Category Archives: Cognitive Neuroscience

Maps and Infinite Homuncular Regression

Although maps provide an excellent structural representation of what is going on the brain, in themselves they have no intrinsic value (Deacon, 2012). Maps do not really tell us about how the brain is interpreting or processing the information, only how it stores and forwards the information. To be fair, even that is only based on visual interpretation not on concrete behaviour. Unfortunately, that brings up the issue of whether or not topological maps are of any value. The retinotopic map may just be a consequence of development and evolution, an attempt to minimise wiring of the brain. Deacon (2012) stresses that there is massive flaw with the current use of mapping; he summarizes it as the ‘infinite homuncular regression.’ Basically, we have come to a point where other maps are just reading maps. The actual perceptual neurology has not been determined. Deacon warns scientists of the dangerous of neuroimaging and maps when trying to prove the existence of neural activity and behaviour. All this boils down to really is the classic argument in psychology; correlation does not prove causation.

1421_sensory_homunculus

Fortunately, Graziano and his colleges (2009) suggest that perhaps a homunculus does exist that can bridge perception and behaviour: the motor cortex. To put it plainly, a motor homunculus represents the sensitivity and innervation dedicated to particular muscles in our body. This homunculus can be mapped onto our motor cortex, and stimulation of these regions leads to an immediate motor response. Hence, we can bridge the gap between map and action. In other words, the motor cortex may in fact put an end to the infinite homoncular regression. A recent study carried out by Bouchard et al. (2013) found that when participants vocalised constants and vowels, scans showed smooth trajectories in the motor cortex.

UPDATED – Neuroimaging: EEG, MRI, fMRI, MEG, PET and TMS

Electroencephalogram (EEG)

EEG

EEGs measure electrical signals generated by the brain through electrodes placed on the scalp (ibid). Gel or a conduction solution is used to connect the electrodes to the scalp. Electrical signals are produced by partially synchronized waves of neural activity measured in Δ voltage/time (up to 2000 Hz). Signals are able to amplify the waves of neural activity so that sense can be made of them. Waves themselves represent stages of conscious; different frequencies represent different stages. Most of the time our brain is emitting alpha waves, which are of a regular frequency (8-12/sec), high amplitude and represent relaxed wakefulness. Should the wave amplitude decrease, it can indicate neural activity further from the cortex.

When EEG waves accompany physiological events, they are known as event-related potential (ibid). Event-related potentials are calculated by averaging the signal trails epochs, averaging reduces the noise of surrounding activity and increases strength of the signal.

Advantages of the EEG:

–       High temporal resolution (accurate at recording fast changes in neural activity)

–       Less subject to motion artifacts

–       Not claustrophobic

–       Portable

–       Can be used on infants

Disadvantages of the EEG

–       Weak spatial resolution

–       Synchronous firing of 10K neurons is required to produce a magnetic field which is large enough to measure

MRI: Structural and Functional

MRIs produce high-resolution, three-dimensional images from the measurement of waves that hydrogen atoms emit when they are activated by radio frequencies waves in a magnetic field (Pinel, 2011). High spatial resolution means MRIs are able to detect and represent different spatial locations. The images produced are far clearer than CT scans; however, fMRIs are seen as even greater improvement.

The fMRI produces images that represent increased oxygen flow in response energy needs of specific brain regions. Oxygenated blood has magnetic properties due to its high iron content making it sensitive to magnetic fields emitted from protons in the MRI. Deoxygenated blood is not sensitive to magnetic fields; as such brightly light portions of the fMRI reflect high-energy consumption. If you want to read, more about the BOLD fMRI click here, BOLD stands for blood oxygen level dependent signal. The job of the fMRI is to record this BOLD signal.

Advantages of the fMRI

–       Accurately depecits structural data

–       Reasonable temporal resolution

Disadvantages of the fMRI

–       Claustrophobic

–       Noisy (literally, not signal noise)

–       Very susceptible to movement artifacts

–       No metal-based equipment can be around the machine

–       BOLD is not a direct measure of neural activity, only oxygen consumption

Magnetoencephalogram (MEG)

EEG2

The MEG measures changes in magnetic fields on the surface of the scalp (ibid). Unlike the fMRI, magnetic fields are produced by changes in neural activity, which activate pyramidal cells of the cortex. Neural activity is not being affected by magnetic fields.

Advantages of the MEG

–       High temporal resolution

–       Acceptable spatial resolution

–       Compared to an EEG, it is less distorted by the scalp

Disadvantages of the MEG

–       Just like the EEG, it requires a high baselines firing rate in order for a magnetic field to be produced

–       Normally it has to be paired with an MRI

–       Expensive

–       Not portable

Positron Emission Tomography (PET)

The PET scan is a bit more controversial than some of the other scans because it involves injecting a radioactive substance. Specifically, 2-deoxyglucose is injected in the carotid artery. This substance is used because of its similarity to glucose, a quality which neurons like very much. Neurons take 2-DG into their system, but cannot metobolise it. The result accumulates in active regions of the brain resulting in measurable levels of radioactivity.

Advantages of the PET:

–       Reasonable structural accuracy

–       Direct reflection of current activity

–       No motion artifacts

–       Not claustrophobic

Disadvantages of the PET:

–       Radioactive substance is involves

–       No temporal resolution and no structural information

–       Poor spatial resolution

–       Expensive and not very portable

Transcranial Magnetic Stimulation (TMS) 

In 1985, Tony Barker invented the TMS, which is now known for its ability to prove a particular brain activity causes certain behaviour. A non-invasive technique, the TMS causes depolarisation and hyperpolarisation of neurons in the brain. Electromagnetic induction causes a weak electrical current in the cortex to evoke synaptic potentials. With the TMS it is possible to create a stimulated temporary lesion of the brain by preventing normal brain function without causing any adverse effects.

Advantages of the TMS

–       Almost portable

–       Can  prove causality

–       Can simulate a lesion

Disadvantages of the TMS

–       Difficult to specify precise regions of the brain

–       Only surface regions are detectable