A Map to Attach Memories to Kate Jeffrey

Niall McLaughlin and Yeoryia Manolopoulou speak to Professor Kate Jeffrey, a cognitive and behavioural neuroscientist at UCL. She is interested in how the brain manifests spatial knowledge: how the hippocampus processes sensory information, and uses it to construct a representation, or ‘map’ of a person’s surroundings. Kate explains that this research is beginning to show a relationship between this ‘map’ and how memories are made and recalled, and that she finds this relationship fascinating: how is the connection formed, at a cellular level? Kate’s particular interest lies in how information is transferred from the primary sensory areas of the brain – e.g. the visual cortex (which receives its information from the retina) – to the hippocampus. Kate views these primary sensory areas as ‘the beginnings of perception’; the brain has received information, but has yet to begin to cultivate an understanding of it. ‘It’s more or less a copy’, she says. Then, as the information passes through various brain structures, ‘each structure does something with it to elaborate the representation’.

To explain how we acquire spatial knowledge, Kate first sets out the means, on a neurological level, by which we amass knowledge generally. Neurons communicate by firing nerve impulses/action potentials, which travel physically along a projection, called an axon. Upon reaching the end of the axon, the impulse jumps across a gap to activate the next neuron down: an active neuron passes information to the next neuron, interacting with various other active neurons in the process, and in doing so, that information is transformed. So, the eye sees something, and the ear hears something, and the brain combines the information to draw a conclusion: ‘that sound must be coming from that object’. Kate elucidates: ‘much of what the brain is doing is making these associations…much of what knowledge is, is making associations between things in the outside world.’

The hippocampus seems to be the place where all of this incoming information is synthesised, and then used by various parts of the brain. According to Kate, it seems that the spatial part of the brain uses this synthesised information to create a map, and the memory part of the brain uses that map to attach memories to. 'When you go back to a place you’ve been to before,' she explains, 'all the familiar landmarks, in their familiar places, reawaken memories of all the things that happened to you the last time you were there' Furthermore, humans have the capacity to mentally visit a place and re-awaken memories: 'in a sense, that’s what memory recall is – you reactivate the place where the memory happened’.

This leads Niall to ask whether memory can occur without 'some conception' of place. Kate says that it’s dependent on the type of memory: to enter a pin number, or drive a car, we don’t engage the hippocampus. Even people without a hippocampus – those with hippocampal dementia, for example – can still complete these procedural tasks, because they depend on a different brain system, called the striatum. Declarative memory – memories we think about, that we can talk about – does depend on the hippocampus, and according to Kate, we still can’t be entirely sure if this is inextricably linked to place or not: ‘Can I remember what I had for breakfast, without mentally transporting myself back to the kitchen with my bowl of cereal?' Without a spatial map, would it still be theoretically possible to retrieve the other parts of the memory?

In discussing neurological associations, Kate introduces the notion of a ‘grandmother cell’: a neuron that fires specifically for one’s grandmother. She explains that while it is unlikely that an individual cell or neuron relates to a particular person, the Cell Assembly Hypothesis proposes that a memory of (or interaction with) a person activates a particular pattern of neurons to fire throughout different regions of the brain, and that any one neuron may participate in various memories. Niall asks if there are dedicated neurons devoted to the ‘grandmother’ – or if the grandmother induces the firing of a particular pattern through a matrix of generally neutral neurons? Kate explains that it is difficult to answer this conclusively – it depends what part of the chain of processing you examine, and means of examination are limited. We know, for example, that a neuron in the retina may respond only to the presence or absence of light – a simple, binary reaction. Further down the path to the hippocampus, there may be neurons that fire only when activated by particular kinds of things, or the convergence of certain qualities. Further again down this route, there may be neurons that fire only when a number of distinct characteristics – of height, appearance, voice, etc. – converge: those of your grandmother, for example. Though a ‘Jennifer Aniston’ cell has been detected in monkeys – a cell which fires for Jennifer Aniston, no matter how old the image of her of the length of her hair – it has yet to be proven that such a cell is not activated by other, very similar looking people; or that the cell would activate upon hearing Jennifer Aniston speak, or thinking of her.

Delving further into the concept, Yeoryia asks if there are different cells for ‘grandmother’ or ‘bedroom’ at different times of day. Kate says that people are starting to try to explore whether the hippocampus incorporates code for time, as well as space – what is its role in episodic memory? Place is static; episodic memory is fleeting. It can be hard to reconcile these two, seemingly opposed, states. Kate explains that there are cells which appear to experience peaks and troughs in activity, and that this may be the temporal system: some neurons may cease to fire in response to a given stimulus after a certain period of time has elapsed. Perhaps the volume of cell reactions is a way for the brain to discern how much time has passed since you were last in a particular place?

Niall asks how our different forms of ‘knowing’ space – including place cell information, egocentric perception and proprioceptive memory – are synthesised: is there a ‘master map’? Kate explains that we aren’t sure, yet, but that she is interested in exploring the notion of a master map, and where this might be in the brain. It may be that this occurs in the hippocampus, but it’s also possible that another part of the brain consults the hippocampus, creating the ‘master map’ somewhere else. In the hopes of better understanding how this works, Kate is starting to look at the retrosplenial cortex, which has significant connections to the hippocampal system. She says that the retrosplenial cortex has been shown to be very involved in navigation, but that as yet, its specific function has not been identified. She suggests that, perhaps, it is responsible for relating the mosaic fragments of your internal ‘map’ together, in terms of their directional relationship to each other.

Kate, Niall and Yeoryia then begin to discuss the less formal, pragmatic aspects of spatial perception. Niall says that architects often think about space as a neutral medium, in its extension, or dimension, or boundary. In truth, however, even a child acquiring its sense of space never does so ‘innocently’: the acquisition is bound into the taboos, desires and permissions around that space. There may be a room she may not go into, a thing she cannot reach, and so on. Adults carry the memory of all these previously acquired spaces and, presumably, it is through that lens that all future spaces are experienced. Kate agrees that while neurons such as place cells seem to be entirely indifferent to the more superficial aspects of experience, there must be others that relate to more associative, less empirical aspects of memory. These are, currently, harder to define or isolate, but our way of placing ourselves in the world is surely influenced by our personal history. 

Kate says that people are just beginning to explore the emotions around spaces. We know that primal responses such as fear are governed by the amygdala, and that the amygdala and hippocampus have lots of connections in both directions. So, if you enter a space and have a fearful experience, the hippocampus is forming a representation that is connected to fear in the amygdala.

In terms of how the brain organises space, Kate explains that well-functioning grid cells arrange themselves in regularly-spaced hexagonal matrices of action potentials. The position of the grid cells in a room are generally determined upon first entry, are specifically aligned to that room, and are subsequently consistent in their position. However, if you continually walk back and forth, back and forth between two rooms, the grid cells for these rooms will gradually adjust into a more harmonious relationship with each other. So, as Yeoryia puts it, our brains are ‘constantly drawing’; continually redrafting our understanding of the world around us.

According to Kate, one of the earliest symptoms of Alzheimer's is often spatial disorientation. In studying cases of Alzheimer’s in animals, it has been found that their spatial representation may be quite messy and undefined: the grid cells lack their characteristic regularity, and the ability to build a reliable map begins to degrade. Weakening synaptic connections may mean that when you re-enter a known environment, the associated cells can’t fire, so you don’t recognise it. Kate says that ‘the capacity of your brain to reconstruct the representation that it had made in service of memory recall’ is often one of the first brain functions to fail.

When designing for those whose spatial abilities are in decline, Kate stresses the importance of facilitating global orientation. Head-direction cells [also featured in the Dialogue with Hugo Spiers] tell you which way you are facing, informed by a combination of movement and visual cues. Kate believes that they tell the grid cells how to line up. If a person whose head-direction cells are damaged enters a new room, and is not immediately oriented through explicit visual cues to the space they were previously in, the brain may construct a flawed representation of the relationship between the two spaces. Even in individuals without dementia, laneways, alleys and extended, spiralling circulation routes can disrupt the functioning of head-direction cells.

Studies are beginning to show that problems with head-direction cells and issues with amnesia often occur in the same regions of the brain. This suggests that there may be a profound link between memory and place. Kate says that this concept is not new: ‘even the ancient Greeks and Romans used spatial strategies to remember things’, but that we are yet to prove why, or how it developed. Kate offers a theory, saying that we evolved memory ‘to help us act in the world; to act again in a place where you’ve been before. Memory, really, is there to help you exploit space – and maybe that’s why they evolved together?’