femme Neuroscience Notes

Some Topics

Memory

Memory involves the retention of experiences by alterations in brain structure and function. Intelligence requires a short-term retention of experience so that links between current events and features stored in long-term memory can be compared. This comparison is the basis for recognizing new events and making quick decisions. The frontal cortex provides a variety of processing areas that hold short and medium term memories and extract features of current events to regulate behavior. There are several short term memory buffers that allow us to carry out routine tasks. For example, we have a buffer function that can store about 7 word or number items plus or minus 2 for a few seconds. A seven digit telephone number is relatively easy to remember but a sixteen digit VISA card number is hard or impossible to recall from this buffer.

The most important characteristic of a short-term store is, clearly, that it retains information for a limited amount of time only. Most definitions of short-term memory limit the duration of storage to less than a minute; no more than about 30 seconds, and in some models as little as 2 seconds. Memory that exceeds short-term memory duration limits is known as long-term memory. All memories decay over time. Short-term means that memory decays over seconds to minutes. If you are trying to remember a phone number, the strategy is to repeat the number out loud or covertly using your selftalk facility. Memory decay is reduced by repeating at regular intervals. When memory tasks involve several components such as digits, words, and pictures there is competition for storage and recall. New content can replace older content unless the older content is refreshed.

Tovee suggested [i] that working memory is temporary information storage that provides continuity between our past experience and our present situation, and allows us to plan ahead for what we are likely to encounter in the immediate future. Deficits in working memory lead to attention deficits and behavioral disorganization. The human frontal cortex is involved in temporary storage and manipulation of information. The ability to sequence tasks is disrupted by forgetting what you just did. Working memory maintains information for a brief period of time, available for all brain modules to use. If you cannot remember what you just did or wanted to do, cognitive performance is impaired; you become disorganized and life becomes difficult. Learning is impaired by the inability to retain an impression of recent events.

Reading, for example, requires words to be held briefly in recent memory so that the meaning of whole sentences and then paragraphs can be assembled. If short-term memory fails to work properly, you cannot understand what you read. Smith and Jonides suggested: [ii] “Working memory includes two components: short-term storage (on the order of seconds) and executive processes that operate on the contents of storage...different frontal regions are activated for different kinds of information. Storage for verbal materials activates Broca's area and left-hemisphere supplementary and premotor areas; storage of spatial information activates the right-hemisphere premotor cortex; and storage of object information activates other areas of the prefrontal cortex.” They [iii] also suggested that there is a: Verbal working memory in the left-hemisphere posterior parietal cortex Subvocal rehearsal component in left-hemisphere speech areas (selftalk), including Broca's area as well as the premotor and supplementary motor areas Spatial working memory mostly in the right-hemisphere spanning posterior parietal, occipital, and frontal cortices, divides into storage and rehearsal regions, with right-hemisphere posterior parietal and premotor regions subserving spatial rehearsal.

In addition they described executive regulation of the processing of working-memory contents. Inhibition of verbal working memory is mediated by the left-hemisphere prefrontal region and that it can be dissociated from verbal storage and rehearsal processes. Ungerleider [iv] et al suggested that working memory actively maintains a representation of events briefly. Jonides et al [v] suggested that parietal regions are part of a network of short-term storage and retrieval of verbal sound representations. They described two divisions of the medial temporal lobe, the perirhinal and hippocampal cortices. The perirhinal cortex judges familiarity. Neurons in the perirhinal cortex respond maximally to first presentations of visual signals but less to subsequent presentations. Individual neurons signal different types of information that may comprise a judgment of prior occurrence. For example, there are recency neurons that know if an event has been seen recently irrespective of whether it has been seen many or few times previously.
Familiarity neurons encode the relative familiarity of an event, making a judgment about how many times the event has been seen previously without regard to recency. Other neurons respond best to novel stimuli. Yet other neurons change their response progressively as initially unfamiliar stimuli are repeated and become more familiar. Wang [vi] stated:: "Delay-period activity of prefrontal cortical cells, the neural hallmark of working memory, is generally assumed to be sustained by reverberating synaptic excitation in the prefrontal cortical circuit… the biophysical properties of cortical synaptic transmission are important to the generation and stabilization of a network persistent state… to achieve a stable persistent state, recurrent excitatory synapses must be dominated by a slow component. If neuronal firings are asynchronous, the synaptic decay time constant needs to be comparable to that of the negative feedback; whereas in the case of partially synchronous dynamics, it needs to be comparable to a typical interspike interval (or oscillation period). Slow synaptic current kinetics also leads to the saturation of synaptic drive at high firing frequencies that contributes to rate control in a persistent state. For these reasons the slow NMDA receptor-mediated synaptic transmission is likely required for sustaining persistent network activity at low firing rates. This result suggests a critical role of the NMDA receptor channels in normal working memory function of the prefrontal cortex."

The prefrontal cortex is important in working memory and damage to this part of the brain causes memory deficits. The prefrontal cortex has connections to most other areas of the brain that process sensory information. Fuster suggests that the prefrontal cortex "serves the overarching function of the temporal organization of behavior" by driving networks that maintain currently important information in an active state. Updating memory is related to mid-dorsolateral prefrontal activation and right inferior parietal region, probably related to a visuospatial strategy used to maintain the information in short-term memory. Information storage within separate sensory systems is brief; less than a second in visual sensory storage. Information that moves into a schematic or conceptual form is maintained for many seconds and can be stored indefinitely in LTM if repeated and recalled in a learning sequence. Visual memory does not involve storing images the way a digital camera stores images. Vision is so information-rich it is real-time processing only. Visual memory consists of feature extractions and a visual equivalent of paraphrasing – a pseudo-image. When you close you eyes, you cannot see a real picture, but you have a sense of what is out there. Recall based on quick exposure to images permits for example the recognition of images already seen in a sequence.

Performance on image recognition tasks decreases with sequence length. Visual recognition depends on features extracted from the visual information and does not require that intact images are preserved in STM and compared in an image processor. Visual working memory involves interaction among the prefrontal cortex and the higher association areas. This activity is modulated by dopamine. Other neurotransmitters, such as acetylcholine play a modulatory role in prefrontal memory function. In monkeys, visual working memory involves posterior areas in visual cortex and anterior areas in prefrontal cortex. In the visual cortex, ventral areas are involved in object vision, and dorsal areas are involved in spatial vision. In the prefrontal cortex, ventrolateral areas are involved in working memory for objects and dorsolateral areas for spatial locations. In humans, object vision has a more inferior location in temporal cortex, and spatial vision a more superior location in the parietal cortex.

[i] Ibid see ref 93
[ii] Edward E. Smith, John Jonides. Storage and Executive Processes in the Frontal Lobes Volume 276, Number 5313 Issue of 2 May 1997, pp. 821 - 824 The American Association for the Advancement of Science
[iii] Edward E. Smith* and John Jonides. Neuroimaging analyses of human working memory PNAS Vol. 95, Issue 20, 12061-12068, September 29, 1998
[iv] Leslie G. Ungerleider* Susan M. Courtney and James V. Haxby Vol. 95, Issue 3, 883-890, February 3, 1998 This paper was presented at a colloquium entitled "Neuroimaging of Human Brain Function," organized by Michael Posner and Marcus E.Raichle, held May 29-31, 1997,sponsored by the National Academy of Sciences at the Arnold and Mabel Beckman Center in Irvine, CA.
[v] John Jonides, Eric H. Schumacher, Edward E. Smith1, Robert A. Koeppe, Edward Awh, Patricia A. Reuter-Lorenz, Christy Marshuetz1, and Christopher R. Willis The Role of Parietal Cortex in Verbal Working Memory . The Journal of Neuroscience, July 1998,18(13):
[vi] Xiao-Jing Wang.Synaptic Basis of Cortical Persistent Activity: the Importance of NMDA Receptors to Working Memory. Jour of Neuroscience, 1999, 19(21):9587-9603