cognitive neuroscience
developingintelligence | June 21, 2007Though anatomically heterogenous, the human prefrontal cortex seems to perform a rather general function: it actively maintains context representations to guide and control behavior. What, then, is the reason for the anatomical diversity within this region of the brain?
Some theories suggest that prefrontal cortex (PFC) is organized to represent increasingly protracted contexts. According to this "temporal cascade" model, the most posterior regions of PFC are responsible for maintaining only the most current contextual information (e.g., "the doorbell rang") whereas progressively more…
developingintelligence | June 18, 2007How does your brain represent the feelings and thoughts that are a part of conscious experience? Even the simplest aspects of this question are still a matter of heated debate, reflecting science's continuing uncertainty about "the neural code." The fact is that we still don't have a clear picture of the ways in which neurons transmit information. Here's a quick guide to current theories, beginning with well-established theories and moving into ideas that are considered more theoretical.
The canonical model: firing rate
Clearly, neurons encode some information in the rate of their firing…
developingintelligence | June 13, 2007What was your 6th birthday party like?
If you successfully retrieved that memory, you may now be ever so slightly less able to remember your other childhood birthdays. A variety of behavioral evidence has shown that such "retrieval induced forgetting" of strongly competing memories is fundamental to memory retrieval. In a new article in Nature Neuroscience, Kuhl et al. provide neuroimaging evidence which ties retrieval-induced forgetting to activity in prefrontal cortex.
In the study, subjects studied a series of 240 word pairs, for example ATTIC-DUST, ATTIC-JUNK, or MOVIE-REEL. During…
developingintelligence | June 11, 2007Symbols redirect attention - in some ways, that is their intended purpose - but this "reorienting" is a surprisingly literal and involuntary effect. Even when we know symbols are irrelevant to our current circumstances, they still influence our behavior.
A simple experiment demonstrates this nicely. Hommel et al. showed that letters appearing in unpredictable locations can be identified more quickly when their location is compatible with a preceeding symbol - even if those symbols are totally irrelevant to the task, and subjects are explicitly told to ignore them!
For example, a letter…
mixingmemory | June 8, 2007About a year ago, there was an article in Seed Magazine titled "Seduced by the Flickering Lights of the Brain," in which Paul Bloom argued that people are too easily seduced by neuroscience, believing that it made for good science, even when it doesn't. At the end of the article, Bloom mentioned a then unpublished study in which participants were more impressed with bad scientific explanations if they contained a bit of irrelevant neuroscience. Well, now the study, which is by Weisberg and a bunch of other people (apparently to write a paper about neuroscience, you have to have as many…
developingintelligence | June 7, 2007The neural processing of color, shape, and location appears to be widely separated in the brain, and yet our subjective experience of the world is highly coherent: we perceive colored shapes in particular locations. How do these distributed representation about visual features get brought or "bound" together to form an integrated percept?
Charles Gray suggests that there are actually many such binding problems - not only between visual features, but between other sensory modaliities as well as between perception and action. The brain may have evolved redundant mechanisms for solving this…
developingintelligence | June 6, 2007Some people experience an intermingling of the senses, known as synaesthesia, in which certain shapes become combined or "bound with" certain colors, or that certain colors are strongly associated with certain sounds. Of course, in healthy normal adults, color and shape become bound together only when visible shapes really are visibly colored - a process which is itself still a mystery of neuroscience. Yet similar mechanisms may be involved in both these kinds of "binding," but are merely more active in synaesthetes than in normal adults.
Based on Patient RM, we know that binding between…
developingintelligence | June 5, 2007How do you know what's where? Cognitive psychologists believe this requires "binding" - the ability to associate certain colors or shapes with each other and with certain locations in space, giving rise to a coherent perception of the world. It turns out that at least one unlucky person lacks this ability we often take for granted: the patient is known as RM, as reviewed by Lyn Robertson in this article on binding.
RM suffered two strokes, damaging both sides of his occipito-parietal cortex (see the image above). This region of the brain is known to be important for spatial computations;…
developingintelligence | June 4, 2007Suppose that one day your computer's hard drive stops working, but everything else about the machine is fine. Your friend has an identical computer in which the hard drive works fine, but the keyboard suddenly stopped working. Based on this "double dissociation" between the two different problems, can you safely assume that the "hard drive system" and the "keyboard system" rely on distinct underlying mechanisms?
For years, cognitive neuropsychologists have felt safe in making equivalent assumptions about brain damage. If one type of damage leads to difficulty on task A, but not task B, and…
developingintelligence | May 31, 2007Imagination allows us to escape our current time, place, or perspective in favor of an alternative context, whether that may be fanciful or mundane. So imagination is a mechanism for specifying and maintaining a context that differs from our more immediate and stimulus-driven experiences or contexts (at least, that is what I mean by "imagination"). According to Buckner & Carroll, this kind of "self-projection" from one context to another is the essential function that underpins the involvement of the prefrontal & medial temporal lobe (MTL) circuit in a variety of tasks, including…
developingintelligence | May 30, 2007While the "modal model of memory" is still widely taught and accepted as a general theory, an enormous amount of recent research has focused on how short-term memory enables higher cognitive processes like those involved in planning, goals, and executive functions. Yet this research has revealed surprisingly intricate links between short- and long-term memory. Increasingly, it appears that interactions among prefrontal areas (traditionally thought to be important for short-term memory) and medial temporal lobe areas (traditionally thought to be important for long-term memory) are important…
developingintelligence | May 29, 2007The capacity to use and manipulate symbols has been heralded as a uniquely human capacity (although we know at least a few cases where that seems untrue). The cognitive processes involved in symbol use have proven difficult to understand, perhaps because reductionist scientific methods seem to decompose this rich domain into a variety of smaller components, none of which seems to capture the most important or abstract characteristics of symbol use (as discussed previously).
So, it's important to specify how the simpler and better-understood aspects of symbol processing may interact and give…
developingintelligence | May 24, 2007Many will agree that algebra is difficult to learn - it involves planning, problem-solving, the manipulation of symbols, and the application of abstract rules. Although it's tempting to imagine a specialized region of the brain for each of these processes, they may actually recruit roughly the same widely-distributed and general-purpose "task network" of brain regions. The individual contribution of each region has been, and continues to be, a matter of much debate.
However, the functional specialization of each brain region may be best understood as fulfilling a particular balance between…
developingintelligence | May 16, 2007The analytic depth of cognitive neuroscience is, in many ways, a curse. Those aspects of high-level cognition most relevant to real-world applications are the least understood at a neurobiological level, and those mechanisms that are well-understood neurobiologically are too simple to inform real-world practices.
The explanatory gaps between these levels of analysis is a result of hyper-reductionism in science, itself rooted in a lasting preference (reverence?) for the simplistic and "parsimonious." But natural phenomena, like the emergence of behavior from the brain, are ultimately more…
developingintelligence | May 15, 2007It could be argued that any single level of scientific analysis is at once too simple (since there are always important emergent phenomena at higher levels) and also too complex (poorly-understood phenomena inevitably lurk at lower levels). If I wanted to kick the sacred cow of science again, as I did yesterday, I'd suggest that parsimony can be a misleading principle here too: whatever data is used to evaluate a theory may not include phenomena occurring at other levels of analysis that may be relevant to the theory. So inevitably, the simplest theory (i.e., the one with the fewest…
developingintelligence | May 9, 2007"Where is it, this present? It has melted in our grasp, fled ere we could touch it, gone in the instant of becoming." - William James, 1890
Extremely dangerous, traumatic, or surprising moments are often accompanied by reports that time seemed to "slow down" or "fly by." The perceptual basis of these subjective temporal distortions is unclear, but not for lack of trying: one recent experiment went so far as to drop subjects off a 400 foot tower while testing their ability to decipher a rapidly flashing string of numbers - a test of perceptual processing speed. Unfortunately, it didn't work…
developingintelligence | May 7, 2007Hemispatial neglect might be the most striking example of brain trauma's cognitive effects: patients with damage to right parietal regions appear unaware of the left half of space. For example, they'll often shave only the right side of their face, will only eat food from the right half of their plate, and when asked to copy a variety of drawings will include only their right half.
As you can tell from these examples, right parietal cortex is particularly important for our understanding of space. Although left parietal cortex may be involved in similar computations, the right-sided region…
developingintelligence | May 3, 2007In his famous essay, Thomas Nagel suggested that science's reductionist methods can never provide a complete understanding of the "subjective qualities" of consciousness. To illustrate this problem, he wrote that there was "no reason to suppose that" we would ever be able to comprehend what it's like to be a bat - because we can't truly understand the subjective experience of, for example, echolocation.
Ironically, scientific advances in "sensory substitution" technology have demonstrated that it's possible to simulate (or stimulate) one modality (sight, hearing, touch) with sensory data…
developingintelligence | May 2, 2007What neural mechanisms underlie "fluid intelligence," the ability to reason and solve novel problems? This is the question addressed by Gray et al. in Nature Neuroscience. The authors begin by suggesting that fluid intelligence (aka, gF) is related to both attentional control and active maintenance of information in the face of ongoing processing (i.e., working memory). Each of these concepts, in turn, has been associated with the functioning of the lateral prefrontal cortex - a region that has been massively expanded in humans compared to even our closest evolutionary relatives.
To…
developingintelligence | April 27, 2007Early neuropsychology research indicated that long-term memory and short-term memory were separable - in other words, long-term memory could be impaired by damage to the hippocampus without any corresponding deficits in short-term memory. However, this idea has come under scrutiny in recent years. Neuroimaging technology has demonstrated that the same network of brain regions is active in both long-term and short-term memory tasks, suggesting that these regions may interact more than previously assumed. As noted in Speer, Jacoby & Braver's 2003 article, estimates of either type of…