Miller (1956) summarized evidence that people can remember about seven chunks in short-term memory (STM) tasks. How- ever, that number was meant more as a rough estimate and a rhetorical device than as a real capacity limit. Others have since suggested that there is a more precise capacity limit, but that it is only three to five chunks. The present target article brings together a wide vari- ety of data on capacity limits suggesting that the smaller capacity limit is real. Capacity limits will be useful in analyses of information processing only if the boundary conditions for observing them can be carefully described. Four basic conditions in which chunks can be identified and capacity limits can accordingly be observed are: (1) when information overload limits chunks to individual stimulus items, (2) when other steps are taken specifically to block the recoding of stimulus items into larger chunks, (3) in performance discontinuities caused by the capacity limit, and (4) in various indirect effects of the capacity limit. Under these conditions, rehearsal and long-term memory cannot be used to combine stimulus items into chunks of an unknown size; nor can storage mechanisms that are not capacity- limited, such as sensory memory, allow the capacity-limited storage mechanism to be refilled during recall. A single, central capacity limit averaging about four chunks is implicated along with other, noncapacity-limited sources. The pure STM capacity limit expressed in chunks is distinguished from compound STM limits obtained when the number of separately held chunks is unclear. Reasons why pure capacity estimates fall within a narrow range are discussed and a capacity limit for the focus of attention is proposed.

/pdf/the-magical-number-4-in-short-term-memory-a-reconsideration-3704b9phuz.pdf

The magical number 4 in short-term memory: a reconsideration of mental storage capacity.

http://www.ece.uvic.ca/~bctill/papers/facerec/HaxHofGob2000.pdf

The distributed human neural system for face perception.

A typical scene contains many different objects that, because of the limited processing capacity of the visual system, compete for neural representation. The competition among multiple objects in visual cortex can be biased by both bot- tom-up sensory-driven mechanisms and top-down influences, such as selective atten- tion. Functional brain imaging studies reveal that, both in the absence and in the presence of visual stimulation, biasing signals due to selective attention can modulate neural activity in visual cortex in several ways. Although the competition among stimuli for representation is ultimately resolved within visual cortex, the source of top-down biasing signals derives from a network of areas in frontal and parietal cortex.

/pdf/mechanisms-of-visual-attention-in-the-human-cortex-1mxnsqjpz6.pdf

Mechanisms of Visual Attention in the Human Cortex

The vertebrate brain continues to produce new neurons throughout life. In the rat hippocampus, several thousand are produced each day, many of which die within weeks. Associative learning can enhance their survival; however, until now it was unknown whether new neurons are involved in memory formation. Here we show that a substantial reduction in the number of newly generated neurons in the adult rat impairs hippocampal-dependent trace conditioning, a task in which an animal must associate stimuli that are separated in time. A similar reduction did not affect learning when the same stimuli are not separated in time, a task that is hippocampal-independent. The reduction in neurogenesis did not induce death of mature hippocampal neurons or permanently alter neurophysiological properties of the CA1 region, such as long-term potentiation. Moreover, recovery of cell production was associated with the ability to acquire trace memories. These results indicate that newly generated neurons in the adult are not only affected by the formation of a hippocampal-dependent memory, but also participate in it.

Neurogenesis in the adult is involved in the formation of trace memories

We propose an “Enhanced Perceptual Functioning” model encompassing the main differences between autistic and non-autistic social and non-social perceptual processing: locally oriented visual and auditory perception, enhanced low-level discrimination, use of a more posterior network in “complex” visual tasks, enhanced perception of first order static stimuli, diminished perception of complex movement, autonomy of low-level information processing toward higher-order operations, and differential relation between perception and general intelligence. Increased perceptual expertise may be implicated in the choice of special ability in savant autistics, and in the variability of apparent presentations within PDD (autism with and without typical speech, Asperger syndrome) in non-savant autistics. The overfunctioning of brain regions typically involved in primary perceptual functions may explain the autistic perceptual endophenotype.

/pdf/enhanced-perceptual-functioning-in-autism-an-update-and-2uxcjjtfx4.pdf

Enhanced Perceptual Functioning in Autism: An Update, and Eight Principles of Autistic Perception

This paper seeks to bring together two previously separate research traditions: research on spatial orienting within the visual cueing paradigm and research into social cognition, addressing our tendency to attend in the direction that another person looks. Cueing methodologies from mainstream attention research were adapted to test the automaticity of orienting in the direction of seen gaze. Three studies manipulated the direction of gaze in a computerized face, which appeared centrally in a frontal view during a peripheral letter-discrimination task. Experiments 1 and 2 found faster discrimination of peripheral target letters on the side the computerized face gazed towards, even though the seen gaze did not predict target side, and despite participants being asked to ignore the face. This suggests reflexive covert and/or overt orienting in the direction of seen gaze, arising even when the observer has no motivation to orient in this way. Experiment 3 found faster letter discrimination on the side the computerized face gazed towards even when participants knew that target letters were four times as likely on the opposite side. This suggests that orienting can arise in the direction of seen gaze even when counter to intentions. The experiments illustrate that methods from mainstream attention research can be usefully applied to social cognition, and that studies of spatial attention may profit from considering its social function.

/pdf/gaze-perception-triggers-reflexive-visuospatial-orienting-3rf2ow1396.pdf

Gaze Perception Triggers Reflexive Visuospatial Orienting

An enduring controversy in neuroscience concerns how the brain "binds" together separately coded stimulus features to form unitary representations of objects Recent evidence has indicated a close link between this binding process and 40-hertz (gamma-band) oscillations generated by localized neural circuits In a separate line of research, the ability of young infants to perceive objects as unitary and bounded has become a central focus for debates about the mechanisms of perceptual development Here we demonstrate that binding-related 40-hertz oscillations are evident in the infant brain around 8 months of age, which is the same age at which behavioral and event-related potential evidence indicates the onset of perceptual binding of spatially separated static visual features

http://science.sciencemag.org/content/sci/290/5496/1582.full.pdf

Gamma oscillations and object processing in the infant brain.

Segmentation, attention and phenomenal visual objects.

Unilateral brain damage frequently produces “extinction,” in which patients can detect brief single visual stimuli on either side but are unaware of a contralesional stimulus if presented concurrently with an ipsilesional stimulus. Explanations for extinction have invoked deficits in initial processes that operate before the focusing of visual attention or in later attentive stages of vision. Preattentive vision was preserved in a parietally damaged patient, whose extinction was less severe when bilateral stimuli formed a common surface, even if this required visual filling-in to yield illusory Kanizsa figures or completion of partially occluded figures. These results show that parietal extinction arises only after substantial processing has generated visual surfaces, supporting recent claims that visual attention is surface-based.

https://science.sciencemag.org/content/sci/275/5300/671.full.pdf

Preattentive Filling-in of Visual Surfaces in Parietal Extinction

Issues concerning selective attention provoke new questions about visual segmentation, and vice-versa. We illustrate this by describing our recent work on grouping under conditions of inattention, on change blindness for background events and the residual processing of undetected background changes, on modal versus amodal completion in visual search, and the differential effects of these two forms of completion on attentional processes, and on attentional modulation of lateral interactions thought to arise in early visual cortex. Many of these results indicate that segmentation processes substantially constrain attentional processes, but the reverse influence is also apparent, suggesting an interactive architecture. We discuss how the 'proto-objects' revealed by studies of segmentation and attention (i.e. the segmented perceptual units which constrain selectivity) may relate to other object-based notions in cognitive science, and we wrestle with their relation to phenomenal visual awareness.

Greg Davis

Papers

Gaze Perception Triggers Reflexive Visuospatial Orienting

Gamma oscillations and object processing in the infant brain.

Segmentation, attention and phenomenal visual objects.

Preattentive Filling-in of Visual Surfaces in Parietal Extinction

Segmentation, attention and phenomenal visual objects.