October 1961
A List of the Sources of Potential Stimulation in the Terrestrial Environment – the Ordinary Causes of Actual Stimulation
J. J. Gibson, Cornell University
The World Wide Web distribution of James Gibson’s “Purple Perils” is for scholarly use with the understanding that Gibson did not intend them for publication. References to these essays must cite them explicitly as unpublished manuscripts. Copies may be circulated if this statement is included on each copy.
A. Surfaces which may come into mechanical contact with an animal (“tangible” surfaces)
1. The earth (surface of support). Exerts local incessant pressure on the skin of an individual in a direction opposite to gravity, i.e., “upward”.
2. a. Stationary bodies during locomotion of an animal (obstacles). Skin depression due to collision.
b. Stationary bodies during manipulation (objects). Skin depression due to “touching”. The pattern and rate of skin depression is specific to the physical properties of the object (rigid or plastic, rough or smooth, large or small).
3. Moving bodies with a stationary animal. Skin depression due to collision.
4. a. Another animal touching the individual (aggressive, sexual or maternal contact).
b. The individual touching another animal (including clasping, clinging, etc.)
B. Mechanical events not in contact with an animal, causing field of sound waves in air (“audible” events). Such wave-trains emanate from a center of vibration, the field persisting as long as the event persists. Wave-amplitude decreases with distance from the center. The wave train at any point in the field specifies the mechanical nature of the event.
1. Natural events.
Examples = scraping, rubbing, breaking, colliding, flowing, etc. waterfall, rolling stone, whistling wind, rustling leaves, crackling fire, footsteps, thunderclap, etc.
2. Vocal events (many animals broadcast their species, identity, and emotional state by vibratory organs, the sound waves usually having some tonal component. The wave-train at any point in the field is specific to the action of the vocal organ).
Examples = cries and calls (birds), squeals (rodents), growls, roars, barking, purring (carnivores) chattering, screeching, screaming (primates) wailing, laughing, crying (man), speaking (man).
3. Non-vocal animate events (animals and men often broadcast noises as a consequence of their behavior, or behave so as to make sounds).
Examples = sounds of locomotion (horses’ hooves, insects’ wings), sounds of tool using, hand-clapping, pounding, humming, musical production from instruments, (Note that “musical” sound usually consist of simple acoustic variables of the wave-train, such as frequency, amplitude, tone, rhythm, etc.)
4. Artificial sound-reproduction (devices manufactured by man to reproduce all the above).
C. Gaseous diffusion causing odor-fields in the air (“odorous” events). Organisms and organic substances normally emit volatile chemical compounds, specific to the organism, which travel outward by diffusion and air-currents until diluted in the air. Propagation is slow, but emission is continuous.
1. Odor fields of plants (the species and state of a plant is broadcast by odor, e.g., its edibility or decomposition).
Examples = flower scents, fruit scents, “essences”, etc.
2. Odor fields of animals (species of animal, individual, and state of the individual are all specified. e.g., alive or dead, male or female, young or old, etc.)
3. Traces of animals and waste-products of animals (animals not only broadcast their identity by odor, but also leave odorous traces of themselves on the ground, i.e., chemical trials).
4. Oxidation of plant substances (e.g., burning).
D. Radiating bodies causing fields of electromagnetic waves of light and heat (“luminous” events). These fields of energy are emitted by hot substances. The wavelength distribution (spectrum) of the radiation is specific to the chemical composition and temperature of the substance. Since propagation is rectilinear, the direction of the center of the field is exactly specified at any point in the field. Hence a photo-sensitive organism can orient itself to the source.
1. The sun in the sky (extra-terrestrial)
2. Fires (terrestrial)
3. Artificial radiators (filaments, gas-tubes, heat sources) (Note that there are natural and artificial sources which broadcast all wavelengths up to radio waves, but these are not sources of stimulation for animals).
E. Scatter-reflecting surfaces causing projective fields in an illuminated medium (“visible” surfaces and events). Except for mirrors, all physical surfaces absorb a good part of the incident light and reflect the remainder in all directions. Different bits of the surface have different reflectances in accordance with the texture of the surface. Consequently a terrestrial surface broadcasts its texture and also its pigmentation, its layout, and its motion to very great distances in a medium of clear air. (The propagation of light by scattered reflection, as contrasted with propagation by radiation, yields the introjection of rays to a point as contrasted with the projection of rays from a point. Hence arises the fact of an ambient optic array at any locus in the world Ð the actual stimulus for visual perception).
1. Physically textured surfaces (rock, leaves, bark, flowers, fruits, water, fur, feathers, skin, etc.) faithfully project their texture-patterns so that in the corresponding sector of an optic array the optical texture is specific to the physical texture (the “form” of the texture is independent of perspective).
2. Partially reflecting surfaces (as above) in a given illumination will project their peculiar reflection-spectra relative to the spectrum of the illumination so that in the corresponding sector of an optic array the relative discrepancy will be specific to the physical albedo and pigment-chemistry of the surface (the form of this discrepancy is independent of the amount and kind of illumination, so long as it is not monochromatic).
3. The geometrical layout of a surface (edges, corners, curvatures, convexities, concavities) is projected so that in the corresponding sector of an optic array the optical form is related to the physical layout by the laws of perspective. The optical form is not specific to the physical “form” (if there is such a thing) but its properties are specific to corresponding properties of physical layout.
4. The transpositions, rotations, and deformations of a rigid or elastic surface (its modes of physical motion) are projected so that in an optic array the transformation is quite specific to the mode of physical motion. The optical transformations arising from rigid motions are mathematically different from those arising from non-rigid motions.
Examples = falling, rotating, or rolling object (rigid) flow of a liquid or viscous substance (non-rigid) movements of animals (non-rigid animate bodies).
5. Textures, colors, forms and motions imposed on a flat surface by human artifice (pictures, moving pictures, or graphic symbols). A common source of stimulation for human animals is a surrogate for an object, i.e., a picture , or a written substitute for a spoken substitute for something.