A transthalamic olfactory pathway to orbitofrontal cortex in the monkey

Abstract

1. Evoked potentials restricted to the magnocellular portion of the mediodorsal nucleus (MDmc) of the thalamus were recorded after stimulation of the olfactory bulb (OB) and the posterior orbital cortex of the frontal lobe (OFC). Potentials evoked by stimulation of OB were probably trans-synaptically elicited, while potentials evoked by stimulation of OFC were probably a result of antidromic activation. 2. The area in which stimulation could elicit antidromic evoked potentials in MDmc was located in the centroposterior portion of OFC (CPOF). This area corresponds approximately to Walker's (80) area 13 and to von Bonin and Bailey's (9) area FF, and is situated medial and just anterior to a previously identified olfactory area, the lateroposterior portion of OFC (LPOF), which receives olfactory impulses through the hypothalamus. 3. Using extracellular microelectrodes, 58 neurons that responded with short latencies to OFC stimulation were identified in MDmc. To determine whether these neurons were activated antidromically by CPOF stimulation, three conventional neurophysiological criteria were applied; 20 of 58 neurons satisfied all the three criteria. Hence, they were concluded to be thalamocortical relay (TCR) neurons. 4. Intracellular recording of MDmc neurons disclosed that CPOF stimulation elicits an antidromic spike potential accompanied by an afterhyperpolarization. This hyperpolarization was presumed to be due to concurrent stimulation of inhibitory orbitothalamic fibers. It was also shown that EPSP-like depolarizations with superimposed spike potentials often occurred in the middle of the afterhyperpolarization. 5. Intracellular recording of MDmc neurons strongly suggested that the remaining 38 neurons that did not satisfy one of the three criteria were also TCR neurons. 6. These studies provide electrophysiological evidence for a transthalamic olfactory pathway from OB through MDmc to CPOF. 7. Using an extracellular recording technique, responses of neurons to eight odors were examined in CPOF and MDmc of unanesthetized awake monkeys. When these results were compared with the responses of neurons to the same odors in OB, prepyriform-amygdaloid area, and LPOF, it was concluded that the newly found transthalamic olfactory pathway to CPOF is very different in function from the previously demonstrated transhypothalamic olfactory pathway to LPOF.

Attention to Odor Modulates Thalamocortical Connectivity in the Human Brain

“…These connections are typically based on previous anatomical data, but the model does not necessarily reproduce every connection, nor does it imply that connections are monosynaptic. We based the architecture of our model on previous anatomical data in rodents and nonhuman primates (Yarita et al, 1980;Price, 1985;Russchen et al, 1987;Ray and Price, 1992;Carmichael et al, 1994), which demonstrate that PC is intimately connected with olfactory OFC and MD thalamus and that MD thalamus has connections with olfactory OFC. Animal studies further suggest that aPC (being more anteriorly located than pPC) receives the majority of afferent inputs from the olfactory bulb and is the major source of direct projections to olfactory OFC (Ray and Price, 1992;Carmichael et al, 1994;Haberly, 2001;Illig, 2005).…”

“…The modulatory effects of attention on each of the connections was obtained by subtracting the modulatory effect of tone attention from that of odor attention and then testing the significance of these differential effects with one-sample t tests ( p Ͻ 0.05, one-tailed). To the extent that aPC, pPC, MD thalamus, and OFC are principally linked via excitatory glutamatergic pathways (Tanabe et al, 1975;Yarita et al, 1980;Carmichael et al, 1994;Haberly, 2001), the use of one-tailed t tests enabled us to specifically test the hypothesis that odor (vs tone) attention would strengthen regional coupling in the positive direction (i.e., faster or stronger connections Ͼ0), in keeping with the known basic physiology of these neural circuits. Finally, to compare modulatory effects of attentional context on the direct and indirect pathways, we computed the modulatory effect for the whole indirect pathway by averaging the modulatory effects for the three connections involved in the indirect pathway (aPC to pPC, pPC to MD thalamus, MD thalamus to OFC) and comparing this with the direct pathway (aPC to OFC).…”

“…These studies suggest that odor information projects from piriform cortex (PC) to olfactory orbitofrontal cortex (OFC) by two different pathways (Tanabe et al, 1975;Yarita et al, 1980;Russchen et al, 1987;Ray and Price, 1992;Carmichael et al, 1994). The principal ("direct") pathway projects directly from PC to OFC and is considered the dominant route for olfactory transmission to neocortical brain areas.…”

Subnucleus-specific loss of neurons in medial thalamus of schizophrenics

“…The nuclei are delineated by cytoarchitecture and distinguished by major differences in cortical and subcortical connections (8)(9)(10)(11)(12)(13)(14)(15). The magnocellular subnucleus (MDmc) occupies the anteromedial aspect of MD and is composed of relatively large, deeply stained neurons.…”

The Hitchhiker’s guide to the rhinencephalon

“…This led to the notion of the orbitofrontal cortex as the isocortical area related to olfaction. These projections from primary olfactory areas have now been extensively studied in rodents, carnivores and monkeys 51,52,53,54 mostly by electrophysiological methods, delimiting the olfactory area in the latero-posterior orbital frontal cortex of the monkey (the posterior part of Area 12 of Walker…”