Responses to moving texture patterns of cells in the striate-recipient zone of the cat's lateral posterior-pulvinar complex

Laminar distribution of cortical projection neurons to the pulvinar: A comparative study in cats and mice

The cortical processing of visual information is thought to follow a hierarchical framework. This framework of connections between visual areas is based on the laminar patterns of direct feedforward and feedback cortico-cortical projections. However, this view ignores the cortico-thalamo-cortical projections to the pulvinar nucleus in the thalamus, which provides an alternative transthalamic information transfer between cortical areas. It was proposed that corticothalamic (CT) pathways follow a similar hierarchical pattern as cortico-cortical connections. Two main types of CT projections have been recognized: drivers and modulators. Drivers originate mainly in Layer 5 whereas modulators are from Layer 6. Little is known about the laminar distribution of these projections to the pulvinar across visual cortical areas. Here, we analyzed the distribution of CT neurons projecting to the lateral posterior (LP) thalamus in two species: cats and mice. Injections of the retrograde tracer fragment B of cholera toxin (CTb) were performed in the LP. The morphology and cortical laminar distribution of CTb-labeled neurons was assessed. In cats, neurons were mostly found in Layer 6 except in Area 17, where they were mostly in Layer 5. In contrast, CT neurons in mice were mostly located in Layer 6 across all areas. Thus, our results demonstrate that CT projections in mice do not follow the same organization as cats suggesting that the transthalamic pathways play distinct roles in these species.

Visual Response Characteristics in Lateral and Medial Subdivisions of the Rat Pulvinar

The pulvinar is a higher-order thalamic relay and a central component of the extrageniculate visual pathway, with input from the superior colliculus and visual cortex and output to all of visual cortex. Rodent pulvinar, more commonly called the lateral posterior nucleus (LP), consists of three highly-conserved subdivisions, and offers the advantage of simplicity in its study compared to more subdivided primate pulvinar. Little is known about receptive field properties of LP, let alone whether functional differences exist between different LP subdivisions, making it difficult to understand what visual information is relayed and what kinds of computations the pulvinar might support. Here, we characterized single-cell response properties in two V1 recipient subdivisions of rat pulvinar, the rostromedial (LPrm) and lateral (LPl), and found that a fourth of the cells were selective for orientation, compared to half in V1, and that LP tuning widths were significantly broader. Response latencies were also significantly longer and preferred size more than three times larger on average than in V1; the latter suggesting pulvinar as a source of spatial context to V1. Between subdivisons, LPl cells preferred higher temporal frequencies, whereas LPrm showed a greater degree of direction selectivity and pattern motion detection. Taken together with known differences in connectivity patterns, these results suggest two separate visual feature processing channels in the pulvinar, one in LPl related to higher speed processing which likely derives from superior colliculus input, and the other in LPrm for motion processing derived through input from visual cortex. SIGNIFICANCE STATEMENT: The pulvinar has a perplexing role in visual cognition as no clear link has been found between the functional properties of its neurons and behavioral deficits that arise when it is damaged. The pulvinar, called the lateral posterior nucleus (LP) in rats, is a higher order thalamic relay with input from the superior colliculus and visual cortex and output to all of visual cortex. By characterizing single-cell response properties in anatomically distinct subdivisions we found two separate visual feature processing channels in the pulvinar, one in lateral LP related to higher speed processing which likely derives from superior colliculus input, and the other in rostromedial LP for motion processing derived through input from visual cortex.

Distribution and Morphology of Cortical Terminals in the Cat Thalamus from the Anterior Ectosylvian Sulcus

Two main types of cortical terminals have been identified in the cat thalamus. Large (type II) have been proposed to drive the response properties of thalamic cells while smaller (type I) are believed to modulate those properties. Among the cat's visual cortical areas, the anterior ectosylvian visual area (AEV) is considered as one of the highest areas in the hierarchical organization of the visual system. Whereas the connections from the AEV to the thalamus have been recognized, their nature (type I or II) is presently not known. In this study, we assessed and compared the relative contribution of type I and type II inputs to thalamic nuclei originating from the AEV. The anterograde tracer BDA was injected in the AEV of five animals. Results show that (1) both type I and II terminals from AEV are present in the Lateral Posterior- Pulvinar complex, the lateral median suprageniculate complex and the medial and dorsal geniculate nuclei (2) type I terminals significantly outnumber the type II terminals in almost all nuclei studied. Our results indicate that neurons in the AEV are more likely to modulate response properties in the thalamus rather than to determine basic organization of receptive fields of thalamic cells.

Blindsight relies on a functional connection between hMT+ and the lateral geniculate nucleus, not the pulvinar

When the primary visual cortex (V1) is damaged, the principal visual pathway is lost, causing a loss of vision in the opposite visual field. While conscious vision is impaired, patients can still respond to certain images; this is known as 'blindsight'. Recently, a direct anatomical connection between the lateral geniculate nucleus (LGN) and human motion area hMT+ has been implicated in blindsight. However, a functional connection between these structures has not been demonstrated. We quantified functional MRI responses to motion in 14 patients with unilateral V1 damage (with and without blindsight). Patients with blindsight showed significant activity and a preserved sensitivity to speed in motion area hMT+, which was absent in patients without blindsight. We then compared functional connectivity between motion area hMT+ and a number of structures implicated in blindsight, including the ventral pulvinar. Only patients with blindsight showed an intact functional connection with the LGN but not the other structures, supporting a specific functional role for the LGN in blindsight.

Bilateral and ipsilateral ascending tectopulvinar pathways in mammals: a study in the squirrel (Spermophilus beecheyi)

The mammalian pulvinar complex is a collection of dorsal thalamic nuclei related to several visual and integrative processes. Previous studies have shown that the superficial layers of the superior colliculus project to multiple divisions of the pulvinar complex. Although most of these works agree about the existence of an ipsilateral tectopulvinar projection arising from the stratum griseum superficialis, some others report a bilateral projection originating from this same tectal layer. We investigated the organization of the tectopulvinar projections in the Californian ground squirrel using cholera toxin B (CTb). We confirmed previous studies showing that the caudal pulvinar of the squirrel receives a massive bilateral projection originating from a specific cell population located in the superficial collicular layers (SGS3, also called the "lower SGS" or "SGSL"). We found that this projection shares striking structural similarities with the tectorotundal pathway of birds and reptiles. Morphology of the collicular cells originating this projection closely corresponds to that of the bottlebrush tectal cells described previously for chickens and squirrels. In addition, we found that the rostral pulvinar receives an exclusively ipsilateral projection from a spatially separate population of collicular cells located at the base of the stratum opticum, deeper than the cells projecting to the caudal pulvinar. These results strongly support, at a structural level, the homology of the pathway originating in the SGS3 collicular cells upon the caudal pulvinar with the tectorotundal pathway of nonmammalian amniotes and contribute to clarifying the general organization of the tectopulvinar pathways in mammals.

Precise discrimination of object position in the human pulvinar

Very little is known about the human pulvinar; suggestions for its function include relaying input from cortical areas, allocating visual attention, supporting feature binding, and other integrative processes. The diversity of hypotheses about pulvinar function highlights our lack of understanding of its basic role. A conspicuously missing piece of information is whether the human pulvinar encodes visual information topographically. The answer to this question is crucial, as it dramatically constrains the sorts of computational and cognitive processes that the pulvinar might carry out. Here we used fMRI to test for position-sensitive encoding in the human pulvinar. Subjects passively viewed flickering Gabor stimuli, and as the spatial separation between Gabors increased, the correlation between patterns of activity across voxels within the right pulvinar decreased significantly. The results demonstrate the existence of precise topographic coding in the human pulvinar lateralized to the right hemisphere, and provide a means of functionally localizing this topographic region.

Distribution, morphology, and synaptic targets of corticothalamic terminals in the cat lateral posterior-pulvinar complex that originate from the posteromedial lateral suprasylvian cortex

The lateral posterior (LP) nucleus is a higher order thalamic nucleus that is believed to play a key role in the transmission of visual information between cortical areas. Two types of cortical terminals have been identified in higher order nuclei, large (type II) and smaller (type I), which have been proposed to drive and modulate, respectively, the response properties of thalamic cells (Sherman and Guillery [1998] Proc. Natl. Acad. Sci. U. S. A. 95:7121-7126). The aim of this study was to assess and compare the relative contribution of driver and modulator inputs to the LP nucleus that originate from the posteromedial part of the lateral suprasylvian cortex (PMLS) and area 17. To achieve this goal, the anterograde tracers biotinylated dextran amine (BDA) or Phaseolus vulgaris leucoagglutinin (PHAL) were injected into area 17 or PMLS. Results indicate that area 17 injections preferentially labelled large terminals, whereas PMLS injections preferentially labelled small terminals. A detailed analysis of PMLS terminal morphology revealed at least four categories of terminals: small type I terminals (57%), medium-sized to large singletons (30%), large terminals in arrangements of intermediate complexity (8%), and large terminals that form arrangements resembling rosettes (5%). Ultrastructural analysis and postembedding immunocytochemical staining for gamma-aminobutyric acid (GABA) distinguished two types of labelled PMLS terminals: small profiles with round vesicles (RS profiles) that contacted mostly non-GABAergic dendrites outside of glomeruli and large profiles with round vesicles (RL profiles) that contacted non-GABAergic dendrites (55%) and GABAergic dendritic terminals (45%) in glomeruli. RL profiles likely include singleton, intermediate, and rosette terminals, although future studies are needed to establish definitively the relationship between light microscopic morphology and ultrastructural features. All terminals types appeared to be involved in reciprocal corticothalamocortical connections as a result of an intermingling of terminals labelled by anterograde transport and cells labelled by retrograde transport. In conclusion, our results indicate that the origin of the driver inputs reaching the LP nucleus is not restricted to the primary visual cortex and that extrastriate visual areas might also contribute to the basic organization of visual receptive fields of neurons in this higher order nucleus.

Simple and complex visual motion response properties in the anterior medial bank of the lateral suprasylvian cortex

The cortical regions surrounding the suprasylvian sulcus have previously been associated with motion processing. Of the six areas originally described by Palmer et al. [J Comp Neurol 177 (1978) 237], the posteromedial lateral suprasylvian (PMLS) cortex has attracted the greatest attention. Very little physiological information is available concerning other suprasylvian visual areas, and in particular, the anteromedial lateral suprasylvian cortex (AMLS). Based on its cortical and sub-cortical connectivity patterns, the AMLS cortex is a likely candidate for higher-order motion processing in cat visual cortex. We have investigated this possibility by studying the receptive field sensitivity of AMLS neurons to complex motion stimuli. Neurons in AMLS cortex exhibited large (mean of 354 degrees (2)) and complex-like receptive fields, and most of them (74%) were classified as direction selective on the basis of their responses to sinusoidal drifting gratings. Most importantly, direction selectivity was present for complex motion stimuli. A subset of the neurons sampled (eight of 38 cells; 21%) exhibited pattern-motion selectivity in response to moving plaid patterns. The capacity of AMLS neurons to signal higher-order stimuli was further supported by their selectivity to moving complex random-dot kinematograms. Finally, 45% of 20 neurons were direction selective to a radial optic flow stimulus. Overall, these results suggest that AMLS cortex is involved in higher-order analyses of visual motion. It is possible that the AMLS cortex represents a region between PMLS and the anterior ectosylvian visual area in a functional hierarchy of areas involved in motion integration.

MMN in the visual modality: a review

The mismatch negativity (MMN) component is an event-related potential (ERP) that can be elicited by any change in the acoustic environment, and it is related to memory-based, automatic processing mechanisms, and attentional capture processes. This component is well defined in the auditory modality. However, there is still a great controversy about its existence in the visual modality. This paper reviews the studies that are relevant with regard to memory-based, automatic deviance detection ERPs in the visual system. The paper discusses the main strengths and limitations of those studies and suggests what directions should be taken for future research.

Higher-order motion processing in the pulvinar

Thalamic nuclei have long been considered as passive relay stations for sensory signals en route to the cerebral cortex, where higher level processing occurs. In recent years, it has been proposed that thalamic nuclei may actively participate in the processing of specific information in conjunction with cortical areas. In support of this hypothesis, we recently discovered that neurons in the main extrageniculate visual nucleus, the pulvinar, exhibit higher-order visual properties that were, until now, only associated with higher-order cortical areas. Pulvinar neurons can indeed code the veridical direction of a moving plaid pattern, indicating that these cells can integrate ambiguous signals into a coherent percept. This finding as well as our demonstration that there are cortico-thalamo-cortical loops involved in complex motion analysis open promising avenues in unraveling the function of the pulvinar complex in normal vision.

Responses of neurons in the cat posteromedial lateral suprasylvian cortex to moving texture patterns

The posteromedial lateral suprasylvian cortex represents a point of convergence between the geniculostriate and extrageniculostriate visual pathways. Given its purported role in motion analysis and the conflicting reports regarding the texture sensitivity of this area, we have investigated the response properties of cells in PMLS to moving texture patterns ("visual noise"). In contrast to previous reports, we have found that a large majority of cells (80.1%) responds to the motion of a texture pattern with sustained discharges. In general, responses to noise were more broadly tuned for direction compared to gratings; however, direction selectivity appeared more pronounced in response to noise. The majority of cells was selective for drift velocity of the noise pattern (mean optimal velocity: 26.7 degrees /s). Velocity tuning was comparable to that of its principal thalamic input, the lateral posterior pulvinar nucleus. In general, responsiveness of cells in the posteromedial lateral suprasylvian cortex increased with increasing texture element size, although some units were tuned to smaller element sizes than the largest presented. Finally, the magnitude of these noise responses was dependent on the area of the visual field stimulated. In general, a stimulus corresponding to roughly twice the size of the receptive field was required to elicit an equivalent half-maximal response to that for gratings. The results of this study indicate that the majority of cells in the posteromedial lateral suprasylvian cortex can be driven by the motion of a fine texture field, and highlight the importance of this area in motion analysis.

Cellular response to texture and form defined by motion in area 19 of the cat

The present study examined the neuronal sensitivity in area 19 of the cat to a motion-defined bar and to texture. Sensitivity was tested in normal, lesioned (areas 17-18) and split-chiasm cats using a kinematogram, as well as a textured bar drifting on a uniform light background and a light bar drifting on a stationary textured background. Texture density was varied. The results indicate that almost all cells of area 19 recorded in the three groups of cats responded to a motion-defined bar or to its edges. Texture density influenced the responses in that the discharge rate increased as density decreased. However, the majority of cells were sensitive to the highest texture density kinematogram. Moreover, the neural responses of all cats were either independent of the density of the textured bar or background, or were modulated by it. These results show that cells in area 19 can signal the presence of a kinetic bar and that the density of either the textured bar, the background or both can influence figure-ground detection. The results are interpreted with respect to how various inputs influence the function of area 19.

Motion integration in a thalamic visual nucleus

Thalamic nuclei have long been regarded as passive relay stations for sensory information en route to higher level processing in the cerebral cortex. Recently, physiological and theoretical studies have reassessed the role of the thalamus and it has been proposed that thalamic nuclei may actively participate with cortical areas in processing specific information. In support of this idea, we now show that a subset of neurons in an extrageniculate visual nucleus, the lateral-posterior pulvinar complex, can signal the true direction of motion of a plaid pattern, indicating that thalamic cells can integrate different motion signals into a coherent moving percept. This is the first time that these computations have been found to occur outside the higher-order cortical areas. Our findings implicate extrageniculate cortico-thalamo-cortical loops in the dynamic processing of image motion, and, more generally, as basic computational modules involved in analysing specific features of complex visual scenes.

Corticocortical communication via the thalamus: ultrastructural studies of corticothalamic projections from area 17 to the lateral posterior nucleus of the cat and inferior pulvinar nucleus of the owl monkey

Electron microscopic anterograde autoradiography has been used to analyze the morphology and postsynaptic relationships of area 17 cortical terminals in the lateral division of the lateral posterior nucleus (LPl) of the cat and medial division of the inferior pulvinar nucleus (IPm) of the owl monkey. Such terminals are thought to arise exclusively from layer 5 in the cat and primate (Lund et al. [1975] J. Comp. Neurol. 164:287-304; Abramson and Chalupa [1985] Neuroscience 15:81-95). All labeled terminals in both nuclei exhibited the morphology of ascending "lemniscal" afferents. That is, they contained round vesicles, were large, made asymmetrical synaptic and filamentous nonsynaptic contacts, and were classified as RLs. These cortical RLs also exhibited the postsynaptic relationships of lemniscal afferents. Thus, they were presynaptic to large dendrites within glial encapsulated glomeruli, where a majority was involved in complex synaptic arrangements called triads. They also were found adjacent to terminal profiles with pleomorphic vesicles but never adjacent to small terminals containing round vesicles. Our results suggest that the layer 5 projection from area 17 provides a functional "drive" for some LPl and IPm neurons. Information carried over this "re-entrant" pathway (Guillery [1995] J. Anat. 187:583-592) could be modified within the LPl and IPm by both cortical and subcortical pathways and subsequently conveyed to higher visual cortical areas, where it could be integrated with messages carried through the well-documented corticocortical pathways (Casagrande and Kaas [1994] Cerebral cortex New York: Plenum Press).

Spatial frequency processing in posteromedial lateral suprasylvian cortex does not depend on the projections from the striate-recipient zone of the cat's lateral posterior-pulvinar complex

It is generally considered that the posteromedial part of the cat's lateral suprasylvian cortex is involved in the analysis of image motion. The main afferents of the posteromedial lateral suprasylvian cortex come from a direct retinogeniculate pathway and indirect retinotectal and retino-geniculo-cortical pathways. Removal of the primary visual cortex does not affect the spatial and temporal processing of suprasylvian cortex cells suggesting that these properties are derived from thalamic input. We have investigated the possibility that the striate-recipient zone of the lateral posterior nucleus-pulvinar complex may be responsible for the spatial (and temporal) frequency processing in posteromedial lateral suprasylvian cortex since these two regions establish strong bidirectional connections and share many visual properties. Experiments were done on anaesthetized normal adult cats. Visual responses in suprasylvian cortex were recorded before, during, and after the deactivation of the lateral part of the lateral posterior nucleus accomplished by the injection of lidocaine or GABA. Results can be summarized as follows. A total of 64 cells was tested. Out of this number, 11 units were affected by the deactivation of the lateral part of lateral posterior nucleus and one cell, by the blockade of pulvinar. For all cells, except one, the effect consisted in a global reduction of the evoked discharge rate suggesting that the thalamo-suprasylvian cortex projections are excitatory in nature. We did not find any significant differences in the optimal spatial frequency, nor in the width of the tuning function, whether the grating was presented at half- or saturation contrast. In addition, there were no significant differences between the low- and high cut-off spatial frequency values computed before and after the deactivation of the lateral posterior nucleus. No specific changes were observed in the contrast sensitivity function of the posteromedial lateral suprasylvian cortex cells. Similar results were observed with respect to the temporal frequency tuning functions. Deactivating the lateral posterior nucleus did not modify the direction selectivity nor the organization of the subregions of the lateral suprasylvian cortex "classical" receptive fields. The absence of strong changes in posteromedial lateral suprasylvian cortex cell response properties following the functional blockade of the lateral posterior nucleus suggests that the projections from this part of the thalamus are not essential to generate the spatial characteristics of most posteromedial lateral suprasylvian cortex receptive fields. These properties may be derived from other thalamic inputs (e.g., medial interlaminar nucleus) and/or from the intrinsic computation of the afferent signals within the lateral suprasylvian cortex. On the other hand, it is possible that the lateral posterior nucleus lateral suprasylvian cortex loop may be involved in other functions such as the analysis of complex motion as suggested by the findings from our and other groups.

Contribution of area 17 to cell responses in the striate-recipient zone of the cat's lateral posterior-pulvinar complex

The cat's lateral posterior-pulvinar complex (LP-pulvinar) contains three main representations of the visual field. The lateral part of the LP nucleus (LPl or striate-recipient zone) is the only region of these extrageniculate nuclei which receives afferents from the primary visual cortex. We investigated the contribution of area 17 to the response properties (orientation and spatial frequency tuning functions) of LPl neurons by cooling or lesioning the visual cortex. Responses of 40 LPl cells were studied before, during and after the reversible cooling of the striate cortex. When tested for orientation, a total of 10 units out of 28 was affected (36%). For most of these cells (eight of 10), cooling the visual cortex yielded a reduction of the cells' visual responses without altering their orientation-selectivity (there was no significant change in the orientation tuning width). For only two cells, inactivation led to an increase in the response amplitude. Also, blocking the visual cortex never modified the direction-selectivity of LPl cells. When tested for spatial frequency, 12 neurons out of 33 were affected (36%) by the experimental protocol. In most cases, we observed a reduction in the responses at each spatial frequency tested, with no change in tuning bandwidth. For only three LPl cells, the effects of inactivation of the visual cortex were restricted to specific spatial frequencies, altering the profile of the spatial frequency tuning function. In five cats, removing area 17 reduced the proportion of visual neurons in LPl and the spared visually evoked responses were noticeably depressed. Despite the reduction in responsiveness, a few LPl receptive fields within the cortical scotoma were still sensitive to the orientation and/or direction of a moving stimulus. This last observation suggests that some properties in LPl could be generated either by circuits intrinsic to the LPl or by afferents from extrastriate cortical areas. Overall, these results indicate that projections from the visual cortex to the striate-recipient zone of the LP-pulvinar complex are mainly excitatory. Despite the strong impact of the area 17 projections, our data suggest that the extrastriate cortex could also play a role in the establishment of response properties in the cat's LPl.

Motion sensitivity and stimulus interactions in the striate-recipient zone of the cat's lateral posterior-pulvinar complex

The cat's lateral posterior-pulvinar complex (LP-pulvinar) establishes reciprocal connections with the anterior ectosylvian visual (AEV) and lateral suprasylvian (LS) cortices; two regions which are believed to be involved in motion analysis. We have investigated the motion sensitivity of neurons in the LP-pulvinar complex by: (1) studying the responses properties of cells in the striate-recipient zone of the LP nucleus (LPI) to the drift of a two-dimensional texture pattern (visual noise); and (2) determining the extent to which the latter stimulus can modify the spatial frequency tuning function of LPI cells. Experiments were carried out on anesthetized normal adult cats. Almost all LPI cells (55 out of 63, 87%) responded to the motion of visual noise. For most units (39 out of 55, 71%), responses varied as a function of the direction of motion (bandwidth of 49 degrees). One-third of the LPI units did not exhibit any preference for drift direction of noise. For practically all LPI cells, responses to noise varied as a function of drift velocity. Optimal velocities were distributed from 2 to 35 degrees/s with a mean value of 27.5 degrees/s (means bandwidth of 2.5 octaves). The influence of visual noise on the spatial frequency tuning function of 22 LPI cells was also studied. For half of LPI cells, responses at all spatial frequencies were reduced when the grating and the texture pattern were moving in opposite directions (anti phase condition). This masking effect of noise was rarely observed when both stimuli were drifted in the same direction (in phase condition). These results suggest that the LP-pulvinar complex may be part of extrageniculate pathways involved in the analysis of motion of visual targets and/or the analysis of the relative movement between an object and its surrounding environment.