University Studies 15A: Consciousness I Memory

Planning for the Last Day of Class: A Concluding Debate It seems like I really ought not be talking at you all the time, so I would like to use the last day of class as an opportunity to set up a multi-sided debate about the usefulness of neuroscience in explaining consciousness. I believe there is a wide range of positions one might take on the matter, and of course for the final paper I will ask you to set out your particular position in relation to visual consciousness, so that you can use the arguments you develop in your paper. I would like you to discuss with your classmates, identify other people who hold similar positions, and form a group to present your arguments To make this easier (?), I have activated the message board for the class:

Memory As Baars and Gage make clear, memory is not a single entity. There are a number of important subtypes made possible through different brain systems: 1. Working memory: usually functions over a period of seconds 2. Episodic memory: first-person memory of singular events 3. Semantic memory: memory about things and events generalized from episodic memory 4. Procedural memory: memory about how to do things

Does anyone recall whose picture appeared at the beginning of the lecture? If I had flashed it at a faster rate and immediately covered it (masked it) with another picture, you still would have what is called an “implicit memory” that could be tested by asking you to guess the picture. There are other forms of implicit memory that mostly focus on patterns extracted from more explicit forms of memory but about which one has no conscious awareness. Knowledge of the grammar of one’s native language is perhaps the best known example. I will not worry about the distinctions between implicit and explicit memory during my presentation.

Working Memory If you read Baars and Gage carefully, you may have noticed a rather different tone to their discussion of memory and a shift into different level of analysis. They become far more tentative: the account of working memory is very much a work in progress. For example:

The abstract for the paper, published this year, states: Working memory refers to keeping track of ongoing mental processes and temporary memory. One hypothesis is that this form of memory consists of multiple domain-specific components…. [Later] advances have often been interpreted as supporting the alternative hypothesis that working memory consists of a single, limited-capacity domain-general system for control of attention…. I argue that the multiple-component perspective and the single- attentional-system perspective are complementary, with each best suited to asking different research questions, and that many areas of contemporary debate regarding the nature of working memory reflect differences that are more apparent than real. 1. Working memory refers to keeping track of ongoing mental processes and temporary memory. 2. One hypothesis is that this form of memory consists of a variety of subsystems that work on different modalities: vision, sound, movement, etc. 3. Another hypothesis is that working memory is a form of executive function.

People consider Working Memory (WM) to be a collection of various subunits because one can disrupt visual WM with a visual task (following a dot moving on the screen) and verbal WM with a verbal task (repeating the word “the”). However, one cannot disrupt verbal WM with a visual task nor visual WM with a verbal task. These are observations at the level of psychophysics, which (to quote Wikipedia ) “quantitatively investigates the relationship between physical stimuli and the sensations and perceptions they effect.” When one shifts to the neuroscience, several questions immediately arise: 1. How does the brain keep that memory (i.e., the pattern of firing of neurons) intact for the required period? 2. What, for that matter, is that memory? What neurons are firing where? 3. How do you store that visual image of Einstein, for example? We know something about the visual system: one cannot simply copy an “Einstein neuron.” Can we copy and store both the image and the knowledge “That’s Einstein?”

And of course there are the lesion studies which show that if one damages the tempero-parietal region, working memory is severely impaired. Why? Baars and Gage present what is in fact a synthesis of the most recent work on WM as a brain system. They follow Ranganath in proposing:

That is, WM requires focused attention to objects and to relations between objects. These are two separate functions: One keeps activating an object (represented in neural networks for high- level representational spaces like IT) The other keeps together a multi-modal pattern of activation that includes multiple networks These two functions are the roles of the dorsal and ventral PFC

This attentional component seems to be the bottle-neck in working memory: only four separate “objects” can be maintained at a time by the ventral prefrontal cortex. For example, I will show you colored squares, masked by a picture, and you will tell me whether the next set of colored squares has the same pattern.

The first component of working memory is the executive regions in the PFC. How the PFC manages to maintain activation over an extended period appears to be through the sorts of recurrent connections we studied in looking at neural network modeling: Note the role assigned to dopamine in activating the initial cueing rule.

The next component of working memory are the sets of neurons that provide the content to be activated by the PFC executive regions. Note a further division of labor: If the objects and/or relations presented by the sensory input that are to be maintained in WM are completely new, these are being processed by the hippocampus, which contributes to the maintenance network If the objects and/or relations from the sensory cortex are available in established memory networks, these are network are linked to the executive functions in the PFC

Either way, we can tentatively answer one major question: researchers are increasingly confident that memories are links binding together patterns developed through the processes of perception. That is, a memory of a house holds together the patterns that captured what the house looked like as created in the high-order visual cortex in the ITL This logic of binding together the networks used in perceptual processing (which, remember, includes complex multi-modal associations) is true for both Working Memory and episodic, semantic, and procedural memory.

The question, then, is how is the binding done. The short answer is, “We don’t quite know yet, but we’re working on it.” Let’s look at a recent article that stresses the importance of the multi-modal high-order association regions of the posterior parietal lobe, a region (with strong connections to the insula) that is receiving ever greater attention:

The abstract reads (in part): The hypothesis that the neural network supporting successful episodic memory retrieval overlaps with the regions involved in episodic encoding has garnered much interest; however, the role of the posteromedial regions remains to be fully elucidated.… Our results provide further evidence that posteromedial regions constitute critical nodes in the large-scale cortical network subserving episodic memory. The article is part of a larger argument that the posterior parietal cortex is central to integrating all the components that comprise complex human episodic memories and that it remains important as the site of binding for both the process of encoding (creating) the memory and for the reactivation of all the components on later retrieval of the memory.

The posterior parietal cortex provides the means to bind components into memory, but it does not do the binding itself. This is the job of the hippocampus and its associated cortices in the medial temporal lobe: Note how this model for the creation of episodic memory looks extremely close to that for Working Memory. How the hippocampus does its job is a subject of intense debate: some argue it is the site for binding of memories; other argue that it facilitates the binding, which happens elsewhere.

It is clear that sleep plays a part in the process of the creation of memories and that the hippocampus displays distinctive activation patterns during sleep that are crucial for the consolidation of memory. This, however, is a topic for next week. For this week, we have one last issue: what gets remembered.

Remember the diagrams from last week: The dopamine SEEKING system, the amygdala FEAR system, and the other emotional networks including the insula all signal the hippocampus. That is, saliency is a strong component in gaining access to the hippocampus’ memory system. The brain tries to remember what matters.