The central nervous system (CNS) consists of the brain and the spinal cord, immersed in cerebrospinal fluid (CSF). Weighing about 3 pounds (1.4 kilograms),

The central nervous system (CNS) consists of the brain and the spinal cord, immersed in

cerebrospinal fluid (CSF).

Weighing about 3 pounds (1.4 kilograms), the brain consists of three main structures: the

cerebrum, the cerebellum, and the brainstem.

See http://www.brainexplorer.org

Cerebrum: divided into two hemispheres (left and right), each consists of four lobes (frontal [at the

front], parietal [in the middle], occipital [at the back], and temporal [at the bottom]). The outer layer of the

brain is known as the cerebral cortex or the ‘grey matter.’ It covers the nuclei deep within the cerebral

hemisphere known as the ‘white matter’.

Cerebellum: responsible for psychomotor function, the cerebellum co-ordinates

sensory input from the inner ear and the muscles to provide accurate control of

position and movement.

Brainstem: found at the base of the brain, it forms the link between the cerebral

cortex, white matter and the spinal cord. The brainstem contributes to the control of

breathing, sleep and circulation.

Neurons use their highly specialised structure to both send and receive signals. Individual neurons receive information from thousands of other

neurons, and in turn send information to thousands more. Information is passed

from one neuron to another via neurotransmission. This is an indirect process that takes place in the area

between the nerve ending (nerve terminal) and the next cell body. This

area is called the synaptic cleft or synapse.

CT (roentgen-ray computed tomography): A beam of x-rays is shot straight through the brain. As it comes out the other side, the beam is blunted slightly

because it has hit dense living tissues on the way through. Blunting or “attenuation” of the x-ray comes from the density of the tissue encountered

along the way. Very dense tissue like bone blocks lots of x-rays; grey matter blocks some and fluid even less. X-ray detectors positioned around the

circumference of the scanner collect attenuation readings from multiple angles. A computerized algorithm reconstructs an image of each slice.

See http://www.med.harvard.edu/AANLIB/hms1.html

MRI (magnetic resonance imaging): When protons (here brain protons) are placed in a magnetic field, they become capable of receiving and then transmitting

electromagnetic energy. The strength of the transmitted energy is proportional to the number of protons in the tissue. Signal strength is modified by properties of each proton’s microenvironment, such as its mobility and the local homogeneity of the

magnetic field. MR signal can be “weighted” to accentuate some properties and not others.

When an additional magnetic field is superimposed, one which is carefully varied in strength at different points in space, each point in space has a unique radio

frequency at which the signal is received and transmitted. This makes constructing an image possible. It represents the spatial encoding of frequency, just like a piano.

SPECT/PET (single photon/positron emission computed tomography): When radio-labelled compounds are injected in tracer amounts, their photon

emissions can be detected much like x-rays in CT. The images made represent the accumulation of the labelled compound. The compound may reflect, for

example, blood flow, oxygen or glucose metabolism, or dopamine transporter concentration. Often these images are shown with a color scale.

Areas of Experimental Work

• Consciousness in vision• Attention• The conscious present• Visual images and inner speech• Thresholds of consciousness• Consciousness and memory• Consciousness as a state: waking, deep sleep, coma,

anaesthesia, dreaming• Empirical theories of the NCC

• The self• Voluntary control

The key idea: treating consciousness as an experimental variable

E.g.:• between conscious and unconscious streams of stimulation• between conscious and unconscious elements of memory• between forms of brain damage that selectively impair

conscious process and those that don’t• between wakefulness and unconsciousness• between new and habituated events

The difficulty has been to discover comparison conditions: science advances by discovering that an apparent constant is actually a variable (e.g. gravity, species).

This means, that to study consciousness, conscious brain events must be sufficiently similar to unconscious ones.

“We can state bluntly the major question that neuroscience must first answer. It is probable that at any moment some active neuronal processes in your head correlate with consciousness, while others do not: what is the difference between them? (Crick and Koch 1998)

This is the problem of the NCC.

Science cannot observe consciousness directly; experiments can only confirm reports of conscious experience. So, for science, consciousness is a theoretical construct based on shared, public observations. But this does not make consciousness unusual in science….

Conscious processes are operationally defined as events that:• can be reported and acted upon,• with verifiable accuracy,• under optimal reporting conditions,• and which are reported as conscious. (Baars 2003, 4)

Unconscious processes are operationally defined as events such that:• knowledge of its presence can be verified, even if• that knowledge is not claimed to be conscious;• and it cannot be voluntarily reported, acted on, or avoided,• even under optimal reporting conditions. (Baars 2003, 5)

Examples of experimental paradigms in vision science:

• blindsight

• on-line (“how”) system vs. seeing (“what”) system

• bistable percepts (gestalts; binocular rivalry)

• electrical brain stimulation (e.g. Penfield)

• selective lesions (e.g. in monkeys)

Two Vision Systems

Milner and Goodale “…go further than their predecessors by proposing that the division of labour is determined by the use to which visual information is to be put, once it has reached the striate cortex. They suggest that a ventral stream, terminating in the inferotemporal cortex, is involved in maintaining an enduring, viewpoint-independent, representation of objects and their behavioural significance (the so-called ‘what’ pathway). In contrast, they suggest that a dorsal stream, terminating in the posterior parietal cortex, is involved in providing an egocentric representation of objects toward which goal directed actions are planned (the so-called ‘how’ pathway).”

Jason B. Mattingley, “Attention, Consciousness, and the Damaged Brain: Insights From Parietal Neglect and Extinction,” Psyche 5 (1999)

State Consciousness

Brainstem mechanisms control the state of consciousness; cortical activity provides the contents of consciousness. The reticular activating system connects lower brain stem neurons to the thalamus (and hence on to the cortex); it is responsible for cortical EEG readings (‘brain waves’). It used (1960s) to be thought that this was the seat of consciousness, but now this is generally doubted, and consciousness is ‘located’ in the cortex itself.

Theories of ConsciousnessA small list of suggestions that have been put forward might include (from Chalmers, online notes):

• 40-hertz oscillations in the cerebral cortex (Crick and Koch 1990) • Intralaminar nucleus in the thalamus (Bogen 1995) • Re-entrant loops in thalamocortical systems (Edelman 1989) • 40-hertz rhythmic activity in thalamocortical systems (Llinas et al 1994) • Nucleus reticularis (Taylor and Alavi 1995) • Extended reticular-thalamic activation system (Newman and Baars 1993) • Anterior cingulate system (Cotterill 1994) • Neural assemblies bound by NMDA (Flohr 1995) • Temporally-extended neural activity (Libet 1994) • Backprojections to lower cortical areas (Cauller and Kulics 1991) • Neurons in extrastriate visual cortex projecting to prefrontal areas (Crick and Koch 1995) • Neural activity in area V5/MT (Tootell et al 1995) • Certain neurons in the superior temporal sulcus (Logothetis and Schall 1989) • Neuronal gestalts in an epicenter (Greenfield 1995) • Outputs of a comparator system in the hippocampus (Gray 1995) • Quantum coherence in microtubules (Hameroff 1994) • Global workspace (Baars 1988) • Activated semantic memories (Hardcastle 1995) • High-quality representations (Farah 1994) • Selector inputs to action systems (Shallice 1988)

Three general types of NCC theory:

• Resonant neuronal assemblies (c.f. neural nets and biological selection theories): e.g. Edelman

• Temporal coordination of large groups of neurons (e.g. 40 Hz oscillation): e.g. Crick

• Global workspace theory (a small central domain of working memory plus a vast distributed set of specialized unconscious processors): e.g. Baars

Binocular Rivalry

Figure 2. Localizer Data: FFA and PPA(a) Two adjacent near-axial slices showing the localized FFA and PPA of one subject (S1). The FFA was localized as the region in the fusiform gyrus that responded more to faces than houses. The PPA was localized as the region in the parahippocampal gyrus that responded more to houses than faces. (These images follow radiological convention with the left hemisphere shown on the right andvice versa.)(b) MR time course on localizer scans showing FFA (blue solid line) and PPA (red dotted line) activity (expressed in percent signal change relative to fixation baseline) averaged across all four subjects. Subjects viewed sequentially presented faces (F), houses (H), or a static fixation point (1).

(Tong, F., Nakayama, K., Vaughan, J.T. and Kanwisher, N., 1998: Binocular rivalry and visual awareness in human extrastriate cortex, Neuron 21, 753-759)

(A) Sagital view showing the orientation of the images and the position of the 10 slices. In all images the left hemisphere is shown on the right-hand side of the image.

(B) Slice taken at 20 mm above the AC-PC line (third slice from the top in A), showing the pattern of increased neuronal activity under the aware condition. Regions of significant increasedactivation are shown in black and include the right dorsolateral prefrontal area (Brodmann area 46).

(C and D) The pattern of activity in two slices taken at z 5 27 and z 5 22 (third and fourth slice from the bottom in A), respectively, showing activity in midbrain centers, including superiorcolliculus in the unaware mode

Sahraie et al., Pattern of neuronal activity associated with conscious and unconscious processing of visual signals, Proc. Natl. Acad. Sci. 94 (1997)

Binocular Rivalry

Neurons in the superior temporal sulcus (STS) and inferior temporal cortex (IT) change with the percept, but most of the neurons in the medial temporal cortex (MT) and V1/V2 do not.

(The STS is the red line)

Rotating Snake: The strong rotation of the “wheels” occurs in relation to eye movements. On steady fixation the effect vanishes.

At the top of the figure are some receptors. Below them are two kinds of synapses (neural connections): Excitation synapses are ones that increase neural activity and inhibitory synapses decrease neural activity.

The concentric circles represent the neural activity recorded with the electrode. when the receptors are stimulated with light. When one or all of the center receptors are stimulated an excitatory increase in neural activity is obtained at the electrode. When the receptors labeled surround are stimulated an inhibitory decrease in neural activity is obtained.

The receptive field that lies at the intersection of the white cross has more light falling on its inhibitory surround than does the receptive field that lies between the two black squares. Consequently, the excitatory center of this receptive field between the squares yields a stronger response than that which lies at the intersection of the white cross.

Close your right eye and look directly at the number 3. Can you see the yellow spot in your peripheral vision? Now slowly move towards or away from the screen. At some point, the yellow spot will disappear.

http://www.michaelbach.de/ot/