Week 10 - Memory

The nature of memory (the fact that it is a 'process' rather than an entity or ‘thing’) makes it difficult to localise to a specific area in the brain.  Further, memory involves several procedural components (including encoding, recall, recognition and retrieval) as well as different modalities (auditory, visual and olfactory).  However, evidence has shown that certain areas of the brain are activated (and therefore associated with) particular parts of this process in the healthy brain. Damage to these regions can lead to deficits in some types of memory (Martin, 2006).
 
The hippocampus is understood to play a part in the consolidation of declarative long-term memory and spatial navigation.  Hippocampal damage has been shown to result in anterograde amnesia – difficulty in forming new memories subsequent to damage - although ‘old’ memories can remain largely intact.  Investigating the distinct role of the hippocampus, Teng and Squire (1999) highlight the case of patient E.P.
   

Patient E.P suffered severe amnesia following a bout of encephalitis which caused near complete damage to all components of his medial temporal lobe, including the hippocampus.  He had profound difficulty recognising people he regularly came into contact with, could only perform at chance level in both verbal and non-verbal recognition memory tasks and had extremely poor recollection of events and facts during the 40 years prior to his encephalitis.  In a series of topographical memory tests, E.P was asked to recall the spatial layout of the neighbourhood he grew up in some 50 years earlier.  E.P performed well, demonstrating “intact memory for remote autobiographical episodes...and intact memory for remote spatial information” (Teng & Squire, 1991, p.676).  Remarkably, despite the fact that E.P could remember the layout and features of a place he had moved away from some 50 years earlier, E.P could not recognise or navigate his way around his current neighbourhood. 

 

Teng and Squire (1999) posit that their own findings concur with evidence in the literature which assigns the medial temporal lobe an essential role in the formation of spatial and non-spatial long-term declarative memory  “but not for the retrieval of very remote spatial or non-spatial memories” (p.676).  

Episode of Scientific American Frontiers featuring Patient E.P

References
Teng, E. & Squire, L. R. (1999). Memory for places learned long ago is intact after hippocampal damage.Nature, 400, 675-677

Martin, G. N. (2006).  Memory. In Human Neuropsychology (2nd ed.). Essex: Pearson Education Ltd

Week 9 - Frontal Lobes

The frontal lobes

The frontal lobes account for approximately one-third of the cerebral cortex and are implicated in a wide range of behaviours including motor control and social behaviour. Additionally, neuroimaging studies have revealed that the frontal lobes have an important role in executive functions (Martin 2006).  

Executive functions

Executive functions can broadly be described as a set of collaborative cognitive tools which facilitate decision making. According to Martin (2006), these  include attention (divided and sustained), processing speed, initiation, sequencing, set-shifting, cognitive flexibility and planning.  Based on Banich (2009), we can elaborate a little on this somewhat definitive list to include inhibition of familiar behaviours, prioritising and the ability to effectively handle novel situations.   Given the complexity of the decision making process, Banich (2009) argues that the potential components of executive function are actually quite extensive, whixh makes testing executive function as a whole somewhat futile. Tests of executive function are therefore best suited to addressing particular components of it.

Some tests of executive function

Tests of executive function include the Wisconsin Card Sorting Test.  Patients are asked to sort cards according to designs based on colour, shape or number.  At random intervals, the experimenter implicitly changes the rule by which the cards are to be sorted.  The patient should ascertain what the new rule is and begin sorting the cards according to this new principle.   Thus, according to Banich (2009) the WCST is testing inference, attentional set creation and set-shifting ability.  Frontal lobe patients and children under the age of four have difficulty in adjusting to the new rule. 


The Stroop Task requires participants to identify the colour of word without saying the word itself.  For example, where the word ‘red’ appears in blue ink, the participant should say ‘blue’ rather than red.  In this case, it is the inhibitory component of executive function that is called upon to ‘override’ the automatic urge to read the word instead of naming the colour.

Demonstration

Finally, the Tower of London Task assesses the ability to plan, strategise and organise (Martin, 2006). Using a set of discs or beads on pegs, participants are required to plan the shortest number of moves from an initial state to the goal state.  Again, frontal lobe patients show impairments on this test, typically beginning the task immediately without any ‘plan’.

Demonstration


Models

In her article, Banich (2009) highlights some differeing models of executive function based on the understanding that there exists several distinct subcomponets of executive function (such as response inhibtion and updating working memory) which are moderated by specific neurobiological regions.  One such theory offered by Petrides (2005, as cited by Baron, 2009) suggests that "inferior lateral regions of the prefrontal cortex ... maintain information in working memory while others, notably mid-dorso-lateral prefrontal regions, perform executive control operations on that information" (p.91).

Banich (2009) suggests that in an effort to further and better explore potential interventions to address executive dysfunction, these competing models might best be integrated to some degree to broaden our understanding of what is a complex and continually evolving field.


References

Banich, M. T. (2009). Executive frunction: the search for an integrated account. Current Directions in Psychological Science, 18, 89-94

Martin, G. N. (2006).  The frontal lobes: cognition, social behaviour and personality. In Human Neuropsychology (2nd ed.). Essex: Pearson Education Ltd

Week 8 - Hemispheric lateralization of function

At first glance, the left and right cerebral hemispheres of the human brain look to be almost perfectly symmetrical.  However, there are in fact anatomical and functional asymmetries.

Functional asymmetry

Functional asymmetry is not simply a case of one half of the brain being solely responsible for a given function.  Rather, there tends to be a predominance of a particular function in either the left (LH) or right (RH) hemispheres.   Language is said to be “the most clearly lateralised higher function” (Martin, 2006, p.160) - Broca’s area and Wernicke’s area in the LH are known to be involved in language processing.  Lesions to either of these areas in the LH are associated with aphasia – a deficit in the communication (Broca’s aphasia) or comprehension (Wernicke’s aphasia) of language.    The LH is also associated with mathematical ability and analysis as well as complex and ipsilateral movement (Sterling & Elliot, 2008).

The RH is understood to be dominant in visuospatial ability, a function said to be more lateralized (in the RH) in men than with women.  This may explain why men appear to be better at mental rotation of objects.  The RH is associated with pattern recognition, perceptual tasks and creativity.  Patients with RH damage are shown to have impairments in spatial orientation, mental rotation and face recognition. 

Anatomical asymmetry

Looking from above, the right frontal lobe is wider than the left frontal region and extends further forwards by several millimetres whilst the left occipital lobe extends further back than the right occipital lobe.   The RH is larger and heavier than the LH.  The planum temporal (which encompasses Wernike’s area) is larger on the left.  Two Heschl’s gyri (primary auditory cortex) are found in the RH as opposed to just one found in the LH.  The Sylvian fissure (the dividing line between the frontal and temporal lobes) is larger in the LH.

The corpus callosum

As briefly outlined above, there is clear evidence of relatively greater localisation of function in either the LH or RH.  However, the two hemispheres communicate with each other through the corpus callosum – the largest bundle of nerve fibres in the nervous system (Colman, 2006).  This communication between the hemispheres is clearly illustrated by looking at visual input to the brain. The right side of the visual field projects to the visual cortex of the LH – the reverse in found of the left visual field.  In spilt brain patients (where the corpus callosum is severed, typically to address epilepsy) the two hemispheres are in effect isolated and unable to communicate. 

Split brain experiments

In a series of experiments with split brain patients in the 1960s, Roger Sperry and colleagues were able to demonstrate task specialisation in each hemisphere.  Patients were asked to fixate on a spot in the middle of a screen.  Words flashed on the right side of the screen (received by the LH) were correctly reported verbally by the patients.  However, words flashed on the left side of the screen (received by the RH) could not be verbalised by the patient.  In fact, patients reported that they hadn’t seen anything at all.  Despite ‘not having seen anything’, they were able to correctly identify and pick up the corresponding item with their left hand when asked to choose from a selection.

References

Martin, G. N. (2006).  Hemispheric localization and lateralization of function. In Human Neuropsychology (2nd ed.). Essex: Pearson Education Ltd

Stirling, J. & Elliot, R. (2008). Lateralisation. In Introducing Neuropsychology (2nd ed.). Hove: Psychology Press

Week 6 - Emotion

Article:
Bechara, A., Damasio, H., Tranel, D., & Damasio, A. H. (2005). The Iowa Gambling Task and the somatic marker hypothesis: some questions and answers. Trends in Cognitive Sciences, 9, 159-162

During an average day, most of us are faced with myriad decisions to make - frequently mundane and simple of course but occasionally more involved and complex. Whatever the issue, choices need to be made and many of us arrive at decisions in a reasonably straightforward way – most often by weighing up the pros and cons. However, an additional element in almost all decision making is that of 'gut feeling' - intuitively ‘knowing’ what the best option is based on how it makes us ‘feel, either consciously or outside of our awareness. This is the cornerstone of the Somatic Marker Hypothesis, proposed by eminent researcher Antonio Damasio. 

With elements of risk, uncertainty, reward and punishment, Bechara et al. (1994) devised the Iowa Gambling Task in an attempt to simulate the ‘real life’ decision-making process. The IGT was employed to test patients with ventromedial prefrontal cortex (VMPC) damage against healthy controls.  Although VMPC patients appear to have normal intellectual function, advantageous decision making is shown to significantly compromised in this task.  Briefly, given that the role of the VMPC in moderating emotional response is well documented in the literature, Bechara et al. argue that (based on SMT) damage to this area leads patients to make poor decisions with no apparent regard for future consequences.

Although a popular paradigm employed by several researchers, the IGT is not without its detractors.  The article by Bechara el al. (2005) listed at the head of this page is a response to one such criticism.  Maia and McClelland (2004), argue that the findings in the original Bechara et al. (1997)  study are really due to VMPC patients’ inability to deal with contingency reversal, a ‘tool’ central to success in the IGT.

Additional reading:

Maia, T.V. and McClelland, J.L. (2004) A reexamination of the evidence for the somatic marker hypothesis: What participants really know in the Iowa gambling Task. Proc. Natl. Acad. Sci. U. S. A. 101,16075–16080