To explore the factors limiting the development of visual sensitivity, we constructed an ideal observer model for the infant macaque visual system. We made measurements of retinal morphology in infant and adult macaque monkeys, and used the data in combination with published optical data to formulate the model. We compared the ideal observer’s ability to detect low-contrast gratings presented either in isolation or in spatiotemporal noise with behavioral data obtained under matched conditions. The ideal observer showed some improvement in visual performance up to the age of 4 weeks, but little change thereafter. Behavioral data show extensive changes over the ages 5–50 wk, after the ideal observer’s performance has become asymptotic. We conclude that the development of visual sensitivity in infant monkeys is not limited by changes in the front-end factors captured by the ideal observer model, at least after the age of 5 weeks. Using noise masking, we also estimated the variability of neural processing in comparison with the photon noise-limited ideal. We found that both additive and multiplicative components of this variability are elevated in infant monkeys, and improve (though not to ideal levels) during development. We believe that these changes all reflect maturation of visual processing in cortical circuits, and that no aspect of visual performance in the regime we studied is limited by the properties of the retina and photoreceptors, either in infant or in adult animals.

*Macaca nemestrina*corresponds to about 0.4 × 0.4 deg. For parafoveal and peripheral samples, photographs were taken near the horizontal meridian 0.5, 1, 2, 4 and 8 mm from the foveola. The negatives were scanned into Adobe Photoshop using an Agfa flatbed scanner. The contrast and grayscale were adjusted to give the clearest images. Using the public domain

*NIH Image*program (developed at the U.S. National Institutes of Health and available on the Internet at http://rsb.info.nih.gov/nih-image/), we counted cones in the windows superimposed on the retinal image. From these counts we calculated the cone density/mm

^{2}and the inter-cone spacing by assuming that the cones are arranged in a perfect hexagonal array. We then converted the densities and spacings into units of visual angle, using measurements of eye size and estimates of posterior nodal distance taken from the individuals whose retinas were measured. Finally, we computed the sampling frequency (Nyquist frequency) for each array.

- Generate “signal” and blank stimuli (including masking noise if needed).
- Filter stimuli by optics and sample with photoreceptors, including the effects of the Poisson noise associated with the quantal nature of light — “photon noise” — to give a vector of quantal absorptions by each photoreceptor .
- Decide which stimulus interval contained the signal by choosing the larger of the computed likelihood values.

*σ*= 1.1 deg); this procedure differed from that of, for example, Banks et al. (1987) and Banks and Bennett (1988), whose stimuli contained a fixed number of grating cycles and whose area was therefore inversely proportional to the squared spatial frequency. The patches were presented alone or in the presence of random spatiotemporal broadband binary noise with a pixel size of 0.16 deg. Presentation duration was 250 msec. Space-averaged luminance was 40 cd/m

^{2}. These conditions matched as closely as possible those used in the comparison behavioral experiments (Kiorpes and Movshon, 1998).

Parameter | 1 week | 4 weeks | >24 weeks |
---|---|---|---|

Line spread function width at half height (min arc) | 2.25 | 1.69 | 1.33 |

Pupil diameter (mm) | 4.8 | 5.3 | 6 |

Posterior nodal distance (mm) | 10.91 | 11.84 | 13.52 |

Cone density (cones/mm^{2}) | 37268 | 110374 | 202905 |

Cone array sampling frequency (Nyquist frequency)(c/deg) | 20.4 | 30.9 | 62.6 |

Outer segment diameter (µm) | 1.94 | 2.09 | 1.79 |

Outer segment length (µm) | 13.6 | 31.8 | 40.0 |

Cone quantum efficiency (Q) | 0.162 | 0.339 | 0.406 |

Retinal coverage (C) | 0.127 | 0.437 | 0.59 |

Relative retinal sensitivity (√QC) | 0.293 | 0.787 | 1.0 |

*Methods*. Our measurements of eye size agree well with those of Blakemore and Vital-Durand (1986). Our values of cone density and outer segment dimensions are the means of values from two individuals at the ages of 1 week and 4 weeks, and of one adult retina. The values are in good agreement with the published reports of Packer et al. (1990) and Hendrickson (1992). In contrast to some earlier reports (e.g. Hendrickson, 1992), we found the data from our 1-week and 4-week infants to be very consistent. There is natural variability in the post-conceptional age at which monkeys are born, and for rapidly-developing functions this could lead to high variability across very young individuals. The consistency of our results may be due to our selection of infants for early study from the center of the normal birthweight range for

*M. nemestrina*(500–550 g), with normal neonatal dentition.

*c*is the threshold contrast,

*N*is the energy (squared contrast) of the added noise, and

*k*and

*N*

_{eq}are constants. The pixellation of the binary noise we used concentrated its power in the spatial frequency band of interest but reduced its power at higher spatial frequencies. To represent the effective contrast of the noise at different spatial frequencies, we normalized the Michelson contrast of the noise by the square root of the noise spectral density (Pelli, 1990) at the spatial frequency of the test target. When working in contrast rather than energy units, it is convenient then to take

*N*

_{eqC}= √

*N*

_{eq}(“equivalent noise contrast”; Kiorpes and Movshon, 1998), normalized as described above.

*k*and

*N*

_{eq}has been considered by Pelli (1990; Pelli and Farell, 1999).

*N*

_{eq}is often called “equivalent input noise” or “intrinsic noise”, because in a simple linear system it corresponds to the magnitude of the system’s internal noise in the same units as

*N*, i.e. as if delivered to the system’s input. For an ideal observer,

*N*

_{eq}corresponds to photon noise.

*k*is the system’s internal signal-to-noise ratio (SNR) at threshold, a measure of the statistical efficiency with which the observer can tell signal from noise (whether intrinsic or extrinsic). The quantity

*k*

^{2}, when given as a fraction of the value for an ideal observer, is sometimes termed the observer’s “efficiency” (Pelli and Farell, 1999); we use the term “central efficiency”. Note that the unmasked threshold contrast is given by

*kN*

_{eqC}and therefore depends on both intrinsic noise and central efficiency.

*N*

_{eqC}), as indicated by a green arrow on the abscissa for each masking function. Comparable functions calculated for the ideal observer for the same spatial frequencies at ages of 1, 4, and 24 weeks, and the corresponding values of

*N*

_{eqC}, appear in red in Figure 3. The form of these functions is the same as for real observers, but there are three important differences between the real and the ideal functions. First, as already indicated in Figure 1, the unmasked contrast threshold of the ideal observer is about 2 orders of magnitude lower than that for the real observer (leftmost points on each function). Second, the values of

*N*

_{eqC}for the ideal observer are between 1 and 1.5 orders of magnitude lower than for the real observer (compare the corresponding red and green arrows on the abscissa). Third, all the masking functions for the ideal observer superimpose at high masking contrasts, indicating that central efficiency for all ages and spatial frequencies corresponds to the same ideal signal-noise ratio. The masking functions for the real observer lie above those for the ideal observer at high as well as low masking contrasts, indicating that central efficiency is less than ideal.

*N*

_{eq}measured behaviorally are not dependent solely on the retinal and pre-retinal factors built in to the ideal observer. Thus, despite its putative origin as “input noise”, there must be a substantial contribution of noise within the CNS to these estimates of

*N*

_{eq}(cf. Pelli, 1990, Graham and Hood, 1992; Kortum and Geisler, 1995; Pelli and Farell, 1999; Beckmann and Legge, 2002). Second, the data for real observers all lie above the diagonal, indicating that the observed levels of intrinsic noise do not completely account for contrast thresholds in these animals. At 4 c/deg, the mean central efficiency measured behaviorally (the square of the ratio between the real and ideal contrast thresholds in high noise) was 0.77% in young animals (the uppermost points, ages ≤ 12 wk) and 6.8% in adult animals (the lowermost points, ages ≥ 46 wk), a developmental change of a factor of 9. At 1 c/deg, on the other hand, central efficiency changed less during development: the mean for young animals was 5.8% and for adults was 21%, a change of less than a factor of 4. The difference in adult central efficiency between 1 and 4 c/deg is probably attributable to our decision to use stimuli of the same size at all spatial frequencies — if we had followed the example of Banks et al. (1987) and scaled stimulus size with spatial frequency, the efficiency of adult observers at the two spatial frequencies would have been more similar.

*lower*than in adults (Rust et al., 2002). We therefore suggest that elevated intrinsic noise and decreased central efficiency in young animals reflect immaturities of cortical computation, probably in areas downstream of V1, and not immature input from the visual front end.