Dorsal stream (Brain areas involved with consciousness)
Even at the very early stage of the retina, an important division arises between two parallel visual streams, the dorsal stream and the ventral stream. The dorsal stream projects to the parietal cortex, and is responsible for movements in relation to objects, many of them of a routine or reflex nature. It is also seen as an answer to the ‘where is it?’ location question. The processing of the dorsal is unconscious, and is faster than the consciousness-related processing of the ventral stream. Speed is adaptive for the dorsal because it has to respond to immediate developments in the external environment. Damage to the dorsal stream is shown to lead to deficits in movements and actions, rather than problems with visual perceptions. On the other hand, damage to the ventral stream results in difficulties in recognising objects, rather than actions such as grasping objects even if they are not recognised.
The ventral stream culminates in an area involved with conscious perceptions. The ventral stream answers the ‘what is it?’ question in respect of identifying new stimuli as objects. Visual processing flows from the retina through the thalamus, and into the primary and secondary cortices. The primary visual cortex (V1) deals with orientation, edges and boundaries. In the secondary visual cortex, there are processes dealing with the separation of objects from their backgrounds and also object features of intermediate complexity, such as geometric shapes.
Within the later ventral stream there are specialised visual modules, most of which are clustered in the inferior temporal region. Different features are processed in these different modules within the ventral stream, and may take different lengths of time to be processed. The ventral stream calculates the size and orientation of objects in relation to one another, rather than against any absolute measure. In contrast, the dorsal stream has an absolute scale. Perception relates to reflectance properties which show the difference between the surface of an object and surrounding surfaces. Within the outline contour, reflectance properties are also important for determining colour and texture. The ventral stream deals with colour, texture, form, brightness, size, shape, orientation and perception of motion. Damage related to any one module in the ventral stream can result in localised deficits such as not being able to recognise faces or landmarks. All this processing, however, happens at a still unconscious stage of the ventral stream. Although the ventral stream is referred to as conscious, consciousness is only involved with the end product of a long line of ventral processing.
Inferior temporal cortex
The inferior temporal cortex is seen as being the terminus of the ventral visual stream. However, it is not just a process for feeding forward from the original external stimulus. Neurons in the inferior temporal cortex feed back to the earlier stages of the visual cortex. This process is suggested to disambiguate the initial image. This secondary process appears to be Bayesian, in comparing the new external stimuli with memories of past stimuli. This reiteration process takes about 100-150 ms. This is in part an explanation of the long time (300-500 ms) it takes for visual stimuli to come into consciousness.
Phase change: The brain goes through what is described as a phase change to support conscious images. Thus the outcome of this feed forward and feedback process between the later and earlier stages of the visual cortex appears to be the stage at which processing goes from being unconscious to being conscious. There is a suggestion here that there is a threshold of neural activity, below which there is only unconscious processing, but above which consciousness is possible. This threshold involves substantial increases in this activity, which utilises massive cortical pyramidal neurons. Some of these neurons have giant cell bodies necessary to support long axons and large dendrites with abundant spines. Such neurons tend to be concentrated in layers II and III of the cortex. These layers are much thicker in the prefrontal, parietal and temporal regions, the areas of the brain most related to consciousness. In general connections in the frontal/conscious areas are long distance, while connections in the back/unconscious parts of the brain are with local neurons.
Substantial changes in these higher visual areas are apparent whenever consciousness is reported. The level of activity can rise by as much as twelve-fold when the signal correlates to consciousness. Studies show that activity levels in the early visual cortex are unrelated to whether a signal is unconscious or conscious. Conscious stimuli pick up strength as they progress through the cortex. In the first 200-300 ms of processing there is no difference between conscious and unconscious processing, but after that unconscious processing faded, but conscious signals move towards the front of the brain. At 400 ms. after the initial stimuli, only the conscious signals are apparent, and these are causing intense activity in the prefrontal, parietal and anterior cingulate. Studies show that activity levels in the early visual cortex are unrelated to whether a signal is unconscious or conscious.
Convergence/hubs: These areas of the brain are also viewed as hubs or convergence zones, in that signals converge on these areas. Brain imaging suggests that consciousness is not involved in the earlier stages of the sensory cortex. In the frontal parts of the brain, multiple sensory inputs are suggested to converge on a single interpretation, which may be fed back so as to alter processing in the earlier unconscious parts of the cortex. Decision-taking and memory formation is argued to require the retention of conscious images in convergence areas/hubs, and these images are seen to be more stable and less transient than unconscious processing. In particular such images can persist in the brain after the original stimuli from the external environment has disappeared.
Transcranial magnetic scanning: Transcranial magnetic resonance (TMS) studies have involved the disruption of this feed forward/feedback reiteration in the ventral stream, and when this was performed conscious images fail to develop. The researchers argue that here one is not looking just at a correlation, but at a physical mechanism that leads to consciousness, and without which consciousness will not happen.
The distinction made here between the dorsal and the ventral is qualified by later research which suggests various interactions between the two, although a raft of studies appear to support the main concept of different dorsal and ventral functions.
Single-neurons in the sensory and prefrontal cortices
Consciousness is seen to involve processing of images (Jennifer Aniston) in single neurons. Imaging suggests that only a few hundred active neurons arranged in patches in the visual cortex are each specialised in the production of particular images. In studies, single-neurons in the temporal lobe fired in response to particular images, such as an individual’s face or a particular building, but not at all to other images, even when it was another face or building.
The global gamma synchrony: The global gamma synchrony is the best known correlate of consciousness. The gamma synchrony is one of several synchronies in the brain. It is important to distinguish between local and global gamma. Gamma synchronisation in local regions of the brain is correlated with unconscious processing. However, it is the global gamma of multiple assemblies spatially separated, but reciprocally signalling across the brain, that is a well-supported correlate of consciousness.
Oscillations are controlled by inhibitory neurons’ inputs to pyramidal cells pushing them to discharge in synchrony. Synchronisation allows interactions between neurons to be more ordered. Excitatory inputs are effective if they arrive on the depolarising slope of the oscillation cycle, and ineffective at other times. Thus groups of neurons that are in synchrony can signal to one another, and those out of synchrony cannot. This type of control can function both within and between neural assemblies. It also improves the energy efficiency of the brain.
Visual stimuli create a wave of gamma activity in the brain for the first 200 ms after stimulus. After this, gamma subsides in the case of unconscious signals, but continues for conscious signals. Widespread gamma activity continues for more than 300 ms after the initial stimuli with many different areas of the brain synchronised is seen as the hallmark of consciousness.
Conscious perceptions correlate both to global gamma synchrony between spatially separated neuronal assemblies and also to high frequency activation in single- neurons. The link between conscious perceptions in single-neurons and synchrony in spatially separated neuronal assemblies is that the individual neurons are supported both by the intense activity of a clump of neighbouring neurons and by the global gamma synchrony, at the times when there is a correlation with consciousness. So the creation of a conscious perception may involve long-range cortical processing, but still only emerges in a small number of localised hot spots, in which there is intense and persistent gamma activity.
Projection to the orbitofrontal
Perceptions in the visual and other sensory cortices appear not to be evaluated, and therefore to be value neutral. It is suggested that this neutral phase would allow physical characteristics of stimuli to be registered in memory without being entirely coloured by sometimes shifting reward evaluations.
Projections then go from the visual areas of the temporal cortex to areas of the prefrontal; one of these areas is the orbitofrontal, which is involved with the evaluating the reward/punisher characteristics of stimuli. Descriptions of evaluative/emotional processing in the brain involve a framework of rewards and punishers together referred to together as ‘reinforcers’. Some stimuli are classed as ‘primary reinforcers’ because they do not have to be learnt. Other stimuli are initially neutral, but become secondary reinforcers because, through learning, they become associated with pleasant or unpleasant experiences.
The assessment of these reinforcers is now seen as being implemented in the orbitofrontal cortex, the anterior cingulate and the amygdala. The reinforcement value of this stimulus is decoded in the orbitofrontal and the amygdala before being projected to other brain regions leading to behaviour and learning. The reinforcer defines a goal but does not specify a particular action. The orbitofrontal integrates inputs from the visual, auditory and somatosensory cortex and other areas allowing it to sample the entire sensory range.
The level of activity in the orbitofrontal correlates to the assessed pleasantness of the stimuli, and not to the strength of the signal in the sensory cortex. Presumably this results from a Bayesian type feedback from the orbitofrontal to the visual cortex to check out previous experiences of these stimuli. Emotions/evaluation may trigger recall of existing memories stored in the cortex by means of projections from the orbitofrontal back to areas in the cortex.
The orbitofrontal is also important for amending responses to stimuli that used to be associated with rewards but no longer produce them. This rapid reversal of response carries through from the orbitofrontal to the basal ganglia. Damage to the orbitofrontal impairs the ability to respond to changes in the reward value of stimuli, and also the ability to deal with longer-term or indirect consequences. Such damage is associated with irresponsible behaviour and difficulty in establishing new preferences. Without the evaluation-based assessments of rewards, rational processing is not by itself adequate to for normal decision-taking or behaviour.
Common neural currency: The orbitofrontal deals with a variety of reward values and this gives rise to the idea of a common neural currency for comparing rewards that have little or nothing in common with one another. In the orbitofrontal single-neurons reflect relative degrees of preference for different stimuli. The subjective experience of one stimuli can be altered by another from a different modality. The impact of words can influence the subjective impression of an odour, and colours can influence the perception of odour.
Learning and memory: The orbitofrontal also helps to signal the significance of events to the hippocampal memory-forming region. Studies indicate that subjects need to be conscious to lay down specific long-term memory traces. Stimuli stabilised and kept active in the brain can be used for evaluation and planning or memorised for future use. Memories may also be strengthened by back projections from the orbitofrontal and amygdala to those parts of the cortex used for representing particular objects. In addition to any direct interaction with hippocampus or stored memories, the orbitofrontal and anterior cingulate, via the basal ganglia, are a major influence on the release of the neurotransmitter dopamine, which also has an important influence on learning and memory.
The anterior cingulate
The anterior cingulate is seen as part of a circuit involving the orbitofrontal, the ventral striatum, the dopamine neurons and also the dorsolateral prefrontal. It is a part of the brain where cognitive and emotional processing may both have an influence.
Positive or negative valuations of stimuli are projected to the anterior cingulate by the orbitofrontal. The anterior cingulate evaluates possible actions in response to stimuli, in particular their costs. Single-neurons in the anterior cingulate can encode the costs and benefits of a decision. This includes selecting for choices of action with short-term negative outcomes, in order to achieve counterbalancing longer-term rewards. The anterior cingulate also has a role in monitoring behaviour and responding to this with error correction.
In a study of single neurons in the anterior cingulate about one third of neurons changed response as the proximity/expectancy of reward increased. This type of response looks to be peculiar to certain evaluative brain regions. Neuronal activity is not correlated to the original strength of the incoming signal from the external environment but instead to the subjective anticipation.
Adaptive advantage: The adaptive advantage of the evaluative system is that responses to all possible situations do not have to be pre-specified. Any attempt to do this would have produced a unmanageable explosion of programmes.
The reasoning/planning functions of the dorsolateral prefrontal depend on the working memory. Stable conscious images influenced by the reward circuit are seen as important for the functioning of working memory. The orbitofrontal and anterior cingulate both project to the dorsolateral prefrontal. Preferences can be projected from the orbitofrontal to the dorsolateral, and thus have an influence on in longer-term planning and reasoning. Where dorsolateral activity reflects preferences, it is found that the orbitofrontal has reflected them first, and then projected them to the dorsolateral. In studies, where an initial trial involves a conflict between various rewards and punishers, the next trial will trigger an increase in dorsolateral activity, in order to improve performance, suggesting a feedback loop between the evaluative functions of the anterior cingulate and the dorsolateral.
The final stage for all this processing is the basal ganglia, the processing of which is immediately upstream of action and behaviour, and also governs the release of dopamine that is crucial to learning and memory. However, processing in the basal ganglia would appear to be unconscious except possibly for the nucleus accumbens which is the pleasure/reward centre of the brain.
The basal ganglia receives strong projections from most parts of the cortex, particularly the orbitofrontal, anterior cingulate and the dorsolateral prefrontal. Structures known as matrisomes and striosomes interact as a form of mixer-tap between the evaluations of the orbitofrontal/anterior cingulate and the reasoning/planning of the dorsolateral.
Striosomes are specialised neurons in the ventral striatum, the area of the basal ganglia involved in modulating emotional arousal. They receive inputs from the evaluative areas of the brain, and project to dopamine producing neurons. At the same time, cortical areas such as the dorsolateral involved with planning project to areas in the striatum known as matrisomes. These latter are often found in close proximity to striosomes, suggesting a link between the planning related matrisomes and the limbic related striosomes. Specialised neurons referred to as tonically active neurons (TANs) are situated where matrisomes and striosomes meet, and are therefore well placed to integrate rational and emotional outputs.
The largest concentrations of dopamine in the brain are found in the basal ganglia, the amygdala and the prefrontal regions, particularly the orbitofrontal. Dopamine producing neurons located in the midbrain appear to be influenced by the size and probability of rewards based on inputs from the orbitofrontal and the amygdala to the basal ganglia. There are projections from the dopamine neurons to the nucleus accumbens, and also back to the the amygdala and the orbitofrontal. Dopamine acting on the ventral striatum reduces inhibition and releases the output of behaviour. Further to this dopamine release is also an important influence on learning and memory.
Attention and working memory
Attention is seen as important for the brain in selecting behaviour-relevant information from the environment. Top-down voluntary attention from fronto-parietal areas of the brain to earlier areas of the sensory cortex are the drivers in giving preference to sensory inputs that are relevant to goal-directed behaviour.
The brain’s reward circuit of the brain, acting partly via the frontal eye field (FEF) in the parietal, influences both the direction of voluntary attention and the determination of which material gets into working memory. Common neural mechanisms in the brain’s reward circuits are suggested to select and maintain images in working memory, and also direct voluntary attention. It should be noted that the influence of conscious processing in the frontal reward circuit feeds back into the unconscious parts of the processing of attention, memory and learning.
Tags: anterior cingulate, basal ganglia, dorsal stream, dorsolateral prefrontal, inferior temporal cortex, orbitofrontal, ventral stream Posted by