The map: the lobes and the rhinencephalon
Each hemisphere has, in the UMN nomenclature, a rostral frontal lobe, a caudal occipital lobe, a lateral temporal lobe, poorly developed in the dog, and a poorly delimited parietal lobe; the primary motor area sits around the cruciate sulcus, and the somatosensory one below the coronal sulcus. The ventral, old part of the hemisphere is the rhinencephalon, the “nose brain”: the olfactory bulb, the peduncle and the lateral olfactory tract, the piriform lobe, separated from the neocortex by the lateral rhinal sulcus. Behind are the cerebellum and the brainstem; below, the pituitary. Other atlases, for example Johnson's cortical atlas (2020), divide the cortex into seven regions; the atlas drawing follows UMN.
The olfactory bulb and the peduncle
The olfactory bulb receives the olfactory nerves through the cribriform plate and sits under the frontal lobes; the olfactory peduncle is the tissue that attaches it to the brain. In the glomeruli, the receptor axons synapse with the mitral cells, about 1,000 axons per second-order neuron (Jenkins 2018); the bulb does the first filtering and discrimination of scents. From the peduncle leave the olfactory striae: the lateral one, the path of conscious olfaction, to the piriform lobe; the medial one, to the septum, for the emotional reaction. In the dog, the bulb is about 0.31 % of brain volume, 10.8 mm wide, and the bulb, tract and striae together measure 48 mm, 72 % of the length of the hemisphere, on five fixed brains (Kavoi and Hassanali 2011). The atlas does not turn the proportions into “X times more than in humans”.
The piriform lobe: what scent is this
The piriform lobe is the paleocortex of the rhinencephalon, below the lateral rhinal sulcus, with a flat rostral part and a bulging caudal one; UMN links it to conscious olfaction. Jia et al. (2014) scanned with fMRI six Labradors to which they delivered a mixture of four odorants: the olfactory bulb and the piriform lobes were activated both in awake dogs and in anesthetized ones, and at a higher concentration the activations were more extensive and bilateral. The authors call for caution until replication: the dogs were not trained on those odorants, and the fMRI signal is not electrical activity.
The amygdala: what the scent is worth
The amygdala is a collection of nuclei in depth, below the piriform lobe, involved in processing and remembering emotions, especially fear. Prichard et al. (2020) scanned 18 awake pet dogs after one scent had been paired with hot dog and another with nothing: the amygdala was the only region that robustly distinguished the rewarded scent, the unrewarded one and their mixture, and the mixture resembled the “no reward” scent, as if it were a new scent. A classifier recognized the scent type from the activity of the amygdala, the piriform cortex and the posterior cingulate, modestly (weighted F1 0.44). Limits stated by the authors: a single mixture, dogs not trained for detection, no behavioral test; the activations may index only how important a scent is.
The entorhinal cortex and the hippocampus: memory
According to the review by Kokocińska-Kusiak (2021), the olfactory cortex, with its anterior, piriform, periamygdaloid and entorhinal areas, passes the signal on: the first three to the frontal cortex and the thalamus, and the entorhinal cortex to the hippocampal formation, involved in recognizing scents from memory. The hippocampus begins in depth, below the piriform lobe, and is essential for new memories, including spatial ones. Andrews et al. (2022) described, by tractography in 23 dogs and dissection of two brains, an olfactory-entorhinal tract, from the bulb through the middle of the peduncle to the entorhinal cortex, and an olfactory-limbic tract; their link to memory and emotion comes from the literature on other species, not from a demonstration in the dog.
The frontal cortex and the thalamus: the decision
The frontal pole, the prefrontal cortex, is associated with planning and executing goal-directed actions; the thalamus is the structure “presumed” to set the detection threshold (Kokocińska 2021). In Jia (2014), the medial, superior and orbital frontal cortex and the cerebellum were activated mainly in conscious dogs: in anesthetized ones, which cannot sniff, the stimulus does not reach the cognitive structures. The cerebellum also responded to scent concentration; the authors link it to the control of sniffing and to the detection threshold, and Berns (2015) calls it a likely candidate for the still unknown “center” of sniffing.
The caudate nucleus: the scent of a familiar human
The caudate nucleus, a deep basal nucleus, takes part in the circuits of voluntary movement and, in fMRI studies, responds to stimuli with reward value. Berns, Brooks and Spivak (2015) scanned 12 awake, unsedated dogs with five scents on gauze: their own, that of a familiar human from the household who was not the handler, a strange human, a familiar dog and a strange dog. The bulb/peduncle reacted the same to all; the caudate responded most strongly to the familiar human, more than to the strange human and to the familiar dog. The authors say they cannot infer an emotion from a single activation and cannot rule out food or play as the explanation; with 12 subjects, the whole-brain analysis is exploratory.
The five tracts and the occipital lobe
Andrews et al. (2022) traced the white matter leaving the bulb in 23 mesaticephalic dogs, under general anesthesia, on a 3 T MRI, and confirmed it by Klingler dissection: five tracts, to the piriform lobe, to the limbic system, to the entorhinal cortex, a ventral tract down to the brainstem and a large, new one between the bulb and the occipital cortex, “the first documentation of a direct bulb–occipital connection in any species”. The authors propose a “highway” between smell and sight, but write that functional inferences are limited, that electrophysiology in the dog is sparse and that tractography has artifacts. In the field, the dog combines scent with sight and with memory of the place; how exactly remains to be researched.
What you see in the field
The target scent, taken from the scent article, passes through the piriform lobe and the amygdala before the frontal lobe “decides” which way: you see the decision, not the path. When the dog lights up at its person's scent, the caudate is the area that responded on fMRI. When, after a run, it stops working, the problem is in the nose, not in the brain: it cannot pant and sniff at the same time. All the studies above are on 6-23 dogs, and the atlas is not a diagnostic tool; for brain diseases, go to the veterinarian.