The growth of cat cerebral cortex in postnatal life: a magnetic resonance imaging study

Intracranial Lesion Detection and Artifact Characterization: Comparative Study of Susceptibility and T2*-Weighted Imaging in Dogs and Cats

Susceptibility-weighted imaging (SWI), an MRI sequence for the detection of hemorrhage, allows differentiation of paramagnetic and diamagnetic substances based on tissue magnetic susceptibility differences. The three aims of this retrospective study included a comparison of the number of areas of signal void (ASV) between SWI and T2^*-weighted imaging (T2^*WI), differentiation of hemorrhage and calcification, and investigation of image deterioration by artifacts. Two hundred twelve brain MRIs, 160 dogs and 52 cats, were included. The sequences were randomized and evaluated for presence/absence and numbers of ASV and extent of artifacts causing image deterioration by a single, blinded observer. In cases with a CT scan differentiation of paramagnetic (hemorrhagic) and diamagnetic (calcification) lesions was made, SWI was performed to test correct assignment using the Hounsfield Units. Non-parametric tests were performed to compare both sequences regarding detection of ASV and the effect of artifacts on image quality. The presence of ASV was found in 37 SWI sequences and 34 T2^*WI sequences with a significant increase in ASV only in dogs >5 and ≤ 15 kg in SWI. The remaining weight categories showed no significance. CT examination was available in 11 cases in which 81 ASV were found. With the use of phase images, 77 were classified as paramagnetic and none as diamagnetic. A classification was not possible in four cases. At the level of the frontal sinus, significantly more severe artifacts occurred in cats and dogs (dogs, p < 0.001; cats, p = 0.001) in SWI. The frontal sinus artifact was significantly less severe in brachycephalic than non-brachycephalic dogs in both sequences (SWI, p < 0.001; T2^*WI, p < 0.001). In conclusion, with the advantages of better detection of ASV in SWI compared with T2^*WI and the opportunity to differentiate between paramagnetic and diamagnetic origin in most cases, SWI is generally recommended for dogs. Frontal sinus conformation appears to be a limiting factor in image interpretation.

Volumetric development of the zebra finch brain throughout the first 200 days of post-hatch life traced by in vivo MRI

The first months of life are characterized by massive neuroplastic processes that parallel the acquisition of skills and abilities vital for proper functioning in later life. Likewise, juvenile songbirds learn the song sung by their tutor during the first months after hatching. To date, most studies targeting brain development in songbirds exclusively focus on the song control and auditory pathways. To gain a comprehensive insight into structural developmental plasticity of the entire zebra finch brain throughout the different subphases of song learning, we designed a longitudinal study in a group of male (16) and female (19) zebra finches. We collected T2-weighted 3-dimensional anatomical scans at six developmental milestones throughout the process of song learning, i.e. 20, 30, 40, 65, 90 and 120 days post hatching (dph), and one additional time point well after song crystallization, i.e. 200 dph. We observed that the total brain volume initially increases, peaks around 30-40 dph and decreases towards the end of the study. Further, we performed brain-wide voxel-based volumetric analyses to create spatio-temporal maps indicating when specific brain areas increase or decrease in volume, relative to the subphases of song learning. These maps informed (1) that most areas implicated in song control change early, i.e. between 20 and 65 dph, and are embedded in large clusters that cover major subdivisions of the zebra finch brain, (2) that volume changes between consecutive subphases of vocal learning appear highly similar in males and females, and (3) that only more rostrally situated brain regions change in volume towards later ages. Lastly, besides detecting sex differences in local tissue volume that align with previous studies, we uncovered two additional brain loci that are larger in male compared to female zebra finches. These volume differences co-localize with areas related to the song control and auditory pathways and can therefore be associated to the behavioral difference as only male zebra finches sing. In sum, our data point to clear heterochronous patterns of brain development similar to brain development in mammalian species and this work can serve as a reference for future neurodevelopmental imaging studies in zebra finches.

Coordinated optimization of visual cortical maps (II) numerical studies

In the juvenile brain, the synaptic architecture of the visual cortex remains in a state of flux for months after the natural onset of vision and the initial emergence of feature selectivity in visual cortical neurons. It is an attractive hypothesis that visual cortical architecture is shaped during this extended period of juvenile plasticity by the coordinated optimization of multiple visual cortical maps such as orientation preference (OP), ocular dominance (OD), spatial frequency, or direction preference. In part (I) of this study we introduced a class of analytically tractable coordinated optimization models and solved representative examples, in which a spatially complex organization of the OP map is induced by interactions between the maps. We found that these solutions near symmetry breaking threshold predict a highly ordered map layout. Here we examine the time course of the convergence towards attractor states and optima of these models. In particular, we determine the timescales on which map optimization takes place and how these timescales can be compared to those of visual cortical development and plasticity. We also assess whether our models exhibit biologically more realistic, spatially irregular solutions at a finite distance from threshold, when the spatial periodicities of the two maps are detuned and when considering more than 2 feature dimensions. We show that, although maps typically undergo substantial rearrangement, no other solutions than pinwheel crystals and stripes dominate in the emerging layouts. Pinwheel crystallization takes place on a rather short timescale and can also occur for detuned wavelengths of different maps. Our numerical results thus support the view that neither minimal energy states nor intermediate transient states of our coordinated optimization models successfully explain the architecture of the visual cortex. We discuss several alternative scenarios that may improve the agreement between model solutions and biological observations.

Neurons in the visual cortex of carnivores, primates and their close relatives form spatial representations or maps of multiple stimulus features. In part (I) of this study we theoretically predicted maps that are optima of a variety of optimization principles. When analyzing the joint optimization of two interacting maps we showed that for different optimization principles the resulting optima show a stereotyped, spatially perfectly periodic layout. Experimental maps, however, are much more irregular. In particular, in case of orientation columns it was found that different species show apparently species invariant statistics of point defects, so-called pinwheels. In this paper, we numerically investigate whether the spatial features of the stereotyped optima described in part (I) are expressed on biologically relevant timescales and whether other, spatially irregular, long-living states emerge that better reproduce the experimentally observed statistical properties of orientation maps. Moreover, we explore whether the coordinated optimization of more than two maps can lead to spatially irregular optima.

Developmental pattern changes of prefrontal efferents in the juvenile gerbil (Meriones unguiculatus)

Previous findings of our group showed that early traumatisation leads to a dysfunctional organisation of prefrontocortical efferents in adulthood. To identify vulnerable time windows during maturation, we labelled either layer III- or layer V/VI-pyramidal cells with biocytin in the prefrontal cortex of gerbils (Meriones unguiculatus) from the age of postnatal day (PD) 15 up to adulthood (PD 90). The density of passing fibres and axonal terminals in distinct cortical columns in specific prefrontal projection areas was assessed by digital image analysis. Following layer III injections, fibre densities reached adult values between adolescence (PD 60) and adulthood (PD 90). However, layer V/VI-fibre densities decreased after eye-opening (PD 15), followed by an increase to adult values after weaning (PD 30). These findings are the first to describe dynamic structural changes even beyond adolescence of functionally diverse prefrontal output systems. External interventions might exert adverse influences on the establishment of integrated prefrontal networks especially during the early phase of re-arranging.