An avian “pyramidal tract” was defined in zebra finches and green finches by making injections of neuronal tracers into the hyperstriatum accessorium (HA) of the rostral Wulst. Extratelencephalic projections of rostral HA traveled in the septomesencephalic tract (TSM) and gave rise to nuclear-specific terminal fields in the precerebellar medial spiriform nucleus of the posterior thalamus, the red nucleus in the mesencephalon, the medial pontine nucleus in the pons, and the subtrigeminal, external cuneate, cuneate, gracile, and inferior olivary nuclei in the medulla. Extensive but more diffuse terminal fields were also present in the stratum cellulare externum of the posterior hypothalamus, the central periaqueductal gray, the prerubral field, and the lateral and ventrolateral tegmentum of the pons and medulla. There was also a sparse projection to the dorsal thalamic nucleus intermedius ventralis anterior, which supplies the somatosensory input to the rostral Wulst, and distinct projections to the intercollicular region surrounding the central nucleus of the inferior colliculus, where they partly overlapped the projections of the dorsal column nuclei. Projections from HA to the cerebellum via the TSM are described separately. In the brainstem the ventral ramus of TSM was situated ventral to the medial lemniscus at the base of the brain, entered the spinal cord in the inner margin of the lateral funiculus, predominantly ipsilaterally, and terminated bilaterally but predominantly contralaterally in the medial part of the base of the dorsal horn of the upper six or seven cervical segments. After injections of tracers into putative targets, numerous retrogradely labeled cells were found in the rostral HA, predominantly ventrally. The results confirm the presence of a major descending fiber system in passerine birds that resembles in its brainstem course and several of its terminations the pyramidal tract of mammals. The reciprocal projections of HA with the hypothalamus suggest that rostral HA may also incorporate neuronal components that in mammals would be considered parts of prefrontal cortex. J. Comp. Neurol. 416:429–450, 2000.

Rostral Wulst in passerine birds. I. Origin, course, and terminations of an avian pyramidal tract

The lateral posterior thalamic nucleus (LP) is one of the components of the extrageniculate pathway in the rat visual system, and is cytoarchitecturally divided into three subdivisions--lateral (LPl), rostromedial (LPrm), and caudomedial (LPcm) portions. To clarify the differences in the dendritic fields and axonal arborisations among the three subdivisions, we applied a single-neuron labeling technique with viral vectors to LP neurons. The proximal dendrites of LPl neurons were more numerous than those of LPrm and LPcm neurons, and LPrm neurons tended to have wider dendritic fields than LPl neurons. We then analysed the axonal arborisations of LP neurons by reconstructing the axon fibers in the cortex. The LPl, LPrm and LPcm were different from one another in terms of the projection targets--the main target cortical regions of LPl and LPrm neurons were the secondary and primary visual areas, whereas those of LPcm neurons were the postrhinal and temporal association areas. Furthermore, the principal target cortical layers of LPl neurons in the visual areas were middle layers, but that of LPrm neurons was layer 1. This indicates that LPl and LPrm neurons can be categorised into the core and matrix types of thalamic neurons, respectively, in the visual areas. In addition, LPl neurons formed multiple axonal clusters within the visual areas, whereas the fibers of LPrm neurons were widely and diffusely distributed. It is therefore presumed that these two types of neurons play different roles in visual information processing by dual thalamocortical innervation of the visual areas.

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Different cortical projections from three subdivisions of the rat lateral posterior thalamic nucleus: a single-neuron tracing study with viral vectors

Everyday, we see thousands of different objects with many different shapes, colors, sizes and textures. Even an individual object – for example, a face – can present us with a virtually infinite number of different images, depending on from where we view it. In spite of this extraordinary variability, our brain can recognize objects in a fraction of a second and without any apparent effort. Our closest relatives in the animal kingdom, the non-human primates, share our ability to effortlessly recognize objects. For many decades, they have served as invaluable models to investigate the circuits of neurons in the brain that underlie object recognition. In recent years, mice and rats have also emerged as useful models for studying some aspects of vision. However, it was not clear whether these rodents’ brains could also perform complex visual processes like recognizing objects. Tafazoli, Safaai et al. have now recorded the responses of visual neurons in rats to a set of objects, each presented across a range of positions, sizes, rotations and brightness levels. Applying computational and mathematical tools to these responses revealed that visual information progresses through a number of brain regions. The identity of the visual objects is gradually extracted as the information travels along this pathway, in a way that becomes more and more robust to changes in how the object appears. Overall, Tafazoli, Safaai et al. suggest that rodents share with primates some of the key computations that underlie the recognition of visual objects. Therefore, the powerful sets of experimental approaches that can be used to study rats and mice – for example, genetic and molecular tools – could now be used to study the circuits of neurons that enable object recognition. Gaining a better understanding of such circuits can, in turn, inspire the design of more powerful artificial vision systems and help to develop visual prosthetics. Achieving these goals will require further work to understand how different classes of neurons in different brain regions interact as rodents perform complex visual discrimination tasks.

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Emergence of transformation-tolerant representations of visual objects in rat lateral extrastriate cortex.

In rodents, the progression of extrastriate areas located laterally to primary visual cortex (V1) has been assigned to a putative object-processing pathway (homologous to the primate ventral stream), based on anatomical considerations. Recently, we found functional support for such attribution (Tafazoli et al., 2017), by showing that this cortical progression is specialized for coding object identity despite view changes, the hallmark property of a ventral-like pathway. Here, we sought to clarify what computations are at the base of such specialization. To this aim, we performed multielectrode recordings from V1 and laterolateral area LL (at the apex of the putative ventral-like hierarchy) of male adult rats, during the presentation of drifting gratings and noise movies. We found that the extent to which neuronal responses were entrained to the phase of the gratings sharply dropped from V1 to LL, along with the quality of the receptive fields inferred through reverse correlation. Concomitantly, the tendency of neurons to respond to different oriented gratings increased, whereas the sharpness of orientation tuning declined. Critically, these trends are consistent with the nonlinear summation of visual inputs that is expected to take place along the ventral stream, according to the predictions of hierarchical models of ventral computations and a meta-analysis of the monkey literature. This suggests an intriguing homology between the mechanisms responsible for building up shape selectivity and transformation tolerance in the visual cortex of primates and rodents, reasserting the potential of the latter as models to investigate ventral stream functions at the circuitry level. SIGNIFICANCE STATEMENT Despite the growing popularity of rodents as models of visual functions, it remains unclear whether their visual cortex contains specialized modules for processing shape information. To addresses this question, we compared how neuronal tuning evolves from rat primary visual cortex (V1) to a downstream visual cortical region (area LL) that previous work has implicated in shape processing. In our experiments, LL neurons displayed a stronger tendency to respond to drifting gratings with different orientations while maintaining a sustained response across the whole duration of the drift cycle. These trends match the increased complexity of pattern selectivity and the augmented tolerance to stimulus translation found in monkey visual temporal cortex, thus revealing a homology between shape processing in rodents and primates.

https://www.jneurosci.org/content/jneuro/39/9/1649.full.pdf

Nonlinear Processing of Shape Information in Rat Lateral Extrastriate Cortex.

Patterns of connections of the visual cortex of the South American opossum, Didelphis aurita, were revealed by using neuronal tracers to identify and characterize visual specializations of the peristriate cortex (PS). The visuotopy of corticotectal connections of the anterolateral portion of PS (PSal) is symmetrical to that of the striate cortex (ST or primary visual area [V1]). Three consecutive bands of commissural connections coincide, respectively, with the ST-PS border, the limit between the caudal and rostral PSal halves (PSc and PSr), and the border of PS with the parietal and temporal cortices. PSc and PSr contain regular commissural rings similar to those present in the peristriate cortex of eutherian mammals. ST projections define in PSc two strings of periodical foci consecutively concentric to V1 and a single focus in PSr. Although they were organized topographically, ascending, descending, and commissural connections between ST and PSal showed a high degree of convergence and divergence. These results conform to the model of a single area homologous to the second visual area (V2) bordering V1. Moreover, they suggest the possibility that PSal includes either one or two additional belt-like areas successively anterior to V2. Along with the finding of alternating bands of high and low cytochrome oxidase activity in PSal, the data further suggest that this region contains modular specializations similar to those of the peristriate cortex of primates and other eutherian mammals. The posterolateral peristriate cortex (PSpl) constitutes another visual area, since it consists of a distinct focus of reciprocal corticocortical and interhemispheric connections and a separate source of corticotectal projections. Finally, a visuomotor function for the orbital cortex is proposed based on its direct projections to optical tectal layers. The close cladistic relationship of opossums to mammalian ancestral forms suggests that the PSal parcelation into belt-like areas that contain modules reflects the primitive organization of the visual cortex. Moreover, a highly diffuse pattern of corticocortical connections may represent a requirement for a brain with few visual areas to perform global processing. J. Comp. Neurol. 416:224–244, 2000. © 2000 Wiley-Liss, Inc.

Patterns of corticocortical, corticotectal, and commissural connections in the opossum visual cortex.

In Experiment I, bilateral ablations of the caudolateral cortex involving Krieg's area 36 impaired discrimination of visual patterns but not delayed alternation. In Experiment II, the same type of lesions retarded postoperative learning to discriminate embedded visual patterns. In rats from the Experiment II tracers of axonal transport gave no signs of damage of the connections of the primary visual cortex. In agreement with this, Nissl stain of the dorsal lateral geniculate nuclei showed no neuronal loss or gliosis. These results suggest that caudo-lateral cortex in rats corresponds to the inferotemporal cortex of primates.

Williams G

Papers

Cortical area in the rat that mediates visual pattern discrimination