related texts back to overview
In higher level, the stimuli vary on many dimensions, e.g how realistic they look, how interesting they are, how attractive they are. Thus any difference in activity may be attributed to differences in the significance of the stimuli, rather than to the way they are processed. These effects must be controlled in some way, before it can be concluded that the activity seen is wholly a result of low-level processes.
Howard et al [1] measured differential metabolic activity, which presumably also means differential neuronal activity. However, differential neuronal activity does not entail functional significance. In other words, the processing of the input is not necessarily done where the highest differential activity is seen.The equivalence of differential activity and functional significance is widely assumed, presumably because it simplifies the analysis. However, there is no evidence for it at the resolution in which Howard et al[1] present their data, and it is logically implausible. This is because the bulk of the information in the brain is not coded in level of activity (how many neurons are active in a specific region), but in the pattern of activity (which neurons are active and when). Thus processing input (or any other process) means changes in pattern of activity, not level of activity. Hence, regions where the processing take place does not necessarily appear as peaks of differential activity. On the other hand, peaks of differential activity can arise from other reasons, including (a) high-level processes, (b) direct effect of the input (rather than processing it), (c) completely irrelevant effects. Thus the equivalence cannot be assumed, and must be supported by evidence.
As the authors themselves point out, the area of differential activity in their data seems to correlate with the size of the visual input. On the other, it does not seem to correlate to the amount of processing that is required for each stimulus. Thus the data itself seems to hint that the differential activity in this case is a direct result of the visual input, rather than anything to do with processing it.
`Specialization`, when applied to body organs, normally means some physiological modification. Even if it will be shown that specific kind of stimuli are processed in a specific region, it does not show that this region is modified in a special way for processing this kind of stimuli. Thus localization is not equivalent to specialization.It can be claimed that localization is a weak form of specialization, but this is a word game that would just lead to confusion. Moreover, it is clearly not what Howard et al [1] mean, as they claim in their conclusions that "there is an extraordinary degree of functional specialization in the visual cortex".
The patterns of differential activity seen by Howard et al [1] do not match any patterns seen in any other investigation, using any method, including, as the authors themselves say, their own previous studies. This argues against specialization. As the argument for specialization is invalid for any of the three flaws listed above, their data does not support specialization.
In summary: The data presented by Howard et al[1] shows some degree of localization of differential activity when subjects are presented with different stimuli. It does not show that this localization is not a result of higher level effects, it does not show that this activity corresponds to functional significance, and it does not show any evidence for specialization, as opposed to localization.
Yehouda Harpaz Harlequin Ltd. Barrington Hall, Barrington Cambridge CB2 5RG UK yh@maldoo.comReferences:
1. Howard RJ, Brammer I, Wright I, Woodruff PW, Bullmore ET, Zeki S: A direct demonstration of functional specialization within motion-related visual and stodgier cortex of the human brain. Current Biology 1996, 6:1015-1019.
------------------------------------------------------------------------------------
Yehouda Harpaz