Distribution and Morphology of Cortical Terminals in the Cat Thalamus from the Anterior Ectosylvian Sulcus

Abstract

Two main types of cortical terminals have been identified in the cat thalamus. Large (type II) have been proposed to drive the response properties of thalamic cells while smaller (type I) are believed to modulate those properties. Among the cat’s visual cortical areas, the anterior ectosylvian visual area (AEV) is considered as one of the highest areas in the hierarchical organization of the visual system. Whereas the connections from the AEV to the thalamus have been recognized, their nature (type I or II) is presently not known. In this study, we assessed and compared the relative contribution of type I and type II inputs to thalamic nuclei originating from the AEV. The anterograde tracer BDA was injected in the AEV of five animals. Results show that (1) both type I and II terminals from AEV are present in the Lateral Posterior- Pulvinar complex, the lateral median suprageniculate complex and the medial and dorsal geniculate nuclei (2) type I terminals significantly outnumber the type II terminals in almost all nuclei studied. Our results indicate that neurons in the AEV are more likely to modulate response properties in the thalamus rather than to determine basic organization of receptive fields of thalamic cells.

Laminar distribution of cortical projection neurons to the pulvinar: A comparative study in cats and mice

“…Our findings corroborate these results showing that labeled neurons in Area 17 were almost exclusively located in Layer 5. Interestingly, the present study and our previous ones (Abbas Farishta et al, 2020; Huppé‐Gourgues et al, 2006, 2019) demonstrated that despite the large number of Layer 5 inputs from Area 17, there is still some contribution from Layer 6 neurons.…”

“…In cats, CT projections from Area 17 in LPl are mostly of Type II with few Type I axons (Abbas Farishta et al, 2020; Guillery et al, 2001; Huppé‐Gourgues et al, 2006; Ojima et al, 1996). Three previous studies from our group characterized the CT terminals from areas 17, 21a, PMLS, and AEV in the cat LP (Abbas Farishta et al, 2020; Huppé‐Gourgues et al, 2006, 2019). These studies revealed a clear distinction between the CT projections from Area 17 and the extrastriate areas PMLS, 21a, and AEV.…”

“…It is generally accepted that Type II terminals (driver) carry the main information that defines the basic receptive field properties of the postsynaptic neurons whereas Type I (modulator) change the neuron's properties (Sherman & Guillery, 1998). Previous studies have shown that Type II terminals belong to CT neurons that are mainly located in Layer 5 whereas Type I terminals are from neurons located in Layer 6 (Abbas Farishta et al, 2020; Bourassa & Deschênes, 1995; Deschênes et al, 1994; Huppé‐Gourgues et al, 2006, 2019). Here, we showed that CT neurons from the cat Area 17 are almost exclusively from Layer 5, and consequently, it should provide the driver input to the LP.…”

“…Experimental and theoretical studies have shown that the nature of the information transferred between the pulvinar and the visual cortical areas varies across the cortical hierarchy (Cortes & Van Vreeswijk, 2012; de Souza et al, 2019; Jaramillo et al, 2019) thus raising the question whether cortico‐thalamo‐cortical pathways exhibit a hierarchical organization that parallels the cortico‐cortical one (Shipp, 2003). Previous studies from our group have shown that the cat pulvinar receives distinct types of CT projection depending on their cortical source (Abbas Farishta et al, 2020; Huppé‐Gourgues et al, 2006, 2019). For instance, projections from Area 17 are almost exclusively comprised of Type II terminals whereas inputs from high‐level extrastriate areas PMLS (posteromedial lateral suprasylvian area) and AEV (anterior ectosylvian visual area) are mostly comprised of Type I terminals, indicating that CT projections to the pulvinar are increasingly modulatory at higher levels of the cortical hierarchy.…”

“…Type I and II CT projections also exhibit distinct laminar origins. Type II projections originate from neurons located mainly in Layer 5 whereas Type I projections are located in Layer 6 (Abbas Farishta et al, 2020; Bourassa & Deschênes, 1995; Deschênes et al, 1994; Huppé‐Gourgues et al, 2006, 2019). These anatomical features of CT projections were found not only in the visual cortex but also in other sensory areas such as the somatosensory and auditory cortices, indicating that the connectivity patterns of these projections are an ubiquitous feature of the CT communication (Guillery et al, 2001; Llano & Sherman, 2008; Ojima, 1994; Ojima et al, 1996; Reichova & Sherman, 2004; Theyel et al, 2010).…”