Tectofugal Visual Pathway Research Articles

Cytochrome oxidase activity reveals parcellations of the pigeon's ectostriatum.

The endogenous cytochrome oxidase activity of the pigeon's ectostriatum, the primary telencephalic structure of the tectofugal visual pathway, was histochemically demonstrated and a heterogeneous distribution of the reaction product was observed. In cross-sections the medial, central and ventrolateral parts of the ectostriatum showed high levels of activity while the centroventral and dorsolateral ectostriatum remained weakly labelled. Only slight left-right and interindividual variations were found in the pattern of labelling. These data demonstrate for the first time anatomical subdivisions within the ectostriatal core and open the possibility of functional parcellations within this structure.

Unilateral optic nerve transection decreases 2-[ 125I]-iodomelatonin binding in retinorecipient areas and visual pathways of chick brain

In chick brain, specific 2-[ 125I]-iodomelatonin-binding was localized primarily in the visual system, i.e., retinorecipient and relay nuclei and fiber tracts of the tectofugal, thalamofugal, circadian and accessory visual pathways. Unilateral transection of the optic nerve (ONX) significantly reduced the binding of 2-[ 125I]-iodomelatonin (75 pM) in many, but not all, primary retinal targets and visual pathways at 7 and 14 days, but not 1 day, postlesion. As measured using quantitative autoradiography, 2-[ 125I]-iodomelatonin binding was decreased by 90% in both the central portion of the lesioned optic tract and one of its targets, the nucleus of the basal optic root (nBOR). Other retinorecipient areas exhibiting substantial decreases (60%) in 2-[ 125I]-iodomelatonin-binding included the optic tectum, lateroventral and dorsolateral geniculate nuclei and tectal gray area contralateral to the lesion. These findings are consistent with the hypothesis that melatonin receptors are located presynaptically on incoming optic nerve terminals in many retinorecipient areas. This localization may account for most of the binding sites in nBOR. In other primary visual areas, however, melatonin receptors also appear to be located on postsynaptic cells and/or non-retinal afferents. ONX had no significant effect on 2-[ 125I]-iodomelatonin binding in two retinorecipient areas, the visual suprachiasmatic nucleus and the dorsolateral anterior thalamus, which are part of the circadian/oculomotor and thalamofugal pathways, respectively. An unexpected consequence of ONX was that 2-[ 125I]-iodomelatonin binding was decreased in certain secondary (nucleus rotundus, isthmi nuclei) and tertiary level (ectostriatum) nuclei along the prominent tectofugal visual pathway. Binding in the tectorecipient nucleus triangularis was not significantly altered, however. Analysis of secondary level relay nuclei in the oculomotor pathway revealed that binding after ONX was decreased in the ipsilateral Edinger-Westphal nucleus but not in the oculomotor nuclei. Selective transsynaptic changes in 2-[ 125I]-iodomelatonin binding after lesion of the visual input most likely reflect activity-dependent regulation and functional plasticity of central melatonin receptors.

Color-reversal learning: effects after lesions of thalamic visual structures in pigeons.

The performance of pigeons on a color-reversal learning task was assessed after thalamic lesions disrupting the thalamofugal and tectofugal visual pathways. Successful performance of a simultaneous color discrimination was accomplished after surgery, and a series of reversals of the original discrimination followed during which the positive and negative consequences associated with the stimuli were interchanged. Shimizu and Hodos (1989) had reported that lesions of two laminae in the visual wulst (IHA and HD), both targets of the avian thalamofugal pathway, resulted in increased errors in a color-reversal learning task in pigeons. This finding suggested that the thalamofugal pathway might play a role in visual discrimination involving stimulus context changes. In the present study, lesions of the OPT complex (the thalamic source of afferents to IHA and HD) were found to have no effect on color-reversal learning performance. Instead, we found that damage to nucleus rotundus (the thalamic component of the tectofugal pathway) resulted in deficits that were far in excess of those that had been obtained after IHA and HD lesions. We suggest that the color-reversal learning deficits after Wulst lesions are not due to the Wulst's connections with the thalamofugal pathway, but rather to its connections with the tectofugal pathway.

Effects of dorsal and medial cortex lesions on reversals in turtles

Two experiments were performed to investigate the effect of cortical lesions on the acquisition and reversal of simultaneous discriminations in turtles. The first experiment examined the effect of cortical lesions on the acquisition and reversal of a spatial discrimination. The results of the first experiment revealed that lesions of the dorsal cortex produced a deficit in spatial learning. The results of the first experiment also revealed that when damage to the dorsal cortex was accompanied by substantial damage to the medial cortex, no deficit was manifest. The second experiment examined the effects of cortical lesions on the acquisition and reversal of a brightness discrimination. The results of the second experiment revealed that damage to neither the dorsal cortex nor the medial cortex produced a deficit. It was suggested that brightness is not represented in the thalamofugal visual pathway but is instead represented in the tectofugal visual pathway in reptiles. It was also suggested that the medial cortex, which is the evolutionary precursor to the mammalian hippocampal formation, functions differently from the mammalian hippocampus.

Flash evoked potentials in the ectostriatum of the zebra finch: a current source-density analysis.

Recent research has demonstrated that ipsilaterally visually evoked potentials (VEPs) can be measured within the ectostriatum, the telencephalic target area of the tectofugal visual pathway in birds. In this paper we systematically measured contra- and ipsilateral VEPs within the ectostriatal complex to obtain more detailed information on the processing of contra- and ipsilateral stimuli. The similarity of neighbouring VEPs at equal depth and a comparison of a one dimensional and a three dimensional analysis of current source-densities (CSDs) for identical coordinates suggested that a one dimensional current source-density analysis might be applicable. The one dimensional current source-density analysis demonstrated largely corresponding patterns in the sink-source sequences of the current source-density depth profiles for the contra- and ipsilateral stimulus responses. The occurrence of a large sink in the centre of the ectostriatal core, together with the results of multiunit recordings, shows that the ectostriatal core is the location of the generators for both the contra- and the ipsilaterally evoked responses. The occurrence of macroscopic sinks and sources and the fact that VEPs can be recorded from the ectostriatum shows that there is a higher degree of order in the ectostriatum than has been previously demonstrated by anatomical methods. The time coincidence between the maximum spike rate of multiunit responses, the negative peak of the evoked potential, and the large central sink demonstrates that the influence of ipsi- as well as of contralateral stimuli is predominantly excitatory.

The sensitive period for the morphological effects of monocular deprivation in two nuclei of the tectofugal pathway of zebra finches

Previous experiments with 2-deoxyglucose (2-DG) suggested the existence of a critical period for the effects of monocular deprivation in the nucleus rotundus of zebra finches. The present study concerns the time course of this sensitive period for the morphological effects of monocular deprivation in two areas of the tectofugal visual pathway of zebra finches, the nucleus rotundus of the thalamus and the telencephalic ectostriatum. Cell size and volume changes were measured in birds subjected to 40 days of unilateral eye closure starting at ages spaced regularly throughout the first 70 days of life. The results show that monocular deprivation markedly affects cell size in both areas if the treatment starts at one or 10 days posthatch. The differences between deprived and non-deprived neurons decline monotonically with increasing visual experience prior to deprivation. However, deprivation onset at day 40 again causes as severe effects as early monocular closure. Deprivation as from day 50 or later no longer leads to abnormalities. The measurements of the volume of the nucleus rotundus parallel the cell size measurements, with the exception that the second increase in sensitivity occurs with deprivation onset at day 50 instead of day 40. These data indicate that the time course of the sensitive period for the effects of monocular deprivation may be double-peaked: the sensitivity for external stimuli declines from hatch until day 30, but has another peak at 40–50 days of life. The definite end of the sensitive period, as determined with this method, can therefore be assumed to be at around day 50–60.

A second ascending visual pathway from the optic tectum to the telencephalon in the pigeon (Columba livia).

Previous studies in the pigeon (Karten and Revzin: Brain Res. 2:368-377, '66; Karten and Hodos: J. Comp. Neurol. 140:35-52, '70) have described an ascending tectofugal visual pathway from the optic tectum to the ectostriatum by way of the nucleus rotundus of the thalamus. This present study used anterograde autoradiographic and retrograde horseradish peroxidase pathway-tracing techniques to investigate another ascending tectofugal pathway in the pigeon. Injections of 3H-proline/leucine confirmed a previous report that the optic tectum projects to the nucleus dorsolateralis posterior of the thalamus (DLP). This projection is predominantly ipsilateral and is confined to a large-celled caudal region of the nucleus (DLPc); the rostral region of the nucleus (DLPr) is not tectorecipient. Injections of horseradish peroxidase in DLPc labeled cells predominantly ipsilaterally in layers 8-15 of the optic tectum. Injections of 3H-proline/leucine placed in the DLPc labeled a discrete region of the ipsilateral telencephalon. Similar injections of DLPr labeled a contiguous, but more rostral, region of the neostriatum intermedium. Nissl- and silver-stained material indicated that the region in which DLP terminates is cytoarchitecturally distinct from ventromedial ectostriatal core and belt. Injections of horseradish peroxidase at various locations in the neostriatal DLP terminal field demonstrated a rostrocaudal ordering of the DLP projection upon the neostriatum intermedium. Single-unit recording demonstrated that cells in DLPc respond to whole-field illumination at the same latency as cells in the nucleus rotundus, indicating that the tecto-DLPc-neostriatal pathway transmits visual information to the telencephalon. We suggest that comparable pathways may exist in both reptiles and mammals.

Discrimination of mirror-image stimuli after lesions of the visual system in pigeons.

Nine pigeons were trained to perform a simultaneous discrimination task with stimuli that were lateral mirror images, vertical mirror images and nonmirror images. All subjects acquired the discriminations rapidly and at approximately equal rates. Following training, bilateral stereotaxic lesions were made in either the visual wulst or ectostriatum, which are telencephalic components of the thalamofugal and tectofugal visual pathways, respectively. After surgery both groups were retrained to their preoperative performance levels or for a maximum of 140 sessions. The performance of the four subjects that received visual wulst lesions was only mildly and transiently impaired and was equally disrupted on each pattern discrimination. The performance of the five subjects that received ectostriatal lesions, however, was markedly and persistently impaired on all pattern discriminations. The impairment was most severe and sustained on the lateral mirror-image discrimination problem in all subjects. Only three of the five subjects with ectostriatal lesions reached their preoperative performance levels on the lateral mirror-image problem, whereas all subjects returned to their preoperative performance levels on the other problems. Possible reasons for this selective deficit in lateral mirror-image pattern discrimination are discussed in relation to interhemispheric pathways and the relative importance of the thalamofugal and tectofugal visual pathways in birds and in mammals.

Interocular transfer in parallel visual pathways in pigeons.

Pigeons were trained to perform intensity, color and pattern tasks monocularly. After their training was completed, a unilateral electrolytic lesion was made either in the nucleus rotundus or in the nucleus opticus principalis thalami (OPT). The lesion was made in the trained hemisphere (contralateral to the trained eye) in half of the subjects and in the untrained hemisphere in the other half. After a 7-day recovery period the birds were retrained on the same tasks with the previously untrained eye. A rotundal lesion, on either side, resulted in the loss of interocular transfer of discrimination, whereas neither contralateral nor ipsilateral OPT lesions affected discrimination. These results suggest that the tectofugal visual pathway plays a crucial role in the interhemispheric transfer of visual information in pigeons.

Near-field acuity changes after visual system lesions in pigeons. II. Telencephalon

Pigeons were trained to perform in a psychophysical task that measured their minimum-separable visual acuity. After their performance stabilized, lesions were made in telencephalic components of the visual system. In one group, lesions were made in the ectostriatum, which is the telencephalic target of the tectofugal visual pathway. These cases showed severe to moderate losses of acuity. The magnitude of the loss was correlated with the extent of ectostriatal damage. In another group, lesions were made in the visual Wulst, a portion of which receives the ascending fibers of the thalamofugal visual pathway. Within this group, only lesions that were large and included all components of the visual Wulst were effective in decreasing visual acuity to a moderate degree. A partial correlation analysis indicated that the components of the visual Wulst that were responsible for the acuity changes were the accessory hyperstriatum and the hyperstriatum ventrale. However, lesions that were generally confined to these regions alone were ineffective. Also ineffective were lesions of the granular components of the visual Wulst, which receive the ascending thalamofugal fibers. The results raise questions about the presumed roles of the tectofugal pathway as a background-vision mechanism and the thalamofugal pathway as a fine-detail vision mechanism.

Effects of unilateral neck deafferentation on the posturo-kinetic limb responses following cortical stimulation in the cat

Pigeons were post-operatively trained to discriminate two colors presented simultaneously. After reaching criterion on this task, they were required to perform a series of reversals of the color discrimination in which the positive and negative consequences of the stimuli were interchanged. Lesions of ectostriatum (the telencephalic target of the avian tectofugal visual pathway) impaired the ability of pigeons to learn a color-reversal task. An examination of the pattern of sparing and loss of performance on this task after lesions of various components of the ascending visual pathways suggests that deficits in color-reversal learning observed after lesions of the visual Wulst are not due to the Wulst's connections with the thalamofugal pathway. Instead, the data suggest that the visual Wulst maintains a role in color-reversal learning through its connections with the tectofugal pathway.

Interactions between components of the avian visual system

Pigeons were trained to perform in a psychophysical procedure for determining luminance difference thresholds. After the psychometric data had stabilized, lesions were made in specific cell populations of the tectofugal visual pathway. In one group of pigeons, the lesions were confined to nucleus rotundus (Rt). In a second group, the lesions included not only Rt, but also the pars ventralis of the lateral geniculate nucleus (GLv). In the third group, the lesions included Rt, GLv and nucleus subpretectalis (SP). The postoperative performance of the birds in the Rt group showed elevations in threshold that represented substantial losses in sensory capacity. In contrast, the birds with lesions of Rt + GLv showed little or no change in their postoperative thresholds. Finally, the birds in the Rt + GLv + SP group were as impaired postoperatively as the birds with lesions of Rt alone. The results are interpreted in the context of the interaction between components of parallel routes within the tectofugal pathway and parallel pathways within the visual system.