Comparison of the Laminar Distribution of Input from Areas 17 and 18 of the Visual Cortex to the Lateral Geniculate Nucleus of the Cat

Abstract

The feedback from area 18 of the cat visual cortex to the lateral geniculate nucleus has been investigated by labeling and reconstructing seventeen axons of known receptive field position and eye preference. The distribution of boutons from each axon was quantified with respect to the compartments of the geniculate complex, and the results were compared with an equivalent analysis of fourteen area 17 axons. Area 18 axons form large, sparse arborizations that extend up to 1.9 mm laterally (1170 Ϯ 85 m; mean Ϯ SEM), with a core of relatively dense innervation spanning on average 600 Ϯ 70 m (mean Ϯ SEM). Thus, they have the potential to influence the transmission of visual information from well beyond their own classical receptive fields. In this respect, they are surprisingly similar to the axons from area 17, despite the fact that the two cortical areas have very different retinotopy. However, there are important differences between the pathways. Area 18 axons project more heavily to the C layers and medial interlaminar nucleus. Whereas the input from both areas to the A layers is biased toward the layer appropriate to the eye preference of each axon, the area 18 input to magnocellular layer C is not. The distribution of area 18 boutons favors the bottom of their preferred A layer, and the area 17 boutons favor the top. These differences mirror those seen in the afferent pathways, suggesting that each cortical area preferentially targets the cells from which it receives input. Finally, their greater diameter suggests that area 18 axons provide the earliest feedback signal in the corticogeniculate loop.

Connections of higher order visual relays in the thalamus: A study of corticothalamic pathways in cats

“…Terminations in the DLGN and PGN are in accord with earlier accounts (Updyke, 1975;Murphy and Sillito, 1996;Murphy et al, 2000). The main focus of termination in DLGN lies within a projection column and is shown in Figure 3 as small shaded areas.…”

“…The axons that form terminals in PGN, TRN 1 , and DLGN (including MIN and the major layers), are axons previously classified as type 1 (Guillery, 1966) and also described by Robson (1983), Murphy and Sillito (1996), and Murphy et al (2000) in the LGN. That is, they are relatively fine, have occasional beads along their course but form most of their terminals at the ends of short, slender side branches, described as "drumstick-like" by LGN, lateral geniculate nucleus; MIN, medial interlaminar nucleus; OT, optic tract; PUL, pulvinar.…”

Corticothalamic interactions in the transfer of visual information

“…There is also good evidence favouring the view that the projection comprises axons of several diameters and conduction velocities. Individual axons, both coarse and fine, seem to innervate both PGN and LGN (Robson 1983;Boyapati & Henry 1984;Murphy & Sillito 1996;Murphy et al 2000). Similarly, estimates of the conduction velocity from latency measurements provide support for two groups of corticofugal afferents (Tsumoto et al 1978;Tsumoto & Suda 1980;Boyapati & Henry 1987;Grieve & Sillito 1995a) projecting to the LGN and PGN.…”

Corticothalamic feedback enhances stimulus response precision in the visual system

“…Thus, feedback effects will be evoked by stimuli that drive layer 6 cells and will be seen only when the cortex is functional. The layer 6 feedback cell axonal arborizations and their pattern of effect in the LGN are retinotopically organized, anisotropically linked to the parent cell orientation, and phase reversed (9,55,56). Framed in this functional and anatomical context, it is hardly surprising that the feedback exerts a high level of control over the response properties and timing of relay cells because they have the capacity to modulate the operation of the entire subcortical circuit.…”