On the Integration of Tactile, Proprioceptive and Visual Signals by the Brain

The integration of different sensory inputs in the brain is crucial not only for taking appropriate motor actions but also for body perception and awareness of the bodily self. Integration occurs in higher areas in the brain usually in areas belonging to the parietal lobe

1) Integrating Vision and Proprioception in Area 5 ( Graziano et. al (2000))

Brodmann Area 5 (or Area 5) is part of the parietal cortex in humans, and in monkeys, is a subdivision of the parietal lobe, occupying primarily the superior parietal lobule. Graziano et. al (2000) studied single neuron responses from Area 5 of monkeys. In their experiments, the arm contralateral to the recording site was outstretched. The real arm of the monkey was covered, and instead a realistic fake hand was kept in view.  The experimental design was 2 x 2

2 x 2 experimental design. Cross indicates fixation spot. The gray arm is the

 fake arm. The real arm is covered in the experiment (Graziano et. al (2000))

Single neuron recordings indicated that the firing rate of individual neurons depends not only on the position of the real arm, but also on the position of the fake arm. The neuron is significantly affected by the  position of the real arm, firing more when he real arm is to the left. Additionally, the firing rate further increases when the fake arm is also to the left. 

Single neuron recording from Area 5.  The firing rate is maximum when 

the real and the fake arms are in congruent position (Graziano et. al (2000))

2) Integrating Vision and Touch in Ventral Intraparietal Area (Duhamel et. al (1998))

The Ventral Intraparietal Area (or VIP) is a discrete area in the depths of the intraparietal sulcus. Duhamel et. al carried out single neuron recordings in the VIP. They found the neurons to possess a bimodal receptive field. Not only did they respond to a combination of visual and tactile stimuli, but the bimodal receptive fields were arranged in an orderly manner.

Bimodal receptive fields of VIP neurons (Duhamel et. al (1998))

Small central visual receptive fields  were associated with small tactile receptive fields on the muzzle, whereas large peripheral receptive fields were associated with  large tactile receptive fields on the side of the head or body. The neurons also demonstrated direction selectivity, in the sense that a visual or tactile stimulus moving in one direction was preferred over the other, and this preferred direction of visual and tactile stimuli coincided in the majority of cells. 

Direction selectivity of VIP neurons (Duhamel et. al (1998))

References

Graziano, M. (2000). Coding the Location of the Arm by Sight Science, 290 (5497), 1782-1786 DOI: 10.1126/science.290.5497.1782

Duhamel JR, Colby CL, & Goldberg ME (1998). Ventral intraparietal area of the macaque: congruent visual and somatic response properties. Journal of neurophysiology, 79 (1), 126-36 PMID: 9425183

 

Scaredy Cat? Blame it on your genes (Again!!)

Serotonin (5-Hydroxytryptamine or 5-HT), as some of us may know, is a neurotransmitter that is involved in modulation of moods such as anger. The level of re-uptake of serotonin by the presynaptic neuron is crucial and abnormal functioning of the serotonin transporter has been implicated in different neurological diseases. The human 5-HT transporter (5-HTT) gene transcription is modulated by a common polymorphism in the upstream regulatory region and the short variant of the polymorphism reduces 5-HTT transcription and hence reduces 5-HT uptake by lymphoblasts. In this paper, the authors (Lesch et. al (1996)) report association studies demonstrating that the presence of the short variant pre-disposes the individual towards anxiety related personality traits.

Neuroticism scores (seperated into eight groups) and percentages of subjects 

from L(n=163) and S(n=342)groups in each of the eight T-score groups.

 Notice that the S percentage is higher than L percentagein all T-score groups

 above 54 (Lesch et. al (1996))

Reference

Lesch, K., Bengel, D., Heils, A., Sabol, S., Greenberg, B., Petri, S., Benjamin, J., Muller, C., Hamer, D., & Murphy, D. (1996). Association of Anxiety-Related Traits with a Polymorphism in the Serotonin Transporter Gene Regulatory Region Science, 274 (5292), 1527-1531 DOI: 10.1126/science.274.5292.1527

Some Thoughts on P-Zombies and the Gap Explanatory Argument

Dedicated to Anna Traußnig...for the several hours of intellectual orgasms, as she likes to call it

Parts of this article have been inspired by the writings of Robert Kirk, William Seager and Daniel Dennett

A philosophical zombie (or p-zombie as it is often called), is a curious creature that is central to the basic idea of the explanatory gap. It is a hypothetical being that is indistinguishable from you and I, or any normal human being for that matter, except for one fundamental detail. Such a zombie, for example, cries in pain (when you kick it), but it feels no pain. In other words it lacks any conscious experience. But, given their definition, this singular fact will have no bearing on the physical processes that the zombie will undergo in it's own world. Put another way, a zombie's behaviour should be indistinguishable from the behaviour and physical state of a genuine human being. This first case is the well known "zombie duplicate" -There exists a philosophical zombie that is physically indistinguishable from me , assuming of course that I can be granted consciousness. Let's stick with that for the moment. We shall come back to this point later.

Now I could argue that a perfectly physical duplicate of me should live in a perfectly duplicate realm of reality. It could also be argued that the only possible world that could have a perfectly physical duplicate of me is one which is perfectly duplicated , which might be the very same possible world. Are zombies nomologically or even physically possible? To state that they are nomologically possible would be to state that in some world that shares all physical and natural laws of this world, there exists a "human being" who lacks consciousness. But wait, what happens if certain natural and physical laws do not bear any causal relation? If all natural laws are physical laws or at least dictate physical laws and if, consciousness is causally related to physical states (and there is some evidence for that), then there cannot exist any world with the same physical and natural laws as ours and yet have a zombie. It's not possible. On the other hand, if physical laws were dissociated from natural laws, then zombies could exist by breaking natural laws but not the physical ones. Remember, we have access only to physical laws (which we assume, by default to be natural laws, although this may not necessarily be true). Going further, if mental states (such as consciousness) were non physical, it seems reasonable to claim that zombies are not only nomologically possible but even logically (or physically) possible. Of course, the question still remains whether such another distinct reality, separate from ours does exist (this in itself is another huge philosophical debate). Now if this were the case, one would have a tough time proving what consciousness really is (or is not). It raises another question. How are we to assume that a zombie that exists in another physical realm sharing the same physical laws as ours but not the natural laws does not have consciousness? It may possess a different kind of consciousness that is drawn from the natural laws that govern it's world.

In 1999, Robert Kirk, presented an argument which attempted to demonstrate that philosophical zombies are logically impossible. He starts by assuming that a zombie suddenly acquires non-physical qualia. A weak definition of qualia would be the property that distinguishes us from zombies (consciousness if you will). Parts of his argument are as follows:

1. My zombie twin is unaffected by anything non-physical
   
2. So he remains unaffected when he acquires non-physical qualia
3. In order to become conscious following acquisition of this non-physical qualia, he must be affected by it in some way
4. But nothing non-physical affects him. So he doesn't become conscious.
5. To tell the difference between the subjective character of two different perceptual experiences, he must be sensitive to them or detect them in some way
6. In any case, nothing non-physical affects the zombie, so he cannot distinguish non-physical qualia from having no qualia
7. Friends of the zombie maintain that a "conscious" being is simply the zombie component with non-physical qualia. This position entails that "conscious" humans cannot tell the difference between tea and coffe, for example.
8. But we can tell the difference between tea and coffee.
9. Hence, we are nothing of the sort that friends of zombies maintain we are and hence zombies in their sense are not genuinely possible

Kirk's paper has been criticized but it presents an interesting case.

One final word on physicalism. In short, there are no kinds of things other than physical things (including consciousness). Daniel Dennett argues that even if consciousness were purely physical, our own conviction that we are not zombies, is simply a product on the external physical world. But, we could still be zombies who think we're conscious, who think we have qualia and who think we feel pain, in ways that are we could never discover. If such were the case, it wouldn't be careless to say that "consciousness" could never be defined or understood or even proved. 

I for one remain hopeful that science may one day provide a good theory on how consciousness works, but Anna and I both think that even if one ends up showing how it works, one would never be able to show what it really is.  

References

Kirk, R. (1999). The Inaugural Address: Why There Couldn't Be Zombies Aristotelian Society Supplementary Volume, 73 (1), 1-16 DOI: 10.1111/1467-8349.00046

Seager, W.E., Are Zombies Logically Possible?-And Why it Matters

http://www.scar.utoronto.ca/~

Take that leg off-It ain't mine

So I recently read this paper from V.S Ramachandran's group at the Salk Institute and came to realize how weird the field of cognitive neuroscience can really get. The paper is on Apotemnophilia (phew!! saying it is a task in itself), which yours truly had never heard of before. In any case, the disease is characterized by the desire to amputate one's own limb (Now how about that). These patients are otherwise mentally normal.

Individuals suffering from apotemnophilia always date the desire for amputation since childhood and often term the limb as being over-present or intrusive. Most obtain an amputation and paradoxically report feeling much more 'complete' and happier. Traditional explanations for the disease range from it being sexual paraphilia, related to the phallic resemblance of an amputee's stump (Someone get Freud in here!) or perhaps the mere sight of an amputee is permanently imprinted in the malleable psyche of a child as the "ideal body representation"

In this paper (McGeoch et. al (2009)), they demonstrate that the disease actually has a neurological basis based on the following observations:

1) Sufferers have no other psychological disorders

2) They desire an amputation of a limb at a specific level

3) There is a left sided bias.

The last observation points a finger to one particular area in the right parietal lobe called the superior parietal lobe (SPL) which receives connections from a host of other areas, namely the visual, primary somatosensory, secondary somatosensory, premotor and motor cortices. Additionally, the right parietal cortex is known to play a vital role in constructing body image and damage can lead to various disorders like somatoparaphrenia (denial of ownership of left arm) and others. The autors postulate that the right SPL may contain a hardwired representation of the body and if a particular limb were missing from the representation, the consequence may be a desire for amputation.

Right hemisphere of averaged control brain (a,b) and subject brain (c,d). Right SPL is outlined in black. Images a and c show touch to the left foot of control and subject respectively while b and d show touch to the right foot of control and subject respectively . This subject wanted right below-knee amputation (McGeoch et. al (2009)) 

This confirms the hypothesis that there is a congenital failure to represent affected limbs in the body image. Since visual and somatosensory inputs are still intact, but there is no corresponding  limb representation, the result would be a mismatch that manifests itself as an intrusive and over-present limb.

References

McGeoch, P.D., Brang, D., Song, T., Lee, R.R., Huang, M., & Ramachandran, V.S. (2009). Apotemnophilia - the neurological basis of a 'psychological' disorder Nature Precedings : 10101/npre.2009.2954.1

Brang, D., McGeoch, P., & Ramachandran, V. (2008). Apotemnophilia: a neurological disorder NeuroReport, 19 (13), 1305-1306 DOI: 10.1097/WNR.0b013e32830abc4d

The Small Network Argument

Not long ago, the problem of consciousness was thought to be scientifically non-tractable. But with a lot more psychophysical and brain mapping data coming in from studies being done on normal subjects and people with brain disorders, the field has opened up. In the last decade, a variety of brain processes were proposed to account for consciousness and several more keep coming in but as of today, no concrete model exists. Typical examples include recurrent computation (Grossberg (1999) and Lamme (2006)), synchronized neural activity (Bachmann (1994), Engel et. al (1999)), winner takes all computation (Grossberg (1999)), closed loop action-perception processing (O'Regan and Noe (2001)). In the midst of the madness, comes a paper by Herzog et. al titled Consciousness and the Small Network Argument (Herzog, Esfeld and Gerstner (2007)). They propose that none of the current models can fully account for consciousness because of what they call the small network argument:

For each of the above models, there exists a very small neural network that fulfills the respective characteristics of the model but does not exhibit consciousness

1) Recurrent Computation (Lamme (2006))

Two mutually connected neurons make up a recurrent systems. Thus if recurrence is sufficient for consciousness, a network of two neurons must create consciousness (to which not may of us may accede).

2) Winner Takes All Computation (Grossberg (1999))

The basic operation of a winner-takes-all (WTA) circuit is to suppress the activity of all neurons except the one having the largest input. Minimal models require only three neurons fully connected to three presynaptic neurons, plus a neuron for vigilance, taking the tally to seven. Such a network can develop states allowing learning and memory. And so are we willing to grant consciousness to a network of seven neurons?

3) Synchronized Neural Activity (Bachmann (1994), Engel et. al (1999))

If synchronized firing in neurons is the main characteristic of consciousness, then a group of three inter connected neurons firing in synchrony is conscious

4) Closed-loop action perception processing (O'Regan and Noe (2001))

Herzog et. al give the example of a thermostat with a temperature sensor ('perception') controlling a heater ('action'). This could be formulated as a two neuron feed-forward network with a sensory neuron connected to an output neuron controlling the switch.

Thus they argue that if one does not want to attribute consciousness to such small networks, other components are needed. Additional characteristics are often imposed. Typical examples being attention, number of neurons and complexity of the network. Yet, none of these additional components necessarily solves the small network argument.

1) O'Regan and Noe (2001) combined their sensorimotor contingency approach with attention based processing. They state that perception occurs in a closed loop of action and information processing but consciousness emerges only when attention comes into play. The most common example of this being a person driving a car in an unconscious and automatic mode (unconsciously perceiving other cars on the road and taking correct measures), but conscious perception, e.g. of a traffic signal, arises only when attention comes into play. However, in this model, attention can be incorporated with just one single extra input arising from a second group containing a very small number of cells

2) It is often proposed that consciousness may emerge if the brain exceeds a certain number of neurons. In a model with a simple linear arrangement of neurons where each neuron is connected only to it left and right neighbors, there is no reason to believe why a network of 10^10 neurons is more capable of generating consciousness than a network of three neurons. hence size alone cannot account for this

3)Other approaches state that a certain level of complexity must be met before a network can yield consciousness. Herog et. al give the example of Tononi and Edelman (1998) who proposed to measure complexity by defining a functional cluster that was loosely connected to the rest of the network but still has a rich repertoire of internal states. But this complexity criterion can be met even by a small network. Herzog et al. propose a network of nine neurons, organized into clusters of three. Assume the following model of interaction. The state of each neuron is described by p bits. Therefore, each neuron has can take up 2^p possible states. Now in the first 100 msec, the three neurons agree on the first bit by a majority vote, in the next 50 msec, on the second bit and so on, until the last 200/(2^p) msec, when they decide on the last bit. Thus for any arbitrary p, three neurons will agree on p bits in 200 msec, showing a large level of complexity in a small cluster.

Hence, Herzog et. al argue that all the above mentioned components are important aspects to understand consciousness but they are only trivially necessary rather than sufficient. Finally, they argue that one way out of the small network argument is to assume that each network, however small, has a vanishingly small consciousness, but this form of "panpsychism" also has it's problems. What happens if two networks, each with it's own vanishingly small consciousness, are connected? Do the two consciousness merge, do they stay separate (as proposed in split brain patients) or are there new coalitions of neurons making up new consciousness?

Overall, the paper seems to have created a bit of a ripple in the community of neuroscientists working on consciousness. Taylor (2007) in "Commentary on 'the small network' argument" states

..paper of this issue (Herzog, Esfeld, & Gerstenr, 2007) has derived an interesting criterion for any neural model of consciousness, with the disturbing result that a whole raft of neural models of consciousness are deficient when looked at by this criterion. The criterion itself is that a neural model of consciousness is suspect if it can be shown to work using only a relatively small network of neurons (of the order of 10–100 or so) 

Reference

HERZOG, M., ESFELD, M., & GERSTNER, W. (2007). Consciousness & the small network argument Neural Networks, 20 (9), 1054-1056 DOI: 10.1016/j.neunet.2007.09.001

The Fuzziness of Boundaries-Max Delbrück

In the summer of 1941, a young man-only 34 years old-showed up for the first time at the Cold Spring Harbor Laboratory Campus. He would ultimately become the leader of a generation of scientists who would establish the field of molecular biology. His name was Max Delbrück. Max was born in Berlin. His father Hans, was a professor of history at the University of Berlin and his mother was the grand-daughter of Justus von Liebig (of chemistry fame).

His early interests were directed towards astronomy. However, during the latter part of his graduate studies in Göttingen, the breakthroughs in quantum mechanics caused him to shift to theoretical physics. He obtained his Ph.D in 1930 following which he went back to Berlin and worked as an assistant to Lise Meitner, It was during this time that he discovered the theoretical basis of gamma ray scattering by a Coulomb field, a phenomenon that to this date bears his name, Delbrück scattering.

In 1937, he moved to Caltech, in the United States, where he teamed up with Emory Ellis doing phage research. Together, they demonstrated that viruses reproduce in one step, rather than exponentially (as cellular organisms do). Following the start of World War II, Delbrück chose to remain in the United States, teaching physics at Vanderbilt University while conducting genetic research. His claim to biological fame came with the establishment of the famous "Phage Group" with Salvador Luria and Alfred Hershey at the CSHL . In 1943, he and Luria demonstrated that genetic mutations in bacteria arise in the absence of selection and not as a response to selection. This work was significantly backed up by mathematical models that they developed which were consistent with experimental results. He shared the 1969 Nobel Prize in Medicine and Physiology with Luria.

His name may not be as familiar as Albert Einstein or Charles Darwin, but Max Delbrück was a scientific giant who changed the world through his research in modern biology. He saw no barriers between the sciences.  He had with him his tools from physics and mathematics and the problems from biology. In an essay commemorating Delbrück's 60th birthday, Luria wrote "Seldom has a group been so richly rewarded as have we, the molecular biologist, whom the physicist Max Delbrück, more than anyone else, guided to the explorations of the deep mysteries of life".

In the words of the great man himself

A Max Sing-Along 

(lyrics by Max Delbrück, 1968)

I was born about 100 years ago 

And there is nothing in this world that I don't know

I have Mendel in the garden

And I steered the ship for Darwin

I showed Otto how the neutrons had to go!

I had Watson grab the helix by the tail

I told Morgan that the flies would never fail

And the prize that went to Feynman

Well the theory was mine man,

And my song and inspiration never stale!

And, my buddy, Linus owes a debt to me,

For the structure of his modern chemistry,

Of the phage I am the father,

But I didn't want the bother,

Of collecting data so statistic-al-ly!

I'm a single-minded scientific man....

The Fuzziness of Boundaries-Introduction

How times change!! A few years back, when I announced my plan of pursuing a research career in fundamental biological sciences, I was labeled by many as a mediocre student who did not want to pursue a degree in engineering for want of insight in the fields of physics, mathematics and engineering. Just a week ago, I was with a colleague (he is an electronics engineer studying at the EPFL) in Lausanne and our discussion soon turned to the evergreen topic of biological neural networks. I impressed upon him, the significance of the ongoing Blue Brain project at the Brain Mind Institute in the EPFL. 

The project uses two huge IBM Blue Gene supercomputers to simulate a neocortical column (the basic functional unit of the brain, containing 10,000 neurons). At the push of a button, the model could reconstruct biologically accurate neurons and automatically connect them in a biological manner, a task than involves positioning 30 million synapses (connections that one neuron makes with another, edges between nodes, if you happen to be a graph theorist). All very well except for one tiny detail. The brain happens to consist of 100 billion neurons and 100 trillion synapses!!! Heck, the Blue Brain doesn't come even remotely close to the actual situation. And it requires two huge supercomputers just for a mere 10,000. 

Is the Blue Brain Project a sham then? The answer is no. It is an engineering feat and it is here to provide more clues on one of the most profound questions in biology, the mammalian brain. The point I'm trying to make here is that physical sciences, engineering sciences and the biological sciences were never mutually exclusive. Biology brings the problems. Big problems, trust me. The physical sciences bring with it the theory, principles and the foundation needed to tackle them. Finally, the engineering sciences bring the tools. 

Having said all of this, in the next part of this series, I shall introduce some great men of science and their efforts in breaking the wall between fields, such that today, I have no idea where one ends and the other starts. The boundaries are seemingly fuzzed up and are getting fuzzier by the day.