The postnatal development of MT, V1, LGN, pulvinar and SC in prosimian galagos (Otolemur garnettii)

Topographic organization across foveal visual areas in macaques

Introduction

While the fovea on the retina covers only a small region of the visual field, a significant portion of the visual cortex is dedicated to processing information from the fovea being a critical center for object recognition, motion control, and visually guided attention. Despite its importance, prior functional imaging studies in awake monkeys often focused on the parafoveal visual field, potentially leading to inaccuracies in understanding the brain structure underlying function.

Methods

In this study, our aim is to unveil the neuronal connectivity and topography in the foveal visual cortex in comparison to the parafoveal visual cortex. Using four different types of retrograde tracers, we selectively injected them into the striate cortex (V1) or V4, encompassing the regions between the fovea and parafovea.

Results

V1 and V4 exhibited intense mutual connectivity in the foveal visual field, in contrast to the parafoveal visual field, possibly due to the absence of V3 in the foveal visual field. While previous live brain imaging studies failed to reveal retinotopy in the foveal visual fields, our results indicate that the foveal visual fields have continuous topographic connectivity across V1 through V4, as well as the parafoveal visual fields. Although a simple extension of the retinotopic isoeccentricity maps from V1 to V4 has been suggested from previous fMRI studies, our study demonstrated that V3 and V4 possess gradually smaller topographic maps compared to V1 and V2. Feedback projections to foveal V1 primarily originate from the infragranular layers of foveal V2 and V4, while feedforward projections to foveal V4 arise from both supragranular and infragranular layers of foveal V1 and V2, consistent with previous findings in the parafoveal visual fields.

Discussion

This study provides valuable insights into the connectivity of the foveal visual cortex, which was ambiguous in previous imaging studies.

The postnatal development of retinal projections in strepsirrhine galagos (Otolemur garnettii)

Here, we describe the postnatal development of retinal projections in galagos. Galagos are of special interest as they represent the understudied strepsirrhine branch (galagos, pottos, lorises, and lemurs) of the primate radiations. The projections of both eyes were revealed in each galago by injecting red or green cholera toxin subunit B (CTB) tracers into different eyes of galagos ranging from postnatal day 5 to adult. In the dorsal lateral geniculate nucleus, the magnocellular, parvocellular, and koniocellular layers were clearly labeled and identified by having inputs from the ipsilateral or contralateral eye at all ages. In the superficial layers of the superior colliculus, the terminations from the ipsilateral eye were just ventral to those from the contralateral eye at all ages. Other terminations at postnatal day 5 and later were in the pregeniculate nucleus, the accessory optic system, and the pretectum. As in other primates, a small retinal projection terminated in the posterior part of the pulvinar, which is known to project to the temporal visual cortex. This small projection from both eyes was most apparent on day 5 and absent in mature galagos. A similar reduction over postnatal maturation has been reported in marmosets, leading to the speculation that early retinal inputs to the pulvinar are responsible for the activation and early maturation of the middle temporal visual area, MT.

A Fine-Scale and Minimally Invasive Marking Method for Use with Conventional Tungsten Microelectrodes

In neurophysiology, achieving precise correlation between physiological responses and anatomic structures is a significant challenge. Therefore, the accuracy of the electrode marking method is crucial. In this study, we describe a tungsten-deposition method, in which tungsten oxide is generated by applying biphasic current pulses to conventional tungsten electrodes. The electrical current used was 40-50 μA, which is similar to that used in electrical microstimulation experiments. The size of the markings ranged from 10 to 100 μm, corresponding to the size of the electrode tip, which is smaller than that of existing marking methods. Despite the small size of the markings, detection is easy as the marking appears in bright red under dark-field observation after Nissl staining. This marking technique resulted in low tissue damage and was maintained in vivo for at least two years. The feasibility of this method was tested in mouse and macaque brains.