Golgi and axonal transport techniques have been used to examine the organization of neurons within primary visual cortex, area 17, of the cat. This organization has been compared to that of the primate cortical area 17 as described in previous studies and it is discussed in relationship to the distribution of afferents from the dorsal lateral geniculate nucleus (dLGN). The visual cortex of the cat and monkey show strong similarities in the laminar positions of neurons projecting extrinsically and also in the restriction of spiny stellate neurons to a central lamina (lamina 4) receiving input from the dLGN. However, lamina 4B in the monkey, which contains spiny stellate neurons but does not receive direct input from the dLGN, has no direct counterpart in cat area 17. Axon projections of spiny stellate neurons in the other divisions of lamina 4 differ in cat and monkey: the small, closely packed neurons in the lowermost division of lamina 4 (4B in the cat, 4Cbeta in the monkey) project chiefly within lamina 4 in the cat whereas in the monkey they have a strong projection to lamina 3. In the cat, spiny stellate neurons of lamina 4A project upon lamina 3 whereas in the monkey those in the apparently equivalent zone, 4Calpha, project upon lamina 4B. Most non-spiny stellate neurons examined have precisely organized interlaminar axonal projections which differ from the axon trajectories of neighboring spiny neurons.

1. The corticotectal, corticothalamic and commissural projections of areas 17 and 18 of the cat have been examined using electrical stimulation techniques. 2. In both area 17 and area 18, almost all corticotectal neurones are C cells and have binocular receptive fields. Some of these cells respond equally well to both small moving spots and elongated stimuli, while others only respond to stimuli of restricted length (cf. Palmer & Rosenquist, 1974). Both types are highly direction-selective. A third type of corticotectal C cell responds optimally to long edges or bars and shows only weak direction selectivity. Corticotectal cells generally have fast conducting axons and the majority are encountered in lamina V. About 25% of all cells recorded in lamina V can be antidromically activated from the superior colliculus. 3. Striate and parastriate cells efferent to the thalamus can have either S or C type receptive fields. Corticothalamic S cells are the most common type of efferent cell in lamina VI and have more slowly conducting axons than C cells. Efferent S cells are almost always direction-selective and about half have binocular receptive fields. 4. It is suggested that there may be at least three subgroups within the corticothalamic cells: lamina V C cells project to the pulvinare complex (the same cells may also send axons to the superior colliculus), lamina VI C cells project to the perigeniculate nucleus and lamina VI S cells provide the cortical input to neurones within the lateral geniculate nucleus. 5. In contrast to the corticotectal and corticothalamic projections, the receptive fields of cells projecting through the corpus callosum forth a heterogenous group. All major striate and parastriate receptive field classes are efferent to the contralateral cortex. Their receptive field centres are located close to the vertical mid line and most cells respond best to stimuli moving towards the ipsilateral visual hemifield. Efferent neurones are mostly encountered in lamina III, within about 1mm either side of the 17-18 border zone. 6. Cells orthodromically excited after commissural stimulation have mostly C or B type receptive fields. Unlike efferent callosal neurones, orthodromically activated cells are encountered up to 3 mm into area 18 and can have receptive fields located up to 9 degrees from the vertical mid line. 7. The results are discussed with regard to the possible functional significance of each of the corticofugal pathways.

Removal of the superior colliculus (SC) in neonatal Wistar rats results in a rapid loss of retinal ganglion cells (RGCs). There is an early twofold increase in RGC death 4-8 hr postlesion (PL) followed by a later 10-11-fold increase in pyknosis about 24 hr PL. We have now used neurotrophic factors (BDNF, NT-4/5, NT-3, NGF, LIF), glutamate receptor antagonists (MK-801, DNQX, CNQX), an antioxidant (N-ace-tyl-L-cysteine), and an NOS inhibitor (L-NAME) to determine whether the early and late phases of lesion-induced RGC death involved similar or different mechanisms. Normal and pyknotic nuclei of tectally projecting RGCs were visualized by injecting the left s.c. of 2 d old rats with diamidino yellow (DY). Two days later the injection site was removed. In most rats, right eyes were injected with factors immediately after the s.c. ablation. Rats were perfused either 6 or 24 hr PL. In the latter group a second intravitreal injection of the appropriate factor was sometimes made 12 hr PL. NT- 4/5 and BDNF significantly decreased RGC pyknosis 6 and 24 hr PL, whereas NT-3 was only protective 6 hr PL. LIF slightly reduced RGC death 24 hr PL, but NGF had no influence on RGC survival at either time point. NT-4/5 also reduced the rate of naturally occurring RGC death. MK-801, DNQX, CNQX, N-acetylcysteine, and L-NAME all prevented the early lesion-induced increase in RGC death but had no significant effect on RGC death measured 24 hr PL; none of these factors significantly reduced the rate of naturally occuring RGC death. Cycloheximide, shown previously to reduce RGC pyknosis 24 hr PL, did not prevent RGC death 6 hr PL. The data indicate that there are at least two mechanisms involved in RGC death after neonatal target ablation. The early increase is related to excitotoxic effects mediated by glutamate receptors and involves NOS and the production of free radicals. We found no evidence that RGC death measured 24 hr PL is dependent on these processes, but the later death does require protein synthesis and, most likely, the activation of an endogenous suicide program. NT-4/5 and BDNF protected RGCs from both types of lesion-induced death.

A laminar distribution of different functional cell types in the striate cortex of the cat is drawn up from the visual responses of single cells recorded in 64 electrode penetrations in 38 cats. In summary, S cells were found to be concentrated in laminae 4 and 6; SH cells in laminae 2, 3 and 4; C cells in laminae 5 and lower 3; B cells in laminae 3 and upper 5 and cells with non-oriented receptive fields in lamina 4. In addition, the nature of afferent innervation to striate neurons was derived from the latency of the orthodromic response to electrical stimulation in the optic chiasm and optic radiations in 19 cats. An analysis of latency values allowed the afferent innervation to a cell to be classed as belonging either to fast or slow conducting streams in the population of dLGN axons and also permitted a decision to be made on whether or not the afferent path passed directly to the cell. Direct afferent innervation from the dLGN was not found to be confined to a single class of striate neuron. Instead, examples of cells with S, SH, C, B and non-oriented receptive fields all had orthodromic latencies that met the requirement for direct innervation. Instances of cells with orthodromic latencies suggestive of indirect innervation were also found for most receptive field classes but these cells were encountered less frequently than those with a direct afferent input. It is argued that a variety of different cell types may act as first order neurons in the striate cortex and that cells occurring at later stages in the sequence of cortical processing may have been incompletely studied because they are more difficult to stimulate either visually or electrically.

1. The receptive field properties, laminar distribution and afferent connectivity of cells in area 18 of the cat are described. 2. Testing with both moving and stationary stimuli revealed three main receptive field types which have been termed S, C and B, respectively (cf. Henry, 1977; Henry, Lund & Harvey, 1978). All three classes may show end-zone inhibition and units exhibiting this property have been designated SH, CH and BH. 3. S cells can be divided into spatially separate lights and/or dark edge response regions when tested with moving edges and usually have separate ON and/or OFF areas when tested with stationary flashing stimuli. They are the most commonly encountered cell type in area 18 and occur most frequently in laminae IIIb, IVa and VI. 4. Both C and B cells have spatially coincident light and dark edge response regions and give mixed ON and OFF discharges when tested with stationary flashing stimuli. Compared to B cells however, C cells have large receptive fields, they are broadly tuned for stimulus orientation and generally have a relatively high rate of spontaneous activity. C cells are more common than B cells and are encountered most often in laminae IVb and V. 5. Electrical stimulation of the optic chiasm (OX) and optic radiation (OR) was used to examine the afferent connectivity of parastriate neurons. Cells driven from both OX and OR have been divided into two main groups and it is argued that group 1 cells are directly, and group 2 cells are indirectly, excited by rapidly conducting afferent fibres. Group 1 cells are found most often in laminae IIIb, IVa, IVb and VI, and their distribution closely follows the anatomically defined laminar disposition of geniculocortical afferent terminals. Group 2 neurones predominate in laminae II-IIIa, IIIA and V. 6. The majority of S and SH cells are directly driven, whereas most C and CH cells have OX and OR latencies suggestive of indirect activation by thalamic afferents. 7. The intrinsic organization and possible functional role of area 18 is discussed in the light of these results.

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