Subcortical structures & cognition

consciousnessSubcortical Structures and Cognition

Leonard F. Kosiol and Deborah Ely Budding

Springer (2010)

INTRODUCTION: This book has some useful angles from the point of view of the role of subjective experience and emotion in brain function. The authors main theme is the underestimated importance for brain function of the subcortical area of the basal ganglia. At times the authors appear to think that their treatise reinforces an automaton model of the brain, against the suspicion of a Cartesian homunculus that might be holding out in the prefrontal cortex. The real implications look rather different. The orbitofrontal and anterior cingulate areas of the prefrontal that are involved with emotional processing have projections to the basal ganglia including the nucleus accumbens (the brain’s reward centre). The basal ganglia system is described as being driven by the neuromodulator dopamine. We only need to be reminded that dopamine is an excitatory modulator present in the experience of orgasm, to see that subjective experience looks to be a driver in the systems described here. As a foot note to this the authors mention that a part of the basal ganglia known as the subthalamic nucleus (STN) appears to have the ability to override actions whose process of execution has already begun. Although the connection is not mentioned in this book, this looks to equate to the ‘free won’t‘ suggested by Libet, as something that could override actions begun by readiness potentials at an unconscious level.

The authors are intent on reducing the cortical emphasis of neuroscience, and instead emphasising the role of subcortical areas and particularly the region known as the basal ganglia. They stress the importance of cortical-subcortical-cortical loops in understanding brain function. In these loops the cortical inputs are always excitatory, with the subcortical for the most part inhibitory. The subcortical areas are seen to project back to the cortex, and to modulate the cortical inputs. They are indicated to have a role in deciding what information is returned to the cortex. Each loop originates in a particular area of the cortex, such as the orbitofrontal and the anterior cingulate. The basal ganglia receive inputs from most cortical regions. This region has traditionally been associated with motor activity, but the authors stress that its connections to the cortex involve it in cognition and emotion.

The authors discuss how brains deal with novel situations. Memory allows perceptual experience to be stored and used later to deal with situations as they arise. The frontostriatal part of the basal ganglia is organised to retain habitual behaviour when situations are no longer novel. The striatum, which includes the nucleus accumbens or reward centre, is the largest structure in the basal ganglia, and receives dopamine connections from the midbrain. The basal ganglia grew in size and specialisation in line with the cortex during evolution. They operate within a reward-driven system based on dopamine. The region is seen as integrating sensory input, motivation and finally motor output. Sensory input is gated in the striatum, while motivation relates to the nucleus accumbens, which is the brain’s reward centre. The nucleus accumbens projects to the ventral pallidum, which is involved in movement. This suggests that movement and habitual responses are based on motivation or reward.Incoming stimuli from the environment to the brain are always excitatory. The thalamus receives the incoming signals, and sends them onto the cortex for processing. This is also primarily excitatory, as are further projections to the frontal cortex, necessary for action. The basal ganglia are seen as important for inhibition. Inhibitory output going back to the thalamus assists the focusing of attention and action. The basal ganglia are here seen as important in both cognitive and emotional functioning. There are projections from the emotion-related parts of the cortex, and the basal ganglia are seen as sensitive to the reward-characteristics of the environment.

Certain regions in the cortex project to parts of the striatum called striosomes. The projecting regions are often involved with emotion, notably the orbitofrontal cortex, the paralimbic regions and the amygdala. This is seen as constituting a limbic-basal ganglia circuit. A region of the basal ganglia known as the SNpc is thought to synthesise dopamine, and thus provide the basal ganglia with the knowledge of rewards and motivation. The limbic system initially evaluates the motivational significance of sensory input. This is projected to the SNpc which regulates the dopamine system. Thus the striatum can read the motivational or reward nature of the environment, and disinhibit or release a behaviour that is appropriate to the environment.

The basal ganglia do not participate directly in cognitive computation, but instead gate or select for elements of the processed information used by the cortex. Novel problem solving requires interaction between the prefrontal cortex, other parts of the cortex and the basal ganglia, with the basal ganglia gating so as to select the right context for solving the problem. The striatum can select both for particular movements or habitual responses, but also for particular cognition or perceptions. A main theme of the book is that functions that the basal ganglia have long been known to perform in inhibiting or releasing movements, also applies to the inhibition or release of cognition. The striatum is also seen as participating in some learning, with responses reinforced on a trial-and-error basis. This is relevant to the important process of categorisation, where aspects of the environment are classified. The basal ganglia are also considered to be important with respect to working memory. This type of memory has to hold a small range of concepts ‘on-line’. The basal ganglia perform the inhibitory role of preventing intrusion or distraction by less important material.

The subthalamic nucleus (STN) has an inhibitory role in stopping behaviours whose execution has already begun. Problems with the STN are related to dysfunction in control of impulsive behaviour. Although not discussed in this book, the function of the STN does seem to relate to the ‘free won’t‘ proposed by Libet, as being capable or overriding behaviours signalled by readiness potentials before there was any conscious awareness of the will to act. The dopamine/reward related nature of the basal ganglia described in this book implies that the ‘won’t’ function involves some element of conscious emotion.

The orbitofrontal cortex and the anterior cingulate areas of the prefrontal are seen as responsible for coordinating emotion and cognition, whereas the dorsolaterial area of the prefrontal is related to more executive functions such as reasoned planning and problem solving. It is remarked that only the dorsolateral activity or deficits are reliably picked up by conventional tests of mental ability. Reports of ‘invoked resistance’ or attempts to use the will to overcome obsessive compulsive disorder (OCD) are related to the dorsolateral becoming more active and counter balancing the orbitofrontal problems.

The orbitofrontal circuit comprises lateral and medial divisions. The medial division projects to the nucleus accumbens and the ventral part of the pallidum, and then back to the thalamus and the orbitofrontal itself, which in turn has connections with the limbic system. The lateral orbitofrontal cortex sends projections to the ventromedial striatum. Deficits in this area are related to personality changes, involving impulsiveness and emotional changeablity. The circuit sustains motivated activity in the absence of external direction or reinforcement. When there are deficits in this area social behaviour is dysfunctional, despite normal performance in many mental tests.

The medial frontal circuit originates in the anterior cingulate and projects to the nucleus accumbens and ventral striatum, and then back to the anterior cingulate. These regions are sometimes referred to as the limbic striatum. Impairments to this area leave patients apathetic, while cognitive and motor aspects are not damaged. Dopamine-based activity in the nuclear accumbens is related to seeking pleasure and avoiding pain. Addictions are related to lack of activity in this area, with drugs of addiction all helping to enhance activity in the area.

The largest concentrations of dopamine in the brain are found in the prefrontal and the basal ganglia. Dopamine projections are mainly to the nucleus accumbens, the amygdala and to the frontal cortex. This is the brains reward circuitry. The prefrontal is suggested to code for the anticipation of reward, notably in the anterior cingulate and ventral striatal. The orbitofrontal and medial frontal are related to reward or reinforcement. The ventral striatum is highly active in anticipation of reward, and remains active during the reward. It is believed to modulate motivation, attention and cognition. Impairment of this area creates a wide range of problems. There are a variety of reward centres influencing behaviour. Within the striatum learning is influenced by dopamine acting on medium spiny neurons, reducing inhibition and releasing or increasing output of activity. By contrast, reduced levels of dopamine lead to increased inhibition and reduced activity. Learning can be reinforced by bursts of dopamine activity. The same principle may apply to emotional circuitry.

Tags: , , , Posted by

Leave a Reply