Then again, the concept image is a global property, the notion of continuity
over an interval, not continuity at a point. There are strange animals like
f (x) = x (x rational)
1- x (x irrational)
ìíî
which are formally continuous at a single point (in this case at 12
) yet conflict
with all three evoked images mentioned above. (It is not in one piece, nor given
by a single formula, nor has it a smoothly varying graph.)
If a student is presented with one of these exotic creatures and it causes
cognitive conflict with his concept image, then he may be in difficulties. When
the teacher is aware of the possible concept images it may be possible to bring
incorrect images to the surface and, by discussion, rationalise the problem.
However, parts of the concept image may be very strong, especially the idea
that the graph is all in one piece over real intervals. This latter mental picture is,
in fact, correct. Even so, it can still cause problems in the formal theory.
In a mental picture of the intermediate value theorem and the extreme value
theorem, the results are patently obvious, but the proofs of those results using
the concept definition are more subtle. Without entering too deeply into another
area, we briefly mention that current investigations are revealing the extent to
which students have problems with the meaning of “all” and “some” and the
manipulation of quantifiers. This means that many students have great difficulty
(as is well known) with manipulating the definitions of limits and continuity.
They are then in the situation where they may have a strong mental picture yet
the concept definition image is weak. They understand the statements of
theorems as being obvious, but cannot follow the proofs.
In such a situation, students who do succeed become very suspicious of ideas
in analysis. At more advanced levels it becomes far more difficult to visualise
the concepts as mental pictures and they can never be sure of the intuitions
suggested by their concept image which may be now a mixture of strong
concept images having potential conflicts with the concept definition. For
instance, how does one visualise a function which is continuous everywhere but
differentiable nowhere? Or what mental picture can one have of a function f
whose derivatives f ¢ exists yet f ¢ may not be continuous? The mental pictures
which served the students well at an earlier stage may now become an
impediment. Bruner suggested that iconic processing limited ideas and urged a
movement onto the symbolic level. But the student, saddled with his inadequate
concept image, may find such a development difficult to achieve. In these and
other ways, the difficulty of forming an appropriate concept image, and the
coercive effects of an inappropriate one having potential conflicts, can seriously
hinder the development of the formal theory in the mind of the individual
student.