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The most conspicuous feature of the rabbit retina is the visual streak that extends along the horizontal azimuth from the nasal margin to the temporal limit of the retina. We believe the streak processes movement vision and that the temporal region (area centralis) is responsible for pattern perception. Both anatomical and behavioural experiments were used to test this hypothesis. Behavioural measures of pattern vision in normal and chiasma-sectioned rabbits revealed both to have the same visual acuity. Using OKN as a measure of movement vision, normal rabbits showed both a directional and velocity-tuned response. The chiasma-sectioned rabbits, with only uncrossed fibre projections remaining, showed a total loss of movement detection. The injection of HRP into the vitreal chamber of one eye in normal rabbits revealed extensive uptake throughout the contralateral thalamus. In the ipsilateral thalamus, there was uptake solely from the ipsilateral retinal projection to a restricted wafer of the lateral geniculate nucleus (LGN). The chiasma cut rabbits showed a very different distribution of HRP in the thalamus. The uptake was restricted to a thin wafer of the LGN, with no contralateral uptake. Thus, the thalamic projections from the retinal area centralis were strictly segregated from the thalamic target areas for the visual streak without any overlap. These findings provide strong evidence for separate retinal origins with anatomically separate pathways for pattern and movement vision in the rabbit.

We used whole brain MRI voxel-based morphometry (VBM) to study the anatomical organization of the visual system in congenitally blind (CB) adults. Eleven CB without a history of visual perception were compared with 21 age- and sex-matched normal-sighted controls (NS). CB showed significant atrophy of the geniculo-striate system, encompassing the optic nerves, the optic chiasm, the optic radiations and the primary visual cortex (BA17). The volume decrease in BA17 reached 25% in both hemispheres. The pulvinar and its projections to the associative visual areas were also dramatically altered, BA18/19 and the middle temporal cortex (MT) showing volume reductions of up to 20%. Additional significant white matter alterations were observed in the inferior longitudinal tract and in the posterior part of the corpus callosum, which links the visual areas of both hemispheres. Our data indicate that the afferent projections to the visual cortex in CB are largely atrophied. Despite the massive volume reductions in the occipital lobes, there is compelling evidence from the literature (reviewed in Noppeney 2007; Ptito and Kupers 2005) that blind subjects activate their visual cortex when performing tasks that involve somatosensory or auditory inputs, suggesting a reorganization of the neural pathways that transmit sensory information to the visual cortex.

Two parallel visual systems, the magnocellular (M) and parvocellular (P) pathways, originate from different types of retinal ganglion cells, and are known to be segregated in different portions of the pregeniculate visual pathways. Their relative contribution to two main cortical streams, dorsal and ventral, is still under discussion, but it is reasonable to suppose that selective damage to the M or P subcortical system might interfere with specific aspects of processing within one or the other cortical system. Using two different apparent-motion tasks, we compared the performance of patients affected by compression of the ventral part of the pregeniculate visual pathways with that of normal controls. In the first task, observers detected small displacements of a low-contrast vertical bar, while in the second task they estimated the visible persistence of moving dots. In the first task, patients were impaired with parafoveal displays, especially in the temporal portion of the visual field. In the second task, patients showed reduced suppression of visible persistence at long, but not at short, exposure durations. Three considerations support the hypothesis that these results represent a selective impairment of the M system. First, M axons are more likely to suffer from compression, particularly in the case of a mass growing from below since they are known to occupy a ventral subpial position in the optic chiasm and tract. Second, the performance of patients with a ventral compression is consistent with the characteristics of the response properties of P ganglion cells, which have previously been shown to exhibit elevated and unmodulated thresholds for displacement detection in the macaque monkey. Third, such patients are less sensitive to the inhibitory signals that suppress visible persistence, which probably originate in the M system.

Auditory brainstem abnormalities affecting decussation patterns and nuclei involved in the acoustic chiasm exist in a variety of albino mammals, suggesting that binaural processes underlying spatial hearing may be disrupted in these mutants. To evaluate this we have compared the contribution of the two ears in albino and normally pigmented guinea pigs to the spatial tuning of auditory neurons in the deep layers of the superior colliculus (SC). Broadband noise stimuli at threshold and at suprathreshold intensities were presented from different azimuthal loudspeaker locations under free-field anechoic conditions, and auditory receptive fields were plotted before, during and after occluding the ipsilateral ear. We show that the deep layers of the albino SC contain a map of contralateral auditory azimuth along its anteroposterior axis, which is aligned with the visual map in the superficial layers above, just as in normal animals. We also show that threshold spatial responses are elicited only via the contralateral ear and at similar stimulus intensities (mean approximately 30 dB SPL) in the two pigmentation phenotypes. The mechanisms that maintain spatial tuning at sound intensities of 10-40 dB above threshold, however, differ markedly in these animals. Plugging the ipsilateral ear in normal guinea pigs caused significant expansions of their auditory receptive fields and loss of directional tuning, but in the albinos occlusion had little effect on these spatial properties. The results suggest that while spatial selectivity for relatively loud sounds among SC neurons is normally maintained via the binaural combination of contralateral excitatory drive and ipsilateral inhibition, it is achieved in albinos almost exclusively by monaural input from the contralateral ear. This finding is consistent with an excessive contralateral ear dominance of higher levels of the albino auditory system caused by anomalies of their acoustic chiasm, analogous to the monocular dominance of the visual system that results from excessive axon crossing at the optic chiasm in these animals.

In convergent strabismus (esotropia) the spatial asynchrony of the two eye inputs unbalances the interocular interactions, leading to the functional advantage of the nondeviated eye and the inhibition of the esotropic eye. It may be argued that the strabismic suppression, if it is the effect of inhibitory interactions between the eyes, could be removed by interrupting the interocular pathways at the optic chiasm. After chiasmatic section, each eye is connected only to the ipsilateral cortex through the uncrossed retinal projections and so the functionality of each eye's input is no longer interfered with by interocular mechanisms. In strabismic cats submitted, as adults, to section of the optic chiasm, we performed electrophysiological recordings from the striate cortex. Results show that in these animals: (1) the cortical responsiveness to the strabismic eye is strikingly higher than in esotropes with an intact optic chiasm; (2) the effectiveness of stimulation of the deviated eye is not different from that of the nondeviated eye in driving neurons of corresponding cortex; (3) surprisingly, a high degree of binocular activation is present in the cortex ipsilateral to the deviated eye, while in the cortex connected to the nondeviated eye the greatest majority of neurons are monocularly driven. Cortical binocularity depends on the corpus callosum, which conveys the input from the nondeviated eye to the opposite cortex (which receives the direct strabismic input), but not vice versa. The asymmetry of callosal transmission parallels the morphological asymmetry of callosal connections that occur in convergent strabismus. All together the findings indicate that the impaired effectiveness of esotropic input does not result from developmental deficit of the strabismic afferents but, rather, from inhibitory influences that are actively exerted through the interocular pathways. Strabismic suppression may be accomplished by the same interocular mechanisms underlying binocular rivalry.

During normal postnatal development, there is an overproduction and subsequent partial elimination of the callosal projections of cortical areas 17 and 18 in the cat. In the present study, we investigated how neonatal splitting of the optic chiasm affects this process. Our results indicate that neonatal splitting of the optic chiasm exaggerates the normally occurring partial elimination of immature callosal projections: it causes a significant reduction in the total number of neurons in the supragranular layers that send an axon through the corpus callosum. It does not, however, cause a significant change in the number of callosally projecting neurons in the infragranular layers. These data suggest that in addition to other factors previously described, the level or spatial distribution of correlated binocular input to visual cortical neurons may influence the stabilization/elimination of immature callosal connections.

The present study was conducted to investigate the therapeutic effects of nilvadipine, a Ca2+ entry blocker, on rat focal cerebral ischemia. Under halothane anesthesia, a 3-0 nylon thread was introduced into the neck internal carotid artery to occlude the left middle cerebral artery. Either nilvadipine (3.2 mg/kg) or vehicle was administered subcutaneously 1, 2, 3, 4, 5 and 6 h following the occlusion (groups 1-6, respectively). Twenty-four hours after the occlusion, the percentage infarct volumes in nilvadipine-treated animals in groups 1-3 (21 +/- 11%, 24 +/- 11%, and 26 +/- 7%, respectively) were smaller than those in the respective control groups (36 +/- 5%, 35 +/- 3%, and 35 +/- 3%; P0.05). Compared with controls, the infarct size of the periphery of the fronto-parietal cortex decreased in nilvadipine-treated animals. The results indicate that nilvadipine decreases the size of infarction when administered up to 3 h after an ischemic insult. Thus, nilvadipine can be considered a potential therapeutic agent for acute focal cerebral ischemia, and may be clinically useful in stroke patients.

Low-level electrical stimulation through electrodes in the medial temporal lobe (MTL) was used to disrupt the performance of chiasm-split macaques working in a delayed matching-to-sample (DMS) visual memory task. The stimulation was below afterdischarge threshold and did not distract the animals. Nonetheless, stimulation caused severe deficits when delivered either during encoding or retrieval stages. Substantially less deficit appeared when stimulation was delivered during the retention interval. Stimulation levels which led to significant disruption on the DMS task had no effect on a discrimination task using the same kinds of images. Unilateral electrical stimulation delivered to MTL in one hemisphere during encoding and to MTL in the other hemisphere during retrieval produced an effective disruption, suggesting that the unilateral stimulation during the encoding period disrupts the encoding on that side while unilateral stimulation delivered to the opposite side during the retrieval period prevents the retrieval of the (now unilateral) memory. This suggestion is supported by control experiments in which significantly less disruption was caused by unilateral electrical stimulation delivered during both the encoding and the retrieval period if the stimulation was delivered to the same side in both periods. The electrical stimulation was further used to determine that interhemispheric access by one hemisphere to memories placed in the other was excellent. This was done, in these split-chiasm monkeys, by using unilateral stimulation to limit memory formation to just one hemisphere, then testing interhemispheric access by routing the test through the "ignorant" hemisphere (using just the ipsilateral eye). Successful interhemispheric access was obtained with either the anterior commissure or with the splenium of the corpus callosum (the other interhemispheric forebrain pathways having been cut). The electrical stimulation was also used to determine that the visual inputs even though entering via just one eye and one hemisphere, lead to bilateral memory formation. In this case each hemisphere was tested separately during retrieval by delivering disruptive levels of the electrical stimulation to the MTL of the other hemisphere.

The distribution of optic chiasm input to different types of neurons in pericruciate cortex of cats agreed with previous work using light flashes. Neuron response times served to differentiate the input pathways to pericruciate cortex, and the types of neurons they influence. Input from the optic chiasm arrived in three distinct surges: the first via the superior colliculus, the second via an unidentified pathway, and the third via the visual cortex. A fourth, diffuse surge arrived in the postcruciate cortex via some unidentified pathway. Stimulation of the contralateral side of the optic chiasm had a weaker effect than stimulation of the ipsilateral side; it evoked activity at a higher threshold, with fewer spikes per response, and at a longer latency. The difference in response latency between the two sides was largest on neurons responding to the first surge, decreasing in later surges, and being least on those neurons responding to the last surge. About 2.3% of the postcruciate and 15% of the precruciate neurons responded only to optic chiasm stimulation; they were isolated in the granular layers, and their responses could not be influenced by prior cutaneous input. It is suggested that much of the visual input to pericruciate cortex serves to modulate on-going cortical output and, thereby, the behavior of the animal.

Binocular disparity, resulting from the projection of a three-dimensional object on the two spatially separated retinae, constitutes one of the principal cues for stereoscopic perception. The binocularity of cells in one hemisphere stems from two sources: (1) the ganglion cells in the homonymous temporal and nasal hemiretinae and (2) the contralateral hemisphere via the corpus callosum (CC). The objectives of this study were, on one hand, to determine whether disparity-sensitive cells are present in a “higher order” area, namely area 19 of the visual cortex, of the cat and, on the other hand, to ascertain whether the CC contributes to the formation of these cells. As in areas 17–18, two types of disparity-sensitive neurons were found: one type, showing maximal interactive effects around zero disparity, responded with strong excitation or inhibition when the stimuli presented independently to the two eyes were in register. These neurons are presumed to signal stimuli situated about the fixation plane. The other type, also made up of two subtypes of opposed valencies, gave maximum responses at one set of disparities and inhibitory responses to the other set. These are presumed to signal stimuli situated in front of or behind the fixation plane. Unlike areas 17–18, however, disparity-sensitive cells in area 19 of the normal cat were less finely tuned and their proportion was lower. In the split-chiasm animal, very few cells were sensitive to disparity. These results, when coupled with behavioral data obtained with destriate animals, indicate that (1) area 19 is probably less involved in the analysis of disparity information than area 17, (2) the disparity-sensitive neurons that are sensitive to disparity are not involved in the resolution of very fine three-dimensional spatial detail, and (3) the CC only determines a limited number of these cells in the absence of normal binocular input.

Mammalian optic nerve axons are organised within a fascicular framework. This pattern changes between the eye and the chiasm. For most of the length of the nerve fascicular patterns are apparent, but close to the chiasm, in a region of major fibre rearrangement, the fascicular configuration is lost. It is not known how this change occurs, or whether there are less obvious systematic changes in the number of fascicles or their geometry along the length of the nerve. In this study these questions have been addressed at successive locations along the length of the cat optic nerve. The number of fascicles varied depending upon the location examined. A relatively high number were found behind the eye. The number declined in the mid-orbital portion before increasing again in the region of the optic canal. Further caudally there was a progressive change in the pattern of fasciculation, with a loss of fascicular structure medially. The remaining fascicles became concentrated around the inferotemporal periphery of the nerve. There was no fascicular organisation at the point at which the two nerves fused at the chiasm. Although the number of fascicles varied along the length of the nerve their geometric pattern did not change.

Binocular disparity, resulting from the projection of a three-dimensional object on the two spatially separated retinae, constitutes one of the fundamental cues for stereoscopic perception. The binocularity of cells in one hemisphere stems from two sources: i) from the ipsilateral ganglion cells in the temporal retina which converge with inputs coming from the contralateral nasal retina; the latter axons cross at the chiasma; ii) from inputs originating in the opposite hemisphere which cross in the corpus callosum. The objective of this study was to demonstrate that interactions from both types of inputs can result in the formation of disparity sensitive neurons and presumably that either type could mediate stereoperception based on disparity cues. Two types of disparity sensitive neurons were found in the normal cat: one type, showing maximal interactive effects around zero disparity responded with strong excitation or inhibition when the stimuli were in register. These neurons are presumed to signal stimuli situated about the fixation plane. The other type, also made up of two subtypes of opposed valencies, gave maximum responses at one set of disparities and inhibitory responses to the other set. These were presumed to signal stimuli situated in front of or behind the fixation plane. In the split-chiasm cat, whose cortical binocularity is presumably assured by converging ipsilateral and callosal inputs, three of the four subtypes of disparity sensitive neurons were found, the uncrossed disparity cells being absent in these animals. Moreover, stimulating each eye individually indicated that nearly 80% of the cells in normal and about 40% in split-chiasm cats were binocularly driven. However, both these figures underestimated the amount of binocular interaction in the callosal recipient zone, since stimulating both eyes simultaneously showed that a proportionately larger number of cells were binocularly driven. Disparity sensitive cells also varied as a function of ocular dominance, i.e., cells signaling the fixation plane tended to have balanced dominance whereas units preferring stimuli situated in front of or behind the fixation plane were dominated by the ipsilateral and contralateral eyes, respectively.

Experiments comparing binocular with monocular abilities of monkeys working on visual mnemonic tasks were performed. First, it was shown that even in split-brain monkeys performance was more accurate when both hemispheres were utilized than when the task was performed with only the single (better) hemisphere. Some form of noncommissural integration is thus possible. However, when the forebrain commissures are present, as in four other animals (with only optic chiasm transected) it was shown that integration occurs via callosal mechanisms as well. This was demonstrated by the fact that here, too, binocular performance was normally more accurate than monocular performance, but when different images to be remembered were presented concurrently to the two eyes, the binocular advantage was lost. Finally, in three monkeys with only the anterior commissure allowing interhemispheric communication the superiority of binocular assessment remained even when the two hemispheres simultaneously received such differing images.

Prior studies of thalamic neurons have demonstrated that they exhibit at least two response modes: a relay mode and a burst mode. During the relay mode, sensory information is faithfully relayed to cortex; during the burst mode, which is caused by a voltagedependent Ca2+ conductance, this relay of sensory information is interrupted. We began in vivo studies of these response modes in neurons from the lateral geniculate nucleus of anesthetized, paralyzed cats. Each of the 9 X and 10 Y cells we recorded intracellularly displayed voltage-dependent, low threshold spikes that were presumably the Ca2+ spikes described from in vitro recording. These spikes were triangular in waveform and typically had 2–7 fast action potentials (interspike intervals of 1.2–4 ms) riding its crest. Furthermore, the cell's membrane had to be hyperpolarized to de-inactivate the low threshold spike before a depolarization could then activate it. We could activate these low threshold spikes in Y cells from EPSPs, whether spontaneous or evoked from activation of the optic chiasm. However, in only one of the X cells could we activate low threshold spikes from chiasm shock; in the remainder, we could activate low threshold spikes only via depolarizing current pulses, possibly because the EPSPs of these X cells were too small to activate these spikes. We also used extracellular recording to study spontaneous activity and responses to chiasm shock from 114 geniculate neurons and, as a control, 57 optic tract axons. We concentrated on periods of bursty responsiveness signifying the burst mode. We define a burst as 2–7 action potentials with interspike intervals 100 ms; from our intracellular recording, we know that such bursts signify low threshold spikes. We found that, during extracellular recording, 20 of the 39 X cells and each of the 75 Y cells displayed evidence of the burst response mode, although burst periods were rare in X cells. Electrical activation of the optic chiasm greatly enhanced the burstiness of Y cells for periods of 500 ms or more. We also electrically stimulated the parabrachial region of the midbrain, which provides a mostly cholinergic innervation to the lateral geniculate nucleus. Although parabrachial activation by itself had no detectable effect on Y cell response modes, prior parabrachial activation prevented the enhanced burstiness caused by chiasm stimulation. This parabrachial effect lasted for roughly 500 ms after stimulation. Neither chiasm nor parabrachial stimulation, singly or in combination, had a noticeable effect on the bursting activity of X cells. Finally, none of the extracellularly recorded retinogeniculate axons (23 X and 34 Y) showed any evidence of burst responses. This supports the conclusion that the burst responses we found for geniculate neurons represent an emergent property of the lateral geniculate nucleus, and this burstiness reflects an interruption of retinogeniculate transmission. We conclude that geniculate X and Y cells do indeed show evidence during extracellular recording of maintaining two very different response modes and that, under our recording conditions, Y cells are much more prone to burst activity than are X cells.

In order to characterize differences in growth patterns of axons as they elongate toward their targets and during the initial stages of terminal arbor formation within the targets, we examined the primary visual system of fetal and newborn hamsters using three morphological methods: the Cajal-deCastro reduced silver method, the rapid Golgi technique, and anterograde transport of HRP. Axons emerge from the retina between the 10th and 11th embryonic days (E10–E11). The front of retinal axons crosses the chiasm, extends over the primitive dorsal nucleus of the lateral geniculate body (LGBd) by E13, and advances to the back of the superior colliculus (SC) by E13.5–E14. The rate of axon growth during this advance is nearly 2 mm/day. Collateral sprouts appear on axons around E15.5. In the LGBd and SC, these sprouts arise from multiple sites along the parent axons. Only one or a few of the sprouts continue to grow and branch, while others are eliminated. The net rate of axon collateral advance in this second phase is an order of magnitude slower than during the stage of axon elongation. Thus, formation of CNS projections may involve two qualitatively distinct modes of axon growth. The arborization mode contrasts with the elongation mode by the presence of branching, a lack of fasciculation and a slower average rate of extension. The Stereotypic direct advance of axons during elongation also differs from the remodelling which occurs during arborization. The delay between axon arrival at targets and onset of arborization could be a reflection of axons “waiting” for a maturational change to occur in the retina or in targets. Arborization in the LGBd and SC is initiated around the same time, implicating the former possibility. However, a slower differentiation of retinal arbors in the SC, in addition to morphological differences of arbors in the two structures, suggests that alterations in substrate factors also play a critical role in triggering the early stages of arbor formation.

Effects of early monocular deprivation on visual response properties of neurons in the nucleus of the optic tract (NOT) were studied in six adult ferrets. Retinal input to NOT was investigated by orthodromic electrical stimulation of optic chiasm and optic nerves. Electrical stimulation of the ipsilateral primary visual cortex was applied to reveal the presence of a cortical pathway to NOT. All 75 neurons studied in the NOT displayed the typical strongly direction-specific response to horizontal stimulus motion; they were activated by ipsiversively directed motion (i.e. motion towards the recorded hemisphere) similar to NOT-cells in animals with normal visual experience. When tested binocularly most of the NOT-cells preferred velocities of 10 or 20 deg/s, revealing no significant difference from animals reared with normal binocular experience. The most pronounced effect of monocular deprivation was observed on ocular dominance: In the hemisphere contralateral to the non-deprived eye, NOT-cells were almost exclusively driven through the contralateral eye. In the hemisphere contralateral to the deprived eye, three of the six animals studied showed a marked dominance of the ipsilateral, non-deprived eye. In the other three animals, most neurons were binocularly activated, but over all they were significantly more strongly activated by the ipsilateral eye than found in normal animals. In four animals, dependence of ocular dominance on stimulus velocity was tested in the NOT contralateral to the deprived eye. In one of them, neurons were almost exclusively driven by the ipsilateral, non-deprived eye, irrespective of stimulus velocity. In the other animals, ocular dominance shifted from contralateral to ipsilateral with increasing stimulus velocities. Electrical stimulation of the optic chiasm revealed a mean latency of 5.53 +/- 0.48 ms. In both hemispheres, NOT-units could only be activated by stimulation of the contralateral optic nerve. Thus, no significant difference in the retinofugal conduction velocities from the deprived and the normal nerve could be detected. Of 52 cells studied, 28 (= 54%) could be activated by stimulation of primary visual cortex, mean latency being 3.9 +/- 1.7 ms. No significant difference in the percentage of cortically excitable cells between the two hemispheres as well as compared to normal animals was found (contralateral to the deprived eye: 67%, contralateral to the non-deprived eye: 53%). Therefore, cortical projections to NOT seem not to be affected by monocular deprivation. The effects of monocular deprivation in the ferret NOT, especially on ocular dominance and cortical input, are compared to the results previously described for the cat.

We obtained good intracellular recording from 5 W cells in the C-laminae of the cat's lateral geniculate nucleus. The recordings were made from an anesthetized and paralyzed in vivo preparation. We found a consistent pattern for the postsynaptic potentials evoked from activation of the optic chiasm: first was an IPSP followed by an EPSP. This is very different from the pattern seen in X and Y cells, for which an EPSP always appears first and is then followed by an IPSP. We interpret the pattern for W cells as follows. The initial IPSP is disynaptic; this involves retinogeniculate conduction over very fast Y axons and a relay through an interneuron. The EPSP is monosynaptic, reflecting retinogeniculate conduction over very slow W axons. A possible implication for this is that activity over the Y pathway may generally inhibit geniculate W cells before these W cells can be excited by their retinal afferents. Finally, we elicited from each of these W cells voltage-dependent, low threshold spikes, which are very similar to those displayed by X and Y cells. These spikes can interrupt normal retinogeniculate transmission, and they are prevented by maintaining relatively depolarized membrane potentials.

Basic properties of responses to visual stimulation with large moving random dot patterns were studied in ferret nucleus of the optic tract. Retinal input to NOT was assessed by orthodromic electrical stimulation of the optic chiasm and optic nerves. Presence of an input from visual cortex was tested by orthodromic electrical stimulation of ipsilateral area 17. All 51 NOT neurons studied displayed a non-habituating, clearly direction-specific response: discharge rate strongly increased with the stimulus pattern moving horizontally in ipsiversive direction (motion directed towards the recorded hemisphere) and decreased with contraversive stimulus motion. Most latencies to visual stimulation ranged from 80 to 100 ms. Velocity tuning was studied using stimulus velocities between 4 deg/s and 100 deg/s. Discharge rates were most effectively modulated at a stimulus velocity of 20 deg/s. A large portion of the cells studied (91%) could be binocularly activated, although for almost all neurons the contralateral eye was dominant. Through stimulation of the optic chiasm 46 out of 51 NOT neurons could be electrically activated with a latency of 5.42 +/- 0.66 ms (mean +/- SD). For 15 fibers stimulated from both optic chiasm and contralateral optic nerve, conduction velocities between 2.5 and 8.9 m/s, with a mean of 5.1 m/s, were obtained. A major direct input from the ipsilateral retina was not found. Furthermore, 65% of all neurons could be activated through electrical stimulation of visual cortex with a mean latency of 3.7 +/- 1.5 ms, indicating a strong cortical projection to ferret NOT. The functional relevance of response properties of ferret NOT neurons for horizontal optokinetic nystagmus is discussed. Parameters that could be related to formation of a cortico-pretectal projection in mammals are considered.

Stimulation of the optic chiasm (OX) or visual cortex (VC) elicited a burst of impulses from visual cells in the superficial layers of the hamster's superior colliculus which was followed by a period of response suppression which lasted from 50-200 ms. During this period responses to normally suprathreshold OX and VC shocks, spontaneous activity and even injury discharges were markedly attenuated. For approximately 50% of the visual cells tested VC stimulation also reduced responses to visual stimuli. No correlations between receptive field properties and whether or not VC shocks diminished a given cell's visual responses were noted. Stimulation of either the cervical spinal cord (SC) or somatic sensory cortex (SMCTX) evoked action potentials from somatosensory neurons in the deep tectal laminae. These responses were followed by a period of suppression identical to that seen in the superficial layers after OX or VC shocks. SMCTX stimulation attenuated responses to tactile stimuli for 30% of the cells tested in the deep layers. Again, no correlation was observed between somatosensory response characteristics and whether or not a given cell exhibited response suppression.

Single-cell recording experiments were carried out to determine whether rearing kittens with surgically induced convergent strabismus (esotropia) alters the development of receptive field (RF) properties of neurons in area 18. In agreement with previous work on kittens with divergent strabismus (exotropia), there was a marked loss of binocularly driven cells in area 18 of esotropic cats. In contrast to the striate cortex of strabismic cats, the spatial properties of area 18 neurons, including receptive-field size and spatial frequency tuning, did not differ from those in normal controls. On the other hand, we found that contrast thresholds, measured at an optimal spatial frequency, were significantly elevated, and that the contrast gain in many cells was reduced in strabismic cats. These deficits were observed in both eyes, though the cells dominated by the deviating eye had a lower response amplitude at all contrasts. Furthermore, temporal frequency tuning curves were abnormal in strabismic cats in that the optimal frequencies and temporal resolutions were shifted to lower values. These effects were also bilateral. Velocity tuning, measured with a high-contrast bar stimulus, revealed that area 18 neurons in strabismic cats were unable to respond to very high velocities compared to normals. This reduced response was more severe when measured with the deviating eye in spite of the bilateral nature of the deficit. Finally, latencies to electrical stimulation of the optic chiasm or the optic radiation were significantly longer in strabismic cats. The magnitude of these effects was virtually the same for both eyes. From these observations, we conclude that the temporal properties of area 18 neurons, particularly the cells abilities to follow fast temporal modulations, are affected by raising kittens with surgically induced convergent strabismus.

1. Quasi-intracellular recordings from neurons in the lateral geniculate nucleus of the cat have been made. From these recordings the excitatory input of these neurons could be determined. 2. The experiments suggest, that the excitatory input of a geniculate neuron is originating from one single optic tract fibre. 3. The experiments were performed on non-anaesthetized paralyzed cats which showed different levels of alertness such as sleep, drowsiness and wakefulness. During these different levels the input of the geniculate neurons remains constant but the output varies considerable. Thetransfer ratio, defined as the ratio between the spike frequency (output) and the EPSP frequency (input) of a neuron is high (0.9–1.0) during wakefulness and low (0.4–0.5) during sleep with intermediate values at intermediate states. 4. The control of the transfer ratio is caused bychanging the amplitude of the EPSPs. During wakefulness nearly all EPSPs are large enough to reach the threshold; duringsleep the EPSPs are smaller than the threshold potential. A mechanism working like presynaptic inhibition might be responsible for the change of the EPSP amplitude. 5. The control of the flow of information to the visual cortex according to the level of alertness is probably one of the functions of the lateral geniculate nucleus.

A suprasylvian lesion removing cortical areas 7 and 21 and portions of area 19 and of the lateral suprasylvian area was placed in one hemisphere of split-chiasm cats. By comparison with the normal side and with cortically intact split-chiasm and split-brain cats, form discrimination learning with the eye on the injured side was severely retarded. This deficit could not be attributed to an unintentional undercutting of areas 17 and 18, since in three cases the laminae of the lateral geniculate nucleus showed little retrograde atrophy; marked degeneration was found in the medial interlaminar nucleus and the pulvinar complex. In addition, interocular transfer of form discriminations to the eye on the injured side was absent or poor, while transfer in the opposite direction was normal. A cat with a suprasylvian lesion undercutting areas 17 and 18 was unable to learn pattern discriminations with the eye on the injured side, in spite of prolonged training with that eye and normal learning with the other eye. Another cat with a suprasylvian lesion selectively removing the anteromedial and posteromedial portions of the lateral suprasylvian area showed no learning deficit on the injured side, but poor transfer to that side. A learning deficit on the side of the lesion emerged in this cat after forebrain commissurotomy. The results support the hypothesis of a major involvement of cortical areas outside of 17 and 18 in the processes of abstraction and generalization of visual information necessary for learning and interhemispheric transfer of form discrimination in the cat.

The spatial separation of the eyes in animals with overlapping visual fields means that parts of a three dimensional object project to slightly disparate retinal points in each eye. This disparity, once interpreted by the brain, is thought to be a sufficient condition for stereoperception. In the present experiment, stereopsis based on spatial disparity cues was evaluated in cats using Julesz random-dot stereograms before and after sections of the optic chiasm, the corpus callosum or both. Normal cats were able to solve the random-dot problem. Optic chiasm transection drastically diminished this ability, callosal section had little effect and combined lesions of these two structures abolished stereoperception. These results suggest that central stereopsis based on spatial disparity is mainly mediated by binocular cortical cells receiving their input via the ipsilateral and the through-the-chiasm contralateral thalamo-cortical pathways.

Visual neurons in the lateral geniculate nucleus (LGN) of the cat may be separated into distinct X and Y classes based on a test of the linearity of spatial summation. Y cells produce nonlinear responses especially when the visual stimulus is a fine spatial grating. X cells exhibit mainly linear summation properties. X cells respond mainly at the fundamental modulation frequency of a contrast reversal grating while Y cells respond at the fundamental and at the second harmonic of the modulation frequency. The spatial resolution of X cells' fundamental responses and Y cells' second harmonic responses is about the same, and both are two to eight times higher than the spatial resolution of the Y cells' fundamental response. The conduction velocity of the Y optic tract afferents is greater than that of the velocity of the X afferents. However, the LGN latencies of the responses of the two classes of cells to optic chiasm stimulation overlap considerably.

This study investigated the response characteristics of single cells in the nucleus of the basal optic root (nBOR) of the accessory optic system of pigeons. One hundred and twenty-one nBOR cells were studied and quantitative data from 87 units indicated that they preferred very large random patterned stimuli, and did not respond to stimuli less than 30 degrees in diameter. In many cases the receptive field encompassed the entire monocular visual field. nBOR cells were directionally sensitive with approximately 70% preferring motion with an upward vector. They responded to very slow velocities in the range of 0.5 degrees to 5.0 degrees s-1, and did not adapt to continuous stimulation. Maximal responses occurred to very large randomly patterned stimuli moving slowly in an upward direction. These response characteristics together with the anatomical connections of the nBOR suggest that this nucleus may be involved in processing self-induced visual motion information required for vertical optokinetic stabilization.

The afferent and efferent connections of omnipause neurons involved in saccadic eye movements were studied electrophysiologically in ketamine anesthetized cats. Omnipause neurons were identified by their characteristic discharge pattern which was qualitatively unaltered by ketamine anesthesia.

It has been shown previously that surgical lesions of the antero-medial thalamus interfere with ocular dominance modifications that normally result from monocular deprivation in young kittens (Singer 1982). The aim of the present study was to determine whether this effect was due specifically to the destruction of the visual cortical projections of the anterior intralaminar nuclei. We report here that large excitotoxin lesions of the anterior dorsal thalamus have no effect on the cortical response to monocular deprivation. These data indicate that the intralaminar projection is not essential for ocular dominance plasticity.

In one of the most-widely used species in pineal gland research, the strongly photoperiodic golden hamster, Mesocricetus auratus, no electrophysiological data on pinealocytes are currently available. To fill this gap, in the present study 185 spontaneously active pinealocytes from male golden hamsters were recorded extracellularly, both during day- and night-time (light: dark cycle 12 ∶ 12, lights on at 07.00 h). As in other species, pinealocytes exhibited action potentials of 1–2 ms duration. An irregular firing pattern was observed in 95% of the pinealocytes, the remainder fired more regularly or showed a phasic discharge pattern. The firing frequencies ranged from 0.2 to 25 Hz and showed clear time-dependent differences. From 07.00 h to 22.00 h the mean firing frequencies were identical, i.e. in the range of 2 Hz; between 22.00 h and 01.00 h mean discharge rate increased to 5 Hz and exhibited a peak of 7 Hz between 01.00 h and 04.00 h, followed by a decrease to 4 Hz between 04.00 h and 07.00 h. Electrical stimulation of the superior cervical ganglion during day-time resulted in an augmentation of firing frequency in some pinealocytes and a decrease in others; during night-time, inhibitory responses only were observed. Photic stimulation, or electrical stimulation of either the optic chiasm or the habenular nuclei mostly decreased the firing rate of pinealocytes. Compared to other mammalian species, the electrophysiological properties of golden hamster pinealocytes appear to be basically similar.

The fibre arrangement in the optic chiasma (OC) and tract (OT) was investigated with anatomical and physiological methods. In silver impregnated material, principal fibre streams can be demonstrated. In the OC, fibre bundles from each eye cross in a regular basket weave pattern, but deviations of single fibres from the predominant stream are often seen. In the OT, fibres run essentially parallel, and crossings of individual fibres are mainly restricted to the periphery of the tract, or around capillaries. Fibres in the upper segment of the OT are of thin, in the lower segments of thick diameter. Individual fibres labelled by HRP injected in the lateral geniculate body (LGB) run essentially parallel over long distances. Ventromedially to the LGB, bifurcations are found with one branch entering the LGB, the other continuing. 57% of OT-fibres had a receptive field (RF) in the contralateral, 43% in the homolateral eye; 60% in the lower, 40% in the upper visual field; and 6% had a RF in the homolateral visual field, mostly near the vertical meridian. Fibres from the central area were underrepresented in our sample. Fibres from the two eyes were mixed. The RFs of consecutively recorded fibres showed a systematic progression only exceptionally. After plotting RFs of a single penetration on a transformed isodensity ganglion cell map of the visual field, the RF's were distributed along elongated paths on this map. In the OT, such paths ran parallel or slightly inclined relative to the horizontal meridian. They were restricted to either the upper or the lower quadrant or to a path along the horizontal meridian. In the OC, the RF-paths mostly crossed the horizontal meridian at an obtuse angle (average 70 degrees). Thus, the visual field representation rotates by nearly 90 degrees from the OC to the OT. In the OC, the central area is located anteriorly, in the OT dorsally, with the upper visual quadrant laterally and the lower medially. Fibres from the two eyes were mixed and, within the range of the scatter, RFs from the homo- and contralateral eye were in register. It is concluded, that the distribution of fibres in the OC and the OT show a basic retinotopic organization superimposed by scatter.

Single unit recordings from 220 units were obtained from the nuclei praetectalis anterior (NPA) and posterior (NPP) of 30 immobilized, anesthetized cats. Quantitative analysis of pretectal (PT) visual activity was mainly based on recordings from the NPP. For comparison, 160 collicular (CS) neurons were studied. A strong sensitivity for moving objects was evident in both samples. The following main types of PT activity were categorized: (A) slow movement, direction-selective units (21%); (B) slow movement, nondirection-selective units (19%); (C) units nonselective for stimulus velocity and direction (24%); (D) jerk movement selective, nondirection-selective units (36%). Latency measurements following single shocks to optic chiasm (OX) and tract (OT) showed mainly slow conducting fiber input to the PT and CS which can be divided into two different groups by conduction properties and synaptic delay: direct W-input and delayed W-input. Fast Y-fiber input of both types, direct and indirect, was recorded at both sites, PT and CS.

We have studied the physiological properties of ganglion cells in the retina of the cat. The experiments were designed to identify those ganglion cells which project to direction-selective cells in the nucleus of the optic tract (NOT), by demonstrating their antidromic activation at low threshold from an electrode in the NOT. These ganglion cells presumably provide the retinal drive to the optokinetic reflex. Altogether, 11 such ganglion cells were identified in a population of 578 cells studied. All 11 were W-cells, with slow-conducting axons. Five of the 11 had on-centre direction-selective receptive fields; the other 6 had a variety of receptive field patterns. Thus, on centre-selective cells form a much higher proportion of the retinal input to direction-selective cells in the NOT than of the overall ganglion cell population. However, their receptive field properties were too varied fully to account for the selectivity of NOT cells for horizontal stimulus movement. In summary the retinal input to the NOT appears to be formed principally or entirely by W-class ganglion cells, including many which are direction selective. It still seems necessary, however, to postulate, some non-retinal mechanism to account for all the receptive field properties of direction-selective NOT cells.

Split-chiasm cats with unilateral or bilateral lesions largely removing the commissurally connected portions of visual cortical areas 17, 18 and 19 showed good interocular transfer of monocularly learned pattern discriminations. The capacity for interocular transfer in these cats was in fact little or not different from that of split-chiasm cats with an intact cortex. Split-chiasm cats with an additional section of the forebrain commissures, as well as two split-chiasm cats with 17-18 lesions also submitted to forebrain commissurotomy after having shown good interocular transfer, were generally incapable of transferring pattern discriminations between the eyes. It is concluded that interocular transfer of pattern discriminations, in split-chiasm cats does not require areas 17, 18 and 19 and must therefore depend on other cortical areas.

The ocular dominance, responsiveness level and receptive field properties of single cortical cells were studied in 12 acute and chronic split chiasm adult cats (729 cells) and in 13 normal controls (544 cells). Recording was made from the border between visual areas 17/18. Responsive cells in the operated cats were obtained exclusively (87.1%) following stimulation of the ipsilateral eye, except for a very few cells (2.5%) which were binocularly driven. In comparison, only few (10.9%) of the cells in the normal control cats were driven ipsilaterally and the majority of them (74.5%) were binocularly driven. Relatively small proportions of cells (46.1%) were visually responsive in the acute (less than 1 week postoperatively) and in the most chronic (greater than 6 months) cats, in comparison to the normal cats (87.3%). No consistent change was found in the responsiveness level of cortical cells as function of length of the survival time (correlation coefficient: -0.45). Only a very slight tendency for a relative increase in binocularly driven cells with survival time was found as well as a reduction in the proportion of nonspecific cells. However, in view of the general absence of binocularity and responsivity in these cats, it was concluded that no recovery was found, even long after the elimination of the contralateral inputs.

The effects of intraocular injection of monensin, a specific inhibitor of membrane glycoprotein transport within the axon, on the development of the chick embryo optic nerve were examined. A proportion of axons growing from the monensin-injected eye were misrouted into the opposite optic nerve at the chiasm and grew to the retina of the opposite eye. These results suggest that formation of the decussation pattern at the chiasm primarily depends on neurite fasciculation mediated by membrane glycoproteins.

Neuronal responses in the pretectum (PT) were analyzed in 4-16 week old kittens after visual and electrical stimulation and compared with adult responses from a previous study. All three retinal fiber types projecting to the adult PT could be electrically activated in kittens from 4 weeks on. There was a dramatic reduction of response latencies to electric shocks to retinal afferents applied at the optic chiasm (OX) and optic tract (OT) in postsynaptic cells as a function of age, involving X-, Y-, and W-fibers. At four through six weeks postnatally the reduction in latency was found to be due to enhanced signal transmission at the axonal terminal region. Latency reduction continued after six weeks of life due to sharp increases in conduction velocity of the afferent fibers. Different steps in the maturation of visual response specifity were found for neurons of different functional types. Possible relationships are discussed between the development of neuronal responses of pretectal cells and the maturation of oculomotor behavior.

While recording extracellularly from single neurons in the Lateral Geniculate Nucleus (LGN) of cats and monkeys, one often observes, in addition to action potentials, smaller and slower potentials which have been called 'S Potentials' (Bishop et al. 1962). Injection of the sodium channel blocker tetrodotoxin (TTX) into the eye of the cat causes spontaneous and light evoked S potentials to disappear. However, electrical stimulation of ganglion cells axons at the optic chiasm still elicits S potentials. Previous studies have shown that retinal ganglion cell impulses elicit S potentials. Our results prove that ganglion cell impulses are the ONLY source of these potentials; therefore, one can use them as a measure of ganglion cell input to the LGN.

The visual acuity (spatial resolving power) and the relative incidence of X- and Y-cells in laminae A and A1 of the dorsal lateral geniculate nucleus (LGN) with receptive fields within 10 ° visual angle to the area centralis have been determined in normal and monocularly deprived cats, using a square-wave grating with logarithmically varying spatial frequencies (0.05–8 c/deg).

The characteristics of the vestibulo-ocular reflex (VOR) of a normal cat can be modified in response to visual demands. Two aspects of the VOR are modifiable independently by a normal cat: the gain and the balance. An imbalance results in a spontaneous nystagmus and an asymmetric VOR. Neither the gain nor the balance of a dark-reared cat's VOR is susceptible to visual modification. A cat whose crossed visual pathways are severed at the level of the optic chiasm is able to modify the gain of the VOR but not its balance. Both dark-reared and split-chiasm cats have only very short-lasting optokinetic after-nystagmus.

The spatiotemporal distribution of excitation and inhibition has been characterized in 283 X-, Y- and W- cat retinal ganglion cells. Cells were classified by their latency from optic chiasm stimulation, responses to moving bars and gratings and responses to flashed gratings. As expected from earlier LGN studies the responses of retinal X-cells and Y-cells had distinct spatiotemporal profiles. The Y-cells had a relatively constant or homogeneous spatial distribution of responses while X-cells had a heterogeneous spatial arrangement. As has been previously reported in LGN, retinal X- and Y-fields had four independent spatiotemporal components or response domains; primary excitation (PE), primary inhibition (PI), secondary excitation (SE) and, finally, secondary inhibition (SI). In contrast, W-receptive fields had a few as a single response domain or as many as eight. An unexpected discovery was that all of these W-receptive fields were constructed from spatiotemporal domains, identical to those found in X-receptive fields. Thus, taking on-center and off-center cells into account we have found a total of 16 different spatiotemporal response components or "building blocks" for X- and Y-cells and the X-building blocks appear to be used in the construction of W-receptive fields.

Visual callosal transfer during development was studied in order to reveal plasticity-related compensation for the absence of direct contralateral inputs. The optic chiasm was midsagittally sectioned in 6–8 weeks old kittens (OCK) and for comparison, in adult cats (OCA). Unit recording was made during adulthood in the border area between visual cortex areas 17 and 18, namely the callosal projection zone. The proportion of cells showing interhemispheric transfer in the OCK group, as indicated by the presence of visual input from the contralateral eye was 10.5%; in the OCA cats their proportion was 4.0%. Moreover, 2.3% of the cells showed a pure transfer of input from the contralateral eye in the OCK, although none was seen in the OCA cats. Thus, during the developmental period, a plasticity induced process, albeit limited, takes place in the enhancement of interhemispheric transfer of visual information.

To test the possibility that glutamate and aspartate are transmitters at geniculo-cortical synapses and to elucidate which type of receptors for the excitatory amino acids (EAA) operate at these synapses, we studied effects of microionophoretic administration of EAA antagonists on the responses of visual cortical neurons to afferent electrical and visual stimulation in the cat. The antagonists used were kynurenate, a non-selective antagonist for all classes of EAA receptors and 2-amino-5-phos-phonovalerate (APV), a selective antagonist for N-methyl-D-aspartate (NMDA)-preferring receptors. The administration of kynurenate suppressed responses elicited by electrical stimulation of the dorsal lateral geniculate nucleus (LGN) and optic chiasm (OX) of 65% of the cells tested. This suppression was more marked for the short-latency responses which were evoked monosynaptically from the LGN, than for the longer-latency responses. In contrast with the effectiveness of kynurenate, APV failed to suppress electrically and visually elicited responses in 66% of the cortical cells. Such differences between kynurenate and APV were particularly prominent in layers IV and VI, which receive direct inputs from the LGN, but were less marked or were not recognizable in layers II + III and V. These results support previous suggestions that EAAs may be excitatory transmitters in the cerebral cortex, at least at geniculo-cortical synapses, and indicate further that EAA receptors of the “non-NMDA type” may be involved in afferent synaptic transmission in the cat's visual cortex.

Physiological and anatomical criteria were used to clearly establish the existence of a pretectal relay of visual information to the ipsilateral inferior olive in the macaque monkey. After injection of horseradish peroxidase into the inferior olivary nucleus, retrogradely labelled neurons were found in the nucleus of the optic tract (NOT) and the dorsal terminal nucleus of the accessory optic tract (DTN). The labelled cells were distributed in a sparse band arching below the margin of the brachium of the superior colliculus between the dorsal and lateral borders of the brainstem at the caudal edge of the pulvinar. Various types of cells could be distinguished. More superficially the cells were extremely spindle shaped, cells deeper within the midbrain had more compact somata. NOT-DTN neurons in the same region were also found to respond with short latencies to electrical stimulation of both the inferior olive and the optic chiasm. All neurons in the NOT-DTN which were antidromically activated from the inferior olive were also found to have direction specific binocular visual responses. Such neurons were excited by ipsiversive motion and suppressed by contraversive motion, regardless of whether large area random dot stimuli moved across the visual field or small single dots moved across the fovea. Direct retinal input to these neurons was via slowly conducting fibers (3-9 m/s) from the monkey's optic tract conduction velocity spectrum. As shown previously for non-primates, NOT-DTN cells may also in the monkey carry a signal representing the velocity error between stimulus and retina (retinal slip), and relay this signal into the circuitry mediating the optokinetic reflex.

In Amphibia all primary retinotectal nerve fibres cross at the chiasma. To investigate why this decussation takes place, a series of embryological operations were performed in which one host eye was replaced by an eye from the opposite side of a donor in Xenopus laevis. The visual projections to the optic tecta were mapped electrophysiologically in these animals when adult to reveal to which side of the brain the nerve fibres had become connected. In the majority of cases the contralaterally grafted eyes had developed primary projections to both tecta, in others all the fibres had innervated the contralateral tectum and in 2 animals optic axons were detected only from the ipsilateral side. In 2 other animals optic nerve fibres from both the normal and operated eyes had innervated both tecta. Control experiments showed that section of the optic nerve and stalk without any misalignment of the eye and stalk did not disturb the normal decussation of the fibres. Neither did enucleation in the embryo affect the decussation of the fibres from the remaining eye. It is concluded that no “side specificity” has been shown to exist to determine the side of the brain into which retinotectal fibres develop; rather it seems that complete decussation depends on the developing optic nerve fibres being able to enter a normally aligned optic stalk as they leave the eye.

The role of the corpus callosum in the mediation of binocular depth perception was examined by measuring monocular and binocular depth discrimination thresholds in cats which had undergone section of the corpus callosum shortly after birth. Three kittens had the posterior callosum sectioned at the age of eleven days. A fourth kitten underwent a sham operation and one additional animal served intitially as an unoperated control. Monocular and binocular depth thresholds were measured for all kittens when they were between three and five months old. Although there was some individual variability, none of the callosum-sectioned kittens showed any deficits of binocular depth perception relative to normal animals. The initially unoperated kitten had its callosum sectioned at five months and was retested following surgery. Its performance did not change from preoperated levels. Finally, the three neonatal callosum-sectioned kittens underwent section of the optic chiasm when they were six months old, causing a complete breakdown in binocular depth discrimination. The results are interpreted to indicate that although the corpus callosum may be a sufficient pathway for the maintenance of stereopsis in cat, it is not necessary.