Active Maintenance and The Visual Refresh Rate

Does the visual system have a "refresh rate," similar to the 24 frames per second of traditional film, or the 60 Hz refresh rate of many computer monitors? In one sense this is an obvious question: film appears continuous precisely because we have a visual refresh rate somewhat below 25 fps. You can also see this in the "strobing" effect of waving your hand very fast in front of a CRT monitor: the distance between successive images of your hand corresponds to the velocity of your hand divided by the difference between the refresh rate of your monitor and the refresh rate of your visual system.

While interesting superficially, this may not seem incredibly important for something like working memory, or other components of higher cognition. But according to one view of working memory, "refresh rate" may actually be critical.

In Kosslyn's cognitive architecture, for example, there is no single component corresponding to Baddeley's "visuo-spatial sketchpad." (The sketchpad is a purely visual storage space in which humans can perform tasks like mental rotation and size comparison, and is known to be distinct from another storage space for auditory information known as the articulatory loop.) Instead of considering the sketchpad a single architectural component, it can be viewed as an emergent property of the system in which the sketchpad is a rapid cycling of Kosslyn's architectural loop: from visual buffer to attentional window, to object/spatial encoding, to associative memory, to information lookup, to attention shifting, and back to attentional window, ad infinitum.

Indeed, there are several reasons to suspect that such "rapid cycling" may be responsible for visual working memory. In visual tasks requiring adaptive responses from one trial to the next (such as the Dimensional Change Card Sort, or DCCS), other measures of processing speed highly correlate with successful switching performance. And neural network models of good DCCS performance consist of a layer corresponding to prefrontal cortex, in which artificial neurons project back to themselves in an explicit loop of recurrent connections. Further, we see an analogous phenomenon in measures of verbal working memory: your digit span is highly correlated with how quickly you can subvocally rehearse the digits, just as you would expect if the auditory system works on the basis of a similar loop.

Of course, it's possible that the "retinal refresh rate" and the "visual sketchpad refresh rate" are two distinct quantities, and if visual working memory span measures (such as mental rotation, size comparisons, and attentional blink measures) do not correlate with measures of retinal refresh rate, this would appear to be the case. But this is still an empirical question.

Supposing for the moment that both visual and verbal working memory are implemented in prefrontal cortex as a kind of cognitive loop with a certain refresh rate, then one particularly interesting phenomenon is the developmental emergence of verbal rehearsal of visual information at around 7 years. Why does it take so long for kids to be able to verbally rehearse information that was originally presented to them visually? Perhaps there is a slow-developing gateway between the articulatory and the visual loops in prefrontal cortex. Corresponding with the hypothesis that both the cognitive loop and the gateway between loops arises from a willful process of active maintenance, some work has shown that kids can be trained to verbally rehearse visual material before 7 years of age. What late-developing prefrontal gating mechanism could accomplish this transfer of information between the two modalities, and can it be modeled with neural networks?


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