Two types of sensory comparison J Mollon1, M Danilova2 |
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1Department of Experimental Psychology, University of Cambridge, United Kingdom |
| Sensory systems are largely designed to compare concurrent or consecutive inputs. Thus an edge is identified by comparing the illumination falling on adjacent retinal regions; and a given chromaticity is judged by comparison with other chromaticities in the nearby field. We suggest, however, that there exist two different types of sensory comparison, with different psychophysical properties and different neural bases. In the case of some comparisons, the precision of discrimination deteriorates rapidly as the spatial separation of the discriminanda increases. Examples are judgements of luminance or of binocular stereopsis (e.g. Marlow & Gillam, Perception, 2011). For other attributes, however, the differential threshold is constant as the targets are increasingly separated in the visual field (Danilova & Mollon, Perception, 2003). Comparisons of the first type are likely to depend on local, hard-wired comparator neurons. A paradigmatic example of such a comparator is a retinal ganglion cell with an excitatory centre and inhibitory surround. But hard-wired comparator neurons are unlikely to account for comparisons over a distance. As an alternative, we raise the possibility of a 'cerebral bus', which, like the man-made Internet, avoids the need for hard-wired connections between every transmitter and every receiver. |
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