| Abney's Law | The principle that the visual effectiveness of a light composed of different wavelengths can be predicted from the sum of the responses to the wavelengths considered separately.
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| action potentials | Brief electrical discharges generated by a neuron and provide a major source of neural communication and computation within the nervous system. See axon and synapse.
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| amacrine cell | A type of cell in the retina that transmits electrical signals from the bipolar cells to the ganglion cells, in the process influencing the rate at which those signals are generated over time. See bipolar cell and horizontal cell.
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| axon | The portion of a neuron over which action potentials are conducted. A group of axons constitute a nerve or a tract. See action potential.
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| bipolar cells | Type of cell in the retina that transforms electrical signals from the photoreceptors into action potentials; these cells play a crucial role in generating ON and OFF responses. Together with amacrine and horizontal cells, bipolar cells form a network that shapes the receptive field properties of ganglion cells. See amacrine cell, horizontal cell.
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| Bloch's Law | The principle that all stimuli in which the product of time and intensity is constant will be equally detectable.
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| brightness | The dimension of visual experience related to the amount of light emitted by an object. See lightness.
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| circadian rhythm | Fluctuation in biological or behavioral activity that coincides with the day/night cycle.
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| crowding effect | The tendency for small letters to be difficult to read when they are in close proximity to one another.
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| dark adaptation | The increase in visual sensitivity that accompanies time in darkness following exposure to light.
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| DoG curve | Difference of gaussian, a category of visual patterns whose limited spatial frequency content makes them useful in the study of spatial vision.
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| duplex | Referring to the co-existence within the eye of two different systems, scotopic and photopic. The scotopic system provides high sensitivity in dim light; the photopic system provides high resolution under daylight conditions.
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| eccentricity | The distance between the center of the retina and the location of the retinal image cast by an object.
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| Hermann grid | A regular, geometric pattern within which illusory spots are seen; the presence and strength of the illusory spots depend on the spacing of the grid's elements.
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| horizontal cell | A type of cell in the retina that modulates the strength of signals generated by neighboring photoreceptors. Together with bipolar and amacrine cells, horizontal cells form a network responsible for shaping the receptive field properties of ganglion cells. See amacrine cell, bipolar cell.
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| K cells | Class of visual cells characterized by small cell bodies; thought to be involved in color vision. See M cells, P cells.
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| lateral inhibition | Antagonistic neural interaction between adjacent regions of a sensory surface, such as the retina.
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| lightness | A perceptual variable that is correlated with intensity of light reflected from a surface.
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| lightness constancy | The tendency for the perceived lightness of an object to remain constant despite variation in the level of its illumination.
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| lightness contrast | Variations in perceived lightness of a surface of invariant intensity, produced by variations in the lightness of regions surrounding that surface.
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| M cells | Retinal ganglion cells characterized by relatively large cell bodies and axons, large receptive fields, strong response to small differences in light levels in receptive field center and surround, and lack of sensitivity to stimulus wavelength. See P cells.
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| Mach bands | Illusory spatial gradations in perceived lightness that occur in the absence of corresponding gradations in the actual spatial distribution of light.
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| mesopic | Referring to vision at levels of illumination at twilight. See photopic, scotopic.
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| microelectrode | A thin wire that can be inserted into brain tissue in order to record action potentials.
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| P cells | Retinal ganglion cells characterized by relatively small cell bodies and axons, small receptive fields, weak response to small differences in light levels in receptive field center and surround, and good sensitivity to stimulus wavelength. See M cells.
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| photon | The smallest unit of light energy.
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| photopic | Referring to vision under daylight levels of illumination. See scotopic.
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| Purkinje shift | Perceptual variation in the relative lightness of different colors as illumination changes from daylight to twilight.
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| receptive field | The region of a sensory surface within which the activity of a sensory neuron can be influenced. In the case of vision, this constitutes a region of the retina which, itself, represents a given region of visual space. In the case of touch, the receptive field corresponds to a delimited region on the skin's surface.
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| resolution | The ability to distinguish spatial details of an object.
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| retinal ganglion cells | Cells within the retina with center/surround receptive field organization; axons of the retinal ganglion cells constitute the optic nerve. See bipolar cell, amacrine cell, horizontal cell.
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| Ricco's Law | The principle that stimuli will be equally detectable if the product of their intensity and area is constant.
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| scotopic | Referring to vision under dim levels of illumination. See photopic.
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| single cell recording | The use of a microelectrode to record the neural activity of individual nerve cells as they respond to stimulation.
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| spatial summation | The process by which neural signals from neighboring retinal areas are combined, thereby increasing sensitivity. See temporal summation.
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| temporal summation | The process by which signals from a neuron or neurons are cumulated over time, thereby increasing sensitivity. See spatial summation.
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| visual acuity | A measure of the smallest detail that a person can resolve visually.
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