Biological explanation for OCD

• What do genetic explanations focus on?

  Genetic explanations focus on whether individuals inherit a genetic pre-disposition to developing OCD.
• What have family studies shown in the role genetics play in developing OCD

  Family studies have shown that relatives of OCD are more vulnerable to developing OCD.

  
  

  For example, Nestadt et al (2000) found that first-degree relatives (i.e. parents, siblings) of OCD sufferers had an 11.7% chance of developing the disorder compared to a 2.7% risk in first-degree relatives of control patients without OCD.
• How have twin studies have been used to investigate the role genetics play in developing OCD

  Twin studies involve a comparison between identical twins (monozygotic - MZ) and non-identical twins (dizygotic - DZ).

  
  

  MZ twins share 100% of their genes and DZ share only 50% of their genes so if genes do play a role in developing OCD we would expect to find a higher concordance rate for MZ than DZ.

  
  

  For example, Carey and Gottesman (1981) found MZ twins has a concordance rate of 87% for obsessive symptoms and features compared to 47% in DZ twins.
• Is OCD caused by a single gene?

  However, OCD seems to be polygenic.

  
  

  This means that OCD is not caused by one single gene but that several genes are involved. Taylor (2013) found evidence that up to 230 different genes may be involved in OCD.
• Define cocordance rate


  The probability that a pair of individuals will both have a certain characteristic, given that one of the pair has the characteristic.
• Evaluate genetic explanations of OCD (supporting evidence)


  There is consistent evidence from twin and family studies which show that genetic factors are important in developing OCD.

  
  

  Van Grootheest et al (2005) reviewed twin studies on OCD and all studies reported higher concordance rates for MZ twins and came to an overall conclusion that there was a moderate genetic influence on the development of OCD.

  
  

  Nestadt et al (2010) reviewed previous twin studies and found that 68% of identical twins shared OCD as opposed to 31% of non-identical twins.
• One limitation is that too many candidate genes have been identified

  Although twin studies strongly suggest OCD is largely under genetic control, psychologists have not yet pinned down all the genes involved.

  
  

  One reason for this is because it appears that several genes are involved and that each genetic variation only increases the risk of OCD by a fraction

  
  

  The consequence is that a genetic explanation is unlikely to ever be very useful because it provides little predictive value.
• Evaluatate genetic explantions of OCD (environmental factors)


  Evidence for the genetic explanation suggests that some people may be more susceptible to OCD however it does not suggest that genes are the sole cause of OCD.

  
  

  It seems that environmental factors also trigger or increase the risk of developing OCD.

  
  

  For example, Cromer et al (2007) found over half the OCD patients in their sample had a traumatic event in their past, and that OCD was more severe in those with more than one trauma.
• Evalutate genetic explantions of OCD (genetic code =complex)

  Our genetic coding is complex and as genome research has developed there has been some suggestion that MZ may have similar genes but are not completely identical.
• What do the neural explanations for OCD include

  the role of the neurotransmitter, serotonin and certain areas of the brain e.g. the orbitofrontal cortex.
• Describe the role of serotonin in neural explanations for OCD

  If a person has a low level of serotonin then normal transmission of mood-relevant information does not take place which means mood, and sometimes other mental processes, are affected.

  
  

  At least some cases of OCD may be explained by a reduction in the functioning of the serotonin system in the brain.
• Describe the role of areas of the brain in neural explanations for OCD

  Research has found sufferers of OCD have elevated levels of activity in the orbitofrontal cortex and the caudate nucleus (located in the basal ganglia).

  PET scans of patients with OCD have shown higher levels of activity in the OFC.

  
  

  The orbitofrontal cortex is part of a brain circuit; one of the functions of this circuit appears to be turning sensory information into thoughts and actions. Primitive impulses, for example to check and clean, arise from the orbitofrontal cortex in response to sensory inputs.
• Describe how the brain circuit relates to OCD (with example)?

  The orbital frontal cortex sends a message of panic to the caudate nucleus.

  
  

  A normal brain would decide whether or not this issue is important and if it is, it would get passed on to the thalamus to take action.

  
  

  If the message isn't important or has already been dealt with it will filter out ending the circuit.

  
  

  However in a brain of an OCD sufferer, the caudate nucleus does not work correctly and send the potentially faulty message of panic to the thalamus which then carries out the action e.g. washing hands.

  
  

  This will keep repeating on a loop which is why someone with OCD performs compulsions which are repetitive rituals.

  
  

  For example, a non-suffer of OCD may have an impulse to wash dirt from their hands; once this is done the impulse to perform the activity stops and so does the behaviour. It may be that the brains of those with OCD have difficulty switching off these impulses so that they turn into obsessions, resulting in compulsive behaviour.
• Evaluate the neural explanations (supporting evidence)

  There is supporting evidence for both neural explanations

  
  

  Hu (2006) compared serotonin activity in 169 OCD sufferers and 253 non-sufferers, finding serotonin levels to be lower in the OCD patients.

  
  

  Zohar et al (1987) gave mCPP (a drug that reduces serotonin levels) to 12 OCD patients and 20 non-OCD control participants, finding that symptoms of OCD were significantly enhanced in the OCD patients.

  
  

  Menzies (2007)found an association between ability to stop a repetitive task and a decrease in grey matter in the orbital frontal cortex in participants with OCD.
• Evaluate the neural explanations (extent of neural abnormality leves)

  To what extent abnormal levels of serotonin and activity within the frontal orbital cortex are actual causes of OCD or merely effects of the disorder has not been established.
• Evaluate the neural explanations (straightforwardness)


  Neural explanations are not straightforward; there is evidence to suggest that anatomical (brain) differences are inherited and these may lead to OCD.

  Therefore this explanation cannot be used to fully explain OCD.
• Evaluate the neural explanations (applications)


  These explanations have led to drug therapies which involve increasing serotonin levels. It could also lead to brain scans being used in the future to detect OCD risk.
• Evaluate the neural explanations (contradictory research)


  The research into the role of serotonin is contradictory; some studies have found taking serotonin enhancing drugs do not always work to relieve symptoms. Therefore it is difficult to draw firm conclusions about the exact role of neurotransmitters in OCD.
• Candidate genes e.g. 5HT1-D


  Researchers have identified specific genes which create a vulnerability for OCD called candidate genes:

  • Serotonin genes e.g. 5HT1-D beta are implicated in the transmission of serotonin across synapses
  • Dopamine genes are also implicated in OCD

  Both dopamine and serotonin are neurotransmitters that have a role in regulating mood
• OCD is polygenic 


  OCD is not caused by one single gene but several genes are involved. Taylor (2013) found evidence that up to 230 different genes may be involved in OCD
• Different types of OCD


  One group of genes may cause OCD in one person but a different group of genes may cause the disorder in another person - aetiologically heterogenous.

  There is also evidence that different types of OCD may be the result of particular genetic variations, such as hoarding disorder and religious obsession
• Low levels of serotonin lowers mood


  Neurotransmitters are responsible for relaying information from one neuron to another.

  For example, if a person has low levels of serotonin then normal transmission of mood-relevant information does not take place and mood (and sometimes other mental processes) is affected.
• Decision making systems in frontal lobes impaired


  Some cases of OCD, and in particular hoarding disorder, seem to be associated with impaired decision making. This in turn may be associated with abnormal functioning of the lateral frontal lobes of the brain. The frontal lobes are responsible for logical thinking and making decisions.
• Parahippocampal gyrus dysfunctional 


  There is also evidence to suggest that an area called the left parahippocampal gyrus associated with processing unpleasant emotions, functions abnormally in OCD