The role of feedback projections in feature tuning and neuronal excitability in the early primate visual system.

A general assumption in visual neuroscience is that basic receptive field properties such as orientation and direction selectivity are constructed within intrinsic neuronal circuits and feedforward projections. In addition, it is assumed that general neuronal excitability and responsiveness in early visual areas is to a great extent independent of feedback input originating in areas higher in the stream. Here, we review the contribution of feedback projections from MT, V4 and pulvinar to the receptive field properties of V2 neurons in the anesthetized and paralyzed monkey. Importantly, our results contradict both of these assumptions. We separately inactivated each of these three brain regions using GABA pressure injections, while simultaneously recording V2 single unit activity before and hours after inactivation. Recordings and GABA injections were carried out in topographically corresponding regions of the visual field. We outline the changes in V2 activity, responsiveness and receptive field properties for early, mid and late post-injection phases. Immediately after injection, V2 activity is globally suppressed. Subsequently, there is an increase in stimulus-driven relative to spontaneous neuronal activity, which improves the signal-to-noise coding for the oriented moving bars. Notably, V2 tuning properties change substantially relative to its pre-injection selectivity profile. The resulting increase or decrease in selectivity could not be readily predicted based on the selectivity profile of the inactivated site. Finally, V2 activity rebounds before returning to it pre-injection profile Our results show that feedback projections profoundly impact neuronal circuits in early visual areas, and may have been heretofore largely underestimated in their physiological role.

Horizontal projections of area 17 in Cebus monkeys: metric features, and modular and laminar distribution

Metric features and modular and laminar distribution of intrinsic projections of area 17 were studied in Cebus apella. Anterogradely and retrogradely labeled cell appendages were obtained using both saturated pellets and iontophoretic injections of biocytin into the operculum. Laminar and modular distributions of the labeled processes were analyzed using Nissl counterstaining, and/or cytochrome oxidase and/or NADPH-diaphorase histochemistry. We distinguished three labeled cell types: pyramidal, star pyramidal and stellate cells located in supragranular cortical layers (principally in layers IIIa, IIIb alpha, IIIb beta and IIIc). Three distinct axon terminal morphologies were found i.e., Ia, Ib and II located in granular and supragranular layers. Both complete and partial segregation of group I axon terminals relative to the limits of the blobs of V 1 were found. The results are compatible with recente evidence of incomplete segreagation of visual information flow in V 1 of Old and New World primates.

Morphometric analysis of intrinsic axon terminals of Cebus monkey area 17

A morphological study of intrinsic projections in area 17 of Cebus monkey was conducted after iontophoretic injection of biocytin. Thirty axon terminals located in supragranular layers were qualitatively and quantitatively analyzed using 3-D automatic microscopy. Three types of axon terminals could be identified: Ia, Ib and II. Group I was characterized by a sparse and/or long-distance branch pattern, while type II presented compact and localized arborization. Ia axon terminals formed "clusters" and "terminaux"boutons while Ib did not. On overage, group II axon terminals tended to present straight or obtuse branching angles and a much more ramified pattern, and occupy a smaller cortical territory with shorter intermediate segments and higher density of synaptic potential sites than group I. The common characteristics of group I included innervation of larger cortical territories, longer intermediate segments, acute branching angles and lower synaptic density compared to group II. The results are compatible with the major subdivisions of neocortical neuronal morphology that classifies them as smooth and spine neurons. Smooth neurons may be related to axon terminals of group II while spine neurons may be related to group I.