Question #8: Explaining the role of gamma oscillations

Lakatos et al have noted that “the role of neuronal oscillations in brain operations has been debated since the discovery of the electroencephalogram.” So this is no small cookie. But the current consensus seems to be that gamma waves with frequencies between 25 and 100 Hz are necessary for sensory processing.

Here’s the theory: Activated groups of neurons have the tendency to oscillate in coherent fashion, affecting the output of the given group and its sensitivity to input. Thus, two groups of neurons will be able to communicate much more effectively if their oscillations are phase-locked. Conversely, neural groups that don’t have this frequency oscillation synchrony will have a much reduced capability to communicate. So, incoming sensory info from the currently attended stimulus will have an advantage during recieving in upstream cortical regions. Also, a “broadcasting center” in the thalamus could distribute the selected rhythm to appropriate cortical regions and prime them to preferentially recieve certain frequencies of sensory input.

The theory implies that the spike-traveling time from sending to recieiving group must be timed correctly and have high fidelity. This is true of afferent thalamocortical axons. Spikes in thalamic neurons arrives in cortical cells in between 1-4 ms and peaks at 2 ms. Indeed, Salami et al (2003) found that the conduction velocity along axons on the thalamocortical tract is 10 times faster than other afferents in mice, due to its selective myelination. So this particular tract must be selected from development to be able to have a low latency interaction with the cortex.

The empirical work I’ve seen backs this theory up. For example, Dockstader et al (2010) used MRI and MEG on healthy participants while administering electrical current stimuli for 0.2 ms just above motor threshold. The participants attended either to the electrical stimuli or a distracting video. Selective attention to the electrical stimuli significantly increased activation in the early phase-locked contralateral primary somatosensory cortex gamma response, starting at 20 ms post-stimuli presentation. This shows that selective attention is likely mediated in neural circuits via gamma oscillations.

Note: Very high oscillations (100-500 Hz) are associated with epilepsy, and those between 250 and 500 Hz are often identified via EEG near the onset of focal seizures. The consensus on the role for lower Hz delta-range oscillations is much less distinct, but it may also be involved in early sensory selection. It is of course possible and likely that there are multiple roles for the gamma waves, and that sensory integration is only one of them.

Inspired by CalTech’s Question #8 for cognitive scientists: “What do you know about high-frequency oscillations (20-50 Hz) in invertebrates or vertebrates? What causes them?”


Salami M, et al. 2003 Change of conduction velocity by regional myelination yields constant latency irrespective of distance between thalamus and cortex. doi: 10.1073/pnas.0937380100 .

Fries P. 2005 A mechanism for cognitive dynamics: neuronal communication through neuronal coherence. doi:10.1016/j.tics.2005.08.011 .

Dockstader C, et al. 2010 Cortical dynamics of selective attention to somatosensory events. doi:10.1016/j.neuroimage.2009.09.035 .

Jirsch JD. 2006 High-frequency oscillations during human focal seizures. doi:10.1093/brain/awl085.

Question #7: Describing primary sensory pathways in humans

Visual: Path goes, photoreceptor cells in retina –> ganglion cells — (via optic nerve) –> optic chiasm (crossing of axons) –> lateral geniculate nucleus (mostly) + superior colliculi (can mediate saccades) + pretectal area (pupillary light reflex: if light shined in one eye, both pupils constrict). Then from lateral geniculate nucleus — (via optic radiations) –> V1 –> V2 –> parietal visual cortical areas (moving objects around in your head) + temporal visual areas (complex perception of patterns and forms). See here.

Auditory: Path starts in the hair cells of the cochlea, specifically the center axis called the spiral ganglion — (via auditory nerve) –> cochlear nucleus –> location of sound detection: (ventral cochlear nucleus –> superior olive of medulla) + quality of sound: (dorsal cochlear nucleus: frequency differences) — (via lateral lemniscus fiber tract) –> inferior colliculus –> auditory nucleus of the sensory thalamus (aka medial geniculate nucleus) –> primary auditory cortex of temporal lobes. See here.

Olfactory: Path is from olfactory receptors of roof of the nasal cavity — (via axons of receptors projecting as first cranial nerve) –> olfactory bulb — (via axons of mitral cells projecting as olfactory tract) –> olfactory cortex — (via mediodorsal nucleus of the thalamus) –> insular cortex (taste integrates with smell to produce flavor) + orbitofrontal cortex (odor-taste association learning at single neuron level). Notice that olfaction is the one sensation that gives info directly to the cortex from receptors without first passing through the thalamus. See here.

However, there are many multisensory connections in the cerebral cortex, superior colliculus, and thalamus, such that our sensations can feedback and “correct” one another well before conscious awareness. For example, the premotor cortex achieves multisensory integration by converging visual, auditory, and somatosensory inputs, and has large amounts of overlap with axons of various sensory systems sending projections to other cortical regions. These integration mechanisms vary by behavioral task as well type of sensory input and context must be taken into account.

Inspired by CalTech’s Question #7 for cognitive scientists: “Describe the main pathways between sensory receptors and cortex (including intra-cortical circuits) for mammalian vision, hearing and olfaction.”


Critchley HD, Rolls ET. 1996 Olfactory neuronal responses in the primate orbitofrontal cortex: analysis in an olfactory discrimination task. Abstract.

Cappe C, et al. 2009 Multisensory anatomical pathways. doi:10.1016/j.heares.2009.04.017.

The Washington University School of Medicine Neuroscience Tutorial, here.

Question #6: Amplification of auditory and olfactory sensation

Auditory: Mechanical basilar membrane displacement in the cochlea opens mechanoelectrical transduction channels in hair cells, allowing an influx of potassium (K+) mediated current. This leads to the downstream neural pathways for hearing. Amplification of this signal across the entire hearing range occurs as a result of one specific subset of these hair cells, the outer hair cells. In response to the transmembrane receptor potential, outer hair cells actively oscillate at the frequency of the incoming sound, in a process called electromotility. It is believed that these voltage-dependent cell vibrations are dependent upon a protein called prestin that is expressed highly in the lateral plasma membrane of the outer hair cells. This is supported based on Liberman et al’s homozygous gene disruption paradigm in mice that led to a greater than 100 fold loss of auditory sensitivity. Its mechanism seems to mediate the electroneutral exchange of two anions across the plasma membrane, chlorine (Cl -) and carbonate (CO2 -3), which causes a direct voltage to displacement conversion. The unique morphology of the outer hair cells allows them  to operate at frequencies higher than 50  kHz if properly stimulated. This amplification via outer hair cells only occurs in mammals and allows for improved frequency selectivity, which is necessary for the complexities of human speech!

Olfactory: The mucous epithelial layer of the nose contains olfactory receptor neurons, each of which has 8-20 whip-like cilia that are each 30-200 microns long, and are where molecular reception of the odor commences. The incoming odor stimulates the transmembrane protein andenylate cyclase which catalyzes the conversion of ATP to 3′,5′-cyclic AMP (cAMP). cAMP is directly connected to an ion channel, allowing an influx of cations (primarily calcium) that depolarize the cell. The influx of calcium leads to an opening of calcium-dependent chloride channels. The chloride conductance is the amplification step and accounts for 80-90% of the odorant-induced depolarizing current, in sigmoidal fashion. Lowe and Gold (1993) had a classic study showing the effects of this. Using a newt olfactory receptor cell, they used flash photolysis of caged cAMP to simulate an upstream odor reception and analyzed the chloride influx. In addition to varying the intensity of the light source to uncage varying amounts of cAMP, and measuring the chloride amplitude, the researchers also used a condition with 5 millimoles of a chloride channel blocker, SITS. Here’s their figure 3D. The normal condition shows a sigmoidal amplification curve (black circles), and the SITS condition of open circles has no such amplification:

Very cool!

Inspired by CalTech’s Question #6 for cognitive scientists: “Every sensory system relies on receptor cells that transduce a stimulus into an electrical signal. This clearly requires some significant amplification. Describe two different sensory receptor cells, with attention to the location(s) and the mechanism(s) of this amplification.”


Liberman MC, et al. 2002 Prestin is required for electromotility of the outer hair cell and for the cochlear amplifier. doi:10.1038/nature01059.

Mistrik P, et al. 2009 Three-dimensional current flow in a large-scale model of the cochlea and the mechanism of amplification of sound. doi: 10.1098/​rsif.2008.0201.

Stephan AB, et al. 2009 ANO2 is the cilial calcium-activated chloride channel that may mediate olfactory amplification. doi: 10.1073/pnas.0903304106.

Lowe G and Gold GH.  1993 Nonlinear amplification by calcium-dependent chloride channels in olfactory receptor cells. doi:10.1038/366283a0

Question #5: Describing the limbic system

The limbic system is a set of brain regions that combine to control long term memory, emotion, and olfaction. There are many areas of the brain that are sometimes considered a part of the system, but the main ones are the amygdala, the hippocampus, the cingulate gyrus, the hypothalamus, the thalamus, and the fornix, all except the last of which are described here.

It has somewhat of a spotted history. As Wikipedia’s article explains, it was originally assumed to control emotion while the neocortex controls cognition, but evidence has since come out stating that the truth is more integrated than that (ie, both the limbic system and the neocortex process at least a little bit of both cognition and emotions). The journal mentions per year (via Scopus) have been declining for the search “limbic system” when normalized to the mentions of the word “brain,” as you can see here:

Nevertheless, the concept of a limbic system is still a useful paradigm for some scientific research. Brankovic (2008) used it as a general brain region where symptoms from patients suffering from depression would manifest themselves. Turner et al (2008) used it as general brain region that was affected when the ventral pallidum was activated in a rat model of Parkinson’s disease. Although it may not totally reflect the reality in the brain, it represents a simplifying assumption that may help spur research.

Inspired by CalTech’s Question #5 for cognitive scientists: “What is the limbic system?”


Turner MS, Gray TS, Mickiewicz AL, Napier TC. 2008 Fos expression following activation of the ventral pallidum in normal rats and in a model of Parkinsons Disease: Implications for limbic system and basal ganglia interactions. Brain Structure and Function 213:197-213. doi:10.1007/s00429-008-0190-4.

Brankovic SB. 2008 System identification of skin conductance response in depressionAn attempt to probe the neurochemistry of limbic system. Psychiatria Danubina 20:310-322.

Question #4: The major areas of the vertebrate brain

There are links to diagrams of the vertebrate brain here and here. These are the fourteen major areas of it with a few remarks to the function of each.

1) Medulla Oblongota: Makes the connection from the spinal cord to the rest of the brain. It controls “autonomic” functions: respiration, blood pressure, pulse, vomiting, defection, reflexes, and swallowing.

2) Cerebellum: It is dorsal to the pons (and medulla oblongota) and inferior (below) the occipital lobe, putting it at the lower back of the brain. It is mainly believed to be involved in sensory and motor control and especially in the timing of movements. Due to its many small granule cells, in humans it has 50% of the total neurons but only 10% of the total mass. However, it also contains some of the largest human neurons in Purkinje cells which are also the only output of the cerebellum. It looks similar across all vertebrate species, which has been taken as evidence for conserved function across the class of animals.

3) Pons: This area is above the medulla oblongota and ventral to the cerebellum but inferior to the rest of the brain. It’s involved in relaying sensory information (not a surprise given its location), helping to regulate breathing and arousal, and possibly is even involved with dreaming. The pons is present in all vertebrates.

4) Hypothalamus: They hypothalalmus is present in all vertebrates just above the brain stem. It is involved with making and secreting hormones, which affect body temperature, hunger, thirst, emotions (anger), and circadian rhythms. If there is bilateral lesion of the ventromedial nucleus of the hypothalamus then the animal will stop intaking food entirely, which would probably be the ultimate diet. Because of all of the hormones, it is a region of high sexual dimorphism. In the vertebrate lineage, it is believed that a common ancestor of lampreys were the first to evolve the hypothalamic-pituitary-gonadal neuroendocrine system, including two gonadotropin-releasing hormones.

5) Thalamus: In humans, this is the main region of the diencephalon. The dorsal thalamus is highly conserved throughout vertebrates. It is believed to play a role in converting prethalamic inputs into a form “readable” by the cerebral cortex, regulating sleep, and various sensory systems.

6) Pituitary Gland: A pea-sized region below the hypothalamus that secretes the hormones produced by the hypothalamus, thus controlling homeostatic processes. Morphologically conserved throughout vertebrates but functionally very specific to the organism. In mammals, the main two neural hormones released are oxytocin, and vasopressin, the latter of which regulates the bodies retention of water.

7) Cingulate Gyrus: Located in the medial part of the brain above the corpus callosum in the cerebral cortex. It is involved in the limbic system so its functions are learning and memory. It gets inputs from the thalamus, neocortex, and sensory systems in the cerebral cortex.

8) Hippocampus: In humans, it is found in the medial temporal lobe and plays major roles in memory consolidation and spatial navigation. Non-mammals do not have a hippocampus, but they do have a homologous pallium. In ray-finned fish and birds, the medial pallium is involved with spatial memory, which is quite robust. Other species are not believed to use the same type of memory storage, although some insects and cephalopods, it involves different areas (ie, for octopuses it is the vertical lobe). In mammals, the hippocampus-to-size ratio increases (not necessarily linearly) with intelligence, as it is twice as large in primates as in hedgehogs.

9) Olfactory Lobes: These are large and were major components of “early” vertebrate forebrains, and although it has increased in relative size throughout the evolution of vertebrates, it has retained the same five layers from fruit flies to the lab mouse. The functions are to enhance the differences between odors, increasing overall sensitivity to smells, filtering out noise, and communicate with higher brain regions. The main inputs to the area are basal dendrites of mitral cells.

10) Occipital Lobe: Recieves raw retinal sensory information and processes it in the primary visual cortex (V1). In humans, it has a structured map of all visible spatial information. The visual cortex has expanded a large amount in primates along with the neocortex in general.

11) Temporal Lobe: In humans, the temporal lobe is involved with audition, olfaction, vision (association and color), memory, and linking past sensory and emotional experiences into a coherent self. There has been lots of change in its function throughout evolution, but a generally trend is an increase in size in mammals.

12) Amygdala: This is involved in memory and emotional reactions (like fear) in humans. Although it has homologues in all vertebrates, it was not until amniotes that two anatonomical regions developed: the posterior dorsal ventricular ridge, plus the lateral nuclei (in reptiles) and the basolateral complex (in mammals). Laberge et al (2006) has suggested that these news regions are capable of modulating the older sections of the amygdala and allowing for more complex types of emotional learning.

13) Parietal Lobe: Superior to the occipital lobe and posterior to the frontal lobes, this region is involved with aggergating sensory information and contextualizing it, especially in terms of spatial sense and navigation. Different sections of the lobe correspond to different types of spatial awareness: important locations, head-based or eye-based reference frames, shape, size, etc. I have heard that frogs have highly developed parietal lobes and that this is what enables them to stick their tongues out and catch flies seemingly at will. Perhaps Kobe Bryant has a highly developed one as well.

14) Frontal Lobe: Located at the most anterior region of the brain (aka the “front”) this is involved with executive decisions like extrapolating future consequences onto future actions, overriding desires based on social considerations, and the like. There is not much variation in relative volume in the area in primates once you adjust for body size (Semendeferi et al, 1997), although that claim is often made. Nevertheless from what I have read it appears that there has been some evolution towards larger volume of area in the frontal lobes in mammals in general.

Overall I would say that there is too much discussion of naming the brain and too little discussion of function and evolution. There are so many different ways to classify each of these areas, but how much does it actually help our understanding? Perhaps we need an IUPAC of brain names.

This was inspired by CalTech’s question #4: “Draw an outline of a vertebrate brain and name its major areas.” Feel free to offer any critiques or your own answers in the comments.


Butler AB, Hodos W. Comparative Vertebrate Neuroanatomy, 2nd Edition. Wiley-IEEE, 2005.

Sower SA, Freamat M, Kavanaugh SI. 2008 The origins of the vertebrate hypothalamic-pituitary-gonadal (HPG) and hypothalamic-pituitary-thyroid (HPT) endocrine systems: New insights from lampreys. Gen Comp Endocrinol. PubMed Link.

Holmes RL, Ball JN. The Pituitary Gland: A Comparative Account. Cambridge University Press, 1974.

UC Davis Biological Science. “The Vertebrate Brain”., Accessed January 2008.

Laberge F, Muhlenbrock-Lenter S, Grunwald W, Roth G. 2006 Evolution of the amygdala: New evidence from studies in amphibians. Brain, Behavior, and Evolution 67: 177-187. doi: 10.1159/000091119.

Semendeferi K, Damasio H, Frank R, Van Hoesen GW. 1997 The evolution of the frontal lobes: a volumetric analysis based on three-dimensional reconstructions of magnetic resonance scans of human and ape brains. Journal of Human Evolution 32:375-88.

Question #3: King Phillip came over from Germany Sunday

Animals are classified by taxonomic considerations within the Kingdom Animalia according to qualitative traits, DNA sequences, and their combination: clades. Phylogenetic taxonomy is the most commonly used method today. It enables taxonomists to show a lot of information in their deceptively simple trees. There are 21 different animal phyla, but I will only talk about the most important 11, and I will neglect the Latin names because they are uninformative:

1) Sponges. These invertebrate animals lack nervous, digestive, and circulatory systems. They rely on water flowing through their bodies to absorb food and oxygen and to remove waste. They have the awesome ability to regenerate parts of their bodies that are broken off. In some bad cases the will produce survival pods called gemmules that remain dormant until things improve and then either form a new sponge or rejoin their parent.

2) Corals and Jellyfish. These invertebrate animals have only a layer or two of cells filled with a jelly-like substance called mesoglea. They also have venomous cells called cnidocytes that allow them to defend themselves and capture prey. They have decentralized nervous systems with collections of interneurons that connect to sensory and motor neurons. The central interneurons act as ganglia, or local coordination centers. Medusae swimming colonies have a sweet technique called the statocyst that allows them to orient themselves correctly and some species have rudimentary visual systems.

3) Flatworms. These invertebrates have bilateral symmetry and no body cavities so they have developed body types that allow them to get oxygen and nutrients through diffusion, which is why so many of them are microscopic. Some of them have a simple nervous system called a nerve net, which is a collection of non-encephalized interneurons that allow the worms to respond to physical contact and do some basic chemosensing.

4) Nematodes: Aka roundworms, these invertebrates live in either soil and water. According to rough numbers from Wikipedia, about 19% of them are parasitic. They have a complete digestive system and both a dorsal and ventral nerve cord, as well as chemosensory amphids and phasmids, which each contain sensory dendrites.

5) Moss animals. Aka sea mats, they build skeletons of calcium carbonate and are mainly located in warm, tropical water. They feed with a lophophore, which is basically just a bunch of ciliated tentacles around the mouth that reminds me of the worm in Star Wars: The Empire Strikes Back. They have a two lobe ganglion at the base of the lopophore that connects to the internal muscles and organs, as well as looping back to a nerve ring that innervates the tentacles of the lopophore.

6) Lamp shells. These are invertebrate marine animals with two valves that pull their valves apart via internal diductor muscles. They cannot move around, and are attached to the “substrate” via a pedicle. Their nervous system consists of a ring of nerves around the esophagus, from which nerves reach out to innervate the muscles, valves, and lopophore.

7) Mollusks. These are highly diverse invertebrates that have a single opening from their mantle used for both breathing and excretion. They have two main nerve chords with scattered ganglia acting as local control centers. These laterally paired ganglia often have commisures linking them. They generally have a pair of eyes, tentacles for both mechanical and chemical sensing, stotocysts in the feet as balance sensors, and a pair of osphradia, chemical sensors located in front of the lungs. Famous examples include snails, octupuses, and squids.

8) Segmented Worms. These invertebrate animals mostly live in wet environments and are bilaterally symmetric. They have a coelem, a closed circulatory system, and are obviously segmented. Their nervous system is pretty cool. There is a main nerve chord through the body that comes into contact with a lateral nerve from each segment. However, each segment is autonomous, so the local regions must coordinate to perform global actions such as locomotion. Examples of these are earthworms and leeches.

9) Arthropods. These invertebrates are characterized by jointed limbs and cuticles which is mainly made by a long polymer chain of N-acetylglucosamine: alpha-chitin. They are also segmented, and make up 80% of all living species, in large part because they are so succesful in dry environments. The stiff cuticles are penetrated by various touch sensors that feed to the nervous system, which often come in the form of setae. They also have chemical and pressure sensors, internal propioreceptors, and antennae to monitor temperature, moisture, and humidity. Ventral nerve chords run through each segment and form paired ganglion in each segment, and their brains are formed by the fusion of the ganglion from some of these segments and encircle the esophagus. Their nervous system has been described as “ladder like.” Examples include the arachnids, insects, and crustaceans.

10) Sea stars. These include the starfich, brittle stars, sea urchins, sand dollars, sea cucumbers, sea daisies, and the sea lilies. They have radial symmetry, and a mesodermal endoskeleton, meaning that their support system is within the tissue of the organism. Their nervous system is made up of interconnected neurons with assorted ganglia. There is a central ring of neurons around the mouth from which neurons radiate into each arm, but no actual brain. Some of them (the sea lillies) can’t move, but most of them can.

11) Chordates. These animals have a dorsal nerve chord, pharyngeal slits for feeding and/or breathing,  an endostyle, a tail, and a notochord. This last one, used for axial support, is the most differentiating trait. If the notochord is present through the lifetime then the animal is an invertebrate, but if it is replaced in the adult stage by a vertebral column then the animal is a vertebrate. The invertebrates are sub-grouped into tunicates and lancelets, but the vertebrates are all in the same subphylum Vertebrata. The most important vertebrates are the jawless fishes, the cartilaginous fish, the bony fish, and then tetrapods, which are divided into the really famous classes: amphibians, aves, and mammals.

Inspired by Question #3 of CalTech’s 100 Questions for Cognitive Scientists: “Do you know how animals are classified? What are the major phyla? What different types of vertebrates exist?”


Ramel, G. “The Phylum Bracipodia.” Accessed January 2008.

Armstrong, WP. “Animal Phyla.” Accessed January 2008.

Question #2: Describing the neuroanatomy of octopuses and owls

Octopuses: The behavior of octopuses is characterized by extreme curiosity, quick adaptation to most circumstances, superb vision, flexible arms (which are sometimes used to mimic other animals), and the ability to learn associatively. Although octopuses are invertebrates, when you normalize the number of neurons to body weight of various species, octupuses contain a ratio similar to that of vertebrates. However, they still have fewer neurons proportionally than mammals and birds.

The ganglionic masses in the octopus (and cephalopods in general) is greater than that of other invertebrates, which is thought to be due to evolutionary encephalization. This increased size decreased the distance between various brain regions and probably increased the speed of neural computation, making octopuses “smarter.”

Of the octupus’s ~ 500 million neurons, 120 to 180 lie outside of the brain capsule in the optic lobes, and 330 million or so lie in the nervous system of the arms, which are fairly autonomous. Only 40 to 50 million lie in the actual “brain”, which is usually less than 8 cubic cm.

Through lesion experiments, researchers have been able to confirm that the vertical lobe (VL) is essential to learning in memory. Octopuses whose VL have been removed retain motor function but will continue to attack a crab despite receiving electrical shocks, while octopuses with intact VL regions will learn to discontinue this behavior. The VL has about 25 million neurons, representing the majority of cells in the octupus brain.

The VL region receives most of its input from the sensory median superior frontal lobe (MSF), which has some axon tracts running through the VL. Small amacrine interneurons from the VL extend neurites which attach along the path of these axons. The interneurons extend to about 65,000 larger neurons, whose axons form the output of the VL.

This is similar to the interactions between Schaffer collaterals and pyramidal cells in the CA1 region of the mammalian hippocampus. The convergent evolution of these systems raises some interesting questions: 1) Is this redundancy of connections through en passant innervation crucial for molecular mechanisms of memory? and 2) Is a large number of small interneurons also essential for encoding memories?

Owls: Owls are an order of birds of prey, and thus are endowed with many characteristics typical of bird brains. Contrary to the conventional wisdom towards a “bird brain”, aves are generally quite smart and as indicated above, they have very high numbers of neurons in their brain once you normalize for body weight. They can learn by observation (or at least contagion), they can recall the exact location of a stored piece of food over a long period of time, and some species can recognize themselves in a mirror, which is one of the weaker criterions for conciousness. Evolutionarily, birds are the only living ancestors of dinosaurs, which makes them interesting to 8 year old kids everywhere.

One of the specialized aspects of owl brains is their ability to detect movement via their auditory system, which is especially useful for owls because they are nocturnal and cannot rely as much on vision. Witten et al recently studied this ability of owls and found that their auditory receptor field updates rapidly based on changes in the interaural time difference and interaural level difference for high frequency wavelengths. The model that the researchers built to account for the prediction allows the receptive field to predict sound movements about 100 ms in the future, but that it needs to listen to the sound for at least 500 ms before it can orient a proper response. I assume that a proper response would either be an attack (gobble gobble) or to orient its head toward the stimulus in order to be able to recieve better auditory feedback.

The neural correlate of this ability is the space map of the optic tectum. This is similar to sensory representation in humans, for whom motion induced perceputal effects are apparent not only in the auditory system but also for luminance and color aspects of the visual system. The predictive abilities in the owl’s auditory system is a microcosm of its nuanced intelligence.

This was inspired by CalTech’s question #2: “Discuss the major features of at least two very different nervous systems (i.e. jellyfish, locust, lamprey, octopus, owl, rat, monkey). In what ways might the features of each system affect neural processing?”. Feel free to discredit my answer or offer your own in the comments.


Hochner B, Shomrat T, Fiorito G. 2006 The octopus: A model for a comparative analysis of the evolution of learning and memory mechanisms. Biological Bulletins 310: 308-317. Link.

Witten IB, Bergan JF, Knudsen EI. 2006 Dynamic shifts in the owl’s auditory space map predict moving sound locations. Nature Neuroscience 9: 1439 – 1445. doi:10.1038/nn178.

Question #1: Describing the theoretical models of the nervous system

There are at least five general spectrums upon which all models of the nervous system must lie, and there are trade-offs to being on either end of each of these spectrums.

Global vs. Local Computations: Global propagation posits that certain computations occur at “lower” levels of the nervous system and lend their suggestions in the form of neural signals to “higher” levels of the nervous system who aggregate and act on these signals. On the other side of the spectrum, we have strictly local computations with no overseer. The global propagation standpoint has the advantage of being easier to explain complex phenomena, and being more intuitive to humans due to its organized framework. The local computation standpoint has the advantages of being more evolutionarily plausible and currently having more evidence, as scientists have yet to locate an obvious supervisor region of the brain. An example of global computation is error recognition in the medial frontal cortex, while an example of local computation is BCM synaptic modification in the visual system. Advantage: Local.

Recurrent vs. One-Shot Feedback: Recurrent feedback models have the advantage of being able to handle more complex situations with fewer nodes while one-shot feedback has the advantage of less computation necessary per input, because the weights don’t have to converge to a steady state. As Douglas Hofstadter explains, recursive computation can lead to strange loops that never converge on an answer, however, our brains must have some way of handling this uncertainty. Ultimately, the number of neurons in the brain is probably the most important bottleneck evolutionarily, and therefore recurrent feedback is more likely. An example of a one-shot feedback system is a linear associator network while an example of a recurrent feedback system is the Hopfield model. Advantage: Recurrent.

Undirected vs. Directed Trees: Markov networks are undirected, while Bayesian networks are directed. Directed networks are built such that through each connection from one node to another must also specify a direction, with a parent and a child node. Undirected networks make no such distinction. The advantages of the directed network are that it can infer extra dependency relationships among the variables based simply on the topology, but the disadvantage is that it cannot logically express all the dependencies that an undirected network can. The causal nature of the directed network makes it more plausible and intuitive to a human audience, but of course neurons themselves are not human. If you really want to know the answer, go ask your local math professor. Advantage: Directed (according to Judea Pearl).

Immutable vs. Plastic Networks: There are many different biologically revelant types of brain plasticity, including new nodes (through neurogenesis), more connections between existing nodes (through the outgrowth of neurites), and altering the weights of existing connections (through long-term potentation). The advantage of including plasticity in your model of the nervous system is that it is more realistic, but the disadvantage is that we are not exactly sure how this plasticity works and you may end up coding processes that would accomplish lots of cool stuff but that don’t actually occur. For example, it currently seems that neurogenesis may yield a benefit cells around them through chaperone effects and then die off themselves, but how would you formalize that? Advantage: Plastic, but approach with care.

Empirical vs. Abstract Pathways: This dichotomy could be applied to science in general; it is not applicable simply to models of the brain. The advantages of predicting abstract pathways is that you can have a more parsimonious answer to complex questions. The disadvantage of attempting to guess without solid proof is that you may waste your time without solving any important puzzles. Most of what gets published in the top journals is empirical, while most of what your roommate tells you at 2:30 am is  abstract. However, there have been many instances where pure, abstract theory has helped to advance the science; for example, see Hebbian Learning. Advantage: Thomas Kuhn.

This was inspired by the first of Caltech’s 100 Questions for Cognitive Scientists: “Models of the nervous system come in many forms and types; from abstract to highly realistic (e.g. Hopfield model vs. biophysical detailed networks). Use specific examples to discuss the tradeoffs.” Feel free to offer your own answer or thoroughly trash mine in the comments.