History’s Top Brain Computation Insights: Day 18

Reaction times for a visual search task illustrating controlled and automatic processing18) Behavior exists on a continuum between controlled and automatic processing (Schneider & Shiffrin – 1977)

During the 1970s those studying the cognitive computations underlying visual search were at an impasse. One group of researchers claimed that visual search was a flat search function (i.e., adding more distracters doesn't increase search time), while another group claimed that the function was linear (i.e., adding more distracters increases search time linearly).

Both groups had solid evidence supporting their view. What were the two groups doing differently that could explain such different results?

As a graduate student working with Shiffrin, Schneider sat the two groups down during a scientific conference to have them figure out why their results differed so much. Needless to say, little was accomplished as both sides talked past one another.

Several years later Schneider & Shiffrin came to the realization that the two groups were practicing their subjects differently. The group with the flat search function allowed their subjects to practice the search task many times before collecting data. In contrast, the group with the linear search function began collecting data as soon as their subjects could perform the task.

This realization lead Schneider & Shiffrin to posit a distinction between automatic (flat search function) and controlled (linear search function) processing. In a landmark set of papers they clearly demonstrated this dual process distinction along with the boundary conditions of controlled and automatic task performance.

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History’s Top Brain Computation Insights: Day 17

17) Reverbatory activity in lateral prefrontal cortex maintains memories and attention over short periods (Fuster – 1971, Jacobsen – 1936, Goldman-Rakic – 2000)

Patient H.M., with his lack of long term memory but largely intact working (short-term) memory, illustrated a dissociation between these two forms of memory. While long-term memory seemed to rely on hippocampus and the neocortical temporal lobes, in the 1960s it was not clear how working memory might be maintained.

Hebb had postulated a distributed and dynamic mechanism for working memory that was quite hard to test. However, a more testable hypothesis had emerged from observations of patients with prefrontal cortex damage. Such patients had trouble making and carrying out plans over time, possibly due to a working memory deficit. Previous work by Jacobsen lesioning primate prefrontal cortex supported this theory, but this work was far from conclusive.

In 1970 Joaquin Fuster cooled the monkey prefrontal cortex, showing a reversible deficit in working memory. The following year he recorded from monkey prefrontal cortex neurons and found cells that maintained activity over delay periods (‘memory cells’). These neurons respond not just to the stimulus presentation and the response, but also maintain activity between the two events (see figure for illustration).

A decade later Fuster found visual memory cells in inferior temporal cortex. Subsequent research has suggested that the prefrontal memory cells drive the temporal cortex memory cells to maintain their activity.

Patricia Goldman-Rakic, another monkey neurophysiologist, was instrumental in elucidating the network properties of working memory function. She showed in 2000 that lateral prefrontal and posterior parietal cortices mutually support the sustained working memory activity discovered by Fuster. She also showed the importance of the dopamine system and thalamus in working memory function.

Since the advent of PET and functional MRI in the 1990s a number of researchers have extended the primate working memory findings to humans. Some of these researchers include Jonathan Cohen, Mark D’Esposito, Michael Petrides, Julie Fiez, and John Jonides.

Implication: The mind, largely governed by reward-seeking behavior, is implemented in an electro-chemical organ with distributed and modular function consisting of excitatory and inhibitory neurons communicating via ion-induced action potentials over convergent and divergent synaptic connections strengthened by correlated activity. The cortex, a part of that organ composed of functional column units whose spatial dedication (determined via local competition) determines representational resolution, is composed of many specialized regions involved in perception (e.g., touch: parietal, vision: occipital), action (e.g., frontal), and memory (e.g., short-term: prefrontal, long-term: temporal), which depend on inter-regional communication for functional integration.
[This post is part of a series chronicling history’s top brain computation insights (see the first of the series for a detailed description). See the history category archive to see all of the entries thus far.]

-MC

History’s Top Brain Computation Insights: Day 16

16) Critical periods of cortical development via competition (Hubel & Wiesel – 1970)

Hubel & Wiesel showed that the ocular dominance columns they had discovered in cortex (see previous post) are organized during a critical period of development. Keeping one eye of an animal shut during the first few months of life made that animal blind in that eye for the rest of its life. Keeping the eye shut for the same amount of time later in life had no such effect.

Hubel & Wiesel found that the ocular dominance columns became lopsided in such a case: The functional eye tended to take over cortical space not used by the blind eye. This finding extended Penfield‘s notion that the amount of cortical space dedicated to a function determines its resolution. In this case, visual acuity was decreased through smaller cortical space for the unused eye.

Importantly, this experiment illustrated that the eyes compete for cortical space, with the most active eye claiming the most space. Generalizing this finding, it appears that many representations in cortex compete for space, whether it is in visual, motor, or somatosensory cortex.

V. S. Ramachandran applied these findings in an interesting case involving phantom limb pain. He found that many patients who still felt pain in their amputated hand also reported feeling a touch on that phantom hand when their face was touched. As can be seen in the second figure of the previous post on the sensory-motor homunculus, the hand and face representations are next to each other in cortex.

Ramachandran interpreted this to mean that the loss of input from the hand allows the face representation to win its competition with the hand representation, allowing it take over the cortical space previously dedicated to the hand. This also suggests that this competition continues into adulthood in some cases.

Implication: The mind, largely governed by reward-seeking behavior, is implemented in an electro-chemical organ with distributed and modular function consisting of excitatory and inhibitory neurons communicating via ion-induced action potentials over convergent and divergent synaptic connections strengthened by correlated activity. The cortex, a part of that organ composed of functional column units whose spatial dedication (determined via local competition) determines representational resolution, is composed of many specialized regions involved in perception (e.g., touch: parietal lobe, vision: occipital lobe), action (e.g., frontal lobe), and memory (e.g., temporal lobe), which depend on inter-regional communication for functional integration.

[This post is part of a series chronicling history’s top brain computation insights (see the first of the series for a detailed description). See the history category archive to see all of the entries thus far.]

-MC

History’s Top Brain Computation Insights: Day 15

A split brain patient verbally reports the left-brain information, while reporting with his hand for right-brain information15) Consciousness depends on cortical communication; the cortical hemispheres are functionally specialized (Sperry & Gazzaniga – 1969)

It is quite difficult to localize the epileptic origin in some seizure patients. Rather than removing the gray matter of origin, neurosurgeons sometimes remove white matter to restrict the seizure to one part of the brain.

One particularly invasive procedure (the callosotomy) restricts the seizure to one half of cortex by removing the connections between the two halves. This is normally very effective in reducing the intensity of epileptic events. However, Sperry & Gazzaniga found that it comes at a price.

They found that presenting a word to the right visual field of a patient without a corpus callosum allowed only the patient’s left hemisphere to become aware of that word (and vice versa). When only the side opposite the one which was presented the word was allowed to respond, it had no idea what word had been presented.

The two hemispheres of cortex could not communicate, and thus two independent consciousnesses emerged.

Sperry & Gazzaniga also found that the left hemisphere, and not the right, could typically respond linguistically. This suggested that language is largely localized in the left hemisphere. (See the above figure for illustration.)

The functional distinction between the left and right hemispheres is supported by many lesion studies. Generally, the left hemisphere is specialized for language and abstract reasoning, while the right hemisphere is specialized for spatial, body, emotional, and environment awareness. The boundary between these specializations has been trivialized in the popular media; it is actually quite complex and, in healthy brains, quite subtle.

Implication: The mind, largely governed by reward-seeking behavior, is implemented in an electro-chemical organ with distributed and modular function consisting of excitatory and inhibitory neurons communicating via ion-induced action potentials over convergent and divergent synaptic connections strengthened by correlated activity. The cortex, a part of that organ composed of functional column units whose spatial dedication determines representational resolution, is composed of many specialized regions involved in perception (e.g., touch: parietal lobe, vision: occipital lobe), action (e.g., frontal lobe), and memory (e.g., temporal lobe), which depend on inter-regional communication for functional integration.

[This post is part of a series chronicling history’s top brain computation insights (see the first of the series for a detailed description). See the history category archive to see all of the entries.]

-MC

History’s Top Brain Computation Insights: Day 14

Neuron staining illustrating column structure in cortex14) Neocortex is composed of columnar functional units (Mountcastle – 1957, Hubel & Wiesel – 1962)

Mountcastle found that nearby neurons in monkey somatosensory cortex tend to activate for similar sensory experiences. For example, a neuron might respond best to a vibration of the right index finger tip, while a neuron slightly deeper in might respond best to a vibration of the middle of that finger.

The neurons with these similar 'receptive fields' are organized vertically in cortical columns. Mountcastle distinguished between mini-columns, the basic functional unit of cortex, and hyper-columns, which are functional aggregates of about 100 mini-columns.

Hubel & Wiesel expanded Mountcastle's findings to visual cortex, discovering mini-columns showing line orientation selectivity and hyper-columns showing ocular dominance (i.e., receptive fields for one eye and not the other). The figure below illustrates a typical spatial organization of orientation columns in occipital cortex (viewed from above), along with the line orientations corresponding to each color patch.

Implication: The mind, largely governed by reward-seeking behavior, is implemented in an electro-chemical organ with distributed and modular function consisting of excitatory and inhibitory neurons communicating via ion-induced action potentials over convergent and divergent synaptic connections strengthened by correlated activity. The cortex, a part of that organ composed of functional column units whose spatial dedication determines representational resolution, is involved in perception (e.g., touch: parietal lobe, vision: occipital lobe), action (e.g., frontal lobe), and memory (e.g., temporal lobe).

Orientation selective cortical columns 

[This post is part of a series chronicling history's top brain computation insights (see the first of the series for a detailed description). See the history category archive to see all of the entries.]

-MC

History’s Top Brain Computation Insights: Day 13

A small man whose proportions represent the amount of cortical space dedicated to each body part13) Larger cortical space is correlated with greater representational resolution; memories are stored in cortex (Penfield – 1957)

Prior to performing surgery, Wilder Penfield electrically stimulated epileptic patients' brains while they were awake. He found the motor and somatosensory strips along the central sulcus, just as was found in dogs by Fitsch & Hitzig (see previous post). The amount of cortex dedicated to a particular part of the body was proportional not to that body part's size, but to its fine control (for motor cortex) or its sensitivity (for somatosensory cortex).

Thus, the hands and lips have very large cortical spaces relative to their size, while the back has a very small cortical space relative to its size. The graphical representation of this (see above) is called the 'homunculus', or 'little man'.

Penfield also found that stimulating portions of the temporal lobe caused the patients to vividly recall old memories, suggesting that memories are transfered from the archicortical hippocampus into neocortical temporal lobes over time.

Implication:  The mind, largely governed by reward-seeking behavior, is implemented in an electro-chemical organ with distributed and modular function consisting of excitatory and inhibitory neurons communicating via ion-induced action potentials over convergent and divergent synaptic connections strengthened by correlated activity. The cortex, a part of that organ whose spatial dedication determines representational resolution, is involved in perception (e.g., touch: parietal lobe, vision: occipital lobe), action (e.g., frontal lobe), and memory (e.g., temporal lobe).

 

[This post is part of a series chronicling history's top brain computation insights (see the first of the series for a detailed description). See the history category archive to see all of the entries.]

-MC

History’s Top Brain Computation Insights: Day 12

Transverse section showing a superior view of the hippocampus of each hemisphere12) Hippocampus is necessary for episodic memory formation (Milner – 1953)

Patient H.M. had terrible epilepsy originating in the medial temporal lobes. His neurosurgeon decided to take out the source of the epilepsy: the hippocampus. Surprisingly, after the operation H.M. could no longer form new long-term memories. He could remember things for short time periods if he wasn't distracted (i.e., he had near normal working memory). Also, he could learn new sensory-motor skills, though he could not recall how he learned them.

Patient H.M. is still alive today, and has no new episodic memories since the early 1950s. He still thinks he's in his twenties, and meets his doctors anew each day.

Some have compared his experience with those dramatized in the movie Memento.

Implication:  The mind, largely governed by reward-seeking behavior, is implemented in an electro-chemical organ with distributed and modular function consisting of excitatory and inhibitory neurons communicating via ion-induced action potentials over convergent and divergent synaptic connections strengthened by correlated activity. A part of that organ, the medial temporal lobe, is essential for memory formation.

[This post is part of a series chronicling history's top brain computation insights (see the first of the series for a detailed description)]

-MC

History’s Top Brain Computation Insights: Day 11

Neuron showing sodium and potasium concentration changes11) Action potentials, the electrical events underlying brain communication, are governed by ion concentrations and voltage differences mediated by ion channels (Hodgkin & Huxley – 1952)

Hodgkin & Huxley developed the voltage clamp, which allows ion concentrations in a neuron to be measured with the voltage constant. Using this device, they demonstrated changes in ion permeability at different voltages. Their mathematical model of neuron function, based on the squid giant axon, postulated the existence of ion channels governing the action potential (the basic electrical signal of neurons). Their model has been verified, and is amazingly consistent across brain areas and species.

You can explore the Hodgkin & Huxley model by downloading Dave Touretsky's HHSim, a computational model implementing the Hodgkin & Huxley equations.

Implication: The mind, largely governed by reward-seeking behavior, is implemented in an electro-chemical organ with distributed and modular function consisting of excitatory and inhibitory neurons communicating via ion-induced action potentials over convergent and divergent synaptic connections strengthened by correlated activity.

[This post is part of a series chronicling history's top brain computation insights (see the first of the series for a detailed description)]

-MC

History’s Top Brain Computation Insights: Day 10

Hebbian reverbatory cell assembly 10) The Hebbian learning rule: 'Neurons that fire together wire together' [plus corollaries] (Hebb – 1949)

D. O. Hebb's most famous idea, that neurons with correlated activity increase their synaptic connection strength, was based on the more general concept of association of correlated ideas by philosopher David Hume (1739) and others. Hebb expanded on this by postulating the 'cell assembly', in which networks of neurons representing features associate to form distributed chains of percepts, actions, and/or concepts.

Hebb, who was a student of Lashley (see previous post), followed in the tradition of distributed processing (discounting localizationist views).

The above figure illustrates Hebb's most original hypothesis (which is yet to be proven): The reverbatory cell assembly formed via correlated activity. Hebb theorized that increasing connection strength due to correlated activity would cause chains of association to form, some of which could maintain subsequent activation for some period of time as a form of short term memory (due to autoassociation).

Implication: The mind, largely governed by reward-seeking behavior, is implemented in an electro-chemical organ with distributed and modular function consisting of excitatory and inhibitory neurons communicating via convergent and divergent synaptic connections strengthened by correlated activity.

[This post is part of a series chronicling history's top brain computation insights (see the first of the series for a detailed description)]

-MC

History’s Top Brain Computation Insights: Day 9

A network that can compute the XOR logic gate9) Convergence and divergence between layers of neural units can perform abstract computations (Pitts & McCulloch – 1947)

Pitts & McCulloch created the first artificial neurons and artificial neural network. In 1943 they showed that computational processing could be performed by a series of convergent and divergent connections among neuron-like units. In 1947 they demonstrated that such computations could lead to visual constancy, in which a network could recognize visual inputs despite changes in orientation or size. This computation is relevant for many topics besides vision.

More profound than the visual constancy network was the proof of concept it represented. As illustrated in the above figure, multi-layered perceptrons (as networks of converging and diverging neuron-like units came to be known) can compute logical functions such as AND, OR, and XOR.

This insight provided the first clear glimpse of how actual computation might be carried out in the brain via the many convergent and divergent connections already found in its anatomical projections.

Implication:  The mind, largely governed by reward-seeking behavior, is implemented in an electro-chemical organ with distributed and modular function consisting of excitatory and inhibitory neurons communicating via convergent and divergent synaptic connections.

[This post is part of a series chronicling history's top brain computation insights (see the first of the series for a detailed description)]

-MC

History’s Top Brain Computation Insights: Day 8

A dog being trained to jump on command over the course of 20 minutes8) Reward-based reinforcement learning can explain much of behavior (Skinner – 1938, Thorndike – 1911, Pavlov – 1905)

B. F. Skinner showed that reward governs much of human and animal behavior. He discovered operant conditioning, a method for manipulating behavior so powerful he could teach a pigeon to bowl (or a dog to jump on command; see figure). This was an expansion of Thorndike's Law of Effect, which says that behaviors associated with a reward will be reinforced, while behaviors associated with no reward will not. Both Skinner and Thorndike expanded on earlier work by Pavlov, which showed that some reflexes can be conditioned using paired stimulus presentation (e.g., a bell with food later causes salivation to a bell alone).

Implication: The mind, largely governed by reward-seeking behavior, is implemented in an organ with distributed and modular function consisting of excitatory and inhibitory neurons communicating via electro-chemical synaptic connections.

[This post is part of a series chronicling history's top brain computation insights (see the first of the series for a detailed description)]

-MC

History’s Top Brain Computation Insights: Day 7

The fluid from the donor heart, which was just stimulated, causes the recipient heart to slow down 7) Brain signals are chemical (Dale – 1914, Loewi – 1921)

Loewi found that electrically stimulating a heart causes it to release a chemical substance which changes the beating of a different heart when exposed to that chemical substance. Dale had already discovered neurotransmitters, one of which (acetylcholine) was the chemical responsible for the change in heart rate. This was eventually generalized to the rest of the nervous system.

Implication: The mind is implemented in an organ with distributed and modular function consisting of excitatory and inhibitory neurons communicating via electro-chemical synaptic connections. 

 A chemical synapse depicting neurotransmitters crossing between the axon and dendrite

[This post is part of a series chronicling history's top brain computation insights (see the first of the series for a detailed description)]

-MC

History’s Top Brain Computation Insights: Day 6

6) Neural networks consist of excitatory and inhibitory neurons connected by synapses (Sherrington – 1906)

Based on his observations in the spinal cord, Sherrington theorized that the brain consists of complex networks of excitatory and inhibitory cells he was the first to term 'neurons', with connection points he was the first to term 'synapses'. His theories turned out to be correct.

Sherrington's insight into the network nature of neural interactions is still at the cutting edge of neuroscience. It's ramifications are vast and complex, and we will still be working to understand the implications of this insight for decades to come.

Sherrington himself was able to prove that opposing muscles on the limbs inhibited each other via network interaction in the spinal cord. Also, it was found that timing of muscle movements is controlled by negative feedback via local neuronal networks. These spinal cord networks were an essential proof of concept for Sherrington, but they pale in comparison to the complexity of network interactions recently discovered in cortex, basal ganglia, and cerebellum.

Implication: The mind is implemented in an electric organ with distributed and modular function consisting of excitatory and inhibitory neurons communicating via synaptic connections.

[This post is part of a series chronicling history's top brain computation insights (see the first of the series for a detailed description)]

-MC

History’s Top Brain Computation Insights: Day 5

Drawing of a Purkinje cell by Ramon y Cajal 5) Neurons are fundamental units of brain computation (Ramon y Cajal – 1889)

Golgi, a prominent 19th century biologist, argued that the brain is one unified reticulum (or web) of neural tissue, much like the circulatory system. However, Ramon y Cajal came to a very different conclusion using Golgi’s very own silver chromate staining technique. He argued that this tissue web was composed of separate cells. Later studies showed (using the electron microscope) that Ramon y Cajal was correct.

Some have argued that Golgi was partially correct since electrical synapses (gap junctions) exist in small number in the brain. However, even with gap junctions the cells’ plasma membranes separate the two sides of the synapse, which Golgi’s theory did not predict.
Looking carefully at his stained neural cells, Ramon y Cajal postulated that nerve signals travel in one direction (from dendrite to axon). The dendrite (top), cell body (dark central spot), and axon (bottom) can be clearly distinguished in the included image of a drawing by Ramon y Cajal. He was unable to test this prediction himself, but he turned out to be correct once again.

Implication: The mind is implemented in an electric organ with distributed and modular function consisting of neural units.

[This post is part of a series chronicling history’s top brain computation insights (see the first of the series for a detailed description)]

-MC 

History’s Top Brain Computation Insights: Day 4

Primary sensory-motor cortical regions 4) Functions can be localized in the brain (Bouillaud – 1825, Broca – 1861, Fritsch & Hitzig – 1870)

Bouillaud and Broca discovered patients with frontal cortex lesions who had speech problems. Fritsch & Hitzig discovered primary motor cortex; a specialized chunk of cortex specifically for motor control. Broca believed that all brain functions would be localized eventually, and held the brain area he discovered (now termed Broca's area) as an example. It was found that all sensory modalities have dedicated chunks of cortex, called primary sensory areas.

This localized function insight immediately clashed with the distributed function insight (see yesterday's post). Flourens and his followers were adamantly against the localizationist view championed by Broca and others.

The origin of the concept of localizing brain function, phrenology, added discredit to the practice. Phrenology claimed that differences in localized brain function were reflected in bumps on individuals' skulls. Once it was clear that phrenological findings were not replicated between individuals the practice was labeled a pseudoscience. Unfortunately, localization of function applied without the fallacies of phrenology was not spared the renewed skepticism of many scientists.

Today localization of function is well established, especially in primary sensory-motor regions. Functions have also been localized within association cortex, though much work remains in understanding how such localization arises. Insights within the last 20 years have led to a more sophisticated view of how functions arise in association cortex involving network interactions (see future insight posts).

Implication: The mind is implemented in an electric organ with distributed and modular function.

[This post is part of a series chronicling history's top brain computation insights (see the first of the series for a detailed description)]
-MC