital nerve were attenuated during movement of that digit but were not altered during movement of 1st digit. Gating of somatosensory activity is a selective phenomenon occurring when movement involves the areas being stimulated.

Reactions of the geniculate cells to extraocular proprioceptive activation in rabbits

The aim of these investigations was to advance our knowledge of the influence of extrinsic ocular muscle (EOM) stretching on cell excitability of the retinocortical pathway. The initial goal was the lateral geniculate; however, histologic analysis indicated that most cells that responded to stretching were located near its superior edge, in an area analogous to the perigeniculate. Rabbits were anaesthetized and prepared for single-cell recordings. The extraocular muscles were detached from the eye and attached to a rigid tungsten hook. The hook was soldered to the pivot of a galvanometer that was controlled by a waveform generator. Precise and repeated length changes were obtained with extension and relaxation ramps of constant velocity. Thirty percent of the units (N = 250) responded to EOM stretching. These stimuli evoked either an excitation or a decrease of the units' spontaneous activity. Several experimental controls provided evidence that the signals that evoked the geniculate responses originated from proprioceptors belonging to the EOM. Most responding cells had their receptive fields located eccentrically (greater than or equal to 50 degrees). Also, cells that reacted to EOM stretching responded to optic nerve stimulation with a significantly shorter latency than cells that were unresponsive. Pairing the EOM stretching with light stimuli produced the following results. In about half of the units the light-evoked responses augmented, but in 28% of the cells the light-evoked discharges decreased. These effects were obtained even though EOM stretching delivered singly failed to elicit a reaction from the cell. Histologic reconstructions indicated that cells were distributed in a discrete region lying rostral and dorsomedial to the lateral geniculate nucleus. This area has been associated with the perigeniculate nucleus in cats and rats.

Conditioned responses in the visual cortex of dogs. I. During wakefulness

Repetitive electrical stimulation or double shocks of the optic chiasm were used as cue stimuli for alimentary lever-press in dogs. Changes in evoked potentials recorded from the visual cortex were examined by using 'matrix' and component analysis. Peak-to-peak amplitude of components 4 and 5 (C4-5) increased preceding initiation of the lever-press, whereas component 1 or 2 remained unchanged. The enhancement of C4-5 amplitude was suppressed during extinction and reappeared during the reacquisition phase. Possible artifacts and relation between enhancement of EP and lever-press behavior were discussed. The results demonstrate that the enhancement of C4-5 amplitude before the lever-press initiation is the conditioned response in the central nervous system.

Brainstem afferents to the lateral geniculate nucleus of the cat

The ascending connections from the brainstem to the dorsal division of the lateral geniculate nucleus were examined using retrograde transport of horseradish peroxidase. Labelled cells were identified in a variety of structures, including the nucleus of the optic tract (NOT), the posterior pretectal nucleus (NPP), the superior colliculus (SC), the parabigeminal nucleus (PBN), the midbrain reticular formation (MRF), locus coeruleus and nucleus sub-coeruleus, the substantia nigra (SN), and parts of the raphe complex. The projections from NOT, NPP, MRF, LC and PBN were all bilateral in origin. The most intense labelling was observed in the nucleus of the optic tract and the superior colliculus. Colliculo-geniculate cells were located primarily in the superficial gray (lamina II1 and II2 of Kaneseki and Sprague (1974), but sparse labelling was also observed in lamina II3 and in statum opticum (lamina III). Consistent with the report of Harrell et al. (1982), these cells represent a morphologically diverse population, which includes stellate cells, granule cells, and both vertical and horizontal fusiform cells. A similarly diverse population of cell types contributes to the geniculate projection arising from NOT. These results confirm and extend earlier descriptions of the brainstem projections to the cat LGNd, and serve to emphasize the diversity of brainstem influences over the geniculate.

Retinotopic organization of extra-retinal saccade-related input to the visual cortex in the cat

Single unit activity of 842 cells has been recorded in cat visual cortex and analyzed with respect to vestibular induced, and spontaneous saccadic eye movements performed in the dark. This study has been done in awake, chronically implanted cats, subsequently placed in "acute" conditions to achieve the precise retinotopic mapping of the cortical areas previously investigated. In areas 17 and 18, respectively, 27% and 24% of the cells tested were influenced by horizontal saccadic eye movements in the dark (E.M. cells). In the Clare-Bishop area, the proportion of E.M. cells was 12%, while only 2% of such cells were found in areas 19 and 21. The distribution of E.M. cells in areas 17 and 18 with respect to retinotopy showed that E.M. cells were more numerous in the cortical zones devoted to the representation of the area centralis (38% in area 17, 27% in area 18) than in the zones subserving the periphery of the visual field (17% and 12%, respectively). Two of the characteristics of E.M. cell activations appear dependent on the retinotopic organization. First, larger number of E.M. cells presenting an asymmetry in their responses to horizontal saccadic eye movements in opposite directions (directional E.M. cells) were encountered in the cortical representation of the peripheral visual field. 53% of E.M. cells recorded in area 17 and 71% in area 18 were directional in the cortex corresponding to the peripheral visual field. This percentage was of 23% and 25% respectively in the cortex devoted to area centralis. Second, E.M. cells were found to have a latency from the onset of the saccade systematically larger than 100 ms (i.e., they discharged at, or after the end of the eye movement) if they were located in the cortical representation of the area centralis, while E.M. cells related to the peripheral visual field displayed a wider range of latencies (0-240 ms). Results obtained in Clare Bishop area, although limited to the representation of the peripheral visual field, were quantitatively and qualitatively similar to those observed in the homologous retinotopic zones of areas 17 and 18. It is concluded that an extra-retinal input related to oculomotor activity is sent to the cat visual cortex and is organized, at least in areas 17 and 18, with respect to the retinotopic representation of the visual field. These data support the hypothesis of a functional duality between central and peripheral vision and are discussed in the context of visual-oculomotor integration.

Influence of background luminance on visual sensitivity during saccadic eye movements

Detection thresholds were measured for a brief test flash projected on a uniform background before, during, and after saccadic eye movement. The amount and duration of threshold elevation during saccades was directly dependent on back-ground illumination; no significant elevations occurred at backgrounds of 2.0 log fl or less. Similar results were obtained during fixation when the backgrounds were "saccadically" displaced. An occipital evoked potential was recorded in association with both eye movements and background displacements at higher background luminances (no test flash). These results may indicate an activation of a selected population of neural elements - probably the "Y" channels - which occurs during saccades in illuminated environments and which renders the channels less responsive to additional, simultaneous, and appropriately structured stimuli.

Neurology Update: “Recent Developments in the Understanding of the Nervous System”

Neural Mechanisms of Sensori-Motor Integration

New methods of investigation have enlarged understanding of the mechanisms underlying activity in the nervous system. Excitable cells transmit impulses by means of their special membrane properties and excitation is transmitted from cell to cell across specialized sites called synapses. In the nuclei of the central nervous system there are many small neurons that have no axon or only very short axons and dendrites. These are referred to as interneurons and the chemical transmitters they release may be excitatory or inhibitory to the cells with which they synapse. Cells with axons ending in a nucleus and cells with dendrites in the nucleus plus the interneurons which may intervene between the input of the message by an axon reaching the nucleus and its transmission onwards, form networks of cells that act as micro-circuits, affecting the nature of the neural signal. Control of transmission by these networks is the means by which sensory and motor impulses may be modified, enhanced, suppressed or facilitated. The integration of many sensory inputs, and the feedback during movement modulate and shape the motor response.

An understanding of the mechanisms of inhibition and facilitation becomes increasingly important for therapists who use techniques based on “sensori-motor integration“.

Operant conditioning of vertical eye movements without visual feedback in the midpontine pretrigeminal cat

An operant conditioning of vertical eye movements was achieved in the midpontine pretrigeminal cat in total darkness by contingent reinforcement of spontaneous eye movements with lateral hypothalamic (LHT) reward stimulation, when each movement (upward direction was chosen in this experiment) exceeded a preset amplitude. However, the response rates in the dark were lower than those in the light and the time to reach the peak response rate was much longer. Recording of evoked potentials to optic chiasma (OC) stimulation revealed enhancement of late components of the visual cortex (VC) and superior colliculus (SC) responses in relation to eye movements. Sequential records of the averaged evoked responses associated with eye movements indicated that the amplitudes of the late components of the VC and SC waves gradually increased in the course of establishment of the operant conditioning, and decreased gradually during extinction. In a yoked control test, increase in amplitudes of the late components was much less significant during non-contingent reinforcement given independently of the eye movements. These results suggest that 'corollary discharge' may play a critical role as a cue in acquisition of the operant conditioning of vertical eye movements when visual feedback is absent in total darkness.

Electrophysiological correlates of optokinetic and reversed postoptokinetic nystagmus in the rabbit

Six rabbits with implanted electrodes in visual cortex, lateral geniculate body, superior colliculus and the mesodiencephalic nystagmogenic centres were exposed to 90 min optokinetic stimulation, followed by 30 to 60 min observation of reversed postoptokinetic nystagmus (RPN) in darkness. The oculographically recorded nystagmus was used to trigger a LINC 8 computer programmed for perisaccadic EEG averaging and for examination of excitability of visual centres by flashes applied at different intervals after the saccade onset. Whereas optokinetic nystagmus (OKN) was accompanied by marked electrical potentials appearing at all recording sites during and for about 100 msec after the saccade, these potentials were considerably attenuated or absent during RPN. Visual evoked responses to flashes applied during the OKN saccades were considerably smaller than responses to flashes delayed by 130-200 msec. No saccadic suppression was observed when the same visual stimuli were applied during RPN saccades in darkness. It is concluded that eye movements generated in the absence of external stimuli during RPN elicit only weak corollary discharge.

Striate cortex potentials related to eye movements in the light and in darkness in the waking human

Potentials in relation to eye movements were studied by means of direct recording of the striate cortex in a waking man. In a lighted environment, the usual evoked potential--lambda response--was obtained and was clearly visible after each eye movement. In complete darkness no individual potential was observable by means of visual analysis after each eye movement, but a slow potential of low amplitude could be obtained by superimposition and averaging of the cortical striate activity time-locked to the start of a series of eye movements. This eye-movement potential showed a longer latency and a lower amplitude than the lambda response. These data are discussed in reference to those obtained in the cat and the monkey; the significance of this eye-movement potential in darkness as a 'corollary discharge' is considered.

Visual cortical neurons influenced by the oculomotor input: characterization of their receptive field properties

A class of cells in cat visual cortex (area 17 and 18) had a tight correlation with spontaneous and electrically evoked PGO waves under reserpine. They tended to have high maintained activity and a large receptive field which was preferentially excited by fast moving slits. They were also direction selective and influenced through both eyes. For these cells, the selectivity for visual stimuli was dramatically altered in the presence of PGO waves induced by pontine stimulation. They were complex cells. A second class of cortical cells showed a moderate correlation with spontaneous PGO waves. Visually evoked activity of these cells was either excited or inhibited by evoked PGO waves in response to pontine stimulation. A third class of cells, the majority, did not seem to have any correlation with PGO waves. The second and third classes of cells could be either one of the 3 main categories of visual cortical cells, predominantly simple cells in area 17 and complex cells in area 18. The present study provided further support to a previous proposal that PGO waves in the visual cortex, as neural correlates of saccadic eye movements, modulate specifically the ongoing activity of a selective type of cortical cell. A type of complex cell in the deeper layers is supposed to integrate visual and oculomotor inputs. It is hypothesized that the consequence of oculomotor inputs to the visual cortex is specified by the type of cells which receive oculomotor inputs, rather than by the nature of the inputs themselves.

Quantitative studies of intracellular postsynaptic potentials in the lateral geniculate nucleus of the cat with respect to optic tract stimulus response latencies

LGN cells were intracellularly recorded with glass micropipettes. Electrical stimuli of different amplitude and frequency were applied to the optic tract close to the optic chiasm. The cells were classified according to stimulus response latencies of action potentials as belonging to class I (1.0-16 msec) Or class II (1.7-3.0 MSEC). Class I EPSPs had shorter latencies (1.0-1.5 msec), durations (4-12 msec), rise times to peak (0.5-1.4 msec), and decay times (3.0-8.5 msec); the synaptic transmission time was on the average 0.41 msec. Class II EPSPs (1.6-2.6 msec latency) had longer durations (10-30 msec), rise times (1.6-3.7 msec), and decay times (9.0-25 msec); the synaptic transmission time was on the average 0.67 msec. With repetitive stimulation the EPSPs of latency class I revealed almost no stimulus frequency dependence between 1 and 120 HZ, while class 22 EPSPs decrease in amplitude between 30 and 70% with increasing frequency. Comparable temporal summation of excitation occurred in cells of both latency classes. Negative serial correlation coefficients of first order were found for consecutive EPSP amplitudes of all cells recorded for sufficient periods of time. The IPSPs were subdivided into two groups according to their optic tract response latency. Group 1 IPSPs had shorter latencies (2.0-2.6 msec), durations (15-50 msec), and times from the onset to maximal hyperpolarization (2.4-4.2 msec) than group 2 IPSPs (3.0-4.8 msec latency, 40-100 msec duration, 2.7-7.5 msec time from onset to extremum). The group 2 IPSPs decreased in amplitude by about 90% when the stimulus frequency was increased form 1 to 50 HZ, while the group 1 IPSPs displayed a comparable decrease in the frequency range between 50 and 120 HZ. Effective temporal summation was found in group 2 IPSPs in the frequency range below 70 HZ, and in group 1 IPSPs at stimulus frequencies between 70 and 120 HZ. The EPSP peak latencies and the latencies to the minimum of IPSPs proved to be invariant with respect to PSP amplitude and stimulus fre quency in individual cells. The latencies to the extrema of EPSPs and IPSPs as well as the amplitude values were symmetrically distributed.

Influence of saccadic eye movements on geniculostriate excitability in normal monkeys

Using permanently implanted electrodes in squirrel monkeys and macaques, transmission through the lateral geniculate nucleus (LGN) was assayed from the amplitude of potentials evoked in optic radiation by and electrical pulse applied to optic tract. Averaging of either individually or machine selected potentials, elicited at 0.3, 1.0, 20 or 50 HZ, in all cases showed a decrease in transmission ranging from 5-60% in the period after saccadic eye movements made ad libitum. The suppression was greater in a patterned visual environment than in diffuse illumination, which in turn was greater than that occurring following saccades in the dark. Demonstration of the effect in darkness always required data averaging and never exceeded 20%. The effect was consistently greater in the magnocellular than parvocellular component. Suppresion was often abruptly terminated and replaced by a facilitation of 5-15% about 100 msec after saccade detection. Comparable effects were observed for excitability of striate cortex tested by a stimulus pulse applied to optic radiation. In addition, sharply demarcated potentials inherently arising in LGN and striate cortex were found in association with saccades made even in total darkness. Neglecting a possible but dubious contribution from eye muscle proprioceptors, the experiments establish the existence of a centrally originating modulation of visual processing at both LGN and striate cortex in ralation to saccadic eye movement in primates. This modulation may partially underlie the phenomenon of "saccadic suppression" and hasten the acquistion of a meaningful visualsample immediately following an ocular saccade. It remains uncertain as to how it may relate to similar or greater effects accompanying changes in alertness, or to fluctuations of unknown origin occurring sometimes semirhythmically at 0.05-0.03 HZ (Fig 7).

Effects of frontal eye field stimulation upon activities of the lateral geniculate body of the cat

Effects of electrical stimulation of the frontal eye field (FEF) upon activites of the lateral geniculate body (LG) were studied in encephale isole cats. In some experiments the effects were examined by recording field responses of the dorsal nucleus of LG (LGd) and the visual cortex (VC) to electrical stimulation of the optic chiasm (OX). Conditioning repetitive stimulation of FEF exerted no significant effects on the r1 wave of LGd responses but had a facilitatory effect on the r2 wave. FEF-induced facilitation of VC responses was prominent in the late postsynaptic components. These effects had latencies of 50-100 msec and durations of 200-500 msec. Transection of the midbrain showed that most of the FEF-effect was not mediated via the brainstem reticular formation. Extracellular unitary recordings were made from 125 neurons, of which 91 were LGd neurons, 23 neurons of the caudal part of the thalamic reticular nucleus (TRc) and 11 neurons of the ventral nucleus of LG (LGv). In 30 to 87 LGd relay neurons FEF stimuli increased response probabilities to OX stimuli and their spontaneous discharges. These FEF-facilitated LGd neurons were distinguished from the non-affected ones in that the former had longer OX-latencies than the latter. The FEF-facilitated neurons probably correspond to "X" neurons of LGd. In 17 TRc neurons the effects were inhibitory. Their time courses were similar to those of the facilitation in the LGd relay neurons. Seven LGv neurons recieved facilitaroy effects from FEF. Among them 5 neurons showed short-latency (6.7-17 msec) responses to FEF single shocks. The FEF sites inducing conjugate lateral eye movements exerted stronger facilitatory effects than those inducing upward or centering eye movements did. It is suggested that the effects may subserve to cancel the inhibitory convergence onto X-cells just after saccadic eye movements so as to improve visual information transmission through LGd during the eye fixation.

Discharges of relay cells in lateral geniculate nucleus of the cat during spontaneous eye movements in light and darkness

1. Discharges of 315 relay cells of the lateral geniculate nucleus (LGN) during spontaneous eye movements were studied in alert cats. 2. When tested in a stationary patterned field, 114 cells showed sustained discharges related to the direction of gaze (S cells) and to local differences in luminance; 109 cells showed transient response to quick shifts of retinal image during saccades (T cells); ninety-two cells showed mixed responses (M cells), i.e. transient responses to rapid shifts of retinal image and sustained firing related to local differences in luminance. 3. Following saccades occurring in the light, T and M cells showed a burst discharge, while spontaneous discharges of S cells were completely suppressed for 150-200 msec. 4. When tested in total darkness, modifications in activity which were apparent in light disappeared completely. This was true for all 315 relay cells. 5. T cells responded to optic chiasm stimulation at shorter latencies (X = 1.15 msec) than S cells (X = 1.77 msec). M cells showed a latency distribution in between those for S and T cells with a mean latency 1.40 msec. 6. When tested with moving grating stimulation, S cells responded in only one manner; with discharges to each stripe of the grating (primary response), while T and M cells showed two different responses: a primary response to a slower motion and a non-specific burst in response to a faster motion. The burst did not reflect the stimulus pattern (secondary response). 7. When tested with diffuse light switched on and off over the tangent screen, S cells showed a sustained response either to light or darkness, whereas T and M cells responded transiently either to the onset or offset of the light, or to both. M cells occasionally showed a mixture of transient and sustained responses either to light or darkness. 8. In over-all response properties, most S cells correspond to X (sustained) cells and most T cells to Y (transient) cells previously known from acute experiments. M cells had intermediate response properties between X and Y cells. 9. Functional roles of these classes of cells in relation to previously proposed functions are discussed.

Depression in the excitability of relay cells of lateral geniculate nucleus following saccadic eye movements in the cat

1. The excitability of relay cells of the lateral geniculate nucleus during a saccadic eye movement was studied in alert cats. Excitability was assessed by the firing probability of the cells in response to electrical stimulation of the optic chiasm. Modifications in the excitability were evaluated during the period following eye movements, by triggering a stimulator from potential shifts in electro-oculogram and altering delays in the stimulus pulse. 2. The cells were classified into S and T cells, based on their response properties and the latencies to chiasmatic stimulation. With a saccade in a stationary patterned field, T cells showed a burst discharge, while the discharges of S cells were completely suppressed. 3. The excitability was depressed in both S and T cells for 150-200 msec after a saccade, when the eye movement occurred in light. However, the depression did not occur in complete darkness. 4. The depression occurred also in the absence of eye movement, when the patterned visual field was moved in a saccadic fashion. 5. The depression in S cells occurred during an inhibitory period. Since S cells do not receive signals on image movement directly from the retina, the depression was due to a recurrent inhibition by signals transferred through the T ganglion-relay cell channel. 6. The depression in T cells occurred concomitantly with the burst discharge. Since the recurrent inhibition was operating less effectively during the period, the depression may be due to a phasic occlusion of the test impulse by coincident high-rate firings in the same cell. 7. The impairment in transmission of visual information through the lateral geniculate nucleus during the period following eye movements has been discussed in connexion with a neurophysiological basis for saccadic suppression.

Eye movement-related inhibition of primate visual neurons

The influence of saccadic eye movements (EM) upon spontaneous neuronal activity was studied in the lateral geniculate nucleus (LGN) and striate visual cortex (VC) of encéphale isolé monkeys. EM were spontaneous and occurred in total darkness to eliminate the effects of retinal image displacement. The activity of LGN cells was not altered in association with EM. In contrast, 76% of cells studied in VC displayed a period of inhibition related to spontaneous EM in total darkness. EM-related inhibition of VC neurons was directionally specific; for each cell there was one quadrant of EM direction for which inhibition was most prominent. The majority of VC neurons showed inhibition in relation to EM directed into only one quadrant of the visual field. Reliable detection of EM-related inhibition required the formation of average histograms of neuronal firing time-locked to EM. For individual EM (even of optimum direction), a consistent degree of inhibition was not seen. The time course of EM-related inhibition of VC neurons is consistent with that reported for saccadic suppression. These results support the concept of a central mechanism (corollary discharge) acting at the cortical level being of significance in saccadic suppression.

Sustained and transient discharges of retinal ganglion cells during spontaneous eye movements of cat

Discharges of 223 retinal ganglion cells during spontaneous eye movements (saccades) across a stationary grating pattern were studied in chronically prepared cats. Of these 83 showed sustained responses to local differences in luminance of the grating stripes (S-units); 84 showed transient responses to saccades and did not register local differences in luminance (T-units); and 56 showed mixed responses, i.e., transient responses to saccades and sustained firings in response to local luminance (M-units). When tested with diffuse light, 93.9% of the S-units showed either ON-sustained or OFF-sustained responses; 95.2% of the T-units showed either ON-transient, OFF-transient, or ON-OFF-transient responses; and 50% of the M-units showed ON-OFF responses. In the overall responses properties, most S-units corresponded to the X-cells, most T-units to the Y-cells of retinal ganglion cells previously known from acute experiments. Under normal conditions of active eye movements, the major function of the S-units would be to register the differences in luminance in their receptive fields, and subserve the mechansim of form recognition. The major function of the T-units would be to register information related to quick image motion, induced either by eye or object movements, and subserve the mechanism of detecting the dynamic aspects of visual stimuli. The other important functions of the T-units are their possible participation in the afferent routes for two recently proposed mechanisms; one for goal-directed saccades and the other for saccadic suppression. The M-units would possess the functions of both S- and T-units.