Morphology of Callosal Axons Interconnecting Areas 17 and 18 of the Cat

Abstract

Seventeen callosally projecting axons originating near the border between areas 17 and 18 in adult cats were anterogradely labelled with biocytin and reconstructed in 3-D from serial sections. All axons terminated near the contralateral 17/18 border. However, they differed in their diameter, tangential and radial distributions, and overall geometry of terminal arbors. Diameters of reconstructed axons ranged between 0.45 and 2.25 microns. Most of the axons terminated in multiple terminal columns scattered over several square millimetres of cortex. Thus in general callosal connections are not organized according to simple, point-to-point spatial mapping rules. Usually terminal boutons were more numerous in supragranular layers; some were also found in infragranular layers, none in layer IV. However, a few axons were distributed only or mainly in layer IV, others included this layer in their termination. Thus, different callosal axons may selectively activate distinct cell populations. The geometry of terminal arbors defined two types of architecture, which were sometimes represented in the same axon: parallel architecture was characterized by branches of considerable length which supplied different columns or converged onto the same column; serial architecture was characterized by a tangentially running trunk or main branch with radial collaterals to the cortex. These architectures may relate to temporal aspects of inter-hemispheric interactions. In conclusion, communication between corresponding areas of the two hemispheres appears to use channels with different morphological and probably functional properties.

Input and Output Gain Modulation by the Lateral Interhemispheric Network in Early Visual Cortex

“…This strengthens our hypothesis that a contrast gain modulation reflects a direct action of the callosal input, because most visual callosal connections terminate in these layers (Houzel et al, 1994).…”

Evolution amplified processing with temporally dispersed slow neuronal connectivity in primates

“…In chimpanzees and humans, the conduction length was estimated by scaling the conduction length in the macaque to the cube root of brain volume (after subtraction of the cerebellum) (12) in each species. After correction for 35% shrinkage of the pathway lengths measured in the BDA material (23), the values obtained for humans to the CC midline are close to those estimated by DTI (13) Statistics, Model, and Network. ANOVA across all samples of BDA-traced axons reported highly significant intergroup differences (P ϭ 1.48 ϫ 10 Ϫ43 ).…”

“…However, because the same shrinkage should apply to the radial dimension of the axon and to its length, this would not affect the axon conduction measurements (below). The shrinkage of the BDA material was previously estimated to be 35% (23).…”

Selective interhemispheric circuits account for a cardinal bias in spontaneous activity within early visual areas

“…Interhemispheric connections between the two primary visual cortices of cats are thought to perpetuate the intrahemispheric network across the visual field's midline, are also patchy (Houzel et al, 1994) and selectively link neurons of similar orientation (Schmidt et al, 1997b;Rochefort et al, 2007) and direction preference (Peiker et al, 2013). As visual interhemispheric connections (VIC) can be easily manipulated by reversible deactivation without directly interfering with the recorded ipsilateral responses, we chose them as a model for a selective long-range connection between homotopic visual areas.…”