Activation of the Right Hippocampus

ELEANOR MAGUIRE, RICHARD FRACKOWIAK AND CHRISTOPHER FRITH’S STUDY ON TAXI DRIVERS (1997)

Introduction
Warning: this study uses a lot of big words.

Ever sat in a car and wondered how your mum or dad (or driver, if you’re posh) can drive you around cities without having to stare at a map? How they can recall the correct route when it seems like every street looks the same?

Well, these abilities are all related to various types of memory that we possess.

When we are recalling this kind of information, different areas of our brain are switching on. The question that Maguire, Frackowiak and Frith wanted to answer was which parts of our brain become activated when we are retrieving topographical information from our mind. For example, when we are recalling a route, which part of our brain is helping us? The researchers conducted PET scans on a group of taxi drivers to examine the neural substrates of topographical memory retrieval.

Important terms:

  • Semantic memory – this is concept-based knowledge in our long-term memory. It helps us to do things like give a meaning to otherwise meaningless sentences or words by recalling information. It is not related to our personal experiences, it simply includes general knowledge (e.g. names of animals, shades of colours, capitals of countries).
  • Episodic memory – this is specific and personal knowledge. It is like a recollection of our personal past experiences in life and specific events that happened (e.g. remembering your first day at work or school, remembering your fifth birthday party, remembering the name of your first pet).
  • Topographical memory –the ability to remember routes previously used and recognise familiar places. So the next time you get lost in Islamabad, blame your topographical memory for failing.
  • Sequencing memory –the recollection of experiences or events in an ordered sequence.
  • Hippocampus – a part of the brain’s limbic system. Humans and other mammals have two hippocampi, one on each side of the brain. It is located in the medial temporal lobe of the brain, under the cerebral cortex. It is involved in memory processes.

The limbic system

Aims

  1. To examine semantic topographical memory
  2. To determine whether the recall of well-established spatial layouts activates similar brain regions found in other studies to subserve episodic topographical memory, particularly to the medial temporal region.
  3. To assess the neural instantiation of landmark knowledge without any location information within a large-scale spatial layout.
  4. To examine topographical memory(landmarks, spatial layouts) and non-topographic semantic memory retrieval to find out if common brain regions subserve semantic memory, regardless of the memory type.

Sample
The sample was made up of 11 right-handed males ranging in age from 38 to 52. They were all qualified, licensed taxi drivers in London. None of them had a history of any psychiatric or neurological illness.

All the volunteers (meaning this was a self-sampling technique) gave written consent to be experimented on and the study itself was approved by a local hospital ethics committee. It was conducted under the Administration of Radioactive Substances Advisory Committee’s certification.

Procedure
Before the scanning and science-y stuff began, all participants completed a questionnaire that asked about the following: areas of London with which they were most familiar, films (from a list of 150 colour films from 1939 onwards) that they would rate as very familiar, individual landmarks (from a list of 20 world-famous ones) that they had visited in person and could visualise in their mind’s eye.

The results of the questionnaire formed the basis of the study: routes from areas in London that all participants were familiar with were used as stimuli in the Routes Recall task; films described as very familiar by all participants were used as stimuli in the Film Plots and Film Frames tasks; famous landmarks selected by all participants as places they had not visited but could visualise were also used.

With regards to the films, the participants were very familiar with them but they could not remember where, when or with whom they actually saw the movie. This prevented spatio-temporal references and allowed focus to stay on semantic memories. This is also why the landmarks were places that had not been personally visited by the participants. There were six experimental tasks, and every task was performed twice. Tasks were either topographical or non-topographical, as well as sequential or non-sequential.

The topographical tasks: 

(a.) The taxi drivers were given a starting point and destination point in greater London. They had to describe the shortest legal route between the two points overtly during the scan. This was topographical and sequential.

(b.) The taxi drivers had to recall and describe the appearance of individual world-famous landmarks outside of London. This was topographical and non-sequential.

The non-topographical tasks: 

(a.) The taxi drivers had to recall and describe the plots of familiar famous films between specific points of the storyline. This was non-topographical and sequential.

(b.) The taxi drivers had to recall and describe individual scenes from famous films. This was non-topographical and non-sequential.

When the actual experiment began, participants were blindfolded and speech was recorded. A baseline task was used as a control for the quality of speech output. In this, participants repeated two four-digit numbers during the scanning. During the whole experiment, participants received 12 infusions of H2 150 given after eight minute intervals.

This experiment had a factorial design; there were two factors of interest: the type of memory (topographical, non-topographical, sequential, non-sequential) and the baseline condition.

During each PET scan, a stimulus item was presented: either a route, film plot, film frame or landmark. The subjects response was recorded for examination.

After all the participants had their brain scanned, they were debriefed.

Results
Here’s a summary of the results to calm your brain:

  • All participants spoke for approximately the same amount of time.
  • There was a high level of accuracy in all tasks (e.g. street names and landmarks were recalled correctly).
  • In the navigation task, the routes chosen by participants were very similar.
  • During tasks, participants said that they could visualise the paths, landmarks and films.
  • In every comparison of an experimental task against the baseline task, the cerebellum and the left temporal lobe became activated regardless of which experimental task was taking place.
  • The right hippocampus was only activated during the Routes Recall task.
  • Retrieval of landmark knowledge and retrieval of route information (both topographical tasks) activated many similar brain regions.
  • The network of brain regions that support semantic topographical memory retrieval supports the learning of new complex spatial layouts along with the retrieval of recently acquired topographical memories.
  • The network of brain regions showing increased activation during retrieval of semantic topographical memory are different to those activated during retrieval of semantic non-topographical memory.
  • There was no change of hippocampal activity in relation to sequencing.

A comparison between brain activity during the Routes Recall task and brain activity during the baseline task revealed that the hippocampus was activated and bilaterial activity occurred in the following:

  • Extrastriate regions
  • The medial parietal lobe
  • The posterior cingulate cortex
  • The parahippocampal gyrus.

A comparison between brain activity during the Landmarks task and brain activity during the baseline task revealed that the hippocampus was not activated but the following regions were activated:

  • The posterior cingulate cortex
  • The medial parietal lobe
  • Occipitotemporal regions (including the parahippocampal gyrus).

A comparison between brain activity during the Film Plots and Film Frames task and brain activity during the baseline task revealed that the following were activated:

  • Left frontal regions
  • Middle temporal gyrus
  • Left angular gyrus.

Detailed comparisons of tasks were made within the factorial design framework. This determined two things: (a.) simple main effects within memory type and (b.) compound main effects across memory type and across the sequencing factor. The interaction between memory type and sequencing was also examined.

With regards to simple main effects, a comparison of brain activity during the Routes Recall task against any other experimental task showed that the right hippocampus was activated significantly more.

The compound main effects showed us that during topographical to non-topographical memory retrieval (e.g. recalling routes and then recalling the four-digit number during the baseline task), the following areas were activated:

  • Bilaterial medial parietal regions
  • The posterior cingulate cortices
  • Fusiform gyri
  • Parahippocampal gyri.

Also, the effects of sequencing on brain activity were only visible in the bilaterial medial parietal regions – nothing to do with the hippocampus.

Type of research method
This was a laboratory experiment that used scientific equipment and scientific procedures.

Independent variable
The independent variables were the stimuli (i.e. a route, landmark, film plot or film scene).

Dependent variable
The dependent variable was the brain activity.

Strengths

  1. Very high level of control: The experimenters had a large amount of control over the variables which lessened the chance of external factors disturbing the findings. All participants were blindfolded, speech was recorded, information not related to semantic memory was excluded, stimuli were the same for all participants, the whole procedure was standardised, etc.
  2. Good ecological validity: Although the setting was a laboratory, the experimenters designed the tasks in such a way that they mirrored real life. This meant that the participants’ behaviour was probably true to real life, although the setting was artificial.
  3. Replicable: The entire procedure was conducted in the same way for all participants. It can therefore be replicated and the reliability of the results can be checked.

Weaknesses

  1. Unrepresentative sample: Probably one of the biggest weaknesses of this study. The study only experimented on middle-aged men from London.
  2. Small sample size: Since the sample consisted of only 11 people, the results cannot be properly generalised to a wider population. Maybe the results would have been different if the study was conducted on a larger scale.

Ethical issues

  1. Informed consent: This was obtained from all participants in writing.
  2. Deception: There was no deception.
  3. Confidentiality: The identities of the taxi drivers was kept private.
  4. Emotional and physical harm: There was no emotional or physical harm inflicted on the sample. They also had approval from an official ethics committee and hospital.
  5. The right to withdraw: There was nothing stopping the participants from withdrawing.
  6. Debriefing: All participants were thoroughly debriefed about the experiment after it took place.

Reference: Maguire, E. A., Frackowiak, R. S. J. and Frith, C. D. (1997). Recalling Routes around London: Activation of the Right Hippocampus in Taxi Drivers.