Rodents are emerging as increasingly popular models of visual functions. Yet, evidence that rodent visual cortex is capable of advanced visual processing, such as object recognition, is limited. Here we investigate how neurons located along the progression of extrastriate areas that, in the rat brain, run laterally to primary visual cortex, encode object information. We found a progressive functional specialization of neural responses along these areas, with: (1) a sharp reduction of the amount of low-level, energy-related visual information encoded by neuronal firing; and (2) a substantial increase in the ability of both single neurons and neuronal populations to support discrimination of visual objects under identity-preserving transformations (e.g., position and size changes). These findings strongly argue for the existence of a rat object-processing pathway, and point to the rodents as promising models to dissect the neuronal circuitry underlying transformation-tolerant recognition of visual objects.DOI: http://dx.doi.org/10.7554/eLife.22794.001

In rodents, the progression of extrastriate areas located laterally to primary visual cortex (V1) has been assigned to a putative objectprocessing 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.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.

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Extensive loss of noradrenaline-containing neurons and fibers is a nearly invariant feature of Alzheimer's Disease (AD). However, the exact noradrenergic contribution to cognitive and histopathological changes in AD is still unclear. Here, this issue was addressed following selective lesioning and intrahippocampal implantation of embryonic noradrenergic progenitors in developing rats. Starting from about 3 months and up to 12 months post-surgery, animals underwent behavioral tests to evaluate sensory-motor, as well as spatial learning and memory, followed by post-mortem morphometric analyses. At 9 months, Control, Lesioned and Lesion + Transplant animals exhibited equally efficient sensory-motor and reference memory performance. Interestingly, working memory abilities were seen severely impaired in Lesion-only rats and fully recovered in Transplanted rats, and appeared partly lost again 2 months after ablation of the implanted neuroblasts. Morphological analyses confirmed the almost total lesion-induced noradrenergic neuronal and terminal fiber loss, the near-normal reinnervation of the hippocampus promoted by the transplants, and its complete removal by the second lesion. Notably, the noradrenergic-rich transplants normalized also the nuclear expression of the transactive response DNA-binding protein 43 (TDP-43) in various hippocampal subregions, whose cytoplasmic (i.e., pathological) occurrence appeared dramatically increased as a result of the lesions. Thus, integrity of ascending noradrenergic inputs to the hippocampus may be required for the regulation of specific aspects of learning and memory and to prevent TDP-43 tissue pathology.

New and noteworthyKnowing the depth and laminar location of the microelectrodes used to record neuronal activity from the cerebral cortex is crucial to properly interpret the recorded patterns of neuronal responses. Here we present an innovative approach that allows inferring such properties with high accuracy and in an automated way (i.e., without the need of visual inspection and manual annotation) from the evoked response potentials (ERPs) elicited by sensory (e.g., visual) stimuli.

Rodents are emerging as increasingly popular models of visual functions. Yet, evidence that rodent visual cortex is capable of advanced visual processing, such as object recognition, is limited. Here we investigate how neurons located along the progression of extrastriate areas that, in the rat brain, run laterally to primary visual cortex, encode object information. We found a progressive functional specialization of neural responses along these areas, with: (1) a sharp reduction of the amount of low-level, energy-related visual information encoded by neuronal firing; and (2) a substantial increase in the ability of both single neurons and neuronal populations to support discrimination of visual objects under identity-preserving transformations (e.g., position and size changes). These findings strongly argue for the existence of a rat object-processing pathway, and point to the rodents as promising models to dissect the neuronal circuitry underlying transformation-tolerant recognition of visual objects.

Knowing the depth and laminar location of the microelectrodes used to record neuronal activity from the cerebral cortex is crucial to properly interpret the recorded patterns of neuronal responses. Here, we present an innovative approach that allows inferring such properties with high accuracy and in an automated way (i.e., without the need of visual inspection and manual annotation) from the evoked response potentials elicited by sensory (e.g., visual) stimuli.