Neural substrates of cognitive capacity limitations

Abstract

Cognition has a severely limited capacity: Adult humans can retain only about four items "in mind". This limitation is fundamental to human brain function: Individual capacity is highly correlated with intelligence measures and capacity is reduced in neuropsychiatric diseases. Although human capacity limitations are well studied, their mechanisms have not been investigated at the single-neuron level. Simultaneous recordings from monkey parietal and frontal cortex revealed that visual capacity limitations occurred immediately upon stimulus encoding and in a bottom-up manner. Capacity limitations were found to reflect a dual model of working memory. The left and right halves of visual space had independent capacities and thus are discrete resources. However, within each hemifield, neural information about successfully remembered objects was reduced by adding further objects, indicating that resources are shared. Together, these results suggest visual capacity limitation is due to discrete, slot-like, resources, each containing limited pools of neural information that can be divided among objects.slot model | interference model | attention | hemisphere D espite the remarkable power and flexibility of human cognition, our working memory-the "online" workspace that most cognitive mechanisms depend upon-is surprisingly limited. An average adult human has a capacity to retain only four items at a given time (1-3). This capacity is fundamental to cognition: Individual variability in capacity is highly correlated with fluid intelligence (4-6) and patients with neuropsychiatric disorders often have a reduced capacity (7,8). Because it is so basic to cognition, capacity limitation has been well studied in humans (9), particularly the capacity limitation of visual shortterm working memory (for reviews see refs. 1 and 4). This work has led to several competing theories about the neural basis of capacity limitations. "Discrete" models suggest that capacity limitations reflect a limit in the number of objects that can be simultaneously represented (3, 10, 11). "Flexible resource" models predict that only the total amount of information available is limited, with information divided among all represented objects (12)(13)(14). It is also not clear whether the limitation is in stimulus encoding or in maintenance (15).To better understand the neural basis of capacity limitations we simultaneously recorded from single neurons in the prefrontal and parietal cortex of two monkeys trained to perform a typical human test of cognitive capacity: change localization (Fig. 1A). Two arrays of objects (colored squares) were separated by a short memory delay. In the second array, the color of a randomly chosen object (the target) was changed. Monkeys were trained to detect this change and make a saccade to it. Cognitive load was increased by varying the number of objects in the arrays from two to five. We recorded simultaneously from multiple electrodes in the frontal cortex [lateral prefrontal cortex (LPFC) and frontal eye fields (FEF)] and ...

Human short-term spatial memory: Precision predicts capacity

“…Here, we propose that the precision of the neural coding of information in short-term memory contributes to variations in memory capacity. This view is consistent with electrophysiological recordings in monkeys showing that visual capacity limitations appear immediately upon stimulus encoding, in a bottom-up manner (Buschman, Siegel, Roy, & Miller, 2011). Accordingly, our behavioral data suggest that human subjects' memory for individual locations is accurate but imprecise.…”

Prefrontal cortex and sensory cortices during working memory: quantity and quality

“…A similar quantity limitation has also been revealed in monkey neurophysiological studies [46,47] . In addition, studies have shown that neurons in the PFC of numerically naive monkeys tune to a preferred numerosity [48] , independent of sensory modality [49] .…”

“…Charron and Koechlin have proposed that the frontal lobes in the two hemispheres represent two concurrent goals [50] . Similar hemispheric limitation has also been proposed through neurophysiological data from non-human primates [47] . Further, the frontal lobe in each hemisphere can be subdivided according to the abstraction Neurosci Bull April 1, 2015, 31(2): 175-182 178 of processed goals, which may result in different capacity limits for each hierarchy [51] .…”

“…This limit has been attributed to activity in parietal areas [41][42][43] . However, further studies have shown that high-capacity individuals are more effi cient at fi ltering out irrelevant items, while low-capacity individuals cannot efficiently filter out such distractions [44] .This fi ltering ability seems to be particularly critical since the high-and low-capacity groups tend to have similar capacity limits counting the number of both targets and distractors [44] .The PFC has been shown to control accesses to WM [45,46] and can then guarantee the quantity of task-relevant items in WM.A similar quantity limitation has also been revealed in monkey neurophysiological studies [46,47] . In addition, studies have shown that neurons in the PFC of numerically naive monkeys tune to a preferred numerosity [48] , independent of sensory modality [49] .…”

Causal Mathematical Logic as a guiding framework for the prediction of “Intelligence Signals” in brain simulations

“…The proposal here is that the hippocampus has a gearing mechanism, except the number of gears is as high as the number of phase timings which can integrate and decouple. Such a number has been defined for STM as correlated to the theta -gamma interaction intitally as 7 +/-2 ( Jensen and Lisman, 1996a) and more latterly as 2 items per hemisphere (Buschman et al, 2011). To bolster the gearing concept as present in critical transition areas between the cortex and limbic system, hippocampal place cells are also proposed as speed-controlled oscillators (Geisler et al, 2007).…”