Attention and normalization circuits in macaque V1.

Attention affects neuronal processing and improves behavioural performance. In extrastriate visual cortex these effects have been explained by normalization models, which assume that attention influences the circuit that mediates surround suppression. While normalization models have been able to explain attentional effects, their validity has rarely been tested against alternative models. Here we investigate how attention and surround/mask stimuli affect neuronal firing rates and orientation tuning in macaque V1. Surround/mask stimuli provide an estimate to what extent V1 neurons are affected by normalization, which was compared against effects of spatial top down attention. For some attention/surround effect comparisons, the strength of attentional modulation was correlated with the strength of surround modulation, suggesting that attention and surround/mask stimulation (i.e. normalization) might use a common mechanism. To explore this in detail, we fitted multiplicative and additive models of attention to our data. In one class of models, attention contributed to normalization mechanisms, whereas in a different class of models it did not. Model selection based on Akaike's and on Bayesian information criteria demonstrated that in most cells the effects of attention were best described by models where attention did not contribute to normalization mechanisms. This demonstrates that attentional influences on neuronal responses in primary visual cortex often bypass normalization mechanisms.

Multiple paternity and sperm storage in captive Hermann's tortoises, Testudo hermanni boettgeri determined from amniotic fluid adhering to the eggshell.

We identified multiple paternity in 52.9% of the clutches of Hermann's tortoise Testudo hermanni boettgeri using polymorphic microsatellite markers. In addition we demonstrated sperm storage across seasons. DNA was extracted from the amniotic fluid adhering to the eggshell's inner surface, a procedure suitable for easy, non-invasive DNA sampling in conservation and breeding programs. To improve the informative value of monomorphic single tandem repeat (STR) markers we additionally analyzed single nucleotide polymorphism (SNP) variability.

Visual response properties of neurons in cortical areas MT and MST projecting to the dorsolateral pontine nucleus or the nucleus of the optic tract in macaque monkeys.

Neurons in cortical medial temporal area (MT) and medial superior temporal area (MST) projecting to the dorsolateral pontine nucleus (DLPN) and/or to the nucleus of the optic tract and dorsal terminal nucleus (NOT-DTN) were identified by antidromic electrical stimulation in five macaque monkeys. Neurons projecting to either target were located in close proximity to each other, and in all subregions of MT and MST sampled. Only a small percentage of the antidromically identified projection neurons (4.4%) sent branches to both the NOT-DTN and the DLPN. Antidromic latencies of neurons projecting to the NOT-DTN (0.9-6 ms, median 2.1 ms) and to the DLPN (0.8-5 ms, median 2.0 ms) did not differ significantly. Visual response properties of the neurons antidromically activated from either site did not differ significantly from those of cells that were not so activated. On the population level only neurons activated from the NOT-DTN had a clear preference for ipsiversive stimulus movement, whereas the neurons activated from the DLPN and neurons not antidromically activated from either target had no common directional preference. These results are discussed in terms of specification of cortico-subcortical connections and with regard to pathways underlying slow eye movements in different visuomotor behaviours.

Retinal projections to the accessory optic system in pigmented and albino ferrets (Mustela putorius furo).

We investigated if a reduced specificity of the retinal projection to the accessory optic system might be responsible for the loss of direction selectivity in the nucleus of the optic tract and dorsal terminal nucleus (NOT-DTN) and, in consequence of this, the optokinetic deficits in albino ferrets. Under electrophysiological control we performed dual tracer injections into the NOT-DTN and the medial terminal nucleus (MTN). Retrogradely labelled ganglion cells were found in the visual streak, the dorsal, and the ventral retina both after injections into the NOTDTN and the MTN indicating that both nuclei receive input from the same retinal regions. The distribution and spacing of labelled ganglion cells did not differ between pigmented and albino ferrets. However, retinal ganglion cells projecting simultaneously to both the NOT-DTN and the MTN occurred only in albino ferrets. These results suggest that a reduced specificity of the projection pattern of direction specific ganglion cells may contribute to the loss of direction selectivity in the NOT-DTN in albino ferrets.

Sensitivity to relative disparity in early visual cortex of pigmented and albino ferrets.

To investigate binocular interactions as the neuronal substrate for disparity sensitivity in the ferret (Mustela putorius furo), we measured the effects of relative horizontal disparities on responses of neurons in areas 17 and 18 of the visual cortex. Stimulation by moving bars and sinusoidal gratings showed that about half of our sample in pigmented ferrets was sensitive to relative horizontal disparity. This also included many neurons, which were classified as only monocularly activated when testing either eye alone. However, the tuning width was about two or three times coarser (median tuning width 4 degrees of visual angle) than that in the cat. In albino ferrets, only 8% of the neurons in the early visual cortex displayed some sort of disparity-dependent binocular interactions, but none could be clearly identified as relative disparity-coding neuron.

Receptor protein tyrosine phosphatases are expressed by cycling retinal progenitor cells and involved in neuronal development of mouse retina.

Receptor protein tyrosine phosphatases (RPTPs) appear to coordinate many aspects of neural development, including cell proliferation, migration and differentiation. Here we investigated potential roles of RPTPs in the developing mouse retina. Using a degenerate oligonucleotide-based reverse transcription polymerase chain reaction approach, we identified 11 different RPTPs in the retina at embryonic stage 13 (E13). Subsequently, the expression patterns of RPTPkappa, RPTPJ, RPTPRR, RPTPsigma, RPTPepsilon and RPTPgamma in the retina from embryonic stages to adult were analyzed in detail using quantitative real-time-PCR, in situ hybridization, immunohistochemistry and Western blotting. At E13, all six RPTPs are expressed in actively cycling retinal progenitor cells and postmitotic newborn retinal neurons. With ongoing maturation, RPTPkappa, RPTPJ, RPTPRR, RPTPsigma, RPTPepsilon and RPTPgamma display a different spatiotemporal regulation of mRNAs and proteins in the pre- and postnatal retina. Finally, in adulthood these six RPTPs localize to distinct cellular compartments of multiple retinal neurons. Additional studies in RPTPgamma(-/-) and RPTPbeta/zeta(-/-) (also known as PTPRZ1, RPTPbeta or RPTPzeta) mice at postnatal stage P1 reveal no apparent differences in retinal laminar organization or in the expression pattern of specific retinal cell-type markers when compared with wild type. However, in RPTPbeta/zeta(-/-) retinas, immunoreactivity of vimentin, a marker of Müller glial cells, is selectively reduced and the morphology of vimentin-immunoreactive radial processes of Müller cells is considerably disturbed. Our results suggest distinct roles of RPTPs in cell proliferation and establishing phenotypes of different retinal cells during retinogenesis as well as later in the maintenance of mature retina.

Deficits of visual motion perception and optokinetic nystagmus after posterior suprasylvian lesions in the ferret (Mustela putorius furo).

We recently described an area in the ferret posterior suprasylvian (PSS) cortex characterized by a high proportion of direction selective neurons. To answer the question whether area PSS subserves functions similar to cat posteromediolateral suprasylvian area (PMLS) and monkey medial temporal area (MT) we investigated the contribution of area PSS to visual motion perception and optokinetic nystagmus. Ferrets were tested on global motion detection before and after bilateral lesions involving area PSS and control lesions of other extrastriate visual areas. Following PSS lesions motion coherence thresholds were significantly increased both in pigmented and albino ferrets, whereas control lesions sparing PSS did not affect visual motion perception. Optokinetic nystagmus was strongly reduced to absent after PSS lesions. These results indicate that area PSS is crucial for global motion processing in the ferret and in that sense may be functionally equivalent to PMLS in the cat and area MT in the monkey.

Motion perception deficits in albino ferrets (Mustela putorius furo).

Albino ferrets contrary to their pigmented conspecifics show no optokinetic nystagmus. Therefore, in this study motion perception was compared between pigmented and albino ferrets (Mustela putorius furo) trained to discriminate between coherently moving random dot patterns and dynamic noise stimuli in a two-alternative forced choice task. Fully coherently versus incoherently moving patterns could be distinguished by ferrets of both phenotypes. Motion coherence thresholds, however, were significantly higher in albinos. These results indicate that albino ferrets are not motion blind as could be expected from their total lack of optokinetic reactions. However, they are severely impaired in global motion perception.

Vitread proliferation of filamentous processes in avian Müller cells and its putative functional correlates.

In order to examine to what extent the neuronal and metabolic activities of avascular vertebrate retinae are reflected in the morphology of their Müller cells we have studied (by using several monoclonal antibodies) the morphology of Müller cells in two species of diurnal birds (chicken, Gallus domesticus, and pigeon, Columba livia) and one species of nocturnal saltwater crocodiles (Crocodylus porosi). In all three species, the outer nuclear layer is relatively thin and the Müller cell trunks divide into rootlets that wrap around the photoreceptors. In both diurnal birds studied, the trunks of Müller cells in the inner plexiform layers invariably divide into numerous fine filamentous processes that terminate in small expansions covering most of the vitreal surface of the retina. Furthermore, the networks of filamentous processes of birds' Müller cells exhibit conspicuous horizontal lamination in the inner plexiform layer. In contrast, the filamentous processes arising from the individual Müller cell trunks of the crocodile, if present, are much less numerous and less widely spread than those of diurnal birds. It is proposed that the splitting of the Müller cell trunks into numerous filamentous processes terminating in small vitreal expansions represents a morphological adaptation for: 1) effective spatial buffering of K+ ions in thick and presumably metabolically highly active, yet avascular, avian retinae, and 2) effective absorption and distribution of nutrients leaking from the vitreally located supplemental nutritive organ, the pecten.

Glia cells of the monkey retina. I. Astrocytes.

Morphology and distribution of retinal astrocytes have been studied in macaque monkeys by immunocytochemical localization of glial fibrillary acidic protein (GFAP). With the exception of the fovea and the far periphery, astrocytes are ubiquitous in the nerve fiber layer (NFL) and the ganglion cell layer (GCL) of the monkey retina. The morphology of NFL astrocytes changes gradually, from star-shaped in the periphery to bipolar close to the optic disc. By contrast, GCL astrocytes maintain their star-shaped appearance throughout the retina. Astrocytes are unevenly distributed in the monkey retina, showing the highest concentration around the optic disc, and particularly low densities in the perifoveal region and the far periphery. The fovea proper is devoid of astrocytes. Employing high-resolution confocal microscopy, we could demonstrate that astrocytes form manifold contacts to blood vessels. In addition, bundles of NFL astrocyte processes are co-localized with axon bundles, individual astrocytes forming contacts to several axon bundles. In contrast, a similar affinity of astrocytes to ganglion cell somata was never observed. Thus, our data confirm and extend the current knowledge of morphology and putative function of astrocytes in mammalian and especially the primate retina.

Müller cells in vascular and avascular retinae: a survey of seven mammals.

Eight monoclonal antibodies were used to label Müller cells in four mammals that have vascular retinae (cats, dogs, humans, and rats) and in three with avascular retinae (echidnas, guinea pigs, and rabbits). Müller cells were found to have a fairly uniform retinal distribution in seven species, with a mean density of 8,000-13,000 cells mm-2. Müller cells in avascular retinae differ from their vascular counterparts in four respects. First, they are shorter than those in vascular retinae. This difference is mainly due to a reduction in the thickness of the outer nuclear layer. Second, the trunks of Müller cells in avascular retinae tend to be thicker, although those in echidnas are an exception to this trend. Third, Müller cell rootlets in avascular retinae follow a more tortuous course than those in vascular retinae, reflecting the fact that photoreceptor nuclei in the two types of retina have different shapes and stacking patterns. Fourth, due to a reduction in the density of photoreceptors in avascular retinae, there are fewer neurones per Müller cell. Although these four features may enable Müller cells to assist the nutrition of neurones in the inner layers of avascular retinae, they are unlikely to be morphological specializations that have evolved for that purpose. Rather, these features appear to be a direct consequence of the fact that avascular retinae are thinner and have a differently organised outer nuclear layer. These features aside, Müller cells in avascular retinae closely resemble their counterparts in vascular retinae.

Callosal and superior temporal sulcus contributions to receptive field properties in the macaque monkey's nucleus of the optic tract and dorsal terminal nucleus of the accessory optic tract.

To assess the functional contribution of the cortical input to the receptive field properties of nucleus of the optic tract (NOT) and dorsal terminal nucleus (DTN) neurons, a first set of experiments evaluated the response properties of NOT-DTN cells in monkeys with split corpus callosum. With respect to visual latency, direction specificity, directional tuning width, velocity tuning, ocular dominance, and binocular interaction, they were indistinguishable from NOT-DTN neurons in normal monkeys. However, a clear difference was found regarding the extent of the receptive fields. Whereas, in normal monkeys, NOT-DTN receptive fields include the contralateral hemifield and the fovea as well as substantial parts of the ipsilateral visual field, receptive fields in callosum-split monkeys stop abruptly at, or close to, the vertical 0-meridian and do not extend into the ipsilateral visual field. In addition, the location of the highest sensitivity within the receptive fields in callosum-split monkeys is shifted away from the vertical 0-meridian in comparison to normal animals. In a second set of experiments, we antidromically identified cortical neurons within the superior temporal sulcus that project to the NOT-DTN. These neurons were found in area MT mostly near the border of MTp or MSTl. All of them are direction selective for ipsiversive stimulus movement, and their receptive fields extend substantially into the ipsilateral visual hemifield. Neurons with other preferred directions did not project to the NOT-DTN. These results contribute to the explanation of the ipsiversive directional deficits in slow eye movements after cortical lesions, as well as the asymmetries in optokinetic nystagmus with hemifield stimulation after transection of the corpus callosum. The more general implication of the results is that a particular function of a cortical area can only be understood by knowing its subcortical connections.

Functional projections from striate cortex and superior temporal sulcus to the nucleus of the optic tract (NOT) and dorsal terminal nucleus of the accessory optic tract (DTN) of macaque monkeys.

The nucleus of the optic tract (NOT) and the dorsal terminal nucleus of the accessory optic tract (DTN) have been recognized to be relevant structures for optokinetic and vestibuloocular reflexes. NOT-DTN neurons relay visual information to the vestibular nuclei via the nucleus prepositus hypoglossi and to the flocculus via the dorsal cap of the inferior olive. It has been previously shown that in carnivores the NOT-DTN receives information from primary visual cortical areas in addition to the direct retinal input. In this study we demonstrate the presence and some functional characteristics such as latency and evicacy of considerable cortical projections to the NOT-DTN in macaque monkeys. In anaesthetized and paralyzed monkeys NOT-DTN neurons were identified physiologically and tested for cortical input by electrical stimulation in various cortical areas. Successful sites of stimulation to activate NOT-DTN neurons orthodromically lie in the primary visual cortex (V1) and in the motion-processing areas in the superior temporal sulcus (STS). In contrast, electrical stimulation in area V4 and in parietal areas in most cases did not yield orthodromic responses. Overall latencies of action potentials elicited by stimulation in V1 were 0.5 ms longer than those elicited from STS. These short latency differences between V1 and STS stimulation suggest a direct projection from both V1 and STS to the NOT-DTN. The physiological results were supported by the results of anatomical experiments by using horseradish peroxidase as anterograde tracer. Both injections into V1 and into the lower bank of STS resulted in anterogradely labelled fibers and terminals around the recording sites of direction-specific NOT-DTN neurons. This paper is a first step in clarifying the significance of corticofugal projections from individual areas involved in the analysis of visual motion for the optokinetic reflex.

Cortical connections of inferior temporal area TEO in macaque monkeys.

In macaque monkeys, lesions involving the posterior portion of the inferior temporal cortex, cytoarchitectonic area TEO, produce a severe impairment in visual pattern discrimination. Recently, this area has been shown to contain a complete, though coarse, representation of the contralateral visual field (Boussaoud, Desimone, and Ungerleider: J. Comp. Neurol. 306:554-575, '91). Because the inputs and outputs of area TEO have not yet been fully described, we injected a variety of retrograde and anterograde tracers into 11 physiologically identified sites within TEO of seven rhesus monkeys and analyzed the areal and laminar distribution of its cortical connections. Our results show that TEO receives feedforward, topographically organized inputs from prestriate areas V2, V3, and V4. Additional sparser feedforward inputs arise from areas V3A, V4t, and MT. Each of these inputs is reciprocated by a feedback projection from TEO. TEO was also found to have reciprocal intermediate-type connections with the fundus of the superior temporal area (area FST), cortex in the most posteromedial portion of the superior temporal sulcus (the posterior parietal sulcal zone [area PP]), cortex in the intraparietal sulcus (including the lateral intraparietal area [area LIP]), the frontal eye field, and area TF on the parahippocampal gyrus. The connections with V3A, V4t, and PP were found only after injections in the peripheral field representations of TEO. Finally, TEO was found to project in a feedforward pattern to area TE and to areas anterior to FST on the lateral bank and floor of the superior temporal sulcus (areas TEm, TEa, and IPa, Seltzer and Pandya: Brain Res. 149:1-24, '78), all of which send feedback projections to TEO. Feedback projections also arise from parahippocampal area TH, and areas TG, 36, and possibly 35. These are complemented by only sparse feedforward projections to TG from central field representations in TEO and to TH from peripheral field representations. The results thus indicate that TEO forms an important link in the occipitotemporal pathway for object recognition, sending visual information forward from V1 and prestriate relays in V2-V4 to anterior inferior temporal area TE.

Development of the optokinetic response in macaques: a comparison with cat and man.

In macaque monkeys, an optokinetic response (OKR) can be elicited monocularly both in temporonasal and, albeit weaker, in nasotemporal direction very early after birth. The further maturation of equal strengths of OKR in both directions depends on stimulus velocity: at low-stimulus velocities (10-20 degrees /s) symmetry is reached at 3-4 weeks of age, at higher-stimulus velocities (40-80 degrees /s) it is reached only at 4-5 months of age. Retinal slip neurons in the NOT-DTN are direction selective for ipsiversive stimulus movement shortly after birth. Most of these neurons receive input from both eyes; many are dominated by the contralateral eye. Electrophysiological and neuroanatomical evidence suggests that the cortical input to the NOT-DTN starts to become functional by postnatal day 14, at the latest. Based on these behavioral and physiological data, as well as on comparison with data from kittens and human infants, we hypothesize that the very early monocularly elicited bidirectional optokinetic response is due to the direct retinal input from both eyes to the NOT-DTN. As the cortical projection matures, it gains more and more influence upon the response properties of retinal slip neurons in the NOT-DTN, and the retinal influence gradually decreases.

GABA content in the retina of pigmented and albino rats.

Reduction of the melanin precursor DOPA associated with albinism leads to spatiotemporal disturbances in retinal neurogenesis and thus seems to be responsible for numerous neuronal alterations found in albino retinae. To investigate whether these cellular alterations are reflected in retinal neurotransmitter concentrations we compared the levels of GABA and glutamate in the retina of adult pigmented Long Evans and albino Wistar rats using reversed phase-liquid chromatography (RP-HPLC). When normalized to retinal weight, GABA levels showed a statistically insignificant trend to be lower and glutamate values to be higher in albinos than in pigmented animals. The ratio of glutamate to GABA was significantly higher in albino than in pigmented retinae. As numerous studies have shown that the balance between GABA and glutamate plays a crucial role for establishing direction selectivity, these results are discussed in relation to direction selectivity and defects in the optokinetic system of albinos.

Interactions of inflammatory mediators and low pH not influenced by capsazepine in rat cutaneous nociceptors.

The rat skin-saphenous nerve preparation was used to record from mechano-heat sensitive C-fibers whose receptive fields were superfused with various solutions of low pH and of bradykinin, serotonin and prostaglandin E2. Only synchronous application of protons and mediators resulted in a significant nearly three-fold augmentation of the nociceptive pH response, and capsazepine (10(-5) M) did not block this short-lived enhancement. Thus, it does not seem to involve the capsaicin receptor (VRI) which is in contrast to a previous finding from cultured sensory neurons.

Early development of the subcortical and cortical pathway involved in optokinetic nystagmus: the cat as a model for man?

The optokinetic reflex undergoes qualitative changes during the first postnatal weeks in kittens or months in human babies. Under monocular stimulus conditions, a clear preference for temporonasal stimulus directions at moderate velocities is replaced by a symmetrical, broad velocity range horizontal optokinetic nystagmus (OKN) over these periods. Evidence is presented for the cat that development changes in OKN can be related to maturation of neuronal response properties in the nucleus of the optic tract (NOT) in the pretectum. NOT cells in 3-week-old kittens are already direction-selective but all of them are exclusively or predominantly driven by the contralateral eye. Only starting with the 4th week of life NOT cells become more binocular and respond to a broader spectrum of stimulus velocities. This step in maturation coincides with the time when the cortical input to the NOT becomes functional.

Differences in calcium signalling in rat peripheral sensory neurons.

Calcium influx and the resulting increase in intracellular calcium concentration [Ca2+]i can induce enhanced sensitivity to temperature increases in nociceptive neurons. Using the patch-clamp technique and simultaneous calcium microfluorimetry we show that experimental elevation of [Ca2+]i using the calcium ionophore ionomycin resulted in a significant potentiation of heat-activated currents. This was not the case when rises in [Ca2+]i were elicited by depolarization of the cell membrane by current injection via the patch pipette. Our data provide first, however, indirect evidence that in sensory neurons calcium ions may be guided into different intracellular microdomains depending on the type of ion channel or pore through which they enter the cell. We conclude that the compartmentalization of sensory neurons for calcium ions may be decisive on further signalling cascades accounting, for example, for neuronal plasticity.

Glia cells of the monkey retina--II. Müller cells.

In this paper, for the first time a quantitative description of the morphology and distribution of Müller cells in the macaque monkey retina using immunohistochemistry and high resolution confocal laser scanning microscopy is given. By their morphological features Müller cells are ideally adapted to their neuronal environment in the various retinal layers, with a dense network of horizontal processes, especially in the inner plexiform layer, and close contacts to neuronal somata especially in the outer nuclear layer and ganglion cell layer. Morphology varies with retinal eccentricity. The thickness of the inner trunk increases significantly with increasing retinal eccentricity. According to the overall thickness of the retina, Müller cells in central retina are longer than in peripheral regions. In the parafoveal region, the outer trunks of Müller cells in the outer plexiform layer are immensely elongated. These Müller fibres can reach lengths of several hundred micrometers as they travel through the outer plexiform layer from the foveal centre towards the foveal border where they enter the inner nuclear layer. Müller cell density varies between 6000 cells/mm2 in far peripheral and peak densities of > 30,000 cells/mm2 in the parafoveal retina. There is a close spatial relationship between Müller cells and blood vessels in the monkey retina, suggesting a role of Müller cells in the formation of the blood-retinal barrier, in the uptake of nutrients and the disposal of metabolites.