Below is the full text of the review of 'Functional specialization in the visual cortex?' . I have added the numbers of the comments in my response at the appropriate position, in square brackets.
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I have read the letter by Harpaz, which was written in response to the paper by Zeki on brain imaging studies of MT. Although there is some formal logic to the criticism, there is, in practice, not much to them.
There are three major criticism:
1) Harpaz argues that the three different class of motion used by Zeki might have different conceptual or emotional significance, which could account for the different patterns of activation. It seems unlikely that MT/MST would be much affected by emotional significance [1], and it is not clear what "conceptual" significance means other than the fact that they are fundamentally different kinds of motion [2] (which is the point of the Zeki paper).
2) Harpaz argues that greater blood flow/oxygenation does not necessarily mean greater involvement in processing a stimulus. He argues that it is the pattern of activity that specifies a stimulus in the cortex and not the overall amount of activity. On the one hand, this argument is not really valid [3]. In the extreme case, switching from auditory to a visual stimulus will cause overall activation to switch from auditory cortex to visual cortex [4]. Although extreme, imaging studies suggest that one gets comparable effects by switching between different visual "submodalities". For example, if cells in a region of the cortex respond only to colored stimuli, this region will indeed show an overall increase in blood cells with colored stimuli compared to an achromatic control [5]. This is the basic idea behind the Zeki study using motion. On the other hand, the criticism in its most general form could be applied to all previous and future studies of brain imaging [6]. What the criticism really means is that a negative result using brain might not mean very much [7], because it is easy to imagine that a given stimulus condition will cause a change in the the pattern of activation in the cortex without causing any change in blood flow. This is why no imaging study, in itself, will ever provide the complete story. Of course, negative results using any one technique are always open to differing interpretations in neuroscience. Zeki, fortunately, has a positive result [8].
3) Harpaz argues that just because one sees local activation caused by viewing a given stimulus, this does not mean that the area activated is specialized for processing that stimulus. An area activated while subjects view biological motion, for example, might be equally activated when subjects view a motion stimulus that was not tested. this is similar to the negative result argument above [9]. The point is that Zeki showed that the three different motion stimuli revealed relative specialization for these three classes of motion. The emphasis is on the relative.
My impression is that Harpaz does not understand that the conclusions of studies in science are not the same thing as logical necessities.