Vertical and torsional optokinetic eye movements in the rabbit

Summary Rabbits were placed inside a striped drum, which was rotated at selected constant speeds around the animal's sagittal or bitemporal axis. Eye position was recorded by means of the scleral search coil system. A regular vertical or rotatory optokinetic nystagmus (OKN) was constantly obtained. The ratioslow phase eye velocity/drum velocity (=gain) amounted to 0.7–0.9 for stimulus velocities up to 1°/sec, and declined progressively for higher stimulus velocities. The overall input-output relations for torsional and vertical OKN were very similar to those found previously for horizontal OKN. Upward and downward motion were equally effective as a stimulus for each eye apart. The same was true for nasal and temporal rotation.In darkness, rotatory and vertical drift of the eye was seen, as described before for the horizontal plane. These findings support the hypothesis that the OKN system stabilizes the eyes on the (non-rotating) visual surroundings.It is proposed that vertical, torsional as well as horizontal OKN are mediated by sub-sets of similar retinal direction-selective cells as described in the literature.     

Responses to moving visual stimuli in pretectal neurons of the small-spotted dogfish (Scyliorhinus canicula)

Single-unit recordings were performed from a retinorecipient pretectal area (corpus geniculatum laterale) in Scyliorhinus canicula. The function and homology of this nucleus has not been clarified so far. During visual stimulation with a random dot pattern, 45 (35%) neurons were found to be direction selective, 10 (8%) were axis selective (best neuronal responses to rotations in both directions around one particular stimulus axis), and 75 (58%) were movement sensitive. Direction-selective responses were found to the following stimulus directions (in retinal coordinates): temporonasal and nasotemporal horizontal movements, up- and downward vertical movements, and oblique movements. All directions of motion were represented equally by our sample of pretectal neurons. Additionally we tested the responses of 58 of the 130 neurons to random dot patterns rotating around the semicircular canal or body axes to investigate whether direction-selective visual information is mapped into vestibular coordinates in pretectal neurons of this chondrichthyan species. Again all rotational directions were represented equally, which argues against a direct transformation from a retinal to a vestibular reference frame. If a complete transformation had occurred, responses to rotational axes corresponding to the axes of the semicircular canals should have been overrepresented. In conclusion, the recorded direction-selective neurons in the Cgl are plausible detectors for retinal slip created by body rotations in all directions.     

Role of the pretectal nucleus of the optic tract in short-latency ocular following responses in monkeys

When a large-field image is suddenly moved in front of an observer, an ocular following response (OFR) with short latency (<60 ms in monkey and <85 ms in human) is observed. Previous studies have shown that neurons in the pretectal nucleus of the optic tract (NOT) of the monkey respond to movements of large-field visual stimuli. To understand the potential role of the NOT in the OFR, we first recorded single-unit activity in the NOT of four monkeys (Macaca fuscata). Sixty-six NOT neurons preferred large-field ipsiversive visual motion. In 86% (49/57) of the neurons, optimal directions were distributed over +/-30 degrees from ipsilateral. NOT units were sensitive to the speed of the visual motion; 54% (27/50) preferred slow (< or =20 degrees/s), 22% (11/50) preferred fast (> or =80 degrees/s) and the remainder intermediate speeds. Their response latencies to the moving visual scene were very short (approximately 51 ms), and 44% of them led the onset of the OFR by 10 ms or more. To characterize the response properties of these neurons, we reconstructed the temporal firing patterns of 17 NOT neurons, using the acceleration, velocity, position and bias components of retinal image slip or eye movements during the OFR by a least squares error method. For each stimulus speed fitting condition, using either retinal slip or eye movements, their firing patterns were matched to some extent although the goodness of fit was better using retinal slip than when eye movements were used. Neither of these models could be applied independently of stimulus speed, suggesting that the firing pattern of the NOT neurons represented information associated with retinal slip or eye movements during the OFR, over a limited range. To provide further evidence that the NOT is involved in generating the OFR, we placed unilateral microinjections of muscimol into the NOT. Following the muscimol injection, we observed a approximately 50% decrease in eye velocity of the OFR toward the side of injection regardless of stimulus speed, while only a weak effect was observed in the OFR during contraversive or vertical image motion. These results suggest that the NOT may play a role in the initiation and support of the short-latency ocular following response.     

Response properties of dorsolateral pontine units during smooth pursuit in the rhesus macaque

1. The anatomical connections of the dorsolateral pontine nucleus (DLPN) implicate it in the production of smooth-pursuit eye movements. It receives inputs from cortical structures believed to be involved in visual motion processing (middle temporal cortex) or motion execution (posterior parietal cortex) and projects to the flocculus of the cerebellum, which is involved in smooth pursuit. To determine the role of the DLPN in smooth pursuit, we have studied the discharge patterns of 191 DLPN neurons in five monkeys trained to make smooth-pursuit eye movements of a spot moving either across a patterned background or in darkness. 2. Four unit types could be distinguished. Visual units (15%) discharged in response to movement of a large textured pattern, often in a direction-selective fashion but not during smooth pursuit of a spot in the dark. Eye movement neurons (31%) discharged during sinusoidal smooth pursuit in the dark with peak discharge rate either at peak eye position or peak eye velocity, but they showed no response during background movement or during other visual stimulation. These units continued to discharge when the target was extinguished (blanked) briefly, and the monkey continued to make smooth eye movements in the dark. The majority (54%) of our DLPN units discharged during both smooth pursuit in the dark and background movement while the monkey fixated. Blanking the target during smooth pursuit revealed that these units fell into two distinct classes. Visual pursuit units ceased discharging during a blank, suggesting that they had only a visual sensitivity. Pursuit and visual units continued to discharge during the blank, indicating that they had a combined oculomotor and visual sensitivity. 3. Ninety-five percent of the units that discharged during smooth pursuit were direction selective. These units had rather broad directional tuning curves with widths at half height ranging from 65 to 180 degrees. Many preferred directions for DLPN units were observed, although the vertical and near-vertical directions predominated. 4. Most units that responded to large-field background movement were direction selective. During sinusoidal movement of a large-field background, half of them also discharged in relation to stimulus velocity, whereas others did not.     

Neuronal responses in the visual cortex of awake cats to stationary and moving targets

Summary Over 300 single units from the visual cortex (within and around the projection of the central area) were recorded from awake and non-paralyzed cats (chronic preparation). Spontaneous activity of 25% of the neurons was below 3/sec, that of 75% above 3/sec (mean 7.65 spikes/sec). Diffuse illumination had only little influence, but nearly all neurons responded to stimulation with some sort of visual contrast. This would be either an irregularly moved shadow on the screen with irregular boundaries (e. g. a hand with moving fingers), a dark stripe moving in a certain direction, stationary parallel gratings with a certain orientation, or saccadic eye movements across a checkerboard. Although some neurons responding to one stimulus type could also be responsive to other stimuli, the majority of units only responded to one stimulus type. The responses to stationary gratings (alternating parallel dark and bright stripes) and to moving dark stripes are described in detail. Responses to stationary gratings showed no adaptation. The orientation of the grating stripes was critical for each neuron, the optimal and minimal response orientation were separated by about 90°. For movement sensitive neurons, the direction of the movement was critical. Most neurons had only one, some had two preferred directions separated by 180°. No statistically significant predominance of certain orientation or direction preferences was found. The preferred target velocity of movement sensitive neurons was between 10 and 60°/sec, above 80–100°/sec only occasional or no responses could be elicited. Neurons which responded to saccadic eye movements (above 300°/sec) in the presence of a checker board, usually did not respond to slower target movements below 100°/sec.The results support the view that the visual system has different channels for the perception of moving and of stationary objects.This work was supported by the Deutsche Forschungsgemeinschaft as a research project in the Sonderforschungsbereich Kybernetik (SFB 31).Dr. R.B. Freeman, jr., was supported by NIH-grant 363-93600-21, MF-428-69 during the period of this research.     

Neural correlates of reafference: evoked brain activity during motion perception and saccadic eye movements

The ability to perceive a stable visual environment despite eye movements and the resulting displacement of the retinal image is a striking feature of visual perception. In order to study the brain mechanism related to this phenomenon, an EEG was recorded from 30 electrodes spaced over the occipital, temporal and parietal brain areas while stationary or moving visual stimuli with velocities between 178 degrees/s and 533 degrees/s were presented. The visual stimuli were presented both during saccadic eye movements and with stationary eyes. Stimulus-related potentials were measured, and the effects of absolute and relative stimulus velocity were analyzed. Healthy adults participated in the experiments. In all 36 subjects and experimental conditions, four potential components were found with mean latencies of about 70, 140, 220 and 380 ms. The latency of the two largest components between 100 and 240 ms decreased while field strength increased with higher absolute stimulus velocity for both stationary and moving eyes, whereas relative stimulus velocity had no effect on amplitude, latency and topography of the visual evoked potential (VEP) components. If the visual system uses retinal motion information only, we would expect a dependence upon relative velocity. Since field strength and latency of the components were independent of eye movements but dependent upon absolute stimulus velocity, the visual cortex must use extraretinal information to extract stimulus velocity. This was confirmed by the fact that significant topographic changes were observed when brain activity evoked during saccades and with stationary eyes was compared. In agreement with the reafference principle, the findings indicate that the same absolute visual stimulus activates different neuronal elements during saccades than during fixation.     

Single-unit responses to whole-field visual stimulation in the pretectum of Rana pipiens

Visually responsive single units were recorded from the pretectal region that includes the large-celled nucleus lentiformis mesencephali (nLM) in the leopard frog, Rana pipiens. During monocular stimulation of the contralateral eye, 60 single units responding to movement of a large-field., random-dot pattern were quantitatively analyzed using horizontal and vertical directions at each of four pattern velocities (0.4–40°/s). All units were spontaneously active, motion sensitive, and the majority showed ‘on’-‘off’ responses. Several different response profiles were observed, including velocity-sensitive units with peak response at 10°/s, most of which showed directional selectivity, and speed-sensitive units that showed increasing spike frequencies as pattern velocity increased, but little or no directional selectivity. About one-third of all units analyzed were direction-selective, and 55% of those responded optimally to the temporal-to-nasal (T-N) direction of motion. T-N units were recorded primarily from an area that lies dorsolaterally between nLM and the optic tectum, in the ‘peri-nLM’ region. The pronounced monocular optokinetic nystagmus (OKN) response asymmetry that occurs in anurans appears to be reflected in the response profiles of the T-N direction selective units.     

A quantitative analysis of the direction-specific response of neurons in the cat's nucleus of the optic tract

Summary All cells in the nucleus of the optic tract (NOT) of the cat, that Bcould be activated antidromically from the inferior olive, were shown to be direction-specific, as influenced by horizontal movements of an extensive visual stimulus. Cells in the left NOT were activated by leftward and inhibited by rightward movement, while those in the right NOT were activated by rightward and inhibited by leftward movement. Vertical movements did not modulate the spontaneous activity of the cells. The mean spontaneous discharge rate in 50 NOT cells was 30 spikes/s.This direction-specific response was maintained over a broad velocity range (<0.1 ° – >100 °/s). Velocities over 200 °/s could inhibit NOT cells regardless of stimulus direction.All cells in the NOT were driven by the contralateral eye, about half of them by the ipsilateral eye also. In addition, activation through the contralateral eye was stronger in most binocular units. Binocular cells preferred the same direction in the visual space through both eyes.An area approximately corresponding to the visual streak in the cat's retina projected most densely onto NOT cells. This included an extensive ipsilateral projection. No clear retinotopic order was seen. The most sensitive zone in the very large receptive fields (most diameters being >20 °) was along the horizontal zero meridian of the visual field.The retinal input to NOT cells was mediated by W-fibers.The striking similarities between the input characteristics of NOT-cells and optokinetic nystagmus are discussed. The direction selectivity and ocular dominance of the NOT system as a whole can provide a possible explanation for the directional asymmetry in the cat's optokinetic nystagmus when only one eye is stimulated.This work was supported by DFG-Grants No 450/3 and 450/7 to K.-P. Hoffmann     

Direction-selective single units in the nucleus lentiformis mesencephali of the pigeon (Columba livia)

Summary The receptive field properties of single units within the nucleus lentiformis mesencephali (LM) of the pigeon were studied using electrophysiological methods. Previous studies have suggested that the avian LM may be homologous to the nucleus of the optic tract (NOT) in mammals. Single units in the pigeon LM are similar to mammalian NOT units in that they are direction-selective, mostly for horizontal directions, velocity-selective, have large visual receptive fields and respond preferentially to large stimuli with many visual contrasts. In contrast to most reports of NOT units of mammals, more than half of pigeon LM units prefer high velocities (>10°/s), a large proportion (0.37) prefer non-horizontal directions, and receptive fields that are retinotopically arranged within the LM. The response properties of pigeon LM units are compared to the response properties of units within the accessory optic nucleus (the nucleus of the basal optic root or nBOR). In the avian brain, nBOR neurons respond at low velocities (0.5–5°/s) and respond predominantly to vertical stimulus movement whereas LM units respond over a broader range of velocities (0.2–80°/s) and respond predominantly to horizontal movements. Thus, the LM and nBOR may play different roles in the control of compensatory eye movements.This work was supported in part by PHS grant EY03638 to BJW and NSF grant BNS 8312571 to SEB     

Neuronal activity during eye movements in a visual association area of cat cerebral cortex

Summary 270 single neurons from the anterior part of the middle suprasylvian gyrus (AMSS) were recorded in awake and non-paralyzed cats (Chronic preparation).10% were unresponsive to visual stimulation, the remainder reacted well to moving visual stimuli. Half of the units tested were directionally selective. Horizontal, or downward preferred directions predominated. Most neurons were relative insensitive to changes of shape, orientation, contrast, and velocity of the visual stimulus. Some neurons preferred rapid (100°/sec) jerky movements, others required complex motions of irregular shapes, a few strongly preferred objects moving towards the animal in the midsagittal plane. 40% of neurons yielded phasic On-Off reaction to flashing stationary spots.Habituation to repeated stimulation was a common feature and occured in 50% of AMSS neurons. In 19% of neurons tested the discharge rate was not affected by saccadic eye movements, when the animal faced a patterned background. Among the remainder two types of saccade associated responses could be distinguished. Type I discharged prior to or simultaneously with the onset of saccades. This early response was usually associated with saccades of particular directions. Saccades in total darkness yielded weaker and less consistent responses. Type II discharged subsequent to the onset of the saccades after a latency of 40 msec (type IIa), 40–80 msec (type IIb) and 80 msec (type IIc). Responses of type IIa are probably consequences of the retinal effects of eye movements.The saccade associated responses of type Ia, IIb and IIc are tentatively interpreted as results of an eye movement-synchroneous subcortical input, which facilitates transmission in AMSS neurons. Presaccadic facilitation, which generates type Ia responses, may be functionally related to shifts of attention prior to eye movements. It is suggested that postsaccadic facilitation, which underlies the reactions of type IIb and IIc, may be a correlate of visual attention during the fixation period.     

Perceived motion direction during smooth pursuit eye movements

Although many studies have been devoted to motion perception during smooth pursuit eye movements, relatively little attention has been paid to the question of whether the compensation for the effects of these eye movements is the same across different stimulus directions. The few studies that have addressed this issue provide conflicting conclusions. We measured the perceived motion direction of a stimulus dot during horizontal ocular pursuit for stimulus directions spanning the entire range of 360 degrees. The stimulus moved at either 3 or 8 degrees/s. Constancy of the degree of compensation was assessed by fitting the classical linear model of motion perception during pursuit. According to this model, the perceived velocity is the result of adding an eye movement signal that estimates the eye velocity to the retinal signal that estimates the retinal image velocity for a given stimulus object. The perceived direction depends on the gain ratio of the two signals, which is assumed to be constant across stimulus directions. The model provided a good fit to the data, suggesting that compensation is indeed constant across stimulus direction. Moreover, the gain ratio was lower for the higher stimulus speed, explaining differences in results in the literature.     

Discharge patterns of neurons in the pretectal nucleus of the optic tract (NOT) in the behaving primate

1. To determine the possible role of the primate pretectal nucleus of the optic tract (NOT) in the generation of optokinetic and smooth-pursuit eye movements, we recorded the activity of 155 single units in four behaving rhesus macaques. The monkeys were trained to fixate a stationary target spot during visual testing and to track a small moving spot in a variety of visual environments. 2. The majority (82%) of NOT neurons responded only to visual stimuli. Most units responded vigorously for large-field (70 x 50 degrees) moving visual stimuli and responded less, if at all, during smooth-pursuit eye movements in the dark; many of these units had large receptive fields (greater than 10 x 10 degrees) that included the fovea. The remaining visual units responded more vigorously during smooth-pursuit eye movements in the dark than during movement of large-field visual stimuli; all but one had small receptive fields (less than 10 x 10 degrees) that included the fovea. For all visual units that responded during smooth pursuit, extinction of the small moving target so briefly that pursuit continued caused the firing rates to drop to resting levels, confirming that the discharge was due to visual stimulation of receptive fields with foveal and perifoveal movement sensitivity and not to smooth-pursuit eye movements per se. 3. Eighteen percent of all NOT units ceased their tonic discharge in association with all saccades including the quick phases accompanying optokinetic or vestibular nystagmus. The pause in firing began after saccade onset, was unrelated to saccade duration, and occurred even in complete darkness. 4. Most (90%) of the visual NOT units were direction selective. They exhibited an increase in firing above resting during horizontal (ipsilateral) background movement and/or during smooth pursuit of a moving spot and a decrease in firing during contralateral movement. 5. The firing rates of NOT units were highly dependent on stimulus velocity. All had velocity thresholds of less than 1 degree/s and exhibited a monotonic increase in firing rate with visual stimulus velocity over part (n = 90%) or all (n = 10%) of the tested range (i.e., 1-200 degrees/s). Most NOT units exhibited velocity tuning with an average preferred velocity of 64 degrees/s.(ABSTRACT TRUNCATED AT 400 WORDS)     

Alterations in response properties in the lateral and dorsal terminal nuclei of the cat accessory optic system following visual cortex lesions

Summary The response properties of cells in the lateral (LTN) and dorsal (DTN) terminal nuclei of the accessory optic system (AOS) were examined in 14 cats which underwent unilateral visual cortex ablation. Following decortication, single units in the LTN and DTN no longer showed the high degree of binocular convergence characteristic of the intact animal, but instead LTN and DTN units became almost completely dominated by the contralateral eye. In addition, responsivity of LTN and DTN cells to high stimulus velocities was abolished by removal of cortical input. This decrement in high velocity response was observed in both the excitatory and the inhibitory components of the velocity response profile.While the incidence of direction selective neurons in both the LTN or the DTN was not affected by decortication, the distribution of preferred and non-preferred directions was dramatically altered in the LTN, and to a lesser extent in the DTN. In the LTN, there was a severe reduction in the number of cells which displayed maximal excitation for upward stimulus motion. Instead, most LTN units in the decorticate cat preferred downward directed stimulus motion. In the DTN, most units still preferred horizontal stimulus motion as in the intact animal, but the overall distribution of preferred directions displayed a clear downward vertical vector component. In other respects, such as receptive field size and position in visual space, on/off responses, and resting discharge rate, LTN and DTN units appeared unaffected by cortical lesions.These experiments demonstrate that the cortical input to the LTN and DTN plays a highly significant role in the formation of response properties of cells located in these nuclei. The results presented in this report indicate that the visual cortex is a major source of ipsilateral eye input, high velocity responses, and upward direction selectivity for the AOS units examined in these experiments.     

Pretectal neurons optimized for the detection of saccade-like movements of the visual image

The visual response properties of nondirectional wide-field sensitive neurons in the wallaby pretectum are described. These neurons are called scintillation detectors (SD-neurons) because they respond vigorously to rapid, high contrast visual changes in any part of their receptive fields. SD-neurons are most densely located within a 1- to 2-mm radius from the nucleus of the optic tract, interspersed with direction-selective retinal slip cells. Receptive fields are monocular and cover large areas of the contralateral visual field (30--120 degrees ). Response sizes are equal for motion in all directions, and spontaneous activities are similar for all orientations of static sine-wave gratings. Response magnitude increases near linearly with increasing stimulus diameter and contrast. The mean response latency for wide-field, high-contrast motion stimulation was 43.4 +/- 9.4 ms (mean +/- SD, n = 28). The optimum visual stimuli for SD-neurons are wide-field, low spatial frequency (<0.2 cpd) scenes moving at high velocities (75--500 degrees /s). These properties match the visual input during saccades, indicating optimal sensitivity to rapid eye movements. Cells respond to brightness increments and decrements, suggesting inputs from ON and OFF channels. Stimulation with high-speed, low spatial frequency gratings produces oscillatory responses at the input temporal frequency. Conversely, high spatial frequency gratings give oscillations predominantly at the second harmonic of the temporal frequency. Contrast reversing sine-wave gratings elicit transient, phase-independent responses. These responses match the properties of Y retinal ganglion cells, suggesting that they provide inputs to SD-neurons. We discuss the possible role of SD-neurons in suppressing ocular following during saccades and in the blink or saccade-locked modulation of lateral geniculate nucleus activity to control retino-cortical information flow.     

Retrograde transneuronal transport of wheat-germ agglutinin to the retina from visual cortex in the cat

Summary The stability of visual perception despite eye movements suggests the existence, in the visual system, of neural elements able to recognize whether a movement of an image occurring in a particular part of the retina is the consequence of an actual movement that occurred in the visual field, or self-induced by an ocular movement while the object was still in the field of view. Recordings from single neurons in area V3A of awake macaque monkeys were made to check the existence of such a type of neurons (called real-motion cells; see Galletti et al. 1984, 1988) in this prestriate area of the visual cortex. A total of 119 neurons were recorded from area V3A. They were highly sensitive to the orientation of the visual stimuli, being on average more sensitive than V1 and V2 neurons. Almost all of them were sensitive to a large range of velocities of stimulus movement and about one half to the direction of it. In order to assess whether they gave different responses to the movement of a stimulus and to that of its retinal image alone (self-induced by an eye movement while the stimulus was still), a comparison was made between neuronal responses obtained when a moving stimulus swept a stationary receptive field (during steady fixation) and when a moving receptive field swept a stationary stimulus (during tracking eye movement). The receptive field stimulation at retinal level was physically the same in both cases, but only in the first was there actual movement of the visual stimulus. Control trials, where the monkeys performed tracking eye movements without any intentional receptive field stimulation, were also carried out. For a number of neurons, the test was repeated in darkness and against a textured visual background. Eighty-seven neurons were fully studied to assess whether they were real-motion cells. About 48% of them (42/87) showed significant differences between responses to stimulus versus eye movement. The great majority of these cells (36/42) were real-motion cells, in that they showed a weaker response to visual stimulation during tracking than to the actual stimulus movement during steady fixation. On average, the reduction in visual response during eye movement was 64.0 ± 15.7% (SD). Data obtained with a uniform visual background, together with those obtained in darkness and with textured background, indicate that real-motion cells receive an eye-motion input, either retinal or extraretinal in nature, probably acting presynaptically on the cell's visual input. In some cases, both retinal and extraretinal eye-motion inputs converge on the same real-motion cell. No correlation was observed between the real-motion behaviour and the sensitivity to either orientation or direction of movement of the visual stimulus used to activate the receptive field, nor with the retinotopic location of the receptive field. We suggest that the visual system uses real-motion cells in order to distinguish real from self-induced movements of retinal images, hence to recognize the actual movement in the visual field. Based on psychophysical data, the hypothesis has been advanced of an internal representation of the field of view, stable despite eye movement (cf. MacKay 1973). The real-motion cells may be neural elements of this network and we suggest that the visual system uses the output of this network to properly interpret the large number of sensory changes resulting from exploratory eye movements in a stable visual world.     

Response properties of single units in the lateral terminal nucleus of the accessory optic system in the behaving primate

1. To determine the potential role of the primate accessory optic system (AOS) in optokinetic and smooth-pursuit eye movements, we recorded the activity of 110 single units in a subdivision of the AOS, the lateral terminal nucleus (LTN), in five alert rhesus macaques. All monkeys were trained to fixate a stationary target spot during visual testing and to track a small spot moving in a variety of visual environments. 2. LTN units formed a continuum of types ranging from purely visual to purely oculomotor. Visual units (50%) responded best for large-field (70 x 50 degrees), moving visual stimuli and had no response associated with smooth-pursuit eye movement; some responded during smooth pursuit in the dark, but the response disappeared if the target was briefly extinguished, indicating that their smooth-pursuit-related response reflected activation of a parafoveal receptive field. Eye movement and visual units (36%) responded both for large, moving visual stimuli and during smooth-pursuit eye movements made in the dark. Eye movement units (14%) discharged during smooth-pursuit or other eye movements but showed no evidence of visual sensitivity. 3. Essentially all (98%) LTN units were direction selective, responding preferentially during vertical background and/or smooth-pursuit movement. The vast majority (88%) preferred upward background and/or eye movement. During periodic movement of the large-field visual background while the animal fixated, their firing rates were modulated above and below rather high resting rates. Although LTN units typically responded best to movement of large-field stimuli, some also responded well to small moving stimuli (0.25 degrees diam). 4. LTN units could be separated into two populations according to their dependence on visual stimulus velocity. For periodic triangle wave stimuli, both types had velocity thresholds less than 3 degrees/s. As stimulus velocity increased above threshold, the activity of one type reached peak firing rates over a very narrow velocity range and remained nearly at peak firing for velocities from approximately 4-80 degrees/s. The firing rates of the other type exhibited velocity tuning in which the firing rate peaked at an average preferred velocity of 13 degrees/s and decreased for higher velocities. 5. A close examination of firing rates to sinusoidal background stimuli revealed that both unit types exhibited unusual behaviors at the extremes of stimulus velocity.(ABSTRACT TRUNCATED AT 400 WORDS)     

Functional coupling of the stabilizing eye and head reflexes during horizontal and vertical linear motion in the cat

Summary The otolith contribution and otolith-visual interaction in eye and head stabilization were investigated in alert cats submitted to sinusoidal linear accelerations in three defined directions of space: up-down (Z motion), left-right (Y motion), and forward-back (X motion). Otolith stimulation alone was performed in total darkness with stimulus frequency varying from 0.05 to 1.39 Hz at a constant half peak-to-peak amplitude of 0.145 m (corresponding acceleration range 0.0014–1.13 g) Optokinetic stimuli were provided by sinusoidally moving a pseudorandom visual pattern in the Z and Y directions, using a similar half peak-to-peak amplitude (0.145 m, i.e., 16.1°) in the 0.025–1.39 Hz frequency domain (corresponding velocity range 2.5°–141°/s). Congruent otolith-visual interaction (costimulation, CS) was produced by moving the cat in front of the earth-stationary visual pattern, while conflicting interaction was obtained by suppressing all visual motion cues during linear motion (visual stabilization method, VS, with cat and visual pattern moving together, in phase). Electromyographic (EMG) activity of antagonist neck extensor (splenius capitis) and flexor (longus capitis) muscles as well as horizontal and vertical eye movements (electrooculography, EOG) were recorded in these different experimental conditions. Results showed that otolith-neck (ONR) and otolith-ocular (OOR) responses were produced during pure otolith stimulation with relatively weak stimuli (0.036 g) in all directions tested. Both EMG and EOG response gain slightly increased, while response phase lead decreased (with respect to stimulus velocity) as stimulus frequency increased in the range 0.25–1.39 Hz. Otolith contribution to compensatory eye and neck responses increased with stimulus frequency, leading to EMG and EOG responses, which oppose the imposed displacement more and more. But the otolith system alone remained unable to produce perfect compensatory responses, even at the highest frequency tested. In contrast, optokinetic stimuli in the Z and Y directions evoked consistent and compensatory eye movement responses (OKR) in a lower frequency range (0.025–0.25 Hz). Increasing stimulus frequency induced strong gain reduction and phase lag. Oculo-neck coupling or eye-head synergy was found during optokinetic stimulation in the Z and Y directions. It was characterized by bilateral activation of neck extensors and flexors during upward and downward eye movements, respectively, and by ipsilateral activation of neck muscles during horizontal eye movements. These visually-induced neck responses seemed related to eye velocity signals. Dynamic properties of neck and eye responses were significantly improved when both inputs were combined (CS). Near perfect compensatory eye movement and neck muscle responses closely related to stimulus velocity were observed over all frequencies tested, in the three directions defined. The present study indicates that eye-head coordination processes during linear motion are mainly dependent on the visual system at low frequencies (below 0.25 Hz), with close functional coupling of OKR and eye-head synergy. The otolith system basically works at higher stimulus frequencies and triggers Synergist OOR and ONR. However, both sensorimotor subsystems combine their dynamic properties to provide better eyehead coordination in an extended frequency range and, as evidenced under VS condition, visual and otolith inputs also contribute to eye and neck responses at high and low frequency, respectively. These general laws on functional coupling of the eye and head stabilizing reflexes during linear motion are valid in the three directions tested, even though the relative weight of visual and otolith inputs may vary according to motion direction and/or kinematics.     

Functional organization of the mechanisms subserving the optokinetic nystagmus in the cat

In the cat both crossed and uncrossed retinal fibres are able to mediate optokinetic nystagmus in both temporonasal and nasotemporal directions. There exists, however, a slight directional predominance of the nystagmus for the crossed fibre system in the temporonasal stimulus direction and for the uncrossed fibres in a nasotemporal direction.During the first 9 days following ablation of the visual cortex the optokinetic nystagmus elicited monocularly is greatly asymmetrical: the nystagmus elicited by a temporonasal stimulus is moderately affected particularly at higher stimulus velocities, whereas the nystagmus elicited by a nasotemporal stimulus is present only at stimulus velocities below 20–30 deg/s and has a low gain.Without the visual cortex, selective stimulation of the crossed retinal fibres of one eye may evoke a weak nystagmus on temporonasal stimulus motion only. In contrast, in absence of visual cortex, the uncrossed retinal fibres do not mediate any optokinetic nystagmus. The behaviour of the vestibulo-ocular reflex in light and of the visual fixation suppression of the postrotatory nystagmus in our lesioned animals provided another means to reach similar conclusions. Twenty-seven units recorded in the vestibular nuclei showed responses to optokinetic stimulations, which were in line with the behaviour of the optokinetic nystagmus.These data suggest that optokinetic nystagmus has two components: (i), a subcortical component in which the temporonasal direction of stimulation is predominant in eliciting the nystagmus and in which both the crossed and uncrossed retinal fibres are involved, although with a different weight, and (ii), a cortical component responsible for a symmetrical optomotor response, which also involves the crossed and uncrossed retinal fibres.     

Properties of rarely encountered types of ganglion cells in the cat's retina and an overall classification

1. In a reference sample of 960 cat retinal ganglion cells, seventy-three had receptive fields departing from the concentric centre-surround pattern.2. Five classes were distinguished among the subset: local edge detectors, direction-selective cells, colour-coded cells, uniformity detectors, edge inhibitory off-centre cells.3. Local edge detectors (forty-five) possessed a radially symmetrical pattern of responses to both centrifugal and centripetal movements of both black and white small targets, an on-off receptive field with a silent inhibitory surround and a low or zero maintained discharge. Their operation could be interpreted as the detection of a contrasting border confined to a small region of the visual field.4. With direction-selective units (eleven) it was possible to find an axis through the receptive field along which sharply different responses could be obtained for opposite directions of movement of small black or white targets.5. Colour units (six) were mostly of the single opponent type having excitatory input from blue-sensitive cones and inhibitory input from long wave-length cones. Both inputs coexisted at the centre of the field and either could be spatially more extensive than the other. One example changed over to rod input under scotopic conditions, another did not.6. Uniformity detectors (five) had a brisk maintained discharge which was reduced or abolished temporarily by all forms of visual stimulation.7. Edge inhibitory off-centre units (three) behaved like uniformity detectors for small targets and fine gratings but like off-centre on-surround units for large targets. Their receptive fields consisted of three concentric regions: a small sized, central edge inhibitory region; a larger zone of off-responsiveness; and an outlying annulus of on-responsiveness.8. It is argued that the above physiological types belong to the morphologically heterogeneous class of cells called gamma cells. The argument is based on similarity in the sizes of receptive fields and dendritic trees and on evidence that the axons are thinner than those of the brisk-sustained and brisk-transient ganglion cells.9. The physiological classification of cat retinal ganglion cells developed in this paper and the preceding one is summarized in a Table.10. It now appears that cat and rabbit possess a qualitatively similar complement of receptive field types among their ganglion cells; the differences reside in the quantitative expression of the various classes.     

The directional effects of passive eye movement on the directional visual responses of single units in the pigeon optic tectum

 We have investigated the visual responses of 184 single units located in the superficial layers of the optic tectum (OT) of the decerebrate, paralysed pigeon. Visual responses were similar to those reported in non-decerebrate preparations; most units responded best to moving visual stimuli, 18% were directionally selective (they had a clear preference for a particular direction of visual stimulus movement), 76% were plane-selective (they responded to movement in either direction in a particular plane). However, we also found that a high proportion of units showed some sensitivity to the orientation of visual stimuli. We examined the effects of extraocular muscle (EOM) afferent signals, induced by passive eye movement (PEM), on the directional visual responses of units. Visual responses were most modified by particular directions of eye movement, although there was no unique relationship between the direction of visual stimulus movement to which an individual unit responded best and the direction of eye movement that caused the greatest modification of that visual response. The results show that EOM afferent signals, carrying information concerning the direction of eye movement, reach the superficial layers of the OT in the pigeon and there modify the visual responses of units in a manner that suggests some role for these signals in the processing of visual information. Received: 17 June 1996 / Accepted: 29 April 1997