What determines the perceived brightness of objects? Luminance is a physical and objective measure of the intensity of light. The sensation elicited by different luminances is called brightness. However brightness is a subjective measure as it is the perceived amount of light emanating from an object. It may seem logical to expect that luminance and brightness are directly proportional and that two objects that reflect the same amount of physical light into the eye will look the same brightness.However, as this essay will discuss, the apparent brightness of objects is not entirely dependent upon the amount of light received from them and other factors are influential in the way we perceive brightness.
Our perception of the brightness of objects often depends more on the luminance of adjacent objects and backgrounds than on the actual luminance of the object itse...
lf. Two surfaces reflecting the same physical amount of light to the eyes typically look differently bright if the surfaces are observed in surrounds that are themselves returning different amounts of light.This phenomenon is called simultaneous brightness contrast. This effect can be seen when two squares with exactly the same physical brightness are each surrounded by a larger square of different brightness. The square on the dark background appears lighter than the square on the light background. This can be seen in figure 1 below.
This effect can be explained by the centre/surround organisation of retinal ganglion cells.The organisation means that the response of the ganglion cell to stimulation of one portion of its receptive field, (the area to which a ganglion cell is sensitive), can be modified by stimulation of a neighbouring area. This interaction between
antagonistic regions is caller lateral inhibition. If an ON-centre receptive field, that is the centre is stimulated by light and the surround is inhibited by light, is placed over the left square, the light in the surround produces an inhibitory mechanism which reduces the neural response rate of the receptors exposed to the central square, making it appear dimmer.
In the right square, the surround exposed to the dark background is less stimulated and therefore the central square appears brighter as it is undergoing less inhibition even though the amount of stimulation from the centre square is the same. Other evidence that the perceived brightness over regions of a surface is related to activity in retinal ganglion cells is that of the Hermann Grid (see figure 2). Here you can see illusory grey spots in the intersections between the horizontal and vertical white stripes.Using an ON-centre receptive field this can be explained by the fact that the cell whose receptive field is centred on the intersection will respond less than the cell whose receptive field is centred between intersections as the cell on the intersection will have more light in its surround and therefore have reduced activity so one will experience dimming at such locales, that is to say the grey spots. The perception of the brightness of objects is also determined by contrast boundaries the object may posses and the tendency of the visual system to emphasise borders.
Figure 3 shows a uniform dark area and a uniform light area with an intermediate zone that gradually changes from dark to light. However a gradual change in brightness flanked by to uniform areas is not seen. Instead
two bands are visible, one is darker than any other part of the figure and one is brighter. This can also be explained in terms of lateral inhibition.
Using on ON-response receptive field, the centre is an excitatory area and the surround an inhibitory area.The receptive fields in the uniformly white and uniformly black areas receive about the same stimulation in their excitatory centres and inhibitory surrounds. Therefore the centre excitations are in balance with the surround inhibitions. The receptive field over the bright Mach Band gives a stronger response in the centre because part of the surround is in the darker area.
Therefore it receives less inhibition from the surround than did the centre at the extreme left and right ends. The receptive field over the dark band receives more surround inhibition because part of the surround is in the brighter area.Therefore, the excitatory response is less and this results in our seeing that the area as darker. Therefore from the example above it seems that the significance of edges and contours and the heightened responsiveness of the retina to contrast boundaries and borders seem to determine how we perceive the brightness of objects. However the physiology of the retina implies that such inhibitory effects should take place over a very limited distance and that objects relatively far away from each other should not be affected.As recent studies show this is not the case (Arend and Goldstein, 1987).
If we have a pattern of squares of varying degrees of lightness, it has been observed that introducing a very light one decreases the apparent brightness of all the others, even if they are a long
distance away. Therefore it seems other factors are also involved in the perception of an object’s brightness. Bruno, Bernardis and Schirillo (1997) suggested a cognitive global mechanism called brightness anchoring could explain this effect.It suggests that the highest luminance in a pattern tends to appear white and serves as a standard by which all other luminance is judged.
When the highest luminance increases, the standard against which the others are judged is raised, and all other surfaces in the scene appear to be darker, not because they have changed, but because they are now darker with respect to the highest. They imply that cognitive factors are important in the perception of brightness. There are other contradictions to the standard simultaneous-brightness-contrast effect.For example, as first shown by the 19th-century physicist Wilhelm von Bezold, an object surrounded by territory of predominantly higher luminance can, under the right circumstances, look brighter than the same target surrounded by territory of lower average luminance. This can be seen in ‘The white illusion’ (figure 4).
Here the grey under the white stripes appears to be brighter than the grey under the black stripes. This is opposite of what the retinal-firing-rate explanation of brightness predicts. Based on the action of lateral inhibition the white stripes should darken the grey rather than lighten it.Shapley and Reid (1985) named this phenomenon ‘brightness assimilation’.
In this effect the brightness of the object seems to be determined by the assumptions that the observer makes about the nature of the scene and even the way in which regions of the visual field appear to be arranged (Agostini and Proffitt 1993). The grey under the white stripes is
perceived as a transparent object on top of a background but the grey under the black stripes looks like an object behind bars so appears less bright. Here cognitive factors seem important as it is how the observer interprets the scene that determines the brightness of the objects.Another example where high level cognitive processing may alter or override the effects of lateral inhibition can be seen in figure 5. Lateral inhibition would lead us to expect that the half of the ring on the white background will be seen as darker than the half on the black background.
But the ring appears to be uniform in brightness. Koffka (1935), claims that because the visual system sees no edges we make assumptions that the object is uniform and this biases the perception of the ring towards a single coloured object and thus no contrasts effects are seen and both sides of the ring appear to be of an ‘average’ grey.There has been some evidence that the way we distribute our attention over a pattern also influences the way we perceive brightness of an object. In general, the part of the pattern that is viewed as the object, rather than the background shows greater brightness contrast ( Coren, 1969). The background regions that are not attended to then show brightness assimilation (Festinger et al 1970).
Brightness perception is also affected not only by stimuli that lie adjacent to the object in space but by events that occur immediately prior in time.If an observer is exposed to a stimulus with a specific attribute for example a spatial frequency of 6 cycles per degree for a long period of
time, the specific group of neurons that respond to that particular frequency will eventually become too fatigued to respond vigorously any longer. This fatigue might last for a minute or two after the exposure to the adapting stimulus and therefore the particular group of spatial frequency channels will be temporarily disabled. This means that there is a depression in sensitivity around the adapted spatial frequency and gratings in this range of spatial frequencies now appear to be less bright.In conclusion there is not one factor alone that determines the perceived brightness of objects.
Lateral inhibition from areas surrounding the object in question plays a part in determining the brightness of the surface and the heightened responsiveness of the retina to contrast boundaries and borders are important in influencing the perception of an object’s brightness. However higher processing of the image also plays a role in the perception of brightness. Cognitive effects such as anchoring and distribution of attention can also explain some brightness phenomena.The way in which a person interprets the scene also contributes to the way the brightness of the object is perceived. These contexts effects are affected by temporal and adaptation effects as well which contribute to the perception of brightness. References Agostini, T.
, and Proffitt, D. R. (1993). Perceptual organisation evokes simultaneous lightness contrast. Perception, 22, 263-272. Arend, L.
E. , and Goldstein, R. (1987). Simultaneous constancy, lightness, brightness. J.
Opt. Soc. Armer. A, 4, 2281-2285. (12).
Bruno, N. , Bernardis, P. , and Schirillo, J. (1997).
Lightness, equivalent backgrounds, and anchoring.Perception and Psychophysics, 59, 643-654. Coren, S. (1969).
Brightness contrast as a function of figure-ground relations. Journal of Experimental Psychology,
80, 517-524. Festinger, L. , Coren, S. , and Rivers, G.
(1970). The effect of attention on brightness contrast and assimilation. American Journal of Psychology, 83, 189-207. Koffka, K. (1935). Principles of Gestalt psychology.
New York: Harcourt, Brace and World. Shapley, R. , and Reid, R. C. (1985).
Contrast and assimilation in the perception of brightness. Proceedings of the National Academy of Science, USA, 82, 5983-5986. .
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