Table 1.

Peeriodic table: elements of perception and illusion.

kinds of phenomena	causes (reception/perception/conception)
bottom-up signals	side-ways rules	top-down knowledge	explicit understanding
nonsense	blindness Loss of signals, or cortical processing damage. Long-term blindness has no sensation—like behind one's head. colour anomaly From loss of colour channels, or spectral shift of cone receptors. There may also be ‘cross-talk’ between channels. channels All perception starts from receptors and neural channels, signalling restricted features of the external world, and states of the organism. Most of the universe is dark matter to the senses.	nonsensical perceptioninappropriate rules Though handled by physical systems, perceptual rules are not laws of physics. They generate perceptions (like Chomskian grammar?) so can generate nonsense, such as paradoxes, from false knowledge or assumptions. When rules are inappropriate, illusions occur with normally functioning physiology. Then explanation must be from rules (or misleading knowledge) and not from the physiology, when this is working normally.	agnosia Lack of visual knowledge. Failure to recognise even familiar things, when visual knowledge not available. change-blindnessirrelevant changes not seen. Perceptual hypotheses may continue until ‘internally’ checked or challenged. inattention-blindness As in conjuring's controlled attention. familiarity blindness Surprise is necessary for information. Low information signals can be ignored as useless.	ignorance Without understanding, the world looks like a conjuring trick. But perceptual experience can be very different from conceptual understanding, and they may conflict. Understanding does not always correct perceptual errors. Many illusions remain in spite of knowing the truth; the truth does not need to be hidden for most experiments on perception, (though explicit knowledge and belief must always be considered.)
context	relative Visual signals code differences, rather than absolute values. Edges and borders carry most information, so are very important. We see brightness, size, distance, etc., by relative judgements, from statistically significant differences. Regions of constant brightness are seen by no change beyond a border.	discrimination Contrast sensitivity is the key to all vision functions. Generally limited by available photons, and neural signal/noise ratio. Discrimination for differences is given by a constant ratio, Weber's Law (ΔI/I=const.). contrast illusions Contrast from surrounding regions may be enhanced by neural interactions: colour constancy, land colours.	clues Visual clues depend on context (like Sherlock Holmes). context-illusions Context can give illusions such as Titchener's circles. (Surrounding dwarfs, make the pyramids look larger!) induced movement Large distant objects are accepted as stationary references—giving induced movement when they move.	meaning Conceptual hypotheses of science are explicit while Perceptual hypotheses are implicit. Context gives meaning for both for perceptions and conceptions. Phenomena cannot ‘speak for themselves’, but need to be interpreted, from knowledge or assumptions, for conceptual significance.
instability	jazzing McKay rays, Op. art jazzing of repeated patterns (stimulate on–off cells with eye tremor.) retinal rivalry Shimmering on polished metal. local drifting At isoluminance. Ouchi illusion. (lack of ’border-locking’?)	grouping Dot patterns group and regroup with Gestalt Laws. Unstable, with inadequate or competing Laws, or Rules: closure, contiguity, common fate, etc. glass effect Random dot pattern, superimposed on itself and slightly displaced, shows lines, or if rotated shows striking circles.	constancy The world generally looks stable, in spite of observer and eye movements. ‘Constancies’ compensate. But when inappropriate, they can generate illusory motion and many other illusions. pseudo parallax Perspective pictures rotate with observer.	conceiving objects Criteria for perceiving and conceiving objects can be very different. Visual objects are not signalled as units, but are brain-constructed, from selected characteristics transmitted by parallel sensory channels, and from past knowledge. Conceptual objects (such as electrons) may be unsensed, but inferred.
confounded ambiguity	distinguishing stimuli Distinguishing differences is limited by neural noise, and lost with overlapping response curves (red+green must look same as monochromatic yellow light, as the R and G responses overlap).	distinguishing objects When their stimuli are the same, different objects must look the same. Ames Room Has the same retinal image as a normal room—so must look the same. But interesting, when objects are inside it.	classifying objects Different kinds of objects are often confounded when not familiar or understood. E.g. fossils, or makes of cars. special knowledge with defined differences is important for classifying.	explaining phenomena As science develops, understanding and appearances separate and often conflict. So classifications change, with new science. This can be circular—as phenomena suggest explanations and explanations interpret phenomena. (Cladistics aims to avoid circularity in evolutionary accounts, by being theory-neutral).
flipping ambiguity	epilepsy The ‘Sacred disease.’ Spontaneous neural activity. Neural nets are dynamic and can be physically unstable, especially when inhibition or negative feed-back fail. migraine Visual disturbances, associated with headache.	figure-ground The most basic decision, is whether there is or is not an object present. This is seen dramatically in flipping figure-ground ambiguity, when the brain cannot make up its mind. Object recognition starts from general rules such as the Gestalt Laws, but when they are inadequate, or conflicting, figure-round is unstable and flip	flipping perceptions Perception flips to alternatives when the brain cannot make up its mind. E.g. Necker Cube, Duck–Rabbit—set by probabilities. hollow face Probabilities normally give stability but can mislead—Hollow face looks convex. stereoscopic vision Resolves ambiguities of distance and form—challenging rules and knowledge.	‘collapsing’ reality Objects do not ‘flip’ to other objects—except in quantum physics. Measurements or perceptions are supposed to ‘collapse’ many possibilities into particular realities. This creation of reality by perception selecting from overlapping possibilities, has been ascribed to consciousness. But this is totally mysterious.
distortion	signal distortions Many visual illusions are due to signalling errors such as cross-talk and lateral inhibition. after-effectsIllusory tilts, curvatures, colours, motion, etc. After-effects may serve to re-calibrate the senses, but can be wrong.	cognitive distortions Inappropriate constancy scaling distortions: Ponzo, Müller–Lyer, Judd, Poggendorff, Hering, Horizontal–Vertical, Harvest Moon, etc. Features signalled as more a distant are expanded, as depth cues set size-scaling to represented distances in pictures. (Scaling is also set by perceived depth, changing with depth-ambiguity.)	anticipation Prediction is essential for cognitive perception—but can mislead size-weight illusion Smaller objects feel heavier than larger objects of the same scale weight. False expectation, as larger objects are usually heavier—setting too much muscle power when actually the same.	reference truths An object cannot itself be distorted, but may differ from accepted references. thus a ruler is bent, or too long or too short by reference to some other ruler, accepted as ‘true’. Reference to non-illusions are essential for measuring illusions; though illusions and errors can show up as internal inconsistencies.
impossible	conflicting signals Neural channels signal various object properties and states of play. Parallel channels can disagree, as when some are adapted differently. Then perceptions may be impossible. The spiral after-effect expands or shrinks, yet without changing size.	impossible objectsThe impossible triangle can exist yet from certain positions it appears impossible. Perceptual hypotheses generated from false assumptions can be paradoxical. The sides of the impossible triangle touch optically at the corners, though some are separated in depth. The false assumption of touching physically generates the paradox.	knowledge conflictsMagritte's painting of the back of a man's head in a mirror, instead of his face. This disturbs, as it is counter to one's implicit visual knowledge of reflections. Conflict of signals with failed predictions, are key to correcting present and evoking new perceptions and, especially, new conceptions.	cognition and physics Though the brain is a physical system, its ‘virtual realities’ are not limited to physics—making it possible to imagine and to see even impossibilities, including paradoxes. The same holds for computer software. Software rules are not laws of physics, and physics is not a guide for cognitive research.
fiction	spurious signals After-images persist, as though from external objects. Projected into space, they appear as external objects though they are retinal ‘photographs’. phi movement Alternating separated lights appear to move, by stimulating normal movement systems, which are tolerant of gaps.	grouping Dots group into object-like shapes, especially with the Gestalt Laws. ghostsKanizsa triangle and many others. Object-shaped gaps can be evidence of nearer occluding objects, here creating fictional surfaces and contours, ‘postulated’ to ‘explain’ the gaps.	phantasmsFaces-in-the-fire; man-in-the-moon; inkblots. These show the creative dynamics of perception, when alternative hypotheses are evoked from minimal evidence. As perception is dynamic, perceptions can break free from stimulus control with a life of their own.	perceptions and conceptions Perceptions are much like predictive hypotheses of science, so may be called ’perceptual hypotheses’, both are derived from interactive experiments; are indirectly related to reality; change with changed knowledge or belief; are subject to errors and illusions—some useful for art, and for investigating perception and mind.