Processing in the Lateral Geniculate Nucleus (LGN)

Layers and maps

The LGN is an elongate layered structure with each layer representing the opposite visual hemifield ( Fig. 29.2 ). Within each layer the opposite hemifield is represented such that the superior and inferior visual fields are located toward the lateral and medial zones of the layer, respectively, and the central (toward the fovea) and peripheral visual fields are located, respectively, at the posterior and anterior zones of each layer. Each layer receives monocular input from the retina and only those layers receiving input from the contralateral eye (nasal retina) represent the entire contralateral visual hemifield since LGN layers receiving input from the ipsilateral eye (temporal retina) cannot represent the monocular portion of the hemifield. This means that the ipsilateral layers are always shorter in all dimensions than the corresponding contralateral layers ( Fig. 29.2 ). The maps in each layer are retinotopically aligned such that each point in visual space is represented along a line perpendicular to the layers. The alignment of the maps is so precise that a cell-free gap, representing the optic disk (see arrows in Figs 29.1B and 29.2 ), exists in all contralaterally innervated layers in order to maintain retinotopic alignment with the ipsilaterally innervated layers. This is because the optic disk contains no receptors and is located in the nasal retina.

There are three types of cell layers, those with large cells (the magnocellular or M layers), those with medium size cells (the parvocellular or P layers) and those with very small cells (the koniocellular or K layers). All primates have at least two P and two M layers with K layers lying below each M and P layer. In humans and some other primates, each P layer can split into two or more layers in the portion of the LGN that represents central vision (∼2–17 degrees). The normal human LGN can contain as few as two or as many as six P layers.

Among primates, the laminar organization and topography of the LGN has been studied in the most detail in the macaque monkey. Malpeli and colleagues ^, performed careful reconstructions of the entire LGN documenting the retinotopic organization and number of M and P cells in each LGN layer (K cells were not counted). These studies have shown that in macaque monkeys the representation of the central visual field is magnified in the LGN ( Fig. 29.2 and see Fig. 29.5 below). The magnified representation of the central few degrees in the LGN is easy to understand given that each LGN cell receives input from approximately one to three retinal ganglion cells and ganglion cell density is much higher in central retina. Only within the portion of the nucleus representing eccentricities from 2 to 17 degrees do the two P layers split to represent four or more P layers, an arrangement which may reflect an increase in the diversity of retinal ganglion cells representing this portion of visual space. Classical textbook sections of the LGN typically are taken from this region of the LGN which shows four P layers and two M layers; as mentioned, we now know that K layers lie below each P and M layer as depicted for the macaque monkey in Figure 29.3 .

Cell classes

All LGN cells can be grouped into two principal cell classes, relay cells that send an axon to visual cortex and interneurons whose axons remain within the LGN. LGN relay cells use the transmitter glutamic acid (glutamate) and interneurons use the transmitter gamma-amino butyric acid (GABA). Relay cells and interneurons occur in a ratio of approximately 4:1 throughout the LGN and exhibit distinct dendritic morphologies. The LGN contains several classes of interneurons whose general role is to regulate signals that will ultimately pass to cortex from the retina via relay cells. The M, P, and K cells mentioned previously are all classes of relay cells and, in addition to size, can be distinguished based on dendritic morphology, calcium binding protein content, physiological properties, and axonal projection patterns to cortex. P and M LGN cells play a major role in setting up the properties of V1 cells, but it remains controversial as to whether P and M cells belong to just two classes. Evidence from retinal inputs, axon projection patterns and physiology indicate that there are subclasses of P and M cells that perform somewhat different functions in V1 (see also below). It is even more likely that K cells are not a single LGN cell class given the diversity of their connections to cortex (see also below).

Inputs: the retina

Classically it was argued that there were two pathways from the retina to the LGN, the P and the M pathways; all other retinal ganglion cells were presumed to project to other subcortical nuclei. Evidence from more recent studies has shown that 10 or more different types of retinal ganglion cells send axons to the LGN based primarily on morphology ( Fig. 29.4 ). This finding complicates the picture because it becomes unclear whether K, M, and P LGN cells, as described above, receive unique inputs from only one class of ganglion cell or more than one class. It is evident from projection patterns of LGN axons in V1 that at least 10 axon classes have been identified that could support the argument that each retinal ganglion cell class has a unique conduit to cortex via a labeled line through the LGN (see also Fig. 29.6 below).

Inputs: extraretinal sources and cortical feedback

In addition to retinal input, LGN cells receive input from a variety of both visual and non-visual extraretinal sources. These extraretinal afferent sources provide a much larger input to the LGN, in terms of synapse number, than does the retina (see review ). These extraretinal inputs also regulate the flow of information in the LGN in a variety of ways given the different transmitters they contain and the number of transmitter receptors that exist on LGN neurons ( Fig. 29.5 ). Input from extraretinal visual sources maintains retinotopic fidelity, meaning, regions representing a common point in visual space are connected. Extraretinal visual input to the LGN has been documented from the following areas in primates with their transmitter type in parentheses: primary visual cortex (glutamate), some extrastriate areas (likely glutamate), superior colliculus (glutamate), pretectum (GABA), parabigeminal nucleus (acetylcholine or ACh), and the visual sector of the thalamic reticular nucleus (GABA). Of these extraretinal visual sources the two major sources, in terms of synapse number, come from the thalamic reticular nucleus and the primary visual cortex. The input from the thalamic reticular nucleus has been studied in great detail, and together with the primary visual cortex, provide retinotopically localized regulation of LGN signals to cortex. The massive cortical feedback pathway is highly specific in that cells in different subdivisions of layer VI of V1 send axons to either the P or M LGN layers but not both. This feedback also targets the K LGN layers and the thalamic reticular nucleus but only via axon collaterals destined for either the P or M LGN layers at least in owl monkeys.

Extraretinal input to the LGN also arrives from several non-visual sources. The most well studied of these afferents are the cholinergic afferents from the pons that innervate all LGN layers and also contain the transmitter nitric oxide at least in the cat. Other afferent sources include a serotonergic input from the dorsal raphé nucleus and a histaminergic input from the hypothalamus.