The Role of Visual Input in Setting up Spatial Filters in the Human Visual System

Abstract At birth the visual system can detect only big objects that are high in contrast. The adult visual system can resolve, with high precision, small objects that are in low contrast. We present a series of studies that provide insight into the role of visual input in the development of the neural architecture underlying human spatial vision. We conclude that the limits to spatial vision during normal development appear to result primarily from retinal immaturities. However, in the absence of early visual experience, the limits to spatial vision appear to be mainly cortical in origin. The early visual input--from the low spatial frequencies to which the infant's vision is known to be limited--sets up the neural architecture that will eventually become ﬁne-tuned to high spatial frequencies. Introduction Spatial vision is the aspect of visual processing that enables us to detect and perceive objects, patterns, and textures of different sizes against a background. Two main factors constrain the resolution of our spatial vision. First, the size of objects limits their detectability. That is, the smaller an object is, the more difﬁcult it is to detect. For example, it is easier to detect the full moon in the sky than it is to detect the small adjacent stars. The second factor limiting spatial vision is contrast, the difference in luminance between objects and their background. High contrast objects are easier to detect than low contrast objects. For example, it is easier to detect the full moon that is bright, against the dark sky than it is to detect the new moon that is dark, against an equally dark sky. The minimum amount of contrast necessary to resolve an object varies with the size of the object. The human visual system is composed of overlapping spatial ﬁlters tuned to different bands of spatial frequency1. The spatial contrast sensitivity function provides a detailed characterization of the limits of spatial vision and its underlying spatial ﬁlters. Spatial contrast sensitivity is an index of the minimum amount of contrast required for one to detect sinusoidal gratings ranging from very low to very high spatial frequencies. The spatial contrast sensitivity function of an adult is characterized by a peak of sensitivity at around 3 – 5 cycles per degree (cpd), with a sharp decline in contrast sensitivity at higher spatial frequencies and a gradual drop in contrast sensitivity at lower spatial frequencies. A typical function for normal adults is shown by the upper curve in Figure 1. To gain insight into the origins of the spatial ﬁlters underlying human spatial vision, the present report reviews a series of studies from our and other labs that investigated the inﬂuence of visual input on development of spatial contrast sensitivity. Speciﬁcally we document the ﬁnal development of spatial contrast sensitivity during childhood, we describe the effects of monocular and binocular deprivation during early childhood on subsequent spatial contrast sensitivity, and we examine how the early deprivation affects the tuning of the spatial ﬁlters by visual input later during childhood.