This paper is a theoretical analysis of eidetic imagery, based upon the author's ten-year study of elementary-school-aged children. The presence of eidetic imagery is inferred from reports of persisting visual images of stimuli when they are no longer in view. According to the criteria for differentiating eidetic images from afterimages, eidetic images should occur even when saccadic eye movements are made during exposure to the stimulus; it should be possible to make saccadic eye movements while one is reporting the image without the image also moving; the image should last long, and it should be positive. The criteria for differentiating eidetic images from nonvisual memorial representations include: reports of seeing an image projected onto a surface in space, the consistent use of present tense when reporting images as opposed to past tense when reporting from nonvisual memory, and the ability to superimpose two images and report the composite image.

Eidetic images are only available to a small percentage of children 6–12 years old, and are virtually nonexistent in adults. However, extensive research has failed to demonstrate consistent correlates between the presence of eidetic imagery and any cognitive, intellectual, neurological, or emotional measure. The negative correlation between eidetic imagery and age has prompted hypotheses to explain eidetic imagery as a developmentally less mature memorial representation, which is gradually replaced by more abstract representations as the child acquires abstract thought, reading, and more advanced cognitive abilities. The evidence in the present review casts doubt on this hypothesis on numerous grounds: an extensive longitudinal study over the entire span of elementary school years found that eidetic abilities remain remarkably stable; there is no correlation between eidetic imagery and abstract thinking or reading performance; there is no higher incidence in preschool ages, among retarded or brain-injured subjects, or among illiterate subjects in crosscultural studies.

It is concluded that we should not abandon work on eidetic imagery or simply force it into a preconceived mold of what memory must be, but rather, expand work on the phenomenological indicators of perception and memory.

Recently a number of complex electrophysiological responses to neurotransmitters have been observed that cannot be described as simple excitation or inhibition. These responses are often characterized as modulatory, although there is no consensus on what defines modulation. Morphological studies reveal certain neurotransmitters stored in what might be release sites without synaptic contact. There is no direct evidence for nonsynaptic release from CNS sites, although such release does occur in the periphery and in invertebrates. Nonsynaptic release might provide a basis for diffuse one-cell-to-many communication, but it might also simply be a means of sending the transmitter to a broader area of a single neuron than occurs in typical synapses. Several kinds of macromolecules have been found to be transported in a retrograde direction – and in some cases transsynaptically. There have been suggestions that some neurons may release more than one type of transmitter. Particularly intriguing is the possibility of release of substances that modulate actions of a primary transmitter. Taken together this range of evidence suggests that neurons may use a variety of forms of molecular communication in addition to traditionally described synaptic transmission.

Several authors have suggested modes of communication distinct from classical synaptic transmission and have classified released substances using terms such as neurohumor, neurohormone, neuroregulator, and modulator. These suggestions have the heuristic value of drawing together diverse kinds of data, but it remains to be established that the pieces fit together in that fashion – for example, that complex electrophysiological effects are associated with substances released nonsynaptically. In order to reduce confusion, a flexible, generic approach to nomenclature for substances released from neurons and for hypothetical modes of communication is recommended. Some behavioral implications of nonconventional transmission are considered.

Drinking and thirst-motivated behaviour have traditionally been explained in terms of the rather simple concept of homeostasis. A homeostatic mechanism readily accounts for responses to acute changes in body-fluid levels. However, there are other factors regulating intake, for example, cues associated with eating, which interact with the time elapsed since last drinking and the availability of water. Future dehydration is avoided by behavioural hysteresis; a sudden reduction in fluid needs is not matched by an equivalent reduction in fluid intake. Another factor not explicable by traditional homeostasis is that, in general, drinking cannot be suppressed by water infusion. Nor are there rigid target values for body-fluid levels independent of the cost of obtaining water; when water is hard to get, a relatively low body fluid level is maintained, thus minimizing loss. On the basis of the results conflicting with traditional homeostatic theory, this paper tentatively suggests certain modifications toward a more realistic model of fluid ingestion.