Directional selectivities of visual afferents to the pretectal neuropil in the fire salamander

Single-unit recordings from visual afferents in the pretectal neuropil of Salamandra salamandra were performed to characterize the properties of direction specific units which are probably involved in the optokinetic reflex. It was shown that afferents from the contralateral eye were more sharply tuned than those from the ipsilateral eye. However, the majority of both were selective for temporonasal movements.     

Visual pathway for the optokinetic reflex in infant macaque monkeys

The horizontal optokinetic nystagmus (hOKN) in primates is immature at birth. To elucidate the early functional state of the visual pathway for hOKN, retinal slip neurons were recorded in the nucleus of the optic tract and dorsal terminal nucleus (NOT-DTN) of 4 anesthetized infant macaques. These neurons were direction selective for ipsiversive stimulus movement shortly after birth [postnatal day 9 (P9)], although at a lower direction selectivity index (DSI). The DSI in the older infants (P12, P14, P60) was not different from adults. A total of 96% of NOT-DTN neurons in P9, P12, and P14 were binocular, however, significantly more often dominated by the contralateral eye than in adults. Already in the youngest animals, NOT-DTN neurons were well tuned to different stimulus velocities; however, tuning was truncated toward lower stimulus velocities when compared with adults. As early as at P12, electrical stimulation in V1 elicited orthodromic responses in the NOT-DTN. However, the incidence of activated neurons was much lower in infants (40-60% of the tested NOT-DTN neurons) than in adults (97%). Orthodromic latencies from V1 were significantly longer in P12-P14 (x = 12.2 ± 8.9 ms) than in adults (x = 3.51 ± 0.81 ms). At the same age, electrical stimulation in motion-sensitive area MT was more efficient in activating NOT-DTN neurons (80% of the tested cells) and yielded shorter latencies than in V1 (x = 7.8 ± 3.02 ms; adult x = 2.99 ± 0.85 ms). The differences in discharge rate between neurons in the NOT-DTN contra- and ipsilateral to the stimulated eye are equivalent to the gain asymmetry between monocularly elicited OKN in temporonasal and nasotemporal direction at the various ages.     

Spatial response properties of contralateral inhibited lobula plate tangential cells in the fly visual system

This study describes the spatial response properties of a particular group of motion-sensitive and directionally selective neurons located in the lobula plate of the fly visual system. Their preferred motion direction is front-to-back (depolarization), and their null direction is back-to-front (hyperpolarization). They receive inhibitory input from the contralateral eye during pattern motion from back to front (regressive). In this study, we call these neurons regressive contralateral inhibited lobula plate tangential cells (rCI-LPTCs). Three physiologic groups of rCI-neurons (rCI-I, rCI-IIa, and rCI-IIb) can be distinguished on the basis of their ipsilateral pattern size dependence and their inhibitory contralateral input. rCI-I neurons depolarize during the motion of small ipsilateral patterns from front to back, but they become hyperpolarized by large ipsilateral patterns moving in the same direction. rCI-IIa and rCI-IIb neurons receive bidirectional inhibitory input from the contralateral eye. rCI-IIa neurons respond best to small ipsilateral pattern sizes, but unlike rCI-I neurons, their net response to large patterns is positive. rCI-IIb neurons respond best to large ipsilateral patterns. The anatomical and physiologic variability of the rCI-neurons suggests that more than three types of rCI-neurons exist. The suggested physiologic groups might be preliminary. We recorded one neuron that could mediate the bidirectional inhibitory input that rCI-IIa and rCI-IIb neurons receive from the contralateral eye. In the case of the rCI-IIa neurons at least one further contralateral inhibitory element has to be assumed. The tuning of rCI-IIa neurons to small ipsilateral pattern sizes is likely to be based on an on-center/off-surround organization of their synaptic input.     

The prenatal development of the cat's retinogeniculate pathway

The prenatal development of connections between the retina and the lateral geniculate nucleus (LGN) was studied by means of the anterograde axonal transport of 3H-amino acids or horseradish peroxidase injected intraocularly in fetal cats older than embryonic day 27 (E27) and in newborn cats. (Gestation is 65 days.) A retinothalamic pathway exists as early as E28, when label can be seen in both ipsilateral and contralateral optic tracts. Afferents from the contralateral eye are the first to invade the anlage of the LGN by E32 with those from the ipsilateral eye following about 3 days later. Initially, the pattern of labeling within the nucleus is uniform, suggesting that the two sets of afferents must share a good deal of territory at early ages. By E47, however, gaps appear in the labeling pattern contralaterally, indicating that afferents from the two eyes are beginning to segregate from each other. Segregation continues so that by E54 it is possible to identify unambiguously regions of the LGN destined to comprise ipsilateral and contralateral eye layers. By birth, afferent input appears adult-like in organization, with the two sets of afferents almost completely segregated from each other into their appropriate layers. Cellular lamination of the nucleus has just commenced, however, thereby lagging the onset of afferent segregation by about 2 weeks. Prenatal development could be followed much more easily in the horizontal than in the coronal plane of section due to the finding here that the LGN is displaced approximately 90 degrees in the horizontal plane between E40 and E60. Measurements of the area occupied by the ipsilateral and contralateral afferents within the LGN indicated that even prior to segregation, the two sets of afferents are not completely intermixed within the LGN. On the contrary, those from the contralateral eye retain almost exclusive control of some territory throughout development. This detail contrasts with development in primates, in which intermixing of afferents from the two eyes is thought to be complete early on (Rakic, P. (1976) Nature 261: 467-471). Nevertheless, in the cat, as in other mammals, development of the retinogeniculate pathway is broadly characterized by an initial period of overlap followed by a period of segregation that gives rise to the adult pattern of afferent input.     

Abnormal development of the retinogeniculate projection in Siamese cats

In the visual system of Siamese cats, the lateral geniculate nucleus (LGN) receives an abnormally large projection from the contralateral eye and a correspondingly reduced projection from the ipsilateral eye. In order to determine how this abnormal pattern of retinal input arises, the prenatal development of the retinogeniculate projection was studied in Siamese cats using the anterograde transport of intraocularly injected [3H]leucine and horseradish peroxidase. Labeled axons from the ipsilateral eye can be detected in the optic tract by embryonic day 30 (E30; gestation is 65 days), several days later than found in normally pigmented animals. The ipsilateral projection is not only apparently delayed but also is reduced in size as compared with normal animals, and this reduction persists throughout development, indicating that the Siamese mutation acts to misdirect growing optic axons to the contralateral side of the brain as originally proposed (Guillery, R. W. (1969) Brain Res. 14: 739-741). The effect of an altered retinal projection on the ingrowth and segregation of retinal fibers to the LGN was also examined. In Siamese fetuses, not until E41 can significant label be seen within the ipsilateral LGN as compared to E35 in normally pigmented fetuses. As in normal animals, in Siamese fetuses, also, the labeled retinogeniculate afferents from the two eyes initially overlap within regions of the LGN before segregating into layers. However, measurements of the area occupied by labeled afferents from the ipsilateral and contralateral eyes indicate that maximum overlap of the two sets of afferents, although close to normal in amount, does not occur until about E51--again several days later than in normally pigmented animals (E47). The time course of segregation is also altered in Siamese cats. The onset of segregation, as signaled by the removal of contralateral eye afferents from territory destined for the ipsilateral eye and by the restriction of ipsilateral eye afferents, does not occur until about E51 in Siamese cats as compared with E47 in normally pigmented animals. Despite this delay in onset, the final segregation of the two sets of afferents in Siamese cats reaches adult-like levels at about the normal time. Thus, the misrouting of axons at the optic chiasm in Siamese cats not only alters the final pattern of innervation from the two eyes within the LGN, but also delays the onset and shortens the total duration of segregation itself.     

Retinal projections and functional architecture of cortical areas 17 and 18 in the tyrosinase-negative albino cat

The visual field representation and functional architecture of cortical areas 17 and 18 in albino cats were studied. In the same animals the distributions of ipsilaterally and contralaterally projecting retinal ganglion cells were determined by injecting horseradish peroxidase into the dorsal lateral geniculate nucleus or optic tract. All cats were tyrosinase-negative albinos (cc), not deaf white cats (W). The proportion of ipsilaterally projecting ganglion cells in the temporal retina of the albino cat was found to be much smaller than in the normal cat or in the Siamese cat. In the albino cat less than 5% of ganglion cells in temporal retina project ipsilaterally. Recordings from areas 17 and 18 provided evidence of a substantial representation of the ipsilateral hemifield in albino visual cortex; cells representing the contralateral and ipsilateral hemifields were often segregated into alternating zones in area 17 and were always segregated in area 18. Cells recorded at the borders of zones representing the ipsilateral and contralateral hemifields often had abnormal properties. Some border cells had two receptive fields separated by as much as 60 degrees of azimuth; one field subserved the contralateral hemifield (contralateral nasal retina) and the other subserved the mirror-symmetric part of ipsilateral hemifield (contralateral temporal retina). Receptive fields of cells subserving the two hemifields did not differ in size. The preferred orientations, preferred velocities, and other characteristics of the two fields were approximately the same; preferred orientation changed gradually and systematically across the borders of zones representing the two hemifields. Our results indicate that afferents representing nasal and temporal regions of retina of the same eye can segregate and form "hemiretina" domains in albino visual cortex. These afferents can also converge upon individual cortical cells in a fashion reminiscent of convergence of afferents from the two eyes upon binocular cells in the normal cortex. The organization of albino visual cortex is therefore different from the organization of Siamese visual cortex. This may be because, in the albino cat but not the Siamese cat, nearly all cells in temporal retina project contralaterally; afferents representing contralateral temporal retina are not at a significant competitive disadvantage in the albino.     

Retinotopic organization in the dorsal lateral geniculate nucleus of the tammar wallaby (Macropus eugenii)

Electrophysiological recordings were made from 187 single cells in the tammar wallaby (Macropus eugenii) dorsal lateral geniculate nucleus (LGNd). The results show that it is topographically organized such that the superior visual field is represented dorsally, the inferior field is represented ventrally, the nasal visual field is represented caudally, and the temporal visual field is represented rostrally. The visual field of one eye ranges from -30 degrees nasal to +179 degrees temporal in azimuth and +73 degrees superior to -49 degrees inferior in elevation. Ganglion cells that had receptive field positions between -9 degrees and +179 degrees projected to the contralateral LGNd while the ganglion cells that projected to the ipsilateral LGNd had visual fields from 0 to +30 degrees. The binocular visual field extends 60 degrees in azimuth. This representation in the LGNd is expanded relative to the monocular representation. There is also an increased representation of the horizon in the temporal field corresponding to the visual streak of retinal ganglion cells. The binocular visual field is located where contralateral and ipsilateral laminae are shown to interdigitate by proline autoradiography. There are nine eye-specific laminae in the LGNd. Four receive afferents from the contralateral eye and five receive afferents from the ipsilateral eye. The lines of isoelevation are perpendicular to the coronal plane of section while the lines of isoazimuth are nearly parallel to the coronal plane. The lines of projection representing one visual direction are inferred to be perpendicular to the tangent of curvature of the laminae as in the LGNd of other mammals. The majority of cells (85%) recorded had on- or off-centre responses. On- and off-centre responses were not apparently segregated in the LGNd but segregation may not have been revealed by the single-unit recording technique.     

Postnatal development of primary visual projections in the tammar wallaby (Macropus eugenii)

The time course and pattern of retinal innervation of primary visual areas was traced in pouch-young wallabies. Tritiated proline was injected into one eye of animals ranging in age from 1 to 72 days after birth. These results are compared to the 11 primary visual areas found in the adult wallaby, seven of which receive binocular input while four are monocular. At birth retinal ganglion cell axons have not reached any visual areas. Two to 4 days after birth, all of the axons are crossing to the contralateral optic tract. Nine to 12 days after birth axons begin to invade the contralateral lateral geniculate nucleus, the superior colliculus, and the medial terminal nucleus. Twenty to 21 days after birth, ipsilateral axons invade the lateral geniculate nucleus and superior colliculus. The contralateral projection precedes the ipsilateral projection in all binocular visual areas. By 25 days, ipsilateral and contralateral afferents share common territory in the lateral geniculate nucleus; however, afferents from each eye are initially concentrated in appropriate areas. Between 52 and 72 days, afferents to the dorsal lateral geniculate nucleus are gradually segregated into nine terminal bands. Four are contralateral while five are ipsilateral. By 72 days, the ipsilateral component to the superior colliculus is clustered beneath the contralateral projection a deeper layer. Projections to four monocular visual areas--lateral posterior nucleus, dorsal terminal nucleus, lateral terminal nucleus, and nucleus of the optic tract--are established later than binocular visual areas, except the suprachiasmatic nucleus. The suprachiasmatic nucleus is the last to be bilaterally innervated even though it is situated closest to the optic chiasm. At the light microscope level a mature pattern of visual development is emerging by 72 days, although the eyes do not open until 140 days.     

Anatomical and physiological characteristics of vestibular neurons mediating the horizontal vestibulo-ocular reflex of the squirrel monkey

The anatomical characteristics of vestibular neurons, which are involved in controlling the horizontal vestibulo-ocular reflex, were studied by injecting horseradish peroxidase (HRP) into neurons whose response during spontaneous eye movements had been characterized in alert squirrel monkeys. Most of the vestibular neurons injected with HRP that had axons projecting to the abducens nucleus or the medial rectus subdivision of the oculomotor nucleus had discharge rates related to eye position and eye velocity. Three morphological types of cells were injected whose firing rates were related to horizontal eye movements. Two of the cell types were located in the ventral lateral vestibular nucleus and the ventral part of the medial vestibular nucleus (MV). These vestibular neurons could be activated at monosynaptic latencies following electrical stimulation of the vestibular nerve; increased their firing rate when the eye moved in the direction contralateral to the soma; had tonic firing rates that increased when the eye was held in contralateral positions; and had a pause in their firing rate during saccadic eye movements in the ipsilateral or vertical directions. Eleven of the above cells had axons that arborized exclusively on the contralateral side of the brainstem, terminating in the contralateral abducens nucleus, the dorsal paramedian pontine reticular formation, the prepositus nucleus, medial vestibular nucleus, dorsal medullary reticular formation, caudal interstitial nucleus of the medial longitudinal fasciculus, and raphé obscurus. Eight of the cells had axons that projected rostrally in the ascending tract of Deiters and arborized exclusively on the ipsilateral side of the brainstem, terminating in the ipsilateral medial rectus subdivision of the oculomotor nucleus and, in some cases, the dorsal paramedian pontine reticular formation or the caudal interstitial nucleus of the medial longitudinal fasciculus. Two MV neurons were injected that had discharge rates related to ipsilateral eye position, generated bursts of spikes during saccades in the ipsilateral direction, and paused during saccades in the contralateral direction. The axons of those cells arborized ipsilaterally, and terminated in the ipsilateral abducens nucleus, MV, prepositus nucleus, and the dorsal medullary reticular formation. The morphology of vestibular neurons that projected to the abducens nucleus whose discharge rate was not related to eye movements, or was related primarily to vertical eye movements, is also briefly presented.     

Functional organization of primary visual cortex in the mink (Mustela vison), and a comparison with the cat

The functional organization of geniculocortical afferents and the visual responses of neurons in primary visual cortex (area 17) were studied in barbiturate-anesthetized, paralyzed minks and cats. Responses of the afferents were studied after silencing intrinsic cortical activity with injections of kainic acid. In both species, afferents were segregated into patches on the basis of eye of origin. In the mink, but not in the cat, there was a further segregation on the basis of center type, with on- and off-center afferents terminating in alternating, partially overlapping patches. The visual responses of cortical neurons in the mink showed many similarities to those in the cat. Nearly all units were orientation-selective, and there was a columnar organization for preferred orientation. Many units were selective for one direction of movement. Within the binocular segment of cortex, although many units could be driven from either eye, there was a marked bias toward the contralateral eye compared to the cat. There was a columnar system for ocular dominance, but contralateral eye columns were wider than ipsilateral. In both species, a quantitative study was made of the responses of cortical neurons to stationary, flashing slits as a function of position in the receptive field. In the mink, and less clearly in the cat, units could be identified as simple or complex on the basis of the spatial separation or overlap of "on" and "off" discharge zones. In both species, simple cells were found most commonly in layers IV and VI, while layer V contained the greatest proportion of complex cells. The relative strengths of the on and off discharges of single cells were also measured. In the mink, many units gave better overall responses to the on or off phase of the stimulus, and 15% showed a strong (greater than 9:1) preference for one or the other, compared to 4% in the cat. In the mink, units with a common preference for the on or off phase of stationary stimuli were arranged in columnar aggregates, a feature of cortical organization that was not found in the cat. These columns probably result from the partial segregation of on-center and off-center geniculate afferents within layers IV and VI of the mink's cortex. On-dominated columns were, however, wider or more numerous than off-dominated columns.     

Bilateral and multimodal sensory interactions of single cells in the pigeon's midbrain

Standard microelectrode recording techniques were employed to monitor single unit activity in the pigeon's nucleus intercollicularis and medial substantia grisea et fibrosa periventricularis in response to visual, tactile and auditory stimuli. Approximately 40% of the units were driven exclusively by visual stimuli, 8% by tactile stimuli, 47% by both visual and tactile stimuli and a very small percentage by auditory stimuli. Visual receptive fields were generally excitatory in the contralateral eye and suppressive in the ipsilateral eye. Most units were movement selective and some demonstrated direction sensitivity, summation and habituation. Units were generally insensitive to stimulus shape or contrast reversal. Somatosensory receptive fields were located on both sides of the body and were either excitatory or suppressive or both. Ipsilateral visual and somatosensory bimodal inputs were most often of the same sign while ipsilateral visual and contralateral somatosensory bimodal inputs tended to be of opposite sign. Visual and somatosensory receptive field locations of bimodal units tended to be in register.     

Neural activity of nucleus reticularis tegmenti pontis — the origin of visual mossy fiber afferents to the cerebellar flocculus of rabbits

Activities of 55 neurons were extra- and intracellularly recorded in the nucleus reticularis tegmenti pontis (Nrt) of anesthetized rabbits. The cells were antidromically activated from the flocculus as well as orthodromically from the optic tracts. They were antidromically activated from either the ipsilateral or contralateral flocculus (latency, 0.94 msec) but not from the flocculi on both sides. There was no preference for projection to either the ipsilateral or contralateral flocculus. Twenty out of 55 neurons were orthodromically activated from the ipsilateral and 15 neurons from the contralateral optic tract. The remaining 20 neurons were excited from the optic tract on both sides. The orthodromic latencies were all in the same range with a mean of 4.7 msec. These findings demonstrate the existence of neurons in the Nrt that transfer visual signals to the flocculus through mossy fiber afferents. The indicate that Nrt neurons may contribute to cotrolling eye movements through the flocculus.     

Latent defect in monocular optokinetic nystagmus after neonatal removal of the lateral suprasylvian area in the cat

Three kittens underwent unilateral removal of the lateral suprasylvian area of cortex at the age of 1 month. After normal rearing for two years, their monocular optokinetic nystagmus was studied. During the experiment one eye was 'seeing' the optokinetic stimulus, but the other eye was 'covered'; by implanting scleral coils on both eyes, we measured movements not only in the 'seeing' eye, but also in the 'covered' eye. The stimulus was moved at a velocity between 1 and 40 degrees/s. Additionally, movements of the both eyes were simultaneously recorded in a normal cat. The previous results on movements of both eyes in normal cats which had been derived from the recordings by one coil (Vision Res., 26: 1311-1314, 1986) were confirmed by this experiment and were compared with the results of the lesioned cats. The gains (slow phase velocity/stimulus velocity) of the 'seeing' eye were not significantly different from the normal values. However, the gains of the 'covered' eye were significantly higher than the normal values when the stimulus was presented in the temporonasal direction, at the velocities between 1 and 40 degrees/s to the eye ipsilateral to the lesion and at the velocity of 40 degrees/s to the eye contralateral to the lesion; in the other conditions of stimulation the gains were not significantly different from the normal values.     

Vestibulo-oculomotor connections in an elasmobranch fish,the atlantic stingray,Dasyatis sabina

In elasmobranch fishes, including the Atlantic stingray, the medial rectus muscle is innervated by the contralateral oculomotor nucleus. This is different from most vertebrates, in which the medial rectus is innervated by the ipsilateral oculomotor nucleus. This observation led to the prediction that the excitatory vestibulo-extraocular motoneuron projections connecting each semicircular canal to the appropriate muscle should use a contralateral projection from the vestibular nuclei to the motoneurons. This hypothesis was examined in the Atlantic stingray by injecting horseradish peroxidase unilaterally into the oculomotor nucleus. It was found that vestibulo-oculomotor projections arise from the ipsilateral anterior octaval nucleus and the contralateral descending octaval nucleus. The same pattern was observed when the trochlear nucleus was involved in the injection. There were no cells labeled in the region of the abducens nucleus, and no candidate for a nucleus prepositus hypoglossus was identified. The presence of compensatory eye movements, the directional sensitivity of the semicircular canals, the location of the motoneurons innervating each eye muscle, and our results indicate that the excitatory input to the extraocular motoneurons is derived from the contralateral descending octaval nucleus, and the inhibitory input is derived from the ipsilateral anterior octaval nucleus. The absence of both abducens internuclear interneurons and a nucleus prepositus hypoglossus suggests that eye movements, particularly those in the horizontal plane, are controlled differently in elasmobranchs than in other vertebrates examined to date. © 1994 Wiley-Liss, Inc.     

Binocular depth vision in the rabbit

The binocular area of the rabbit's right visual cortex was explored. A composite plot of all the single-unit responses of this area showed that the ipsilateral component of the animal's binocular visual field was nasotemporally wider than the contralateral component. The receptive fields of individual units of the ipsilateral eye were larger with less well-defined boundaries than those the contralateral eye. Forty-one percent of the population of the ipsilateral receptive fields showed light response. Movement sensitivity was more common in the units of the ipsilateral eye: 56.6% showed sensitivity to a random target movement, 34.2% showed sensitivity to unidirectional movement of a target, 7.9% to bidirectional movement of a target, and 1.3% were insensitive to movement of any kind. Though no columnar organization of the cortical units was observed in the rabbit's binocular area, clusters of binocularly responding units were encountered at different depths of the animal's visual cortex. As an electrode was driven deeper into the cortex, the units of such a cluster showed a pronounced temporal shift of individual receptive fields in their localization in the ipsilateral part of the animal's visual field. Such a shift was less pronounced in the contralateral field. Based on these observations, a model was produced and a possible means for the rabbit to achieve useful binocular vision was discussed.     

Time course for the reappearance of vibrissal motor representation following botulinum toxin injection into the vibrissal pad of the adult rat

The present study investigates the time course and pattern of movement representation recovery in the motor cortex during the recovery after a peripheral paralysis. To this end a transitory flaccid paralysis of the vibrissae muscle was induced in adult rats that underwent two unilateral injections of 8 U of botulinum toxin (BTX) into a vibrissal pad, at a duration of 12 days from one another. The compound muscle action potential (MAP) of the vibrissae muscle began to reappear 4 weeks after the first BTX injection. Intracortical microstimulation (ICMS) was used to map rat motor cortices 4, 5, 6, 7 and 8 weeks after the first BTX injection. Findings demonstrated that: (i) contralateral vibrissae movement reappears in the medial part of its normal cortical territory when the MAP is almost 10% of the control value; in the remaining part, ICMS elicits eye, ipsilateral vibrissae, neck and forelimb movements; (ii) the contralateral vibrissae movement reappears in sites where ipsilateral vibrissae and/or neck movement are co-represented; (iii) as MAP recovers, the vibrissae representation expands until it recovers the 90.8% of its territory after 7 weeks, when the MAP was almost 43.4% of the control value; (iv) from 4 to 7 weeks, the ICMS-evoked contralateral vibrissae movement shows a significantly higher electrical threshold vs. the control group; (v) recovery of the baseline excitability uniformly involves the vibrissae representation 1 week later, after its cortical territory has recovered 93.1% of the control value and the MAP has returned to 78.8% of the baseline value.     

Recovery of the ipsilateral oculotectal projection following nerve crush in the frog: evidence that retinal afferents make synapses at abnormal tectal locations

The ipsilateral oculotectal projection in the frog is a topographic mapping of the binocular part of the visual field of one eye on the ipsilateral tectal lobe. The underlying neuronal circuitry consists of the topographic, crossed retinotectal projection and an intertectal pathway which relays information from a given point in one tectal lobe to the visually corresponding point in the other. During optic nerve regeneration, there is a period when the terminals of retinotectal afferents are found at abnormal locations in the opposite tectal lobe. Whether they form functional synapses at this time is not known. If so, one would expect to observe correlated abnormalities in the ipsilateral oculotectal projection. To determine whether such abnormalities exist, we have made parallel electrophysiological studies of the recovery of the retinotectal and ipsilateral oculotectal projections following crush of one optic nerve. The earliest stage of recovery was characterized by a lack of significant topographic order in the retinotectal projection and by the absence of a physiologically observable ipsilateral projection. Within a short time, the retinotectal projection became topographically organized and a similarly organized ipsilateral projection appeared. While topographic, the retinotectal projection at intermediate times was abnormal in that the multiunit receptive fields recorded at individual tectal loci were greatly enlarged. Multiunit receptive fields were similarly enlarged in the ipsilateral projection. In addition, some ipsilateral fields included areas of visual space not normally represented in the projection. The abnormalities in both projections subsequently disappeared over the same time course. Throughout recovery there was a high correlation between multiunit receptive field sizes in the contralateral tectal lobe and those at visually corresponding points in the ipsilateral tectal lobe. Enlarged multiunit receptive fields in the contralateral tectal lobe could not be accounted for in terms of optical or retinal abnormalities since single unit receptive field sizes were normal. Nor could they be accounted for in terms of changes in recording characteristics since simultaneously recorded fields activated by the undisturbed eye were normally sized. We conclude that the enlarged fields in the contralateral tectal lobe indicate the presence at individual tectal loci of afferents from wider than normal retinal regions. Similar considerations ruled out optical, retinal, and recording abnormalities as the explanation for the enlarged multiunit receptive fields in the ipsilateral tectal lobe.(ABSTRACT TRUNCATED AT 400 WORDS)     

Analysis of local and wide-field movements in the superior temporal visual areas of the macaque monkey

The middle temporal (MT) and medial superior temporal (MST) areas of the macaque cortex have many cells that respond to straight movements in the frontoparallel plane with directional selectivity (D cells). We examined their responses to movements of a bar, of a wide dot pattern, and to combined movements of the two in anesthetized and immobilized animals. D cells in MT showed a wide variety in the strength of the inhibitory field surrounding the excitatory center field. Responses of SI+-type cells to a bar moving across the excitatory field were suppressed when a wide dot pattern moved over the surround field in the same direction and at the same speed as the bar. Inhibition was selective to the direction and speed of the surround movement, and the effective area for inhibition occupied a wide area, which expanded in all radial directions. Responses of SI- -type cells to a center bar movement were changed little by a conjoint movement over the surround field. Consequently, SI- -type cells responded to wide-field movement as well as to stimuli confined within the excitatory field. Although D cells in MST commonly had large excitatory fields, a proportion of them (Figure type) responded to bar movement much more strongly than to wide-field movement. Their responses to a bar movement were suppressed direction-selectively by a conjoint movement of a wide dot-pattern background. The effective area for inhibition coexisted with the excitatory field in these cells. MST cells of the Nonselective type responded comparably well to the two stimuli, and those of the Field type responded much more strongly to wide-field movement than to bar movement. It is thus suggested that MT cells of the SI+ type and MST cells of the Figure type can detect a difference between movements of an object and its wide background, whereas MST cells of the Field type can detect a conjoint movement of a wide field, neglecting the movements of a single object.     

A population of ascending intersegmental interneurones in the locust with mechanosensory inputs from a hind leg

A population of some 35 intersegmental interneurones with somata in the metathoracic ganglion has been characterized by intracellular recording and staining. These interneurones integrate signals from extero- and proprioceptors on a hind leg. The somata are clustered in an anterior and lateral region of the dorsal cortex, and the axons project to more anterior ganglia in either the ipsilateral or contralateral connectives. Some of these interneurones are excited by afferents from a proprioceptor at the femorotibial joint, the femoral chordotonal organ. An afferent spike evokes a chemically mediated EPSP in an interneurone with a latency and consistency that suggest that the connection is direct. An individual interneurone codes particular features of the movement about the femorotibial joint, responding to flexion, extension, or both directions of movement with either phasic or tonic responses. These interneurones have an extensive field of fine branches ipsilateral to the hind leg from which they receive input. These branches are in lateral and intermediate regions of neuropil to which the afferents of the chordotonal organ also project. Axonal branches, from either an ipsilateral and contralateral axon, are sparse and varicose and occur in dorsal neuropil. Other interneurones are excited by afferents from exteroceptive hairs (trichoid sensilla). An individual interneurone is excited by a particular array of hairs on specific regions of a hind leg. The connections between the afferents and the interneurones appear direct. These interneurones have a dense and compact array of fine branches ipsilateral to the hind leg from which they receive input. These branches are in the most ventral region of neuropil, to which the hair afferents also project. Branches from the ipsilateral axons are sparse and varicose and occur in more dorsal neuropil. The interneurones can thus provide the more anterior ganglia with precise information about the movement of a joint in a hind leg and of the location of an exteroceptive stimulus. This information would be of importance in ensuring the correct co-ordination of the legs during walking.     

Basic optokinetic-ocular reflex pathways in the frog

Frogs (Rana temporaria) have two midbrain nuclei that receive contralateral retinal afferents, and whose neurons respond to optokinetic stimulation. The basal optic nucleus is composed of direction-selective neurons with different response types. One type is activated exclusively by upward moving optokinetic targets; another type is activated only by downward moving targets. Two other types of basal optic neurons show this vertical preference, but each is also activated by patterns moved horizontally from the nasal to temporal visual field. No activation of these cells was found with patterns moved horizontally from the temporal to nasal visual fields. Rather, cells in a discrete pretectal region have this type of sensitivity: they increase their resting rate with temporal to nasal stimulation and decrease it with nasotemporal stimulation. Oculomotor neurons (antidromically identified) have similar optokinetic sensitivities. As with basal optic neurons, these cells have exclusively upward or downward sensitivity, and some also have nasotemporal sensitivity. An additional type of oculomotor neuron and abducens motoneurons are activated by temporonasal pattern movement. In general, the extraocular motoneurons have similar velocity and pattern size preferences, as have the sensory nuclei. Investigations of the connectivity between the sensory and motor nuclei were primarily restricted to the relation between the pretectum and the abducens. A monosynaptic connection between the pretectum and the abducens is suggested by four points: (1) excitatory postsynaptic potential onset latency in antidromically identified abducens motoneurons, following optic nerve stimulation, is consistent with the interpretation of a disynaptic pathway to the abducens from the retina; (2) pretectal cells, sensitive to optokinetic stimulation, can be activated antidromically from stimulation of the abducens nucleus; (3) horseradish peroxidase injections into the pretectum result in labeling of axons, which terminate in the abducens nucleus; (4) horseradish peroxidase injections into the abducens result in labeling of cells in the pretectal region, where optokinetically sensitive cells are found. In the frog, there seem to be three-neuronal retino-ocular reflexes mediating optokinetic slow phase behavior as there are three-neuronal vestibulo-ocular reflexes that also mediate compensatory spatial behavior. It is suggested that these direct connections act to initiate ocular movements and accelerate the eye, whereas more indirect pathways may act to maintain eye position.