Detecting a deviation from a schema is most often perceptual. The process of accounting for the deviation, on the other hand, is not mandatory and might require post-perceptual processing. I agree with Laurent (2002) that recognizing as left-right inversion the deviation of a mirror view from the schema of frontal encounter might sometimes require post-perceptual processing, especially when it concerns the viewer's own body, let alone when it is done for the first time.
2. I tend to agree. In the target article I have been somewhat elliptic with respect to this question, mainly because I thought discussing it could distract readers from the major issue. Accordingly, I have used disjunctive phrases like "you can harness either your imagery or your perceptual schemata" (Navon, 2001, paragraph 14). Now that the subject is raised, I can be a little more specific.
3. In the times that the "schema" concept made its first steps in psychology, Woodworth (1938) coined the term "schema with corrections" to refer to the idea that in human perception any individual stimulus is perceived by mapping it to one of a number of schemata as well as noting its distinctions with respect to the general schema. Evidently, identifying the schema is the main task, hence gets first priority. The process that detects deviations or distortions is logically subjugated, hence probably comes later. Deviations may be glaring, small, or hard to notice. That must affect the nature of both the process that detects them and the process that determines what they are.
4. I think it is common wisdom that unless the deviation is unnoticeable, just noting its existence is probably a direct product or by-product of perceptual processing. The phenomenal experience could in some cases be no more than "something is not right here" or "something is a bit unusual", which attests that the deviation has been noted albeit not yet accounted for.
5. The case is probably different with trying to ascertain what the nature of the deviation is. Quite often, that is not an easy task, not one that a busy information processing system would undertake without a voluntary act of commitment that is typically ascribed to post-perceptual processing. For example, when we meet a work mate who has shaved his moustache, we immediately sense the change in appearance, yet it might take some time until we spot what it is specifically due to. When we see an impossible object of the sort illustrated by Penrose & Penrose (1958), it might take even more time until we realize that there is no familiar transformation to rectify the distortion.
6. Recognizing left-right inversion of an upright stimulus is usually not very difficult. Cooper & Shepard (1973) found that it takes no more than a few hundred msec to manually respond to it, when the stimuli are familiar characters. I guess nobody would be willing to bet that recognizing such an inversion for what it is takes post-perceptual processing. On the other hand, recognizing the inversion of one's own body is certainly harder, not just because of its overall lateral symmetry, but primarily because we rarely have the opportunity to frontally encounter it (cf Navon, 2001, paragraph 14). No doubt, a person who recognizes her inversion for the first time (perhaps because she has never before noticed the deviation) must be using some reasoning processes that are extra-perceptual. However, most of us have been doing it several times before. When doing it in the hundredth time, are we still using the same slow inferential processes as in the first time? I doubt it. But my hunch may be no better than Laurent's.
Cooper, L. A. & Shepard, R. N. (1973). Chronometric studies of the rotation of mental images. In W. G. Chase (Ed.), Visual information processing (pp. 75-176). New York: Academic Press.
Laurent, E. (2002). From Piaget's assimilating mind to Navon's Clockland: Towards a categorical account of mirror vision: Commentary on Navon on mirror reversal. Psycoloquy, 13(005). http://www.cogsci.soton.ac.uk/psyc-bin/newpsy?13.005
Navon, D. (2001). The puzzle of mirror reversal: A view from Clockland. Psycoloquy, 12(017). http://www.cogsci.soton.ac.uk/cgi/psyc/newpsy?12.017
Penrose, L.S., & Penrose, R. (1958). Impossible objects: A special type of illusion. British Journal of Psychology, 49, 31.
Woodsworth, R. (1938). Experimental psychology. Holt, Reinhart and Winston.