The visual system is not fully developed at birth
1 and relies on normal levels of balanced visual input during postnatal maturation.
2,3 For example, imbalanced visual input from monocular deprivation during infancy due to congenital cataract, anisometropia, and strabismus results in deficits in spatial vision (e.g., contrast sensitivity, acuity) and motion perception (e.g., direction discrimination) that persist throughout life in both the deprived and nondeprived eye.
4–12 Considering that motion stimuli intrinsically possess spatial attributes, it is difficult to determine which aspect of vision underlies motion processing deficits in these disorders. In fact, one study has shown that motion processing deficits in amblyopia are related to contrast sensitivity impairments rather than poor local motion perception.
13 To determine how spatial vision and motion perception develop following visual deprivation, it may be helpful to examine populations that are impaired on only one of these aspects of vision, specifically a monocularly deprived population with intact contrast sensitivity.
One such population does exist, those who have experienced early monocular enucleation (ME; surgical removal of one eye) due to cancer of the retina (retinoblastoma), a disease with onset generally before 2 years of age. Enucleation provides a valuable model for examining the effects of early monocular deprivation on visual system development since it results in a more complete form of monocular deprivation compared with other forms, such as strabismus or amblyopia, where disrupted patterned visual input is received by the deprived eye. Enucleated individuals demonstrate intact spatial vision compared with binocular viewing (BV) controls for contrast sensitivity,
14 luminance-defined (LD) contrast letter acuity,
7,15,16 and LD global shape discrimination.
17 Further, enucleated individuals actually have enhanced ability for the majority of these tasks compared with controls viewing monocularly with a patch over one eye.
7,15–17 Moreover, participants who underwent enucleation before 4 years of age have enhanced contrast sensitivity at 4 cpd compared with BV controls.
14 These data suggest cortical reorganization, which may compensate for the early loss of one eye.
18,19 It is important to note, however, that enucleated individuals exhibit mild deficits in face perception, suggesting that higher level spatial vision is somewhat disrupted in this group.
20
Despite the generally intact or enhanced spatial vision ability following ME, small motion perception deficits are found for motion-defined form
21 and the perception of motion in depth.
22 This dissociation in visual ability suggests a greater vulnerability during atypical postnatal visual development of cortical regions associated with motion processing, likely the middle temporal/medial superior temporal (MT/MST) complex (MT+).
23 However, not all aspects of motion are disrupted, enucleated participants demonstrate intact relative velocity detection
24 and direction discrimination for coherent motion
21 compared with controls. Binocular, more than monocular, congenital cataracts (corrected during the first year of life) disrupt global motion perception, suggesting that normal visual input from one eye spares some vision.
9 Thus, normal visual input from the remaining eye in the enucleated population may result in a relative sparing of motion perception for some tasks, but not others. Nonetheless, response asymmetries have been shown in enucleated individuals favoring lower over upper hemifield motion for relative velocity discrimination,
24 as well as nasalward over temporalward motion for coherent motion discrimination
21 and for eye movement responses to full field motion, specifically optokinetic nystagmus (OKN).
25,26 The OKN asymmetry indicates a disruption in the earlier developing subcortical regions that mediate OKN (e.g., nucleus of the optic tract [NOT]), whereas motion asymmetries point to a higher level cortical disruption and are consistent with evidence for an earlier maturation period for the processing of nasalward motion.
27,28
Given the inconsistent nature of previous findings, it is worthwhile to further examine the effects of ME on motion perception development. One aspect of motion processing not yet studied in this population, the perception of speed, has real world applications such as playing sports, crossing the road, and driving. While research implicates visual areas such as cortical area MT in the coding of speed perception,
29,30 other studies suggest extrastriate visual areas V3 and V3a are also involved.
31,32 Further, speed processing occurs in cortical regions distinct from those for processing coherent motion.
32 The development of speed perception appears to have a number of sensitive periods with fast speeds (e.g., 6°/s and 9°/s) maturing earlier than slow speeds (e.g., 1.5°/s).
33–37 Consistent with developmental data, individuals who experienced binocular and monocular congenital cataracts exhibit stronger deficits in direction discrimination for slow compared with fast speeds,
11 indicating an earlier critical period for the development of neural regions sensitive to speeds that have not yet reached their optimal sensitivity.
We compare speed discrimination and luminance contrast detection and discrimination performance of a group of early ME participants to that of controls viewing binocularly and monocularly. We predict speed discrimination deficits as well as the presence of nasotemporal asymmetries in the enucleated group compared with controls, similar to disruptions observed in other tasks of motion processing with this population.
21 We also predict that these impairments will be more pronounced for slow than high speeds, consistent with developmental
33–37 and cataract
11 research. However, since some aspects of motion processing are not impaired in enucleated participants,
21,24 we may find no speed perception deficits. For luminance contrast perception, we predict intact performance compared with controls, consistent with previous findings.
14–17 Finally, we correlate speed and luminance contrast perception performance within individuals to determine whether spatial vision and motion perception development are affected independently by enucleation as the previously described dissociation in visual ability would suggest. Data from these experiments will contribute to our understanding of the role that binocular vision plays in visual system development.