Department of Physiology, Anatomy & Genetics, University of Oxford, Oxford, UK
The middle temporal (V5/MT) visual area of the macaque monkey is specialized for visual motion and binocular depth. From single unit studies, V5/MT has been proposed to have a columnar organization for direction of motion (Albright et al, J Neurophysiol 51:16-31, 1984) and a clustered organization for binocular disparity (DeAngelis et al, J Neurosci 19:1398-415, 1999). Although connections to and from V5/MT to cortical and subcortical areas have been identified anatomically, the intrinsic columnar organization has been difficult to define anatomically in the rhesus macaque. Using small tracer injections into area V5/MT, we have revealed a clustered pattern of intrinsic connections within area V5/MT.
In anaesthetized macaques (Sufentanil, i.v., and 0.3-0.5% Isoflurane), we recorded neuronal activity with a tungsten electrode attached to a glass capillary tube containing the tracer, Cholera Toxin subunit b (CTb, 1% low salt soln, List Biological Labs). V5/MT was identified based on stereotaxy, sequences of grey and white matter on approach, and receptive field properties (e.g. size and direction tuning). We pressure injected 50-100nl of CTb at neurophysiologically identified sites within V5/MT in three animals. After 48-72 hours, animals were perfused and the tissue processed. Parasagittal sections were stained with SMI-32 antibody and Gallyas to confirm the location of V5/MT, and were histologically examined for CTb labelled cells. Labelled cells were drawn and analysed using Neurolucida (Microbrightfield Ltd).
Labelled cells were found within layers 2, 3, 4 and 6, extending throughout most of the dorso-lateral extent of V5/MT ( 12 mm), with a clear gap between superior and inferior layers. In the latero-medial axis, labelled cells were found up to 2 mm on either side of the tracer injection. Inspecting the parasagittal sections, we found a clustered pattern of labelled cells on both sides of the injection site and in sections medial and lateral to the injection site. We observed between 1-3 such clusters in most sections. We also analysed the pattern of label quantitatively in two hemispheres from two monkeys. We carried out a Fourier analysis and an autocorrelation analysis on the density of labelled cells along layer 6 and layers 2-4, separately within each section. The analysis confirmed clusters of labelled cells at 1-2 mm intervals. We conclude that V5/MT has a distinct local circuitry of connections, both within the superior layers and in layer 6.
Supported by BBSRC, Royal Society, Wellcome Trust