Contrast Enhancement at Borders

Spatial Contrast Enhancement at Borders

Visual spatial contrast is the perceived difference in brightness or color between two or more locations in the visual scene. Perceived contrast is determined by the physical difference in intensity and color between two adjoining areas. Thus, contrast is not [Page 319]a physical quantity but a perceptual comparison between two areas. The neural computation underlying the perception of contrast is partly carried out by lateral inhibition circuits in the visual system. Lateral inhibition is the process by which an excited neuron suppresses the activity of its neighbors across visual space. Lateral inhibition circuits are found in all levels of the visual system (such as in the bipolar and ganglion cells of the retina) as well as in other sensory systems. One perceptual consequence of lateral inhibition is that stimuli to both sides of a luminance border are differentially enhanced in an illusory fashion (as in Figure 1).

Figure 1 Mach Bands

Source: Chevreul, M. E. (1987).

Notes: This Mach band demonstration was originally designed by Chevreul in 1839. Notice how each vertical stripe appears to be lighter on the left than on the right. This illusory effect is caused by contrast enhancement at the borders.

Another corollary of contrast enhancement at borders is that non-borders (such as the interiors of objects) are perceptually suppressed by the same lateral inhibition circuits that enhance the borders.

Indeed, neurons in early visual areas are activated only by the borders (edges or corners) of stimuli, not by their interiors. The process is metabolically efficient in that it reduces the metabolic demand required to respond to the presentation of a stimulus. Only the neurons at the edges of surfaces require energy to respond, whereas the neurons in the interiors do not require as much energy (because they are quiescent). Thus, our visual system sees the edges of surfaces, and then uses this information to create the middles, through a process called filling-in. The neural circuits underlying filling-in have been localized to cortical visual areas beyond area V1 (such as areas V2 and V3).

Temporal Contrast Enhancement at Borders

Contrast is increased at their temporal borders (that is, their onsets and terminations) as well as at the spatial borders of stimuli, also because of lateral inhibition. And just as the interiors of spatial stimuli are suppressed when compared with their borders, so too are the temporal mid-lives of stimuli suppressed when compared with their onset and termination. The result is that the perceived contrast of a stimulus is highest just after it turns on and then again after it turns off. Visual masking (the effect in which the visibility of a target stimulus is reduced by a masking stimulus that does not overlap the target in space or time) occurs perceptually when the neural responses to the target onset or termination are inhibited.

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