BIOLOGY : MECHANISM OF THE BRAIN

The outside is always an inside

Neuronal assemblies

Neurons in primitive animals like hydra have nerve nets. These respond by coordinated action from sensory stimuli like food particles to engulf them .In higher animals these nerve nets multiplied in number for unrelated purposes as ganglions. The problem occurs in coordination of the various nerve nets as they frequently have common inter neurons serving different purposes.

     In higher animals brain has a hierarchy of nerve nets. In mammals the basic nerve net is a map in cortex called hippocampus. The hippocampus has grid cells connected to afterent sensory inputs from propriceptive receptors which give inputs sequentially increasing with muscle movements and distance. It also has inputs from vision for objects, auditory sensations which occur linearly with time. A line of grid cells respond to all inputs but with different intensity depending on place of occurrence on the line. There is another layer of neurons the place cells which have input from all the grid cells. In a combinatorial manner the inputs from grid cells lead to place cells being activated by sensations at a particular place.The place cells thus form a linear map of occurrence of objects,proprioceptive sensations. The grid cell and place cell map also occurs in two dimensions. This is possible because of presence of head direction cells. These respond to angle of head and eyesight. Combined with place cells they give two dimensional location of an object

   In primates the linear occurrence of objects and events in a line of place cells correspond to episodic memory. Further the two dimensional occurrence of events corresponds to pathways which are at intersection of many event sequences. They correspond to semantic memory where the exact time of occurrence is lost but the meaning remains.

Brain rhythms and neuronal assemblies

While in simple networks neurons coordinate by action potentials in a brain with multiple nets the conduction time of action potentials is too long and they cannot occur simultaneously. However if the brain is activated by a regular rhythm , the elevation of excitability of the neurons occur simultaneously. Now action potentials can travel across long distances as the membranes are near their tipping point of firing simultaneously.A rhythm has the advantage that it is energetically conservative and less costly. If the potential has to be raised all the time the energy cost becomes very high for the brain.

     Rhythm in hippocampus has another important function of maintaining time. While moving the sequential action potentials of proprioception or images can get activated only during trough of the rhythm .Thus a time line is created where images have definite position with each other.

 Brain rhythms are created by special centres in septum,hippocampus in response to unexpected occurrences and events. The stimuli originates in brain stem and midbrain dopamine centres probably mediated by new events detection in basal ganglia

    Autonomous  trajectories in hippocampus and perception action cycle

Trajectories of place cell sequential activation form in hippocampus while the animal is moving along a path. The place activations activate sequentially as they have a gradient in their activation where the next place cell requires lesser activation than former. Thus pathways can be reproduced if activated by an image. While this occurs during actual movement it has been observed in the hippocampus that spontaneous pathways are formed in the two dimensional place cell map while the animal is asleep and cannot move.The importance of this phenomena is that the animal does not have to actually discover all pathways by exploration. It can select the best pathway by suitably activating it.The brain is thus autonomous and selects an internal state triggered by outside inputs.

     The rest of the brain has readers of the various hippocampus trajectories. These readers decide which path to activate in which context. The coordination of hippocampus and readers occurs during learning when theta rhythm allows action potentials to bridge the various readers with the hippocampus.

 The readers or control mechanisms constantly learn to adjust response in relation to the output. This is called the perception action cycle.The output goes to muscles in the form of train of action potentials. The same train is received by the control centre as a collateral flow. The sensory change in place cells is received by the control centre again as a train of action potentials. If reward occurs the two are matched as a mapping. If unexpected loss occurs dopamine secreted by reward centres inhibit matching. Similar map formation occurs in control centres hierarchically. Higher centres inhibit lower control centres if response is inadequate till invariance in mapping occurs for all input and output. This form of knowledge thus occurs only as a result of active search and action and not by passive sensory input

      Attention and search system

This system consists of superior colliculus at lower level and Frontal eye fields,orbito  frontal cortex at higher cortical level of control.It is is triggered by theta waves in response to unusual events or search for resources. The superior Colliculli has a map for moving eye towards an object based on activation of relevant mucle.Reward centres in OFC and object signals from parietal cortex converge on FEF. These activate the superior Collicutt to attend to the object of interest.The attention system not only searches for objects but in the process activates the relevant trajectory in the hippocampus on which the object lies.

 The motivation system

At the lowest level  consists of midbrain and brain stem centres secreting dopamine,serotonin. They are involved in regulation of automatic reflex of body to sweet or bitter taste, fear,rage.Fear ,rage are complex combinations of motor and autonomic actions.The lower centres also adjust level of arousal  as per external conditions.The next level is of limbic cortex or amygdala. It associates by conditioned reflex emotions as fear .However these actions may be inappropriate at all contexts. The Orbitofrontal cortex provides contextual adjustment of emotions.It also influences all actions requiring reward . It has a centre nucleus accumbens which provides positive stimulus to prefrontal dorsal and ventral  cortex concerned with planning thoughts and actions.The latter act through premotor  cortex to choose actions.

The planning and thinking system 

These are respectively the dorsal and ventral PFC.The dorsal PFC has same trajectories of events as in hippocampus. They are however more stable and resistant to distractive stimuli.Along with orbital PFC which provides reward signals to contexts it creates sequences of these contexts. Moreover if reward is missing contexts are inhibited and deleted.It thus leads to future planning of  action by following contexts planned by it.

  The ventral PFC associates sensory bits to create categories. The categories are chosen based on reward signals from OFC.When a category is different the associated bits are different. These contextual signals originate in ventral PFC.Thinking such as deduction or induction is based on similar or different categories. Based on results of deduction or induction hippocampus trajectories are selected by the ventral PFC.In humans categories are formed based on rules of association instead of sensory bits to form images.Thus ‘Large size avoid’ is a rule used to form category. Rules originate in generalisation from multiple associations and inhibition of unfavourable associations as occurs in PFC of humans.

At the lower sub cortical level muscle action corresponding to thinking is  the association of muscle movements for particular tasks. This association is carried out in basal ganglia. It is controlled by dopamine signals from put amen which inhibit actions which are inappropriate and unexpected to perceptual requirements. The creation of action sequences corresponds to planning at action level. Sequences for movements are created in cerebellum based  on time map similar to hippocampus. Here the sequence of movements are linked on a time line .

Imagination,symbolic speech and imitation,remote memory,Self awareness

There are certain control centres which are unique to the human brain. These work on the same action perception cycle as in other control centres. However they have functions which increase the capability of planning and reasoning.

Visual imagination is unique to humans. There is a control centre in ventrolateral polar PFC.It selects hippocampal trajectories by attention . However the actual translation into action is inhibited. This allows counter factual thought and planning.

The inferior caudal PFC  is a c control centre for imitation. Imitation is a complex sequence of movement copied by joint attention. The same centre is used to copy speech.The meaning of speech comes from association of speech to visual imagery and this can select particular hippocampal trajectories.. 

  Speech has infinite combinations and possibility of creation and selection of hippocampal trajectories. Morever they have nested character. Speech allows counter factual thought with greater precision. It allows categorical reasoning based on contexts.

Humans have capacity for remote episodic memory as well as recalling them. This is the function of medial PFC along with hippocampus which still maintain episodic memory for recent episodes.

Meta cognition is an ability to be aware of a perception or intention of action of others or self awareness.These centres become aware by the perception action cycle through control centres in medial PFC.

Ack:Understanding the prefrontal cortex,The brain from inside out, Rhythms of the brain,